Tuatara: Volume 32, April 1993
A Review of Interaction between Naturalised Woody Plants and Indigenous Vegetation in New Zealand
A Review of Interaction between Naturalised Woody Plants and Indigenous Vegetation in New Zealand.
The ecology of common woody plants naturalised in New Zealand is reviewed from the point of view of successional interactions between these plants and the indigenous flora. The reduction of fires, often associated with plantation forestry, has allowed pioneer stands of naturalised woody plants to grow to closed stands and to open up enough for indigenous plants to establish, at least in higher rainfall areas.
The naturalised plants are trees, shrubs and vines, grouped below in accordance with their ability to encourage indigenous trees and shrubs.
Groups 1–5, in higher precipitation areas, and often close to indigenous forest remnants are:
1: Pinus radiata, Ulex europaeus, Buddleja davidii,
Cytisus scoparius, Crataegus monogyna.
These plants, all light-demanding, initiate successions containing mesophyll (broadleaf) shrubs and small trees. The earlier indigenous nanophyll(small-leaved) shrub and small tree stages of Leptospermum scoparium and Kunzea ericoides are thus eliminated from successions started by naturalised woody plants. The presence in these successions of tall indigenous forest regeneration is not common, and depends on proximity of seed trees and bird vectors for the seed.
2: Berberis darwinii, being relatively shade tolerant, but still capable of establishing in open sites, initiates an indigenous mesophyll scrub succession but persists under the canopy of the resulting low forest.
3: Calluna vulgaris. The density and youth of stands of this shrub do not yet allow a positive opinion on its role in succession. However, more open stands contain small Leptospermum scoparium, growing higher than the Calluna.
4: Erica lusitanica. This low shrub can give way to the taller Leptospermum but because of the reproductive vigour of E. lusitanica its stands may remain monodominant. Their youth does not yet allow an assessment of the nature of succeeding vegetation.
5: Hakea salicifolia, Hakea sericea, Hakea gibbosa. These three species invade very leached soils. Succession has been recorded in stands of H. salicifolia and H. sericea, usually containing nanophyll indigenous shrubs that would have been present in the absence of these two Hakea.
6: Pinus contorta, Rubus fruticosus, Clematis vitalba. No succession has been seen in stands of this group of one tree and two vines. Pinus contorta stands remain very dense for at least 40 years so the likelihood of entry of indigenous plants is very low. The density and physical weight and consequent collapse of the two vines effectively blocks succession.
Groups 5 and 6 form dense stands in areas of low soil moisture:
7: Rosa rubiginosa. Thymus vulgaris: these shrubs form dense stands in semi-arid areas, distant from any indigenous tree seed source; the only invaders are grasses and herbs, some indigenous.
8: Lupinus arboreus, Lycium ferocissimum. Found on edaphically dry and exposed coastal sites. Lupinus is sometimes followed by large tussocks of Phormium and Cortaderia and the small tree Coprosma repens.
New Zealand, naturalised woody plants, succession to indigenous vegetation.
Plant names used here are those of Allan (1961), Brownsey et al. (1985), Connor and Edgar (1987) and Webb et al. (1988). Clmate records are from New Zealand Meteorological Service (1980)
New Zealand's relatively equable climate has allowed the vigorous growth of many plants naturalised from parts of the world where climates are more extreme. Fortunately almost all the woody plants naturalised in new Zealand are light-demanding and there are few cases of invasion of intact forest, such as is the case in Hawaii (Gerrish and Mueller-Dombois, 1980). However, much indigenous seral woody vegetation has been displaced, in its early stages, by naturalised shrubs and page 33 trees, although later stages in these seres may return to a completely indigenous composition.
Cockayne, (1928) describes communities of naturalised shrubs, such as Ulex europaeus, but does not describe any succession beyond the dense stands he found. It is probable that fires, both intentional and accidental, were more frequent than today and that there had not yet been sufficient opening up with age to allow entry of indigenous plants.
The first detailed account of succession beyond Ulex europaeus is by Druce (1957), where he details change at c. 40 years from Ulex europaeus scrub to low forest of Pseudopanax crassifolius.
Since then an increased appreciation of the role played by naturalised woody plants in succession to indigenous vegetation has resulted in far more information being available on the interaction between naturalised and indigenous woody plants. This increased attention to naturalised plants also has two other causes:
Firstly, the increased areas of distribution of many of these plants has made them more obvious, and a matter of concern in protected natural areas.
Secondly, the stricter control of fire, particularly in areas of expansion of plantation forestry since 1925, has allowed development of naturalised shrub stands to the stage where successful establishment of other plants is possible in these stands.
Most of the woody plants whose ecology is described below were introduced intentionally in the earlier years of European colonisation in the 19th century. A full account of dates of first records in this country is given in Webb et al. (1988). Some common examples are: Pinus radiata for timber, P. contorta for soil stabilisation, Ulex europaeus for hedges, Rubus fruticosus for its fruit, Calluna vulgaris as shelter for naturalised game birds, Thymus vulgaris as a culinary herb, Clematis vitalba as an ornamental.
New Zealand is now a country with an array of domestic and feral mammal browsers. None of these mammals were here before the late 18th century. Only two indigenous shrubs (Discaria toumatou and Urtica ferox) are spiny, and only one of many indigenous vine genera is similarly armed (Rubus). Many of the most aggressive and successful naturalised shrubs and vines are spiny, for instance Ulex europaeus, Rubus fruticosus and Rosa rubiginosa. This spininess contributes to their success in an environment now having a range of browsers.
The naturalised plants to be dealt with here are only those which have spread over considerable areas and densely enough to give their own physiognomy to the vegetation. Almost all the naturalised shrubs and vines are found almost exclusively in the warm temperate or montane /cool temperate zones. The bioclimatic zones are those of Wardle (1991). Only Calluna vulgaris and Pinus contorta are found in quantity in the subalpine and lower alpine zones, having been introduced in areas where open land is continuous to the alpine areas. In higher rainfall areas forested at the time of European arrival, restriction of the rest of the shrubs to lower altitudes is partly a function of very limited forest clearance in the subalpine zone, and the consequent existence of forest as buffer belts between lowland and mountain environments. East of the South Island mountains, largely deforested before Europeans arrived, it is probable that the more continental climate there imposes limitations on the upward migration into the subalpine zone of most naturalised shrubs.
Common New Zealand Naturalised Woody Plants
The list below is arranged in approximate climatic order, from coolest and wettest, to driest climates. Distributions and common names come from Webb et al.(1988), with the distributions in countries of origin in italics. The botanical page 34 authorities are shown as a convenience to overseas readers.
(1) Upper montane to subalpine, high to medium precipitation.
Calluna vulgaris L. (Hull). (Ericaceae) Europe, Scandinavia, Asia minor, N. Africa. North, South, Campbell Islands. Heather
Pinus contorta Loudon (Pinaceae) North America. North and South Islands.
(2) Warm and cool temperate/montane
Pinus radiata D.Don. (Pinaceae)California, Radiata pine.
Ulex europaeus L. (Leguminoseae) western Europe, Italy. North, South, Stewart, Chatham, Auckland, Campbell Islands. Gorse.
Rubus fruticosus L. (Rosaceae) northern hemisphere temperate North, South, Stewart, Chatham Islands. Blackberry.
Hakea gibbosa Cav., Proteaceae. E. Australia. Northern North Island. Downy hakea
Hakea salicifolia (Vent.) Burtt, Proteaceae. E. Australia northern North Island and northwestern South Island. Willow leaved hakea.
Hakea saricea Schrader et Wendl. E. Australia N.Z. distribution similar to Hakea salicifolia. Prickly leaved hakea
Buddleja davidii Franchet, China. North, South and Stewart Islands. Buddleia.
Berberis darwinii Hook. (Berberidaceae) southern South America. North (localised), South and Stewart Islands. Darwin's barberry.
Clematis vitalba L. (Ranunculaceae) Europe, SW Asia. North and South Islands. Old man's beard.
Erica lusitanica Rudolphi (Ericaceae) southwestern Europe North and South Islands. Spanish heath.
Cytisus scoparius L. (Leguminoseae) Europe. Asia Minor, Russia. North, South, Stewart, Chatham and Campbell Islands. Broom.
Crataegus monogyna Jacq. (Rosaceae) Europe. North and South Islands. Hawthorn.
(3) Cool temperate, low precipitation.
Rosa rubiginosa L. (Rosaceae) Europe, N. Africa. North, South, Stewart and Chatham Islands, but more common in drier climates. Sweet brier
Thymus vulgaris L. (Lamiaceae) Mediterranean. South Island, dry inland areas. Culinary thyme
(4) Coastal, high to medium precipitation.
Lupinus arboreus Sims. (Leguminoseae) California. North, South, and Stewart Islands. Tree lupin.
Lycium ferocissimum Miers. (Solanaceae) Southern Africa. North and South Islands. Boxthorn.
The Ecology of Naturalised Shrubs and Trees in Relation to Indigenous Vegetation.
Calluna vulgaris (Heather).
The most extensive and obvious invasion by Calluna vulgaris is within Tongariro National Park, which surrounds and includes New Zealand's active mainland volcanoes. Much of the landscape here, from 600 m up to c. 1600 m is dominated by Chionochloa rubra, a tussock up to 1m high and with orange leaf ends, which impart a golden glow to the landscape. This tussockland spread from naturally poorly drained sites and alpine areas because of forest destruction by pre-European fires. The soils, of low pH, and often compact and moderately drained, were ideal for Chionochloa rubra, whose dense stands appear to slow down invasion by shrubs. The tussocklands were, when not reburnt for early farming, being invaded only slowly by indigenous plants. Species included Leptospermum page 35 scoparium at lower altitudes and Cassinia vauvillersii, Dracophyllum spp. and Phyllocladus alpinus at higher altitudes.
Calluna vulgaris was introduced intentionally in 1912 in the northwest sector of the Park, as food and shelter for the introduced game bird, the grouse (Lagopus lagopus scoticus). The grouse did not survive, but Calluna vulgaris did and is now expanding its area at the expense of the tussock lands and other more open subalpine zone vegetation. In an area along the Mangatepopo Rd., tussock with subalpine scrub, at higher altitudes, was burnt in 1948. The tussock had regenerated all over by the late 1950s, and was invaded by Calluna vulgaris in the 1960s. The cover of Calluna now is almost complete with only sparse suppressed tussocks surviving. Away from the area of the 1948 fire, in most of the north and west of Tongariro National Park, Calluna vulgaris has also spread extensively into tussock and indigenous heath communities which had not been burnt in the mid-20th century. Up to about 1200m, where the Calluna vulgaris stands are not completely closed, Leptospermum scoparium grows through more open stands of Calluna and is already (1992) higher than it. This could be the beginning of succession back to the forests which once occupied the area.
Chapman and Bannister (1990) did not find rapid invasion by Calluna vulgaris of Dracophyllum subulatum heath in the study area. Dracophyllum subulatum has a similar physiognomy and stature to Calluna vulgaris. The least spread of Calluna recorded by these authors has been in boggy ground. Although Calluna vulgaris produces seed only below 1200m altitude (Chapman and Bannister, 1990), the lightness and quantity of this seed has enabled it to establish at 1600m, above the treeline, either blown or adhering to mountaineers' muddy boots. The capacity of Calluna vulgaris for vegetative reproduction ensures its spread there.
In a similar way the distribution of Calluna vulgaris along highways and railways near Tongariro National Park has been aided by man. A plant, found in the Tararua Conservation Park (and dug out !) 250 km south of Tongariro National Park could easily have travelled there on the boots of a mountaineer.
Calluna vulgaris is also locally distributed at The Wilderness Scientific Reserve, near Lake Te Anau. Although only small plants remain in the reserve, after control operations there are more mature stands nearby (P.Bannister, pers. comm.) There is still a risk that Calluna, uncontrolled, may suppress the ground flora beneath the tall bushes of Halocarpus bidwillii in the reserve.
Calluna vulgaris is one of our most unfortunate introductions. In Tongariro National Park Calluna was introduced into a mountain area of great natural beauty and with a unique indigenous vegetation. Calluna is easily dispersed naturally, and its dispersal is assisted in an area of high recreational use. The same trampers often go to other, distant mountain areas, and seed can easily go with them.
Pinus contorta (Contorta pine)
This plant matures in relative shelter to a small tree and many of its invasive attributes are shared with naturalised shrubs. It seeds precociously, as early as seven years of age, in New Zealand. Its early growth is often as a multibranched shrub and it resprouts when cut back, even at ground level.
The first infestation of Pinus contorta to spoil a natural landscape was on the southeast slopes of Tongariro National Park. In contrast to the formerly tussock covered northwest area of this Park, much of which is now covered with Calluna vulgaris, the southeast side of Mt. Ruapehu, from 1000–1800 m altitude was of a desert aspect with sparse vegetation. There is a series of alluvial fans of loose sandy tephra, where substrate instability is maintained by the strong fohn winds in the rainshadow of Mt. Ruapehu, by frost at any time of the year and, near the Whangaehu River, by episodic extensive lahar floods. Rain is adequate for forest growth, 1100mm annually at 823m altitude, and wetter with altitude. Substrate page 36 instability and early fires have allowed only a few relicts of tall scrub to survive on pedestals of clayey, older tephra. Otherwise the vegetation is of a desert aspect, of indigenous plants adapted to an environment of shifting, abrasive sand
On the edge of this “desert” landscape a plantation of P. contorta was established in the 1930s. By the 1960s saplings were establishing on non-vegetated surfaces. By the 1970s there was a dense scrub of P. contorta advancing into intact tussock grassland 10–15km north of the original plantation. By this time also there were isolated plants established in the lower alpine zone open vegetation, often well above the altitude of the nearby treeline. Control measures today in the National Park include hand pulling and grubbing out, much done by volunteers from mountain clubs. To the southeast of the National Park lies the Waiouru military reserve, an extensive area of tussock grassland, of Chionochloa rubra and, at lower altitudes Festuca novaezelandiae, the only extensive area of the latter tussock in the North Island. There are interspersed forest patches, mostly of Nothofagus. Over the last 10 years the Army has undertaken an extensive cutting and burning programme, hoping to eliminate Pinus contorta. The native tussocks respond to burning by good new regrowth.
The second phase of P. contorta introduction started in the 1950s, when foresters decided that, as the South Island's tussock covered mountains east of the Southern Alps had been forested until fires of the 13th and 14th centuries, they should reforest these mountains. This planting was done with the aim of reducing erosion, much of which was then believed to have resulted from pastoral activity in the time since European arrival. While the vegetatively reproduced Salix spp. were used for torrent stabilisation, and Alnus viridis for scree planting, the most commonly used plant on open slopes was Pinus contorta. Hand planted at times, but also directly seeded from the air, Pinus contorta has become established not only on unstable slopes, but also in intact tussock grassland and indigenous open scrub. Allen and Lee (1989) found that, of the three conifers spontaneously establishing in tall tussock grassland, (Larix decidua, Pinus nigra, P. contorta), the latter showed greatest tolerance of the proximity of tussock bases of Chionochloa rigida. This result was ascribed to a possible greater shade tolerance of Pinus contorta, a supposition that could explain its ability to invade indigenous vegetation with a range of canopy densities, from semidesert to open scrub. None of the plants of these communities can survive in the pine's shade.
At about 40–50 years age the Pinus contorta plantations on the south side of Mt. Ruapehu are dense and dark beneath with no growth under them, indigenous or naturalised. They were felled around 1980. In its native territory, of inland North America, Pinus contorta stands can remain at a dense stage for over a century, with little or no self-thinning. This character may be related to the slight shade tolerance noted by Allen and Lee (1989). It seems likely then that this conifer will not provide a starter for indigenous successions for a very long time
In some South Island areas of conservation and tourist interest, attempts are being made to remove P. contorta, but the job is even more vast than in the earlier invasion of Tongariro National Park.
Extensively planted in large blocks, for timber and as windbreaks, this tree, which can reach 40m height in 30 years is now the most noticeable naturalised plant in New Zealand.
From planted trees the wind dispersed seed travels into more open indigenous communities, and trees may establish and ultimately suppress the local vegetation. Julian (1983) described P. radiata establishment, from seed from nearby trees on to the site of Leptospermum scrub, burnt 10 years previously. Although there was vigorous scrub regrowth, this was accompanied by the establishment in the first 3 years after the fire of clumped stands of P. radiata saplings, whose lateral growth page 37 would ultimately suppress the indigenous scrub.
From this stage an estimated 30–40 years has to elapse before the establishment of a dense indigenous understorey, generally of mesophyll (Specht, 1979) shrubs.
Many plantations of Pinus radiata were established, after burning, on the sites of nanophyll (Specht, 1979) scrub or heath. After 25–30 years of pine growth, a tall mesophyll and tree fern scrub develops, very similar in appearance to the understorey of an indigenous forest. The understorey which developed under Pinus radiata is tall, up to 10m high and composed of large, generally soft leaved plants, a strong contrast to the 1m high heath or the 4m high scrub of small leaved plants on the site before the pine was planted. This change in vegetation was first noted by Henry (1954), and studied in detail by the present author (McQueen, 1961).
Under 25–30 year old Pinus radiata, near Tokoroa, with 1600 mm of annual rain, a dense indigenous understorey developed. The soils are sand to silts from the Taupo Pumice eruption of the second century A.D., overlying deep, clayey, fine tephra of earlier Holocene age, Previous to planting, the vegetation burnt was a mosaic of Leptospermum Kanea scrub on hills and a heath of Dracophyllum subulatum and Poa cita in flat pumice-filled valley bottoms After a period of dense pine canopy, lasting 15–20 years, the pine stands were opened up by group mortality. Their dense litter mineralised, and the formerly black greasy organic topsoil of the scrub and heath changed to a brown mull. A soil fungus appeared to be an active agent in this soil change. Vucetich et al. (1960) analysed soils before and after the passage of the hyphal “front” of the bleaching fungus. They demonstrated rises in phosphorus and exchangeable bases in soils changed by the fungus.
Under the changed light and soil conditions, the first plants of the understorey were tree ferns. By 30 years of pine age, on slopes where the pumice was shallow, older moister tephra were in root range, and a dense understorey developed. This understorey was of broadleaf shrubs together with tree ferns, up to 10m high Together with most of the other shrub species in this understorey, the tree ferns were completely absent from the scrub and heath that was burnt before planting. On deeper pumice soils of infilled valleys, the tree fern understorey persisted, with little invasion by shrubs.
Close to an area of intact indigenous podocarp forest a few seedlings of Dacrycarpus dacrydioides and Beilscmiedia tawa were noted, but the absence of any podocarp seedlings within the main pine forests suggested that bird vectors of mdigenous seed (Beveridge, 1964) did not, at least at the time of the survey (1958–1960), penetrate the pine forests. Today, with regular thinning of the pine plantations following the felling of the original trees, such a dense indigenous understorey does not have a chance to develop and is replaced by more light demanding plants, such as Pteridium esculentum.
Following winter clear-felling of the pines, with lack of heat for pine seed release ther was little establishment of pine seedlings. Many of the species of the crushed understorey resprouted, and in the first years the light-demanding nitrogen fixer Coriaria arborea increased from a few scattered plants to form a locally dense cover.
When the pine was felled in summer, its cones opened and its dense seedling growth soon suppressed any indigenous regrowth.
As the aim of these plantations is the continuing production of wood, all indigenous and pine regrowth is now defoliated and burnt before replanting with selected stock.
|Understorey in 25–30 year Pinus radiata||Changes in scrub on cutover land after felling|
In the Rai-Whangamoa forests of the northern South Island, Jelinek (1977) reported vigorous invasion by indigenous species of a Pinus radiata forest after thinning at 30 years of age. The deep humus under Pinus radiata would not only have been mineralised by increased temperature but macerated by machine activity, thus forming an ideal seed bed. The soils here are much older and more leached than those formed from Taupo pumice. Of the 34 trees and shrubs recorded by Jelinek, Weinmannia racemosa was the most abundant. A species of low fertility soils, and forming an acid litter (Wardle, 1966), its abundance reflects the lower intrinsic nutrient status of these older soils of the stands described by Jelinek. Most of the other understorey trees and shrubs in the Rai-Whangamoa forests were also recorded by Mc Queen (1961) in the Tokoroa forests. In addition Jelinek recorded the potentially tall forest trees Dacrydium cupressinum, Podocarpus totara, Metrosideros umbellata and Prumnopitys ferruginea in the Rai-Whangamoa forests. Their presence there is probably due to the fact that Jelinek's sample was only 200m from undisturbed indigenous tall forest and thus within bird seed vector range.
On former Nothofagus sites, on deep weathered loess near Upper Hutt, with rainfall of 1298 mm annually, Vella (1984) decribed an open understory under Pinus radiata about 45 years old. Weinmannia racemosa was common, probably resprouted from the previous Leptospermum seral vegetation (cf. Druce, 1957), as were other species common in lowland Nothofagus sites. These were Cyathodes juniperina, Myrsine australis, Coprosma lucida, Pittosporum tenuifolium, and the ferns Asplenium lucidum and Phymatosorus diversifolius. Although this stand had not been thinned there were abundant and vigorous Dacrydium cupressinum saplings, up to 3m high. There were remnant trees of D. cupressinum on lower slopes near the pine plantation and Vella (1984) recorded the presence of the native pigeon, a vector of podocarp seeds (Preest, 1963). In adjacent remnant forest of Nothofagus truncata, on similar midslope sites and with canopy density similar to that of the pine plantation, there were no Dacrydium cupressinum saplings or seedlings. It was found in a laboratory experiment that the organic soil layers beneath Pinus radiata held water longer than those beneath Nothofagus truncata. The success of Dacrydium cupressinum in the moister Pinus radiata litter, and beneath an unthinned canopy accords with its known need for a moist soil environment and its shade tolerance as a young plant (McEwen, 1983).
From Wellington, Smith (1979) described understorey composition in pine plantations of 45 years, with rainfall of 1240 mm. These plantations are on steep slopes with skeletal soils. On east to south slopes, the following species were abundant, reaching to about 3m height, but not forming a dense scrub: Coprosma page 39 grandifolia, C. robusta, Brachyglottis repanda, Melicytus ramiflorus, with the ferns Asplenium lucidum, Phymatosorus diversifolius and Ploystichum richardii. The shrubs are some of the same species as those recorded as abundant by McQueen (1961) in the Tokoroa forests on tephra soils, but had not reached the height or density of the Tokoroa stands. The rainfall at Tokoroa is higher and soil formed from Taupo Pumice has far greater moisture retention than in soils formed from greywacke (Packard, 1957). Some of the oldest Pinus radiata plantations around Wellington, however have dense and tall understorey, at close to 90 years of pine age (Fig. 3). It appears that time and increasing soil organic matter allows the growth, in about double the time, on Wellington greywacke soils, of an understorey as tall and dense as that on Taupo Pumice.
The Wellington shrub assemblage under P. radiata reflects the fertility of a skeletal young soil. The site, on analogy with indigenous forest remnants in Wellington probably carried a mixed forest, with some podocarps, in a Beilschmiedia tawa matrix. The Wellington stands have a completely different shrub understorey from the Upper Hutt stand, described above, by Vella(1984), where the older soil, having previously supported Nothofagus forest, is more leached.
In areas of reasonable moisture supply, where Pinus radiata has been felled after the development of an indigenous understorey and no attempt is made to regenerate the pine, a mesophyll scrub of indigenous plants results. For example such scrub is very visible from the interisland ferry, on the east of Picton Harbour, the pines there having been felled in the last 10 years. Dependent on the proximity of seed trees, such scrub and its counterparts in other areas, could eventually lead to tall indigenous forest, where commercial forestry does not go beyond a first rotation.
Ulex europaeus (Gorse) (Fig. 4)
This fearsomely spiny shrub, with bright yellow flowers is one of the most widespread and visible of New Zealand's naturalised shrub flora. Newsome (1987) records 254 000ha of land occupied by Ulex europaeus although only on 20 000 ha is Ulex europaeus dense enough to be called scrub. Otherwise it is scattered through low quality pasture. Its main areas of distribution as dense scrub are on either side of Cook Strait, inland of the Bay of Plenty and in the hills surrounding Dunedin. (Hunter and Blaschke, 1986)
As scattered clumps in pasture, Ulex europaeus is common throughout New Zealand, from high precipitation areas of the South Island west coast, down to c.600 mm of annual rain in the east. Introduced last century as a hedge plant, its spread on to pastures is only checked by browsing of domestic stock when pasture growth is inadequate, mostly in drier areas. It is in these areas that gorse hedges are still successfully maintained.
In higher rainfall areas, as stock numbers decrease with weaker grass growth on unfertilised pastures, Ulex europaeus is able to expand and often, after repeated fires, displaces indigenous seral plants such as Leptospermum scoparium.
Ulex europaeus is a plant with strong reproductive capacity. It bears seed young, and the seed is catapulted from the pod by sun or fire heat, and can last up to 30 years in the ground. Fire stimulates its germination (Zabkiewicz, 1976). Plants of all ages can reproduce from root-shoots after fire and can coppice from cut stems. Its growth rate is high, Lee et al.(1986) describe rates of 20cm/yr. for height and 0.5cm/yr for diameter growth. The maximum height recorded by them was 7m with diameter (at 10cm height) of 21.7cm. This is certainly more vigourous growth than the 1–2m high plants of Ireland or Portugal.
Ulex europaeus is relatively short-lived in New Zealand. Druce (1957) recorded 46 years as a maximum near Wellington. Usually by 40 years Ulex europaeus had been replaced by indigenous shrubs developed beneath it.
Druce (1957), Healey (1969) and Oates(1988) all record, from central New page 40 Zealand, replacement of Ulex europaeus scrub by indigenous species, generally broadleaf shrubs within time spans from 15 to 30 years of Ulex age. By this time Ulex europaeus has been overtopped or is senescent.
Ulex europaeus stands were studied by Oates (1988) on former Nothofagus sites, with leached soils, east of the Hutt Valley. Indigenous plants establishing in Ulex around 22 years old, were: Melicytus ramiflorus, Geniostoma rupestre, Pteridium esculentum. Other stands on younger soils in the Wellington area have as emer-gents: Brachyglottis repanda, Coprosma robusta, Pittosporum crassifolium, (itself an introduction from northern New Zealand). All the shrubs and small trees found are capable of forming a closed canopy and, if seed of taller indigenous trees is available it is likely that tall forest could establish.
All three studies cited above are in areas of moderate rainfall of 1000–1200mm annually, and have mild climates. Ulex europaeus litter breaks down under these conditions, allowing seeds of indigenous plants to germinate.
Ulex europaeus enriches the soil with more nitrogen than under an equivalent indigenous sere (Egunjobi, 1971). The addition of this nutrient can result in a richer soil, with consequent changes towards more fertility-demanding species than were present in the original forest.
Under the cooler conditions of the southern South Island, near Dunedin, Lee et al. (1986) reported that Ulex europaeus litter breaks down slowly and can reach 75cm in depth. Under these conditions there are few plants, indigenous or naturalised, in the Ulex europaeus understorey. Indigenous plants did however establish when the Ulex europaeus stems were tall and at a low density, or where litter was shallow or where there was bare ground or bryophyte cover. Under these conditions the more frequent indigenous species, at 25–30 years of Ulex age, were: Myrsine australis, Melicytus ramiflorus, Griselinia littoralis, Kunzea ericoides, Pittosporum tenuifolium, Coprosma propinqua. In comparing successions to Ulex near Wellington with those near Dunedin several features of the species composition emerge:
1. Melicytus ramiflorus is present in both areas.
2. Griselinia littoralis occurs only in stands near Dunedin, reflecting the cooler climate of that study area.
3. Kunzea ericoides occurs only in stands near Dunedin, no doubt reflecting the openness which allows decomposition of the deep Ulex litter there.
Apart from its role in succession on old forest sites Ulex europaeus also invades open ground in many circumstances. On an intact shoreline forest edge south of Haast, Ulex europaeus occupied in a space of 20 years a previously bare gravel strand, between the storm driftwood zone and a zone of Phormium tenax the first indigenous plant encountered inland of the sea. (Author's observation.) Similarly, widespread colonisation of aggraded river beds is common in higher rainfall parts of New Zealand.
Although Ulex europaeus grows well on leached, former forest soils, it will not invade the poorly drained pakihi heathlands of the South Island west coast, growing only along artificial drainage channels through these wet areas.
The high reproductive capacity of Ulex europaeus has enabled it to displace indigenous, early stage woody seral vegetation. This displacement has always been aided by fire in the indigenous woody vegetation, as Ulex europaeus does not seem shade tolerant enough to invade any but low herbaceous and grass cover. Ulex europaeus is highly inflammable, more so because of its dry litter, both hanging on branches and on the ground. Each successive fire renews stands of Ulex europaeus and further eliminates any remaining indigenous seral shrubs. If, however, an area of Ulex europaeus is left unburnt for some decades there is a good possibility that the vegetation will change to an indigenous broadleaf scrub or low forest. It is encouraging to note a network of firebreaks in stands of Ulex europaeus on hills page 41 around Wellington, and also to note over a period of 20 years that the former dark cover of Ulex europaeus has been changing, from gullies up towards spurs, to a lighter green indigenous scrub and low forest.
Hakea gibbosa(Downy hakea), Hakea sericea(Prickly hakea)Fig. 5)
These two similar needle-leaved shrubs to small trees are widespread on the North Auckland peninsula in the very leached soils of gumland scrub, on old Agathis sites. Hakea sericea is, as well, abundant on leached soils of northwest Nelson.
Enright (1989) found that the two species rarely co-occur in the gumland heath of North Auckland, Hakea sericea being characteristic of very leached sandstone soils and in young vegetation on clays, while Hakea gibbosa was found on less leached sandstone soils. Neither species was found on more fertile volcanic soils.
Fire opens their persistent woody seed capsules and accelerates the invasion of the resulting clear ground. Beever (1988) reported, from a site burnt five years previously, young plants of Hakea gibbosa and Hakea sericea, with mean densities of 1.6/m2 for Hakea sericea and 0.7/m2 for Hakea gibbosa.
From northwest Nelson, in the Abel Tasman National Park, Esler, (1961) reported that Hakea sericea excludes all other plants of seral scrub and that it and Hakea salicifolia (below) invade unburnt scrub. Williams, (1992, in press), from Northwest Nelson reported that Hakea sericea lives to about 15–20 years before stand collapse by windthrow. When Hakea sericea plants are growing with Leptospermum and Kunzea shrubs these species continue a succession towards indigenous forest. When, however, Hakea sericea is in dense pure stands, stand collapse is followed by invasion of Pteridium esculentum, which can in turn initiate a sere back towards forest.
Hakea salicifolia (Willow-leaved hakea)
This shrub to small tree, up to 5m high, has flat leaves, to 11 cm long. It is widespread and often dense in Leptospermum and Kunzea scrub in northwest Nelson, including a serious infestation of “several thousand acres” (Esler 1961) within the Abel Tasman National Park. Here the soils are leached, although well-drained, deep granitic clays. Indigenous scrub succession on them is often very slow or even apparently static (Gabites, 1979).
Williams (1992), in this National Park, found evidence of succession on to indigenous vegetation, following invasion of scrub by Hakea salicifolia. As Hakea salicifolia does not produce annual rings he aged stands from Kunzea ericoides associated in the same stands. Hakea salicifolia reaches maximim height of 8m at 30–40 years and is then overtopped by Kunzea ericoides. On deeper soils, at an estimated 50–60 years of age, the Hakea has been mostly suppressed and indige-nous small trees and shrubs are becoming established. Some of the more common indigenous species recorded with Hakea salicifolia were: Cyathea dealbata, Melicytus ramiflorus, Leucopogon fasciculatus, Carpodetus serratus, Coprosma robusta.
On the shallowest soils, however, Hakea salicifolia is still invading the mixed Leptospermum-Kunzea scrub, a scrub which has shown very little change since first described by Dumont d'Urville in 1827.
Rubus fruticosus (Blackberry).
R. fruticosus grows as a scrambling shrub, capable of layering. It has relatively soft leaves, up to 16 cm long, and intensely barbed stems. Introduced as a fruit plant last century its spread by seed has been accomplished by defecation by birds, pigs and humans. As well, its vegetative reproduction allows broken fragments to establish new plants on stream beds after floods.
Fortunately R. fruticosus is not tolerant of the shade of New Zealand's evergreen forests, so it has not invaded intact forest. Its main distribution is in areas of higher rainfall, throughout the Auckland province and western side of both page 42 main islands. However it is also found in drier areas, down to c. 800mm of annual rain, in swamps and along streams. It is a species which invades quite fertile pasture (Hunter and Blaschke, 1986). Control methods now include spraying with herbicides. Goats, extensively used in the past to eat the Rubus, are now being used again, as free ranging animals. They present a hazard to indigenous vegetation when not controlled themselves. Rubus fruticosus grows well on leached poorly drained soils. As such it can be a serious hindrance to the regeneration of Nothofagus forests on the west coast of the South Island.
The density and tendency to collapse, of the low canopy (2m) of R. fruticosus prevents invasion by indigenous shrubs.
Buddleja davidii (Buddleia) (Fig 6)
Although limited in distribution at present in its invasion of indigenous communities, this garden species' strong colonising ability in protected natural areas is a matter of concern. Smale(1990) reports on its occurrence on gravel river beds in Urewera National Park, and Brown (1990), in the Rimutaka Conservation Park, near Wellington. Dobson (1979) also reported its spread on gravel riverbeds near Kaikoura. There is a range of precipitation in these three sites, in the same order, from 2400 mm down to 888 mm annually. Erosion is active on shattered grey-wackes of the three areas, and the resultant depositional surfaces are colonised by Buddleja davidii. It is a short lived plant, from 15 to a maximum of 20 years. Within this time its initially dense stocking declines and seedlings of indigenous, broadleaf woody plants establish. Among these Smale (1990) recorded Pseudopanax arboreus, Melicytus ramiflorus and Aristotelia serrata. Brown, (1990) in the Orongorongo valley found that the commoner seedlings of woody plants beneath Buddleja were Macropiper excelsum, Hedycarya arborea, Melicytus ramiflorus, D.J. Campbell(pers.comm.) however has found little evidence of succession on to indigenous scrub from Buddleja davidii in this area. He ascribes this lack of succession to the frequency of floods over the area occupied by Buddleja davidii and the renewal of its stands. Both Brown and Smale found that Buddleja davidii displaces indigenous early colonisers, both herbaceous and woody. Among those displaced is the nanophyll shrub or small tree, Kunzea ericoides, which can form a canopy for over 50 years, then be succeeded by broadleaf trees and shrubs listed above. Thus Buddleja davidii has accelerated succession to forest. Such acceleration may seem an advantage, but not in areas reserved for their indigenous vegetation. Smale (1990) forecast that Buddleja. davidii populations will persist, shifting from one new surface to another in the unstable landscape.
This spiny leafed shrub, which can grow to 5 m tall, is a common forest understorey and seral scrub plant in Nothofagus forests of southern South America, in climates very similar to New Zealand. Eskuche (1968, 1969) records Berberis darwinii as common in both evergreen tall forest and in shorter deciduous forests of Nothofagus. As well, Berberis darwinii can dominate scrub following fire in these forests(Eskuche, 1969; Hildebrand, 1983). The spread in New Zealand of Berberis darwinii, from garden plants to localised dense patches has only been recorded since 1946 (Webb et al. 1988). It has small fleshy fruit and is easily transported by both naturalised and indigenous birds (Allen, 1991). As well as by seed establishment, Berberis darwinii spreads vegetatitively by suckering. So far it is present as an invader of open land from the southern end of the North Island down to Stewart Island in areas of regular rainfall. A study by Keller (1983) near Wellington showed that Berberis darwinii colonises low fertility pastures, resists browsing by stock, and forms a tight divaricating form in wind exposed areas. As the Berberis darwinii scrub closes canopy and this canopy gets taller, indigenous plants will establish. In Wellington Keller (1983) recorded, beneath Berberis scrub about 2–3m high, seedlings and saplings of the small trees Carpodetus serratus, page 43 Hedycarya arborea, Melicytus ramiflorus and the shrub Coprosma rhamnoides. Lianes recorded were Parsonsia heterophylla, Metrosideros perforata, M. diffusa, and Muehlenbeckia complexa. The floor has a low cover of ferns, including Polystichum richardii, Phymatosorus diversifolium and Pyrrosia serpens. In a low forest, c.5m high there is a tight, windshorn canopy of Melicytus ramiflorus, Hedycarya arborea, Carpodetus serratus, Pennantia corymbosa. Beneath this canopy Berberis darwinii is abundant, with long branches reaching up to the canopy of the low forest (author's observations).
Near Dunedin Allen (1991) recorded, as principal associated indigenous plants: the light-demanding Kunzea ericoides, (whose quantity has an inverse relation to that of the Berberis); more shade tolerant plants present were Melicytus ramiflorus, Pittosporum eugenioides, P. tenuifolium, and Griselinia littoralis.
As the Berberis canopy becomes taller and begins to open the indigenous plants emerged and form a low secondary forest. In this forest Berberis darwinii persists as small lianoid trees beneath the canopy. Allen and Wilson (1992) found that these forest conditions are better suited for germination of Berberis darwinii seeds and establishment than open condition. Therefore Berberis darwinii is likely to remain as a constituent of the forest, as it does in its natural distribution in South America.
There has not yet been a study of the chronology of successions involving Berberis darwinii. It is impossible to say whether Berberis darwinii simply persists in an indigenous forest resulting from succession through Berberis darwinii or whether Berberis darwinii in fact actively invades an intact indigenous forest.
There are few shade tolerant naturalised plants in New Zealand, but the presence of Berberis darwinii, considering its spread within half a century, is a danger to the indigenous purity of forests, particularly to remnant and reserved patches of lowland forests in farmland.
Clematis vitalba (Old man's beard) (Fig 9)
This liane, originally a garden plant, is normally of modest growth in its Northern Hemisphere climates, which have more severe limitations of temperature and precipitation than New Zealand. Here the seeds are easily carried by wind, so Clematis vitalba has spread far from settled areas into open forest remnants and on to the edges of closed forest. The growth habit of Clematis vitalba in New Zealand enables it to reach canopies at 30m from the ground then to spread its foliage so as to eliminate light from the foliage of the indigenous trees. The author's observations over 30 years in the south of the central North Island show that a healthy small patch of podocarp forest of about 1 ha. has suffered mortality of the two tallest tree species, Prumnopitys taxifolia and Dacrycarpus dacrydioides. Their skeletons are now densely covered with Clematis vitalba. Smaller trees of Hoheria and Sophora were completely engulfed in a shorter time. As Clematis vitalba is deciduous, the aspect of a once evergreen wooded slope in winter is very grim.
Clematis vitalba thus presents a very real threat not only to the smaller forest patches already scarce in the New Zealand lowlands, but even more to those farm areas where smaller trees form an attractive woodland and summer shade for stock. Control of Clematis vitalba, in its worst areas of infestation, is being done by cutting and poisoning, but the problem remains of complete elimination of seed source plants.
Erica lusitanica (Spanish heath).
This nanophyll shrub, which can grow to 3m high, is common in milder climates of both main islands. Mather and Williams (1990) from whom this account is drawn, reported that it cannot tolerate frost at above 400m altitude in Canterbury, nor can it grow in areas with low rainfall. Erica lusitanica can tolerate leached and degraded soils in a similar way to Leptospermum scoparium but has reproductive page 44 advantages. Erica lusitanica has copious seed production, of up to 9 million seeds annually from a 4–6 year plant. These seeds, in a soil seed bank, can retain their viability for at least four years. Leptospermum scoparium is also a copious seed producer, but its time of viability is shorter. Erica lusitanica can reproduce by coppice and epicormic shoots after fire or stock damage, while Leptospermum scoparium depends only on seed reproduction. Erica lusitanica, in Canterbury, had a maximum number of growth rings of 44. If these were annual, Erica lusitanica has a shorter life span than the 70 or so years of Leptospermum scoparium. In an undisturbed succession the latter could outlive Erica lusitanica, as appeared to be the case in a succession described by Mc Queen (1991). However, under conditions of repeated fires, Mather and Williams (1990) considered that Erica lusitanica may completely replace Leptospermum scoparium at lower altitudes, as a woody pioneer.
Cytisus scoparius (Broom)
Cytisus scopurius is widely distributed throughout New Zealand, but reaches its greatest densities on freely drained alluvial or tephra-derived soils, (Hunter and Blaschke, 1986). In this respect its edaphic tolerance parallels its distribution in Europe, where it is regarded as an indicator of deep friable soils. (author's observation.) The distribution of Cytisus scoparius in New Zealand excludes the driest areas, and those areas considered to have a marginally Mediterranean type climate (McQueen, 1984).
This distribution accords with its European distribution in climates without a pronounced summer drought; Cytisus scoparius is not, as Williams (1981) believes, a plant of truly Mediterranean climates (Braun-Blanquet 1951, Fournier 1961).
Cytisus scoparius is a fastigiate, almost leafless shrub, which grows to 2.5 m high. It spreads easily, by seeds catapulted from the pod in high natural temperatures, and the seedlings will grow in as little as 10% of full light (Willams, 1981). Growing with a single leader Cytisus scoparius has a competitive advantage over the frequently associated Ulex europaeus.
Where there are forest remnants close by, in areas above c.1000mm of annual rain, indigenous broadleaf shrubs and small trees can establish under Cytisus scoparius. Williams (1983) describes one such succession where mixed stands of Ulex europaeus and Cytisus scoparius were followed within 50 years by low forest of Melicytus ramiflorus. Here an intermediate phase of the locally naturalised Sambucus nigra facilitated the succession by early suppression of Cytisus scoparius.
Cytisus scoparius has a wide range of soil fertility tolerance, particularly to phosphate (Williams, 1981), which enables it to grow equally as well on a leached former forest soil as on a young volcanic deposit. It also actively colonises the broad gravel beds of aggrading rivers, and the old gold dredge tailings of the southern and western South Island.
Crataegus monogyna (Hawthorne)
A shrub or small tree, Crataegus monogyna has spines on its stems, but younger plants are browsed back by stock (Williams and Buxton, 1986.) These authors descibed its establishment and spread in the eastern foothills of the Southern Alps, in about 1500mm of annual rain. Here Crataegus monogyna establishes in the protection of the indigenous spiny shrub, Discaria toumatou, and can grow in this protection until above browse height. In pastures where Discaria toumatou is absent, the Crataegus monogyna seedlings are eaten back before they become too spiny. Crataegus monogyna is also abundant near forest remnants, but, provided these are not opened up by fire, cattle browsing or firewood cutting, Crataegus monogyna does not spread into the forest.
Rosa rubiginosa (Sweet brier) (Fig 10)
A spiny shrub, to 4 m tall, this deciduous shrub of medium height is is widely distributed in New Zealand pastoral land. Rosa rubiginosa is mapped as dominant (Hunter and Blaschke, 1986) in the northeast of the South Island, with c.700mm of annual precipitation and in the driest parts of the intermontane basins of the central South Island, with down to 340 mm annual precipitation.
Rosa rubiginosa reproduces by seed in a fleshy fruit capable of bird transport and favoured by wild pig as an autumn food. The shrub also reproduces vegetatively, by rhizomes and root suckers. Consequently Rosa rubiginosa, with its curved spines, forms thickets impenetrable by man, stock or the dogs used to move the stock.
Molloy (1976), describes its ecology on the northern area of dominance on Molesworth Station, with precipitation around 600–800mm annually. He shows that Rubus rubiginosa is mostly distributed on actively eroding soils, up to the upper limit of Festuca novaezelandiae low tussock grassland (750–1200m). Plants aged from their rootstocks show that their first establishment was in about 1937. The virtual elimination of rabbits in the 1950s had little apparent effect on recruitment of Rosa rubiginosa, and the absence of new plants after the early 1960s was attributed to competition from the vigorous, aerially sown pasture plants sown for the reclamation of depleted rangeland.
Eastwards of the area described by Molloy (1976) in higher rainfall Williams (1989) describes dense Rosa rubiginosa scrub, in an area where there has been little land management, except the rabbit control of the 1950s. Rosa rubiginosa has been in this area since earlier than 1953, when it was mapped as dominant by McQueen (1954). Here Rosa rubiginosa grows in an impenetrable scrub, 4m high, sometimes with Discaria toumatou and the divaricating Coprosma propinqua. The scrub is densest on alluvial fans and lower slopes, merging to shrubland over tussock grassland on slopes up to 900m altitude.
Further south, in the intermontane basin of Central Otago, Partridge et al. (1991), in a study of vegetation on the slopes above a major gorge, found that Rubus rubiginosa was most commonly in a species-poor community of steep rocky faces, with the indigenous low-growing divaricate shrub Melicytus alpinus. Away from this detailed study area, the more gentle slopes and flat areas of Central Otago have become increasingly covered with open scrub of Rubus rubiginosa, during the 25 years of the author's observation. The indigenous tussock grassland vegetation, probably a climatic climax, was destroyed by sheep-grazing, fire, and rabbits. Cockayne (1928) described the area as man-made desert. Open conditions and eroded soils are ideal for Rosa rubiginosa, particularly as, since the rabbit control programme of the 1950s, another later resurgence of rabbit populations has occurred and is so far, at 1991, uncontrolled. Other competing growth, which may delay or prevent Rosa rubiginosa establishment, has been almost completely demolished by rabbits.
Thymus vulgaris (Culinary thyme) (Fig 11)
A low (to 60cm high), multibranched shrub, aromatic and with small grey leaves, this naturalised plant has a restricted but dense occurence in Central Otago, ia region of 340mm of annual precipitation. The altitudinal range of Thymus vulgaris there is from 250–750m. The texture of the schist-derived soils beneath Thymus vulgaris varies from gravels of old gold workings to remnants of the silty loess of the remaning topsoil. These soils are usually near neutral acidity, because of the aridity of the area.
Wilkinson et al. (1979) give the following account of its biology. Originally introduced in the 1860s as a culinary herb, Thymus vulgaris has spread to occupy some 2 000 ha densely, and a much larger area as scattered plants. It flowers and seeds prolifically and the seeds, within calyces, are easily wind-carried as well as page 46 adhering to fur, wool and feathers. Also the seeds are ejected some 10 cm in hot conditions, accounting for the clumping found in invaded areas.
Much of the land which Thymus vulgaris has occupied is that which Cockayne (1928) described as man-made desert (see above, Rosa rubiginosa). The main cover was of the indigenous mat plants Raoulia spp. After the rabbit populations had been severely reduced in the 1950s, the vegetation surveyed in 1967 (McQueen, 1981) included breaking up mats of Raoulia, prostrate species of the indigenous grass Rytidosperma, and an ephemeral cover of naturalised spring green herbs and grasses. By 1990 Thymus vulgaris had spread on to many of the sample points of 1967. In all areas invaded by Thymus vulgaris there is little trace of the previous vegetation. Plants commonly found with Thymus vulgaris are the indigenous grass Elymus tenuis, and the naturalised Vulpia spp., Bromus tectorum, Trifolium arvense and Rumex acetosella. Rosa rubiginosa is found in some areas as a higher stratum over the Thymus vulgaris low scrub. In any direction away from the centre of the arid area occupuied by Thymus vulgaris, precipitation increases. Where there is adequate rain for the growth of introduced pasture grasses. Thymus vulgaris is suppressed by their taller growth. Wilkinson et al. (1979), however, described an area of depleted indigenous tussock grassland where Thymus vulgaris has established in the large inter-tussock gaps.
Thymus vulgaris is one of New Zealand's more agreeable invaders. It is not spiny, it smells nice, makes good honey and its low greyish growth harmonises with an arid landscape.
Coastal Naturalised Shrubs
There are two shrubs which have modified the landscapes of New Zealand sand or gravel coasts. These are Lupinus arboreus and Lycium ferocissimium.
Lupinus arboreus (Tree lupin) (Fig 12)
This low shrub, up to 2 m high is widespread on coastal dunes, introduced intentionally to stabilise the backs of the foredunes on aggrading coasts, or where pastoral attempts had caused renewed sand movement. Lupinus arboreus has suppressed most indigenous plants of the inland sides of foredunes. Cortaderia spp., Phormium tenax. and Coprosma repens may in higher rainfal areas establish under the dense cover of Lupinus arboreus, but its shade is too great for the natural successors towards forest, the shrubs Leptospermum scoparium and Kunzea ericoides. Together with the spread of Lupinus arboreus, the substitution on the seaward face of the foredunes of the introduced grass Ammophila arenaria for the indigenous Spinifex hirsutus and Desmochoenus spiralis, has left very long stretches of New Zealand dunes with a vegetation dominated by naturalised plants.
Lupinus arboreus is also common on steeper scree slopes in some coastal areas Fuller (1985), and on gravel and sand riverbeds stretching to some scores of kilometres inland.
Lycium ferocissimum (Boxthorn) (Fig 13)
A tall shrub at times 5m high, this spiny, tightly branched plant was introduced for hedging in coastal areas. Its fleshy fruit and probable bird transport have resulted in its spread along many New Zealand coasts, both on sand dunes and on gravel storm beaches. Once established it layers from lower branches and can form large dense clumps. Here the indigenous pioneer vegetation is dominated by the mat forming Muehlenbeckia complexa. Lycium ferocissimum is usually scattered in this simple community, and may spread further inland into coastal pasture.
Both Lupinus arboreus and Lycium ferocissimum occupy edaphically dry sites in coastal stands. As well their stand ages have not allowed sufficient accumulation of organic matter to ameliorate the soil moisture retention enough to allow indigenous tall shrub successors to establish.
Naturalised woody plants in New Zealand occupy a wide range of habitats, but the majority are light-demanding pioneer plants and thus prone to be eventually part of succession back to indigenous forest. This succession is governed by availability of indigenous seed, and it is only in areas of adequate rainfall and closeness of forest remnants that the naturalised plants can give way to indigenous vegetation. Such succession has been documented for Pinus radiata, Ulex europaeus, Berberis darwinii, Cytisus scoparius, Erica lusitanica, Buddleja davidii. In most cases succession through this group of trees and shrubs results in the following indigenous vegetation being of mesophyll shrubs. The presence of the naturalised plants usually leads to the omission of initial nanophyll indigenous successors, such as Leptospermum and Kunzea. However Hakea sericea and Hakea salicifolia and Ulex europaeus allow entry of Kunzea at times; Calluna vulgaris when not too dense can lead on to Leptospermum scrub.
In two cases of widely distributed woody plants there is evidence of local differences in the composition of the successional vegetation:
1. The understorey of Pinus radiata varies with soil conditions; on rich, young soils the understorey plants are large-leafed and mull-forming, on older leached soils many species of the pine understorey are smaller-leafed and mor-forming plants, often those of indigenous forests on older leached soils.
2. Under stands of Ulex europaeus and Berberis darwinii, studied near Wellington and near Dunedin, the successional indigenous species differ to some extent. Notable is the presence of Griselinia littoralis in the more southern successions, this medium sized tree does not grow at sea level near Wellington and its presence in successions further south may lead to different secondary forests following Ulex and Berberis there.
In higher rainfall environments, in forest remnants are two shade-tolerant species: Berberis darwinii which can survive within a secondary forest, and Clematis vitalba which can cause canopy dieback by physical suppression of large trees in forest remnants.
The naturalised shrubs of semi-arid areas, Rosa rubiginosa and Thymus vulgaris, show that much of the grassland areas found by Europeans last century are capable of supporting woody growth. All except the driest of these areas have evidence of pre-13th century forest, and in fact still have remnant areas of indigenous shrubs. The distance from indigenous forest seed sources, in these oftburned lanscapes, so far prohibits any succession back to indigenous forest.
The coastal spread, on rocky beaches and sand dunes of the erect shrub Lycium ferocissinum as a primary coloniser, and Lupinus arboreus on sand dunes are examples of the introduction to New Zealand of life forms not previously indigenous in these environments. Neither of these species appear yet to be initiating successions towards indigenous vegetation.
I am very grateful to the post-graduate students, cited in the text, who have provided much of the detail of dynamics of naturalised woody plant successions in the Wellington area. I should also like to thank co-workers who have provided photographs and the School of Biological Sciences, Victoria University of Wellington for assistance in photographic processing. Pamela Searell, Drs. Mary McEwen and Bruce Sampson provided excellent editorial criticism. Much of the travel involved was financed by the Internal Research Grant Commitee of Victoria University of Wellington, and the Miss E. L. Hellaby Indigenous Grasslands Trust.
Allan, H.H., 1961, Flora of New Zealand. Volume I. Government Printer, Wellington
Allen, R.B., 1991, A preliminary assessment of the establishment of and persistence of Berberis darwinii Hook., a naturalised shrub in secondary vegetation near Dunedin, New Zealand. New Zealand journal of botany, 29: 353–360.
Allen, R.B., Wilson, J.B., 1992, Fruit and seed production in Berberis darwinii Hook., a shrub recently naturalised in New Zealand. New Zealand journal of botany, 27: 45–56.
Allen, R.B., Lee, W.G., 1989. Seedling establishment microsites of exotic conifers in Chionochloa rigida tussock grassland, Otago, New Zealand. New Zealand journal of botany, 27: 491–498
Beever, R., 1988. Gumland scrub. Auckland botanical society journal, 43:12–16.
Beveridge, A.E., 1964. Dispersal and destruction of seed in central North Island podocarp forests. Proceedings New Zealand ecological society, 11: 48–55.
Braun-Blanquet, J., Roussine, N., Nègre, R., Emberger, L. 1951. Les groupements végétaux de la France Méditerranéene, C.N.R.S., Paris.
Brown, K.D., 1990. The weed status and ecology of Buddleja davidii in the Orongorongo Valley. (Tararua ecological district) B.Sc. Honours project, Botany. Victoria University of Wellington.
Brownsey, P.J., Given, D.R., Lovis, J.D. 1985. A revised classification of New Zealand pteridophytes with a synonymic checklist of species. New Zealand journal of botany, 23: 431–490.
Chapman, H.M., Bannister, P. 1990: The spread of heather, Calluna vulgaris into indigenous plant communities of Tongariro National Park. New Zealand journal of ecology, 14: 7–16.
Cockayne, L. 1928. The vegetation of New Zealand. (Reprinted 1958). H.R. Engelmann (J. Cramer) Weinheim/Bergstr.
Connor, H.E., Edgar, E. 1987. Name changes in the indigenous New Zealand flora, 1960–1986 and nomina nova IV, 1983–1986. New Zealand journal of botany, 25: 115–170.
Dobson, A.T., 1979. Vegetation, In, Ecology of Kowhai bush, Kaikoura. Maui Ora special publication, 2.
Druce, A.P. 1957. Botanical survey of an experimental catchment, Taita, New Zealand. New Zealand Department of Scientific and Industrial Research Bulletin 124.
Egunjobi, J.K. 1971. Ecosystem processes in a stand of Ulex eruopaeus L. II. The cycling of chemical elements in the ecosystem. Journal of ecology, 59: 669–678.
Enright, N.J. 1989. Heathland vegetation of the Spirits Bay area, far northern New Zealand. New Zealand journal of ecology 12: 63–76.
Eskuche, U. 1968. Fisonomía y sociologiía de las bosques de Nothofagus dombeyi en la región de Nahuel Huapi. Vegetatio, 16: 192–204.
Eskuche, U. 1969. Berberitzengebüsche und Nothofagus antarctica wälder in Nordwestpatagonien. Vegetatio, 19: 263–285.
Esler, A.E. 1961. Botanical features of Abel Tasman National Park. Transactions of the royal society of New Zealand, Botany 1: 297–312.
Fournier, P. 1961. Les quatre flores de la France. Lechevalier, Paris.
Fuller, S.A. 1985. Kapiti Island vegetation. Department of Lands and Survey, Wellington.
Gabites, H. I. 1979. A study of slow succession in Leptospermum shrubland to Nothofagus truncata forest near the Anchorage, Abel Tasman National Park. B.Sc. (Hons) Botany. Victoria University of Wellington.
Gerrish, G., Mueller-Dombois, D. 1980: Behaviour of native and non-native plants in two tropical rain forests on Oahu, Hawaiian Islands. Phytocoenologia 8: 237–295.
Healey, A.J. 1969. The adventive flora in Canterbury. In The natural history of Canterbury. A.H. and A.W. Reed, Wellington.
Henry, J.E. 1954. The development of native vegetation on pumice country and its relationship with exotic pine forests. New Zealand journal of forestry, 7: 79–82.
Hildebrand, R. 1983: Die Vegetation der Tieflandgebüsche des sudchilenischen Lorbeerwaldgebiets unter besonderer Berücksichtigung der Neophytenproblematik. Phytocoenologia, 11: 145–223.
Hunter, G.G., Blaschke, P.M. 1986: The New Zealand land resource inventory vegetation cover classification. Water and soil miscellaneous publication, Wellington.
Jelinek, L.R. 1977. Letter to the editor. Newsletter of the New Zealand institute of foresters, 9: 24–25.
Julian, A., 1983. Pinus radiata establishment in secondary scrub, Stokes Valley. B.Sc., Honours project, Botany. Victoria University of Wellington.
Keller, Jeannine, 1983. The spread and association of Berberis darwinii in shrubland on the northern hills of Karori and Wilton. B.Sc. Honours project. Botany. Victoria University of Wellington.
Lee, W.G., Allen, R.B., Johnson, P.N., 1986. Succession and dynamics of gorse (Ulex europaeus) communities in the Dunedin Ecological District. New zealand journal of botany, 24: 279–292.
McEwen, W. Mary, 1983. Some aspects of the seed development and seedling growth of rimu, Dacrydium cupressinum. Lamb. PhD. Thesis, University of Waikato.
McQueen, D.R., 1960. Indigenous-induced vegetation and Pinus radiata on volcanic ash soils. Proceedings, New Zealand ecological society, 8:1–14.
McQueen, D.R., 1981. Ecological profiles technique applied to modified indigenous grasslands, Central Otago, New Zealand. In Ecological gradients and boundaries. Proceedings of the Australian ecological society, 11: 131–142.
McQueen, D.R., 1984. New Zealand approximations to Mediterranean climates and their vegetation. Proceedings of the 4th International conference on Mediterranean Ecosystems, Perth: 103–104.
McQueen, D.R., 1991. Plant succession and species diversity in vegetation around Lake Pounui. Tuatara, 31: 43–65.
Mather, L.J., Williams, P.A. 1990. Phenology, seed ecology, and age structure of Spanish health (Erica lusitanica) in Canterbury, New Zealand, New Zealand journal of botany, 28: 207–216.
Molloy, B.P.J., 1976. An analysis of sweet brier on Molesworth In Moore, L.B. The changing vegetation of Molesworth station, New Zealand, 1944 to 1971. New Zealand Department of scientific and industrial research. Bulletin 217
Newsome, P.F.J. 1987. The vegetative cover of New Zealand. Water and soil miscellaneous publication No. 112.
New Zealand Meteorological Service., 1980. Summaries of climatological observations to 1980. N.Z. met.s. misc. pub 177.
Oates, M. 1988: Dynamics of gorse (Ulex europaeus) dominated communities on the eastern Hutt hills. Dissertation for Dip. App. Sc. Botany. Victoria University of Wellington.
Packard, R.Q.A. 1957. Some physical properties of Taupo pumice soils of New Zealand. Soil science, 83: 273–289.
Partridge, T.R., Allen, R.B., Johnson, P.N., Lee, W.G. 1991. Vegetation/environment relations in lowland and montane vegetation of the Kawarau Gorge. Central Otago, New Zealand. New Zealand journal of botany, 29: 295–310.
Preest, D.S. 1963. A note on the dispersal characteristics of the seed of New Zealand podocarps and beeches and their biogeographical significance. In Pacific basin biogeography. Bishop museum press, Honolulu: 415–424.
Smale, M.C., 1990: Ecological role of buddleia (Buddleja davidii) in streambeds in the Urewera National Park. New Zealand journal of ecology, 14: 1–6.
Smith, C.V E. 1979. Understorey vegetation types in Pinus radiata D. Don. plantations in the Wellington Town Belt-with reference to the effect of light intensity. B.Sc. Honours project, Botany. Victoria University of Wellington.
Specht R.L. 1979. Heathlands and related shrublands. Ecosystems of the World 9A. Elsevier.
Vella, W.A. 1984. Rimu regeneration, Maidstone Park. B.Sc. Honours project. Botany, Victoria University of Wellington.
Vucetich, C.G., Leamy, M.L., Poppelwell, M.A. 1960. Soils, forestry and agriculture of the northern part, Kaingaroa State Forest and the Galatea basin. Department of scientific and industrial research, soil bureau bulletin 18. Wellington.
Wardle, P. 1966. Biological flora of New Zealand 1. Weinmannia racemosa Linn. f. (Cunoniaceae) Kamahi. New Zealand journal of botany 4: 114–131.
Wardle, P. 1991. Vegetation of New Zealand, Cambridge University Press.
Webb, C.J., Sykes, W.R., Garnock-Jones, P.J. 1988. Flora of New Zealand. Volume IV. Botany Division, Department of Scientific and Industrial Research, Christchurch.
Wilkinson, E.L., Dunn, G.M., Smith, G.J.S., Thyme in Central Otago. Tussock grasslands and mountain lands institute. Lincoln college. Special publication No. 14.
Williams, P.A. 1981. Aspects of the ecology of broom (Cytisus scoparius) in Canterbury. New Zealand. New Zealand journal of botany 19: 31–43.
Williams, P.A. 1983. Secondary vegetation succession on the Port Hills, Banks Peninsula, New Zealand. New Zealand journal of botany, 21: 237–241.
Williams, P.A., Buxton, R.P. 1986: Hawthorn (Crataegus monogyna) populations in mid-Canterbury. New Zealand journal of ecology 9: 11–18.
Williams, P.A. 1989. Vegetation of the Inland Kaikoura Range, Marlborough. New Zealand journal of botany, 27: 201–220.
Williams, P.A. 1992. Hakea salicifolia: biology and role in succession in Abel Tasman National Park, New Zealand. Journal of the royal society of New Zealand, 22:1–18.
Williams, P.A. 1992. Hakea sericea: seed production and role in succession in Golden Bay, Nelson. (in press, Journal of the royal society of New Zealand.)
Zabkewicz, J.A. 1976. The ecology of gorse and its relevance to New Zealand forestry. New Zealand forest service, Forest research institute symposium 18: 63–68page 50
Fig. 1. Pumice country of the central North Island, as it would have looked before the extensive Pinus radiata plantations of the mid 1920s. The flat valley floor, prone to summer frost, is dominated by Dracophyllum subulatum and the nearer slopes by Leptospermum scoparium. This landscape had been repeatedly burnt, and its last fires in the 1960s were followed by introduced pasture. Tihoi, 1959. D. R. McQueen
Fig. 2. On the same type of country as Fig. 1, Pinus radiata has grown here for 40 years. On the Taupo Pumice soils, with a high moisture retention capacity, the pines have further ameliorated the soil. Protection from fire and frost, along with this soil amelioration induced by the pine, have allowed the development of a tall indigenous understorey up to 8m high, beneath the pines. They are up to 30m high, with diameters close to 1 m. Tokoroa forests, 1976, B. Bay.
Fig. 3. On soils with a lower moisture capacity than those of Taupo Pumice, it takes longer for an understorey to develop beneath Pinus radiata. The pines in Central Park, Wellington are 80–90 years old, and have an understorey up to 7m high, of Cyathea medullaris, Melicytus ramiflorus, Corynocarpus laevigatus and a plant not native to the Wellington area, Pittosporum crassifolium. Wellington, 1992, D.R.Mc Queen.
Fig. 4. A northwest facing slope in Wellington, with the yellow flowers of Ulex europaeus showing as white. The lower slope, once dominated by Ulex has a dense cover of Melicytus ramiflorus up to 5m high with Coprosma robusta and Brachyglottis repanda common. Happy Valley, Wellington, 1992, D.R.Mc Queen.
Fig. 5. Hakea sericea, 30–40 years old, now collapsing with age. The Kunzea ericoides trees in the background are of the same age, and plants of Pseudopanax arboreus and Pteridium esculentum are occupying the gap in the foreground. Abel Tasman National Park. P.A.Williams.
Fig 6. An early stage of Buddleja davidii invasion of an actively aggrading stream bed, Rimutaka Mountains. The older stand, left of centre is well established and seedlings are starting to appear on the more recent gravels, ultimately to suppress the native pioneer, Cassinia leptophylla. Photo. D.J. Campbell.
Fig. 7. Along a freshly recut track, the succession from Berberis darwinii is started by Melicytus remiflorus and Pseudopanax arboreus, their larger leaves emerging through the small-leafed Berberis. Karori, Wellington, 1992, D.R.Mc Queen.
Fig. 8. In older vegetation close to the locality of Fig. 6, Berberis darwinii filling the foreground has persisted under a dense low forest canopy, 6m high. This forest is composed of Melicytus ramiflorus, Hedycarya arborea and Cyathea dealbata, and the Berberis is reacting to shade by producing long lianoid branches reaching up into the canopy. Karori, Wellington, 1992, D.R.Mc Queen.
Fig. 9. A heavy infestation of Clematis vitalba. In the foreground most trees of a secondary indigenous forest have died under the cover of the naturalised liane, whose conical masses are supported on dead trunks. Mangawharariki valley, Rangitikei basin. Carol West.
Fig. 10. A semi-arid landscape changing under the influence of two naturalised shrubs. In the foreground and in the mid-distance, Rosa rubiginosa and on the further slopes Thymus vulgaris. Alexandra district, 1991. D.R.Mc Queen.
Fig. 11. Thymus vulgaris with an almost complete cover. In 1967 this quadrat had a partial cover of Raoulia and Rytidosperma, some remnants of which can be seen to the right of the tape. Alexandra, 1991. D.R.Mc Queen.
Fig. 12. Lupinus arboreus. A dense stand regrowing on coastal sands. The light-coloured dead branches may have been frost killed after a severe winter. Te Horo, 1992. D.R.Mc Queen.