© 1994 Heron Publishing—Victoria, Canada
Genotypic and phenotypic variation as stress adaptations in temperate tree species: a review of several case studies
Marc D. Abrams
School of Forest Resources, The Pennsylvania State University, 4 Ferguson Building, University Park, PA 16802, USA / Received July 14, 1993
Summary
Species that occupy large geographic ranges or a variety of habitats within a limited area deal with contrasting environmental
conditions by genotypic and phenotypic variation. My students and I have studied these forms of ecophysiological variation
in temperate tree species in eastern North America by means of a series of field and greenhouse experiments, including controlled
studies with Cercis canadensis L., Fraxinus pennsylvanica Marsh., Acer rubrum L., Prunus serotina Ehrh. and Quercus rubra L., in relation to drought stress. These studies have included measurements of gas exchange, tissue water relations and leaf
morphology, and have identified genotypic variation at the biome and individual community levels. Xeric genotypes generally
had higher net photosynthesis and leaf conductance and lower osmotic and water potentials at incipient wilting than mesic
genotypes during drought. Xeric genotypes also produced leaves with greater thickness, leaf mass per area and stomatal density
and smaller area than the mesic genotypes, suggesting general coordination among leaf morphology, gas exchange and tissue
water relations. Leaf phenotypic plasticity to different light environments occurred in virtually every study species, which
represented a wide array of ecological tolerances. In a study of interactions of genotypes with environment, shade plants,
but not sun plants, exhibited osmotic adjustment during drought and shade plants had smaller reductions in photosynthesis
with decreasing leaf water potential. In that study, sun, but not shade, plants had significant genotypic differences in leaf
structure, but with certain variables phenotypic variation exceeded 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 phenotypic variation as stress adaptations in temperate tree species among both distant and nearby sites
of contrasting environmental conditions.
Keywords:
Acer rubrum, Cercis canadensis, drought, ecophysiological variation, Fraxinus pennsylvanica, gas exchange, irradiance, leaf morphology, osmotic adjustment, Prunus serotina, Quercus rubra, tissue water relations.
