Foliage area and crown nitrogen turnover in temperate rain forest juvenile trees of differing shade tolerance

Abstract

Summary 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.