Photosynthetic response to flooding of Acer rubrum seedlings from wet and dry sites

Seeds were collected from three red maple swamps (wet sites) and three upla nds (dry sites) near Ithaca, New York, and the resulting seedlings were use d in flood tolerance studies to investigate if red maple's broad habitat ra nge is due to ecotypic differentiation. One-year-old seedlings were flooded while still dormant (spring flooding study) and net photosynthesis, growth and chlorophyll levels vi-ere measured at 1 mo intervals for 3 mo. Floodin g reduced net photosynthesis, growth and chlorophyll levels in seedlings fr om both sites, but survival of both wet and dry site seedlings was near 100 %. After 1 mo of flooding net photosynthesis of wet and dry site seedlings were similar, but after 3 mo, flooded wet site seedlings had higher photosy nthetic rates than did dry site seedlings. Control wet rite seedlings were significantly larger than dry site seedlings and had significantly higher p hotosynthetic rates and chlorophyll levels. These differences suggest eithe r genetic variation between seedlings from the two habitats in response to the growing conditions or the influence of seed size differences and confou nd the spring flooding study results. Flooded wet site seedlings had higher , final, net photosynthetic rates than did dry site seedlings, but the resp onse to flooding was greater for wet site seedlings than it was for dry sit e seedlings. In a second study (summer flooding study), 1-y-old seedlings in full leaf w ere flooded for 22 d and then drained to determine if recovery from floodin g stress differed for wet and dry site seedlings. Again, flooding decreased net photosynthesis for seedlings from both habitats but, when the trees we re drained, net photosynthesis for wet site seedlings recovered more quickl y and to a higher level than it did for dry site seedlings. Flooding also c aused a drop in chlorophyll level for seedlings from both habitats, but chl orophyll levels of seedlings from neither habitat recovered when the seedli ngs were drained. There were no significant differences between wet site an d dry site control seedlings for net photosynthesis; therefore, the quicker and larger recovery of photosynthetic potential in wet site seedlings in t he summer flooding study suggests that ecotypic differentiation has occurre d and that genetic differences, in part, account for red maple's occurrence on contrasting edaphic sites.