Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Life History of the New Zealand Conifer Broadleaf Forest
Life History of the New Zealand Conifer Broadleaf Forest
There are a number of theories on this topic.
Linear Succession (Climax) (Fig. 64)
A postulated sequence63,64,65 which leads to the present forests, with their overstorey of scattered podocarps and canopy of flowering trees, is as follows:
Figure 64 Diagrams illustrating linear and cyclic forest successions. (Reproduced with permission from To Save a Forest—Whirinaki.)
Within this association, broader-leaved shrubs and small trees establish, including five-finger (Pseudopanax arboreus), lancewood (P. crassifolius)Coprosma lucida, Pittosporum tenuifolium, juvenile rewa rewa (Knightia excelsa), juvenile kamahi (Weinmannia racemosa), and in moister places the tree ferns mamaku (Cyathea medullaris) and ponga (C. dealbata), mahoe (Melicytus ramiflorus), wineberry (Aristotelia serrata), tree fuchsia (F. excorticata) and pate (Schefflera digitata).
When these broad-leaved plants form a canopy the bracken, manuka and kanuka gradually die out as the light falls below the level required for their continued recruitment. It is at this stage, about 50 years from the beginning of the sequence, when the forest floor is sheltered but still quite well-lit, that the podocarp conifers become established. After a further 50 years or so the podocarps begin to grow above the broadleaf canopy and by the time they form a higher stratum themselves, the lower layer of flowering trees is now dominated by kamahi (Weinmannia racemosa), often initially epiphytic on the ponga tree fern, with scattered rewa rewas (Knightia excelsa), hinau (Elaeocarpus dentatus) and maire (Nestegis cunninghamii and N. lanceolata).
Finally, shade-demanding species enter and tawa (Beilschmiedia tawa) largely replaces kamahi in the forest canopy. The forest floor is now strongly shaded and the podocarps, light-demanding to varying degrees, no longer establish. The podocarp trees which are already present, often in combination with epiphytic northern ratas (Metrosideros robusta), gradually die out and the climax, according to this hypothesis, is a forest with the canopy dominated by tawa and without emergents. The tawa and other shade-demanding species are able to maintain themselves indefinitely until some catastrophic event destroys the forest to initiate a new succession.
Cycles (Fig. 64)
Field observations by a number of botanists have led to some questioning of the above sequence of events. Some have suggested that, rather than there being a succession leading to a climax, there may in fact be repetitive cycles. For example, on the volcanic plateau of the central North Island it has been observed in forests with emergent podocarps page 108over a main canopy of kamahi, or kamahi and tawa, that where a podocarp falls the gap is occupied by tree ferns. Kamahi establishes epiphytically on the tree ferns and podocarps then regenerate under the kamahi and eventually overtop them.66,67 A similar sequence has been observed in south Westland.68
Probably because of their importance as timber trees, the lack of regeneration of podocarps in some New Zealand conifer broadleaf forests has received particular attention. According to the successional and cyclic hypotheses just reviewed this would be a normal consequence of forest development.
Holloway69 suggested that the general lack of regeneration of podocarps since the time when the present mature to aging emergents established is due to a climate change to colder and drier conditions from about 1300 A. D.
Holloway based his climate change hypothesis on forest patterns in western Southland, in which he saw evidence of a downward movement of vegetational zones, and on historical and other evidence of a worldwide colder interval, the 'Little Ice Age', during the last millenium.
Wardle70 investigated a number of podocarp-dominated stands on the west and east of the South Island and in Stewart Island. He supported Holloway's hypothesis but concluded that the 'regeneration gap' started later than Holloway suggested, from about 1600-1800 A. D., and that it was most marked in the drier eastern South Island and least marked on the wetter west and in Stewart Island. Since about 1800 the regeneration of podocarps has resumed at some localities.
Angiosperm Forest Dominance
According to a more dramatic interpretation of the dynamics of the New Zealand conifer broadleaf forest put forward by Robbins,71 it is not in fact one forest, but two in competition — one comprising the conifers and the other the flowering trees. The conifers belong to a more ancient and less specialised group of plants, which along with ferns and their allies formed forests in New Zealand before flowering plants became dominant throughout the world. Following the establishment of the more specialised flowering trees in New Zealand the less specialised conifers, he suggests, have been on the road to extinction. page 109Their present poor regeneration is seen as a reflection of this trend. Only where raw new soil conditions, unsuitable for most flowering trees, are brought about by volcanism or glaciation can the conifers still form dense forests and even these give way to flowering trees as more mature soils develop.
The validity of this interpretation can be questioned. Firstly, conifers have coexisted with angiosperms in New Zealand for 100 million years, so it seems unlikely that they will disappear for some time to come. Secondly, if the conifers are doomed to extinction because they are more ancient and less specialised, wouldn't this be even more true for the ferns, an older and less specialised group than the conifers? In fact ferns are abundant in New Zealand forests and give no cause for any belief that they are on the road to extinction. The probable truth of the matter is that when a more specialised plant or animal group becomes dominant throughout the world, many members of the preceding dominant group become extinct; but there is no reason why the survivors could not evolve new forms suited to the changed conditions. This would certainly appear to be true for the ferns as most forest ferns belong to an advanced group, which came into existence at about the same time as the now dominant angiosperms and diversified with them. The tree ferns and filmy ferns are more ancient groups but they give every indication of being permanent components of the conifer broadleaf forests.
As far as conifers are concerned New Zealand is not the only place where apparently inadequate regeneration in mature forests has been noted. It has been observed in forests of Melanesia with species of Araucaria72 and Agathis,73 and similar extinction hypotheses have been proposed. Studies in both areas have been carried out to test the validity of these hypotheses and they have all concluded that the conifers concerned have a permanent role in the forests as a result of recurring natural disturbances. In New Zealand a similar investigation74 has been made into the role of the mountain cedar (Libocedrus bidwillii) in a number of montane rain forests in the South Island and the same conclusion has been drawn.
This seems to suggest then that the failure of emergent conifers (as indeed of many tropical angiosperm emergents75,76) to regenerate under a dense canopy is a normal feature of rain forest development, and that special climatic and evolutionary hypotheses are unnecessary.