STOMATAL AND NONSTOMATAL LIMITATIONS OF PHOTOSYNTHESIS IN RELATION TOTHE 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 su n and shade phenotypes during drought. Throughout the study, photosynt hesis was highest and mesophyll limitations of A (L(m)) lowest in the intolerant species in both open and understory habitats. The shade tol erant species exhibited the only drought-related decreased A and incre ased L(m) in the open, and the greatest drought-related decreased A an d 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 associat ed with large increases in stomatal limitations to A (L(g)). Combined changes in L(m) and L(g) resulted in increased relative stomatal limit ation 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 as sociated with shallower slope of the l(g) versus g(c) relationship, an indication of greater A capacity. Photosynthetic capacity tended to b e greater in the intolerant species than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant spec ies in the understory. Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynth etic light reactions. If so, reduced carbon reduction capacity of shad e tolerant species or shade phenotypes may predispose them to drought conditions, which suggests a mechanism behind the well-recognized trad eoff between drought tolerance and shade tolerance of temperate tree s pecies.