Quantifying stomatal and non-stomatal limitations to carbon assimilation resulting from leaf aging and drought in mature deciduous tree species

Gas exchange techniques were used to investigate light-saturated carbon ass imilation and its stomatal and nonstomatal limitations over two seasons in mature trees of five species in a closed deciduous forest. Stomatal and non -stomatal contributions to decreases in assimilation resulting from leaf ag e and drought were quantified relative to the maximum rates obtained early in the season at optimal soil water contents. Although carbon assimilation, stomatal conductance and photosynthetic capacity (V-cmax) decreased with l eaf age, decreases in V-cmax accounted for about 75% of the leaf-age relate d reduction in light-saturated assimilation rates, with a secondary role fo r stomatal conductance (around 25%). However, when considered independently from leaf age, the drought response was dominated by stomatal limitations, accounting for about 75% of the total limitation. Some of the analytical d ifficulties associated with computing limitation partitioning are discussed , including path dependence, patchy stomatal closure and diffusion in the m esophyll. Although these considerations may introduce errors in our estimat es, our analysis establishes some reasonable boundaries on relative limitat ions and shows differences between drought and non-drought years. Estimatin g seasonal limitations under natural conditions, as shown in this study, pr ovides a useful basis for comparing limitation processes between years and species.