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Why are there leaves at the lowest part of a tree?
On C-gain, C-costs and the value of playing games. |
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Hendrik Poorter,
Ecophysiology of Plants, Utrecht University, The Netherlands.
H.Poorter@bio.uu.nl
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In this talk, I will first present results of a model that calculates the optimal Leaf Area Index (LAI) and leaf area distribution of a monostand of plants at a given availability of N. Using game theory, and allowing plants to modulate their Specific Leaf Area while they keep the nitrogen concentration of the leaves constant, it is then shown that the evolutionary stable strategy can be quite remote from the optimal solution. In all cases, LAI in the evolutionary stable strategy is higher and stand photosynthesis is lower than what could be considered optimal.
Taking this approach somewhat further, we considered the forest at the Swiss Canopy Crane site near Basel. There is a marked difference in the leaf area distribution of different tree species, with trees like Pinus sylvestris having branches only high up in the tree, whereas Fagus sylvatica has leaves down to very low levels. One may wonder what exactly the use of these lower leaves is for a plant. Are they attributing positively to the carbon balance of the plant? Or are they just there to shade competitors? And does that differ a lot for different species? To assess this question we tried to make a - necessarily rough - calculation of C-gain and C-costs of the uppermost light-exposed sun leaves and lowest shade leaves of 10 different tree species. We estimated the light climate, the rate of photosynthesis and respiration during day and night, as well as the chemical composition of these leaves. From these data we derive the pay-back time, the time a leaf needs to meet its construction costs.
At the moment of writing this summary, we still have no idea what the outcome will be.
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