Leaf function in tropical rain forest canopy trees

In this thesis the effect of constant and fluctuating light availability on several leaf traits was studied for naturally growing trees of different sizes, i.e. from sapling to adult canopy tree, of five species in a tropical rain forest in French Guiana. Leaf acclimation responses were examined throughout the life time of leaves in order to evaluate whether leaves can profit from these adjustments in terms of carbon gain. The five species, arranged in order from most shade-tolerant to pioneer, were: Duguetia surinamensis, Vouacapoua americana, Dicorynia guianensis, Pourouma bicolor spp. digitata, and Goupia glabra.

For Duguetia, Vouacapoua, Dicorynia and Goupia, it was shown that tree height and light availability had independent effects on photosynthesis and other features of leaf function. Direction and magnitude of the variation in leaf variables tended to be similar among species. The morphological variable leaf mass per unit area seemed to be a key variable as it determined most of the variation in other leaf variables.

The time needed to increase the photosynthetic rate (up to 90% of its capacity) to a sudden increase in light was between 7 to 11 min for shade and sun growing saplings of Vouacapoua, Dicorynia and Pourouma. The readiness to exploit the next lightfleck was substantial in these plants as the induction loss was moderate to low, except in gap saplings of Dicorynia. The time needed to reach 75% of the maximum carboxylation efficiency (Vcmax) was used to separate the relative importance of biochemical and stomatal limitation during the time course of photosynthetic induction.

The mean leaf life span of different-sized trees for Vouacapoua was 61 months (range 27-101) and for Dicorynia 32 months (range 17-54). The variation in traits in response to leaf age was low. Photosynthetic capacity and nitrogen concentration were relatively constant with time; leaf mass per unit area increased during the first 18 months. Simulations of the daily carbon gain with a low- and highlight regime showed differences among both sun- and shade-leaves and trees of different sizes. Leaf construction cost was independent of leaf life span. Leaf payback time was relatively short (4 to 40 days); it was constant during a wide range of irradiance, but increased sharply in a narrow range of low light. The rate of net return on carbon investment was slow in long-lived leaves of Vouacapoua. Leaf mass per area decreased with increased leaf life span, which could be explained by a light- and height-dependent selection pressure for leaf life span and leaf mass per area.

The integration of the results with those at higher organisation levels, such as branch and tree crown, is briefly discussed, and the applicability in silvicultural systems in which light is manipulated to enhance growth and production of timber species is evaluated.

Key-words: tropical rain forest, leaf morphology, photosynthesis, leaf age, tree height, shade tolerance, leaf construction costs, leaf payback time, ecophysiology.

Toon Rijkers graduated December 1, 2000.

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