GENOTYPIC AND PHENOTYPIC VARIATION AS STRESS ADAPTATIONS IN TEMPERATETREE SPECIES - A REVIEW OF SEVERAL CASE-STUDIES

Species that occupy large geographic ranges or a variety of habitats w ithin a limited area deal with contrasting environmental conditions by genotypic and phenotypic variation. My students and I have studied th ese forms of ecophysiological variation in temperate tree species in e astern North America by means of a series of field and greenhouse expe riments, including controlled studies with Cercis canadensis L., Fraxi nus pennsylvanica Marsh., Acer rubrum L., Prunus serotina Ehrh. and Qu ercus rubra L., in relation to drought stress. These studies have incl uded measurements of gas exchange, tissue water relations and leaf mor phology, and have identified genotypic variation at the biome and indi vidual community levels. Xeric genotypes generally had higher net phot osynthesis and leaf conductance and lower osmotic and water potentials at incipient wilting than mesic genotypes during drought. Xeric genot ypes also produced leaves with greater thickness, leaf mass per area a nd stomatal density and smaller area than the mesic genotypes, suggest ing general coordination among leaf morphology, gas exchange and tissu e water relations. Leaf phenotypic plasticity to different light envir onments occurred in virtually every study species, which represented a wide array of ecological tolerances. In a study of interactions of ge notypes with environment, shade plants, but not sun plants, exhibited osmotic adjustment during drought and shade plants had smaller reducti ons in photosynthesis with decreasing leaf water potential. In that st udy, sun, but not shade, plants had significant genotypic differences in leaf structure, but with certain variables phenotypic variation exc eeded genotype variation. Thus, genotypic variation was not expressed in all phenotypes, and phenotypes responded differentially to stress. Overall, these studies indicate the importance of genotypic and phenot ypic variation as stress adaptations in temperate tree species among b oth distant and nearby sites of contrasting environmental conditions.