ECOPHYSIOLOGICAL RESPONSES OF FAGUS-SYLVATICA SEEDLINGS TO CHANGING LIGHT CONDITIONS .1. INTERACTIONS BETWEEN PHOTOSYNTHETIC ACCLIMATION AND PHOTOINHIBITION DURING SIMULATED CANOPY GAP FORMATION

Natural regeneration of European beech (Fagus sylvatica L.) establishe s under shade, but sudden exposure to high irradiance may occur due to openings in the canopy. To elucidate ecophysiological mechanisms asso ciated with survival of European beech seedlings, the gas exchange, ch lorophyll concentrations, and chlorophyll a fluorescence parameters of two different beech populations were studied under changing light con ditions. Plants were grown both in a growth chamber and at a natural s ite (one population) where the seedlings were raised in containers pla ced in understory and in simulated canopy gaps. Upon exposure to high light in the growth chamber, photosynthetic rates of shade-acclimated leaves of seedlings from both populations increased severalfold and th en decreased over several days to the rates of the low-light control s eedlings. High-light seedlings always had the highest photosynthetic r ates. Initial fluorescence displayed a trend opposite that of photosyn thesis; it increased over time, and relative fluorescence and half-tim e rise declined continuously until the end of experiment to very low v alues. Exposure to high light of shade-acclimated seedlings resulted i n a shift in chlorophyll concentrations to levels intermediate between high-light and low-light seedlings. The light treatment effects were statistically greater than population effects; however, seedlings from the Abetone population were found to be more susceptible to changing light conditions than seedlings from Sicily. Reciprocal light treatmen ts on plants growing at the natural site confirmed the results obtaine d in the growth chamber experiment. Overall, beech seedlings grown in the field appeared to have a fairly large acclimation potential achiev ed by plasticity in the photosynthetic apparatus. The lack of pronounc ed acclimation to high light in seedlings grown in the growth chamber was ascribed to a threshold-type relationship between the acclimation capacity and the level of damage. These observations on the limited po tential for acclimation to high light in leaves of European beech seed lings which show a clear capability to exploit sunflecks, are discusse d in relation to regeneration following canopy gap formation and reinf orce the view of the central role of gap formation in forest dynamics. We conclude that small forest gaps (in which sunflecks play a major r ole) may present a favorable environment for survival and growth of be ech because of their limited ability to acclimate to a sudden increase in irradiance and because of the moderate levels of light stress foun d in small gaps.