Stomatal and nonstomatal limitations of photosynthesis in 19 temperate tree species on contrasting sites during wet and dry years

A unique approach was used to evaluate stomatal and nonstomatal constraints to photosynthesis in 19 naturally occurring, deciduous tree species on xeric, mesic and wetmesic sites in central Pennsylvania, USA, during relatively wet (1990) and dry (1991) growing seasons. All species exhibited significantly decreased stomatal conductance to CO₂ (g_c) in 1991 compared to 1990. The mesic species had drought related decreases in photosynthesis (A) attributed primarily to increased absolute stomatal limitation to A (L_g), whereas in the wet-mesic species, the absolute mesophyll limitation (Lm) was at least as important as L_g in limiting A during drought. The xeric species maintained relatively high A during drought despite decreased g_c. In the xeric and mesic species, L_m decreased and L_g increased during drought due to stomatal closure. From xeric to mesic to wet-mesic, the relative stomatal limitation (I_g) generally decreased faster, and relative mesophyll limitations to A increased faster, with increasing g_c suggesting greater photosynthetic capacity (i.e. greater potential maximum A) with increasing drought tolerance rank of species. Few species exhibited a significant drought-related decrease in photosynthetic capacity. The results of this landscape-based study indicate that the interaction of stomatal and nonstomatal limitations of A vary in a manner consistent with species' drought tolerance and site conditions, and that nonstomatal constraints to A in field plants during a moderate, season-long drought were generally not as severe as reported in controlled studies.