Leaf traits of moist forest tree species differing in shade tolerance

Tropical rain forests are remarkably diverse in tree species. The maintenance of such a high biodiversity is, amongst others, possible through niche differentiation. Niche differentiation occurs when species have different life history traits, and exploit limiting resources in different ways. Insight in the life-history trade-offs that drive species coexistence is far from complete, but pertinent to our understanding of rain forest functioning and diversity. Species coexistence in wet forest there might be a trade-off to tolerate shade and to compete for light.

The shade tolerance of a species is determined by the ability to persist in the shade. Species that differ in shade tolerance differ often markedly in their morphological and physiological leaf traits. Pioneer species that establish in the high resource environment of gaps realize fast growth rates to compete with their neighbours and maintain a position in the top of the regrowing vegetation. They do so by producing short-lived leaves with low construction costs, a high specific leaf area, high nutrient concentrations and high photosynthetic rates. Shade-tolerant species establish and persist in the low resource environment of the forest understory. Investment in leaves is costly as carbon gain and nutrient uptake proceed at low rates in the forest understorey. Leaves should therefore be well protected and long lived to increase the nutrient residence time, and pay back the initial construction costs of the leaves.

The aim of this study is to screen fifty moist forest species for functional leaf traits related to shade tolerance. It is hypothesized that the leaf longevity and leaf toughness increase, and the specific leaf area, nitrogen content, and water content decrease with the shade tolerance of the tree species.

Fifty species are selected that differ in shade tolerance and maximal adult stature. Leaves will are collected for 5 sun trees, and 5 shade trees per species. For each individual leaves are analyzed for a number of traits. The leaf dry matter content is a proxy for the construction costs of the leaves. The specific leaf area indicates the leaf surface for light capture per unit biomass invested. The leaf toughness is an indicator for the resistance of plants to herbivory, the nitrogen content is a proxy for the photosynthetic capacity of the leaf, and the delta 13C is an indicator of the long-term water use efficiency of the leaves.