Seasonal variation in stomatal conductance and physiological factors observed in a secondary warm-temperate forest

This study quantified stomatal conductance in a CO2-fertilized warm-temperate forest. The study considered five items: (1) the characteristics of the diurnal and seasonal variation, (2) simultaneous measurements of canopy-scale fluxes of heat and CO2 and the normalized difference vegetation index (NDVI), (3) the stomatal conductance of sunlit and shaded leaves, (4) a stomatal conductance model, and (5) the effects of leaf age on stomatal conductance. Sampled plants included evergreen and deciduous species. Stomatal conductance, SPAD, and leaf nitrogen content were measured between March and December 2001. Sunlit leaves had the largest diurnal and seasonal variation in conductance in terms of both magnitude and variability. In contrast, shaded leaves had only low conductance and slight variation. Stomatal conductance increased sharply in new shooting leaves of Quercus serrata until reaching a maximum 2 months after full leaf expansion. The seasonal changes in the canopy-scale heat and CO2 fluxes were similar to the change in the canopy-scale NDVI of the upper-canopy plants. These seasonal changes were correlated with the leaf-level H2O/CO2 exchanges of upper-canopy plants, although these did not represent the stomatal conductance in fall completely. Seasonal variations in the leaf nitrogen content and SPAD were similar, except leaf foliation, until day 130 of the year, when the behaviors were completely the opposite. A Jarvis-type model was used to estimate the stomatal conductance. We modified it to include SPAD as a measure of leaf age. The seasonal variation in stomatal conductance was not as sensitive to SPAD, although estimates for evergreen species showed improvements.