Stomatal and nonstomatal limitations of photosynthesis in relation to the drought and shade tolerance of tree species in open and understory environments

Light saturated photosynthesis (A) in field saplings of shade tolerant, intermediate, and intolerant tree species was analyzed for stomatal and nonstomatal limitations to test differences between species and sun and shade phenotypes during drought. Throughout the study, photosynthesis was highest and mesophyll limitations of A (L(m)) lowest in the intolerant species in both open and understory habitats. The shade tolerant species exhibited the only drought-related decreased A and increased L(m) in the open, and the greatest drought-related decreased A and increased L(m) in the understory. Few species exhibited significant habitat or drought-related differences in stomatal conductance to CO2 (g(c)), but even slight decreases in g(c) during drought were associated with large increases in stomatal limitations to A (L(g)). Combined changes in L(m) and L(g) resulted in increased relative stomatal limitation to A (l(g)) in several species during drought. Nevertheless, the overall lack of stomatal closure allowed for nonstomatal limitations to play a major role in reduced A during drought. Higher leaf N was associated with shallower slope of the l(g) versus g(c) relationship, an indication of greater A capacity. Photosynthetic capacity tended to be greater in the intolerant species than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant species in the understory. Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynthetic light reactions. If so, reduced carbon reduction capacity of shade tolerant species or shade phenotypes may predispose them to drought conditions, which suggests a mechanism behind the well-recognized tradeoff between drought tolerance and shade tolerance of temperate tree species.