The influence of water and light on the physiology and spatial distributions of three shrubs in the Southern Appalachian Mountains

In order to understand vegetational gradients which develop in response to environmental gradients, the physiological capabilities of each species must be examined in relation to the observed environmental gradient. The distributions of three temperate zone shrub species which occupy different positions on the spur ridges of Brush Mountain may be influenced by their tolerances to light and moisture. Greenhouse studies indicate that R. maximum has a 60 % reduction in photosynthesis at water potentials below -1.0 MPa. R. nudiflorum has only a 30 % reduction and K. latifolia has less than 10 % reduction.

Seasonal pressure volume curve determinations, conductance measurements, and water potential measurements of plants in the natural environment indicated that photosynthesis is not affected by water potential in K. latifolia. R. maximum has a significant reduction in conductance during drought which may limit photosynthesis. R. nudiflorum appears to begin senescence prior to severe drought in this area.

Greenhouse experiments of adaptability to increasing light intensity show that K. latifolia and R. nudiflorum can significantly increase their light saturation point and maximum rate of photosynthesis in high light environments. R. maximum does not increase its photosynthesis rate in high light environments and appears to undergo chloroplast degradation when grown in high irradiance.

Gradient analysis of vegetation on Brush Mountain shows that R. maximum reaches its highest importance in low light, high moisture sites. K. latifolia is most important in low moisture, high irradiance sites. R. nudiflorum is infrequent in the study area but occurs mostly in moderate moisture and irradiance sites. The distributions of R. maximum and K. latifolia appear to match their experimentally determined physiological tolerances. The results for R. nudiflorum are inconclusive.