ENVIRONMENTAL AND PHYSIOLOGICAL FACTORS INFLUENCING THE NATURAL DISTRIBUTION OF EVERGREEN AND DECIDUOUS ERICACEOUS SHRUBS ON NORTHEAST AND SOUTHWEST SLOPES OF THE SOUTHERN APPALACHIAN MOUNTAINS .1. IRRADIANCE TOLERANCE

The segregated distributions of Rhododendron maximum (valley positions), Rhododendron periclymenoides (NE slopes), and Kalmia latifolia (SW slopes) were compared to the respective irradiance environments. R. maximum, an evergreen species, was most sensitive to high radiation; quantum yield, light saturated photosynthetic capacity, photosynthesis per chlorophyll, and water use efficiency decreased at high ambient irradiance. These characteristics limit growth on high irradiance SW slopes. Both K. latifolia and R. periclymenoides improve photosynthetic performance at high ambient irradiance. The deciduous R. periclymenoides continued increasing leaf conductance at high irradiance without an increase in photosynthesis indicating a possible limitation by water in high light environments such as SW slopes. The evergreen K. latifolia had reduced photosynthetic capacity and reduced water use efficiency when grown in low irradiance conditions which coincides with its higher abundance on high light, SW slopes. -from Authors