Summary1 Angiosperm trees often dominate forests growing in resource-rich habitats, whereas conifers are generally restricted to less productive habitats. It has been suggested that conifers may be displaced by angiosperms except where competition is less intense, because conifer seedlings are inherently slow growing, and are outpaced by faster-growing angiosperm species. Here we investigate whether competition with ferns and deeply shading trees also contributes to a failure of conifers to regenerate in resource-rich habitats. 2 We examined how changes in soil nutrient availability and drainage affected vegetation along the retrogressive stages of a soil chronosequence in southern New Zealand. Vegetation composition shifted from angiosperm-tree dominance on 'recent' alluvial terraces (< 24 ky), via coniferous-tree dominance on older marine terraces (79-121 ky), to coniferous-shrub dominance on the oldest marine terraces (291 ky). Soil drainage deteriorated along the sequence, and N : P leaves and N : P soil indicate increasing Plimitation. Conifer species appear to be adapted to persistence on infertile and poorly drained soils. 3 The floor of the relatively fertile alluvial forests was deeply shaded ( ∼ 1% light transmission) by dense groves of tree-ferns and ground-ferns, and by large-leaved subcanopy trees. Few seedlings of any type were found on the forest floor, even in tree-fall gaps, and establishment was restricted to rotting logs and tree-fern trunks. Angiosperms were particularly successful at colonizing these raised surfaces. 4 Less shade was cast by the conifer-dominated forests on infertile marine terraces ( ∼ 5% light transmission), which lacked tall ferns. There were many opportunities for conifer establishment, with high seedling densities recorded on the forest floor and on logs. By contrast, angiosperm seedlings were mainly restricted to logs. 5 Our results suggest that several mechanisms act in concert to reduce regeneration opportunities for conifers in productive habitats. In particular, we suggest that tall ferns and deep shade are responsible for a restriction of regeneration opportunities in relatively productive forests in New Zealand, diminishing the opportunities for conifers to escape the competitive effects of fast-growing angiosperms. Thus 'crocodiles' may alter the outcome of the race between 'hares' and 'tortoises'.
Summary 1.A central concept in forest ecology is that differences in the growth rates and shade tolerances of tree species determine patterns of secondary succession. The most shade-tolerant tree species are the competitive dominants in late-successional forests, while species with fast growth rates persist through rapid establishment after disturbance. There is ample support for niche differentiation along the shade-tolerance axis, at least for temperate forests, but less thought has been given to the range of shade tolerances and growth rates encountered within a community and to how it might vary along environmental gradients. 2. We hypothesized that a wider range of growth rates and shade tolerances are found on nutrientrich soils, because such soils not only support fast-growing species with high metabolic rates, but also species capable of tolerating the very deep shade cast by forest canopies growing where nutrients are plentiful. We test our hypothesis by quantifying light transmission through two neighbouring forests in southern New Zealand, one on phosphorus-rich alluvial soil and one on phosphorus-depleted marine-terrace soil, and comparing the growth rates of saplings on these contrasting sites. 3. Less light was transmitted to the forest floor on alluvial sites than on marine terraces (2.5% vs. 7.5% daylight, on average) and neighbourhood analyses within mapped stands indicated that large-leaved subcanopy species were responsible for intercepting that extra light. Sapling growth was strongly inhibited by shade in the understorey of the alluvial forests, but was less inhibited under the terrace forests. 4. Fast-growing subcanopy species were common on the alluvial sites and these species had characteristically soft leaves and high foliar-nutrient concentrations. Slow-growing shade-tolerant species were also abundant on these sites. Therefore, the interspecific variance in growth rates was greater on nutrient-rich sites, supporting our hypothesis of a greater range of shade tolerance niches on better soils. Of the five species found on both forest sites, all five had greater high-light growth rate on the alluvial sites. 5. Synthesis : A wider range of growth rates was observed in the nutrient-rich forests. This wider range may translate into a greater number of shade tolerance niches and thereby provides an explanation for the greater numbers of species commonly found on nutrient-rich soils when compared with neighbouring nutrient-poor sites.
Question: What are the composition, structure and extent of contemporary, common woody vegetation communities in New Zealand? How do the woody plant communities we describe, based on representative sampling, compare to those of previous New Zealand classifications?Methods: We used cluster analysis to classify data from 1177 systematically located vegetation plots, calculated spatial extent and ecological statistics for each alliance defined, and combined forest alliances into groups to assess correspondence with earlier mapped classifications. We used plot-based data on vegetation structure to infer potential community changes.Results: Twenty-four alliances were recognized, each of 19 to 105 plots, with estimated extents of 144 000-794 000 ha. Stand structure data suggest that 15 alliances are largely stable whereas nine may undergo compositional change. Among alliances, the proportion of exotic species ranged from 0 to 50%. Our forest and shrubland classification is another stage in progressively revising such classifications (i.e. forest class maps, vegetative cover map, ECOSAT woody classification and land cover database) produced over the last five decades.
Saplings of canopy tree species frequently undergo alternating periods of suppression and release before reaching canopy size. In this study, we document the effects of periods of suppression and release on current responses to variation in light by saplings of the 11 major tree species of northwestern, interior British Columbia. We were specifically interested in the degree to which increasing length of suppression had long-term effects on subsequent response to release in gaps or following partial cutting, and the degree to which the effects of suppression were ameliorated with time following release. At least some saplings of all 11 species had undergone alternating periods of suppression and release. The most shade-tolerant species generally did not show either a decline in growth over time during suppression or a gradual increase in growth at a given light level over time during release. The least shade-tolerant species exhibited significant declines in growth rate during suppression; however, in all of the species except trembling aspen (Populus tremuloides Michx.), the effects of suppression disappeared over time during release. Failure to account for the effects of past suppression and release leads to significant overestimates of the initial responses of shade-intolerant species to release. Our results suggest that competitive balances between species shift substantially over time as a result of growth history and that these shifts have significant effects on successional patterns.
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