Tuatara: Volume 29, Issues 1 and 2, August 1987
Secondary Succession In Windy Gully, Mt Rimutaka
Secondary Succession In Windy Gully, Mt Rimutaka
Vegetation of a burnt Nothofagus forest site was sampled in Windy Gully, Mt Rimutaka between 591m and 714m. Exposure to wind is a major factor in vegetation structure and succession. Comparisons were made with the results of A. White's 1951 study and showed 1) succession to Weinmannia racemosa forest at lower altitude and more sheltered sites; 2) lower altitude but exposed vegetation in poorly drained sites is still dominated by Leptospermum scoparium; 3) at higher altitude and more exposed sites the persistence of Dracophyllum filifolium dominance shows that little change in vegetation composition and structure has occurred in 35 years.
Keywords. Ecology; succession; wind; Dracophyllum filifolium; Leptospermum scoparium; Weinmannia racemosa.
Windy Gully is on the northern flank of Mt Rimutaka close to the summit of the Rimutaka Hill Road (S.H.2) between the Hutt Valley and the Wairarapa (see Figure 1a). The gully forms a headwater tributary to the Rimutaka Stream.
The original vegetation of the area is thought to have been Nothofagus forest, as evidenced by Nothofagus found throughout the area by White (1951), and by small relicts of Nothogagus menziesii and N. fusca forest still present near the Rimutaka Hill Road Summit. From burn scars located on several trees White assumed that two fires had occurred in the area, one about 1893 and the other about 1915, but she could find no record of these fires.
Fire has destroyed most of the original vegetation near the Rimutaka Hill Road. White noted that in 1951 this vegetation had been largely replaced by gorse and pasture, with indigenous scrub in Windy Gully.
The soils of Windy Gully have developed on loess which is deeper in the lower part of the study area. The soils of the area are classified as Rimutaka soils of moderately clay illuvial. steepland central yellow-brown earths. These soils formed under mor-forming beech or kamahi forests with high rainfalls, cool temperatures, very low fertility and slow vegetation growth (N.Z. Soil Bureau, 1968). The loess deposits mostly overlie greywacke colluvium derived from upslope, but on more topographically prominent sites overlie basement greywacke.
There are no rainfall data for the Windy Gully area, but mean annual rainfall (1941-70) at Kaitoke, 223m above sea level, (see figure 1a) is 2141mm (N.Z. Met. Service. 1984). Rainfall in Windy Gully, near the crest of the Rimutaka Range, is therefore assumed to be in excess of 3000mm annually, fairly evenly distributed throughout the year.
Fig. 1. Locality maps. (a) summit area of Rimutaka Hill, (b) Sketch map of study area showing physiognomic sections and sample plot locations.
|2.||Leptospermum scrub of lower height than section 1.|
|3a.||Leptospermum / Weinmannia — Pseudopanax — Hebe scrub.|
|3b.||Leptospermum / Griselinia — Olearia — Cortaderia scrub.|
|3c.||Dracophyllum — Leptospermum — Weinmannia / Cortaderia scrub.|
|3d.||Dracophyllum — Cortaderia / Leptospermum — Weinmannia heathland.|
|3e.||Dracophyllum / Leptospermum heathland.|
|3f.||Dracophyllum / Olearia — Coprosma / Astelia — Phormium heathland.|
|4.||Leptospermum / Weinmannia — Griselinia tall scrub.|
The present study was undertaken in the upper part of Windy Gully, between the road (591m) and Mt Rimutaka trig (714m) to describe variations in the present scrub composition in relation to environmental factors, and to compare the present stand composition and structure with that of 1951 as recorded by White (op. cit.).
Field work in Windy Gully was carried out from 24 to 27 February 1986 inclusive. There was a new-cut track up the north-west facing side of the gully from the page 36 roadside to the trig. This track was followed and an initial subjective assessment made of physiognomic variations in the vegetation. White (op. cit.) identified eight physiognomic sections in the gully (see Figure 1b) which were still recognisable in the present survey. This study was restricted to White's sections 1, 2, 3 and 4 on the north-west facing slope of the gully. Road-widening work in recent years has resulted in the loss of some of White's section 1. In this study White's section 3 was subdivided into six subsections (see Fig. 1b).
Eight temporary quadrats were established, (Fig. 1b) to sample a range of vegetation types. Quadrats were established at random within each section or subsection. Each quadrat measured 12m by 3m with a 2m wide surround zone.
A general site description and a list of plant species present was made for each quadrat.
One soil pit was dug per quadrat, and a description of the soil profile and its ecological characters noted. Soil samples were taken from each horizon for pH and macroporosity determinations in the laboratory. Soil texture and structure and humus form were determined by field observations.
Ten wood disc samples per quadrat of Leptospermum scoparium were taken for age and growth measurements. Samples of Dracophyllum filifolium and Weinmannia racemosa were also collected when available. Rings were counted in the laboratory and mean growth rates calculated for each sample. Mean radial growth per ten year period was calculated and cumulative growth for each species graphed. It was assumed that the growth rings were annual.
Minimal sampling area was determined for each quadrat by relating cumulative number of species with cumulative area as a check of plot size and homogeneity, (Mueller-Dombois and Ellenberg, 1974).
A field point height intercept analysis (PHI) was carried out following the methods of Park (1973). The data were then analysed on the IBM 4341 to obtain vegetation parameters. The % Crown Space of each species in each quadrat thus obtained was then analysed by a CLUSTAN programme on the IBM 4341 and a dendrogram obtained which grouped the quadrats by species % Crown Space as a weighting of species presence.
In order to compare stand parameters of the 1951 study with those of the present study, a point height intercept analysis was carried out in the laboratory on line transect diagrams drawn to scale by White (op. cit.). One hundred regularly spaced points along each line transect were sampled, using appropriately scaled height intervals and the edge of a ruler. It is not known exactly where White sited her transects, but since her physiognomic sections were recognisable in and formed a base for the present study, the transect of each of her sections 1 to 4 was subjected to point height intercept analysis. These four 1951 “plots” were included in the CLUSTAN analysis and resulting dendrogram.
Results And Discussion
A list of all plant species recorded in the present study was made (Appendix A). The PHI top height intercept data, ranked, were plotted to a canopy surface height curve for each 1986 quadrat and the laboratory analysis of each 1951 transect (Fig. 2).
For quadrat 4/1 (a sheltered site) only 76 points were recorded in the field, instead of the normal 100, but a marked increase in vegetation height and change page 37 in vegetation structure in this section since 1951 is apparent. The very steep gradient below 1.8m of the 1986 curve (Fig. 2d) indicates that very few low plants are now contributing to the canopy, with none below 0.5m. and that the vegetation has now formed a closed canopy. The irregularity of this canopy surface is indicated by the steep gradient of the whole curve, which is steeper than that for the 1951 plot in the section. The 1951 transect did not have a closed canopy, as shown by some ground level points having no vegetation above them.
Quadrat 2/2 (also sheltered) (Fig. 2b) has a similarly closed canopy, nearly as irregular as 4/1. The height and gradient of the curve for 2/1 (more exposed site) are considerbly less than for 2/2. and the curve is almost identical to that of the higher altitude 3d/1 (Fig. 2c). This reflects the exposed position of 2/1. where there has been little height increase compared with the 1951 transect 2 and with 1986 2/2. however canopy closure (except at one point) has occurred.
Section 1 (Fig. 2a) has shown almost complete canopy closure and considerable height growth since 1951.
Fig. 2. Canopy surface height curves for 1986 and 1951. (a) section 1, (b) section 2, (c) section 3, (d) section 4.
Fig. 3. Dendrogram of Rimutaka Summit plots (1986 and 1951) which have been clustered by species abundance in the crown. Plots connected by low coefficients of dissimilarity are similar in species composition, 4/1 refers to the 1986 quadrat in section 4; 4/51 refers to the 1951 transect in section 4.
From Figure 2 it appears that the canopy height and structure of the vegetation in all map sections in 1951 was almost completely within the range exhibited in section 3 in 1986. Height growth and canopy closure have occurred in vegetation at lower altitudes and in less exposed situations, but canopy surfaces have become more irregular with this growth.
The co-efficient of variation (obtained from PHI analysis) showed a decrease in value with the growth from irregular canopy to a smoother surface. Thus it appears that the very low canopy heights of the heathland sites, on the skeletal soils at higher altitudes, have an uneven canopy with low vegetation space and low canopy surface height. As the Vegetation Space increases from site to site, the mean canopy surface height increases, but at the same time the proportion of the Vegetation Space occupied by the crown decreases as understorey development occurs. A site with low mean canopy surface height will ultimately have a low Vegetation Space.
From the CLUSTAN analysis (Fig. 3), plots showing close similarity with each other fell into two groups: 1) section 3 plots; 2) plots in sections 1, 2 and 4. Section 3 plots (3/51, 3d/1, 3d/2, and 3f/1) indicated a species similarity within the section. Quadrat 3f/1 was not grouped quite as closely as the other three, reflecting some species difference because of its higher altitude (694m). quadrat 3e/1 was not grouped with the other section 3 plots, and it appeared that its very exposed site, thin soil and open stunted vegetation have given rise to a very different composition from other quadrats in the section.page 39
|Sample No.||Horizon||Mean pH||Depth to lowest Macroporosity||Depth of root zone||Macroporosity||Slope|
|1/1/3||B||4.405||12.25||39 cm||39 cm||8°|
|2/1/3||B||4.705||10.91||30 cm||30 cm||23±|
|2/2/3||A2||4.815||12.87||40 cm||33 cm||29°|
|3d/1/3||A||4.295||10.64||21 cm||24 cm||>38°|
|3d/2/3||A||3.505||19.17||13 cm||12 cm||>38°|
|3f/1/3||A||3.445||7.53||15 cm||13 cm||15°|
|4/1/4||B||3.935||8.03||43 cm||43 cm||8°|
The grouping of lower altitude plots in sections 1, 2 and 4 (1/51, 2/51, 2/2, 4/51 and 4/1) reflected the similarity of species presence between these sections. The 1986 section 1 quadrat was not grouped with these plots, indicating a change in crown species and species % Crown Space proportions since 1951, and also reflecting the possibility that some or all of White's section 1 site may have been lost to road-widening work. Quadrat 2/1 was not included in this grouping. This is attributed to the more exposed nature of this site.
Soil pH determinations showed that the more acidic soils were the skeletal soils of the higher altitude plots. This was possibly a reflection of the slower litter breakdown due to the more exposed situations of the plots. These higher plots also showed shallow soil development with subsequent shallow root depth, and generally poorer drainage indicated by lower macroporosity values compared with lower altitude plots. (Table 2). Dracophyllum filifolium is predominantly found on these shallow, acidic, poorly drained soils at higher, more exposed locations, and its litter is considered to contribute to soil acidity.
Depth of litter decreased with increasing altitude and exposure.
The lower altitude plots showed greater soil development with better drainage, indicated by higher macroporosity values, and less acidity. Consequently greater root depth was encountered reflecting greater subaerial vegetation growth at the surface.
Yellowish to reddish brown mottles in lower horizons were indicative of iron deposition derived by leaching in the evenly distributed high rainfall, facilitated by the mor type humus, from upper horizons which were generally pale in colour. These features indicated early stages of podsolisation. Colluvium in the C horizons was compacted with loam of a higher clay content than horizons above. Soils at all sites were classed as silty loams.
From each quadrat set of ten wood discs a mean annual radial growth rate was calculated for each species sampled. These growth rates are presented in Table 3. Quadrats are ordered altitudinally, the highest on the left.
A comparison of the growth rates of the dominant species at each site shows a steady decrease with increasing exposure and altitude. Such a decrease in growth rate may also be a result of different growth rates for different species as Dracophyllum filifolium at higher altitudes replaces Leptospermum scoparium. However the most exposed and most sheltered sites show that growth rate correlates with the degree of exposure (see Fig. 4). irrespective of species.
In comparing the 1986 and 1951 data, a change is evident to taller, more even canopy vegetation in the less exposed sites on deeper skeletal soils. However in the more exposed higher sites there has been little physiognomic change in the vegetation. Considered together, all four sections in 1951 resemble the 1986 3d/1 vegetation, with more L. scoparium in the lower altitude sections (1 and 4). Floristically the sites have changed very little in 35 years although the lower altitude more sheltered. L. scoparium — Weinmannia racemosa stands appear to be at a later stage in succession.
The ages of the L. scoparium trees indicate that the oldest L. scoparium is between 30 and 45 years old, which indicates it colonised the area fairly uniformly. This does not however match White's suggestion that the last fire here was in 1915. Possibly after the 1915 fire this area could have been grazed for quite a few years before being left to revert to scrub. W. racemosa in 4/1 is also much older than L. scoparium in 4/1 and. in fact, any of the other L. scoparium sampled from the gully. It is possible that after the gully was allowed to revert to scrub there was a fire that destroyed much of the vegetation, although there was no direct evidence of this. However any W. racemosa present would resprout from a trunk base after such a fire and so would be of a greater age than the L. scoparium.
It would appear that the sheltered parts of the gully are succeeding quickly towards a closed canopy L. scoparium — W. racemosa shrubland with W. racemosa gradually becoming dominant. However on the more exposed sites little change towards a closed canopy has occurred since 1951, and it may be that the extreme conditions in 3e/1 and 3d/1 could preclude eventual succession to forest, except by slow marginal invasion.
Since 1951 vegetation growth has been considerably greater in lower altitude and more sheltered areas of the gully. Lower altitude areas are on deeper soils and show highest growth rates in the most sheltered positions. Growing conditions here also allow greater variation in growth rates between individuals giving rise to a canopy surface that is closed but irregular and less dense. Lower altitude areas that are very exposed show some wind suppression of the vegetation, however, the lower altitude and deeper soil here offsets the exposure effect compared with higher areas. Higher altitude areas show slower growth rates and maintain a low, dense, even but not fully closed canopy surface. This slow development can be linked with wind supression, and shallower soils.
D. filifolium dominates the scrub at higher altitudes, with continued recruitment of new individuals. L. scoparium dominates lower altitude scrub, and also shows continued recruitment. There is a zone of transition between these two species. W. racemosa has established under L. scoparium and in the most sheltered area appears to be emerging through the L., scoparium. Younger individuals of W. racemosa occur occasionally in all other areas except those very close to the ridge crest. It appears likely that L. scoparium and D. filifolium scrub will persist for some time in Windy Gully, but that in sheltered sites at lower altitudes this vegetation will eventually be replaced by W. racemosa if undisturbed.
Thanks are given to Dr Ross McQueen for his help in the field and in editing. These results were obtained from the 1986 field and laboratory work of Botany 314. We thank our classmates for their cheerful and accurate assistance.
Thanks are also given to Dean Stotter for carrying out soil pH and macroporosity determinations. Sarah Adams for field and computing assistance, and Dr Geoff Rogers for computing assistance.
Allan. H.H. 1961. Flora of New Zealand Vol I. Government Printer. Wellington.
Moore. L.B. and E. Edgar 1976. Flora of New Zealand Vol II, Government Printer, Wellington.
Mueller-Dombois. D. and H. Ellenberg. 1974. Aims and Methods of Vegetation Ecology. Wiley. New York.
N.Z. Met. Service. 1984. Rainfall observations for 1984. N.Z. Met. S. Pub. 110.
N.Z. Soil Bureau. 1968. Soils of New Zealand. Soil Bur. Bull. 26(1).
Park. G.N. 1973. Point height intercept analysis. N.Z.J. Bot. 11:103-14.
White. A. J. 1951. The vegetation surrounding Mt Rimutaka, with particular reference to wind. Unpublished M.Sc. thesis. Victoria University of Wellington.page 43
Plant species list, Windy Gully, Mt Rimutaka (* = adventive species)
* Present address: Department of Horticulture. N.Z. Technical Correspondence Institute, Private Bag. Lower Hutt.