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1 Shade-tolerant angiosperm trees are reported to require relatively fertile soils in temperate forests. We explored the possibility that high foliage allocation in shade-tolerant species could result in higher whole-plant nitrogen demands than in light-demanding trees of comparable diameters. We measured foliage mass and area, leaf life span, and nitrogen (N) content of fresh foliage and leaf litter for juveniles of 11 evergreen angiosperms in a Chilean temperate rain forest. This permitted estimation of annual nitrogen losses in leaf litter fall by individuals of a given diameter.
2 Leaf life spans were longest in shade-tolerant species. The highest leaf N levels were found in small short-lived early successional trees, whereas among longer-lived species there was no general relationship of leaf N with shade tolerance level. Shade-tolerant species had lower N resorption efficiencies, and therefore higher leaf litter N concentrations on an area basis, than light-demanding associates.
3 Foliage mass, foliage area and total crown N pool were strongly positively related to shade tolerance level.
4 Interspecific variation in annual N losses in leaf litter fall was more closely related to foliage area (R2 = 0.52) than to N concentration of leaf litter (R2 = 0.31) or leaf lifetimes (R2 = 0.01). Although the short-lived early successional species Embothrium coccineum had the highest annual crown N losses of the 11 species, shade-tolerant species had higher annual losses than light-demanding overstorey dominants of comparable longevities.
5 The results are consistent with the proposal that the costs of obtaining enough N for crown maintenance and expansion may constrain the fitness of shade-tolerant angiosperm trees in late successional stages on infertile sites, when soil nutrient availability is reduced by increased uptake and sequestration in biomass and litter. On the other hand, high N loss rates may be more sustainable for short-lived early colonists that complete their life cycles in the initial stages of secondary succession, when nutrient availabilities are often relatively high.
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- Materials and methods
Shade-tolerant late-successional tree species typically have low maximal growth rates (Bazzaz 1979; Pacala et al. 1994) and might therefore be expected to have low nutrient requirements (Grime 1979; Reich et al. 1995). However, a number of studies have shown that highly shade-tolerant angiosperm trees are actually scarce on infertile soils, a pattern reported both from northern temperate forests (Spurr & Barnes 1980; Keddy & MacLennan 1990; Franklin et al. 1993) and also from temperate rain forests in the coast ranges of southern Chile (Lusk 1996a,b). As shade-tolerant trees are also often drought-sensitive (Smith & Huston 1989), and as availabilities of nutrients and water are highly correlated in many landscapes, it is difficult to rule out the possibility that this pattern is primarily a response to site water balance. However, rainfall is high in the Chilean coast range forests (Almeyda & Saez 1958) and most of the trees that are common on infertile sites in this region are drought-sensitive species confined to humid maritime climates (Weinberger 1973; Weinberger et al. 1973), making water limitation unlikely in this case.
Studies of leaf physiology provide few clues as to why shade-tolerant angiosperm trees might be averse to low soil fertility. Instantaneous photosynthetic nutrient-use efficiency at leaf level is generally low in shade-tolerant species (Seemann et al. 1987; Pons et al. 1994; Reich et al. 1995a) yet their leaf nitrogen and phosphorous levels are usually similar to, or lower than, those of shade-intolerant trees (Popma et al. 1992; Reich et al. 1995a).
A consideration of nutrient demand at crown level, rather than leaf level, may help understand this pattern. Comparative studies of plant nutrient loss rates have usually focused on the main components of mean residence time of nutrients in the foliage biomass, i.e. leaf life span and nutrient resorption efficiency (Berendse & Aerts 1987; Escudero et al. 1992; Aerts 1996). Few if any studies have addressed the possible nutritional implications of species differences in biomass allocation to foliage. Shade-tolerant trees tend to have deeper crowns and greater foliage areas than light-demanders of comparable diameter (Ellenberg 1978; Chapman & Gower 1991; Canham et al. 1994). As synthesis of photosynthetic enzymes and pigments requires large nutrient inputs, leaves have higher concentrations of most mineral nutrients (especially nitrogen) than other vegetative organs. Furthermore, although foliage comprises only a small fraction of total standing biomass in trees, it is subject to more rapid turnover than woody tissues, and therefore accounts for an important fraction of annual biomass allocation (King 1991). As foliage turnover is often the principal mechanism of nutrient loss for woody perennials (Miller et al. 1976; Berendse et al. 1987), development and maintenance of a high leaf area by shade-tolerant trees may imply significantly higher whole-plant nitrogen demands than in less-tolerant species. However, a number of studies in evergreen forests have reported long leaf life spans in shade-tolerant trees (Williams et al. 1989; King 1994; Reich et al. 1995a), so although their crown nitrogen pools are likely to be larger than those of light-demanding associates, they may turn over more slowly. Nevertheless, few angiosperms appear to be capable of producing leaves that live more than 4–5 years. (Reich et al. 1995b). This constraint on angiosperm leaf life spans, coupled to high leaf areas in shade-tolerant trees, may therefore result in inevitably high nutrient requirements in these taxa.
We measured foliage mass and area, leaf longevity, and nitrogen concentrations of foliage and leaf litter for 11 evergreen tree species of varying shade tolerance in the Chilean coast range forests, and examined how these traits interact to determine annual nitrogen losses in leaf litter fall. We hypothesized that maintenance of high leaf areas in the most shade-tolerant angiosperm species would be associated with greater nitrogen losses in leaf litter fall than for shade-intolerant species of comparable diameters.