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

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Abstract
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(gc) in 1991 compared to 1990. The mesic species had drought related decreases in photosynthesis (A) attributed primarily to increased absolute stomatal limitation toA(Lg), whereas in the wet‐mesic species, the absolute mesophyll limitation (Lm) was at least as important asLgin limitingAduring drought. The xeric species maintained relatively highAduring drought despite decreasedgc.In the xeric and mesic species,Lmdecreased andLgincreased during drought due to stomatal closure. From xeric to mesic to wet‐mesic, the relative stomatal limitation (Ig) generally decreased faster, and relative mesophyll limitations toAincreased faster, with increasinggcsuggesting greater photosynthetic capacity (i.e. greater potential maximumA) 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 ofAvary in a manner consistent with species' drought tolerance and site conditions, and that nonstomatal constraints toAin field plants during a moderate, season‐long drought were generally not as severe as reported in controlled studies.