Tree Size- and Age-Related Changes in Leaf Physiology and Their Influence on Carbon Gain

Abstract

Understanding how leaf-level physiology changes with tree size and age is important for scaling single leaf measurements to the whole plant and stand level and for quantifying carbon fluxes from forest ecosystems. This chapter reviews what is known about the influences of tree height and age on gas exchange and foliar structure in both gymnosperm and angiosperm trees. We address how the key physiological processes, photosynthesis, respiration and stomatal conductance vary with tree height and age. To help explain the observed patterns, the underlying factors that can be responsible for the changes in leaf physiology are assessed, including tree size- and age-related trends in foliar anatomy, morphology and chemistry. In addition to modifications in foliar morphology and chemistry, biochemical limitations to net assimilation rates associated with the diffusion of carbon dioxide from the atmosphere to the sites of carboxylation are examined. Our review emphasizes that a variety of factors collectively are responsible for tree height- and age-related decline in net photosynthetic rates, and that the importance of different limitations varies for different species and between gymnosperm and angiosperm trees. While there is still much to be learned, what is clear from our current understanding is that more integrated studies that consider the simultaneous roles of leaf structure, chemistry and stomatal and mesophyll factors are needed to disentangle and assign importance to the various factors responsible for decreases in carbon gain with tree age and size.