Optimization of light use by tropical canopy trees through acclimation of leaf physiology and orientation

The goal of this dissertation was to understand how physiology, spatial orientation, and availability of light of individual leaves change within crowns of adult canopy trees. First, I describe how frequency distributions of instantaneous photosynthetic photon flux density (PPFD) above the forest changed as a function of cloudiness. Clouds greatly modified the frequency distributions of PPFD by reducing PPFD transmittance through the atmosphere. Frequency distributions of transmittance changed predictably with daily cloudiness and were described with digamma probability distributions. I show that it is possible to predict frequency distributions of instantaneous PPFD from simple measurements of daily cloudiness.;The second objective was to determine how leaf physiology, morphology, and orientation changed as a function of long-term (9--17 days) PPFD incident on leaves. In order to characterize leaf acclimation, five trees from three species were selected. Measurements of instantaneous PPFD on leaf adaxial surfaces were taken every two seconds from 15 leaves per tree. The frequency distributions of PPFD indicate that most measurements were below 500 mumol m-2 s-1, independent of species identity or leaf position in the canopy. Leaf thickness, leaf mass per area (LMA), leaf nitrogen per area, and leaf maximum assimilation rate (Amax) increased with increasing total PPFD. However, the rate of change in these traits decelerated with increasing total PPFD suggesting that photosynthetic acclimation was constrained at high PPFD. In contrast, leaf diurnal net photosynthesis showed a near-linear increase with PPFD availability. This near-linear response was attributed to greater leaf inclination at high total PPFD, which maximized instantaneous PPFD in the linear portion of the photosynthetic light response curves.;The third objective was to determine if leaf physiological acclimation and changes in orientation were coordinated to maximize leaf light-use efficiency (LUE). Most of the time, instantaneous PPFD incident on individual leaves was in the linear region of the photosynthetic light response curves where LUE was highest. Thus, leaf physiology and orientation acted in concert to maximize leaf-LUE. I propose that maximization of leaf-LUE is the basic mechanism behind the widely reported linear relationship between canopy photosynthesis and long-term light absorbed by the canopy.