Temporal Variation in Photosynthetically Active Radiation (PAR) in Mesic Southern Appalachian Hardwood Forest with and without Rhododendron Understories
Understanding spatial and temporal variation in, the understory light regime of southern Appalachian forests is central to understanding regeneration patterns of overstory species. One of the important contributors to this variability is the distribution of evergreen shrub species, primarily Rhododendrun maximun L, We measured photosynthetically active radiation (FAR) in the underscory of closed forest canopies in areas with and without R. maximum. Measurements were taken from March through November 1993 wirh a frequency of approximately 3 weeks. In 1994 measuremeants resumed from March through May. PAR under sub-canopies of R. maximum averaged 77% (e.g., 14 vs. 65 µmol m-2s-1) lower than in areas without R. maximum during the growing season. and 70% (e.g., 179 vs. 641 µmol m-2s-1) lower in the dormant season. Variation in PAR during the growing season ranged from 73% to 86% lower in R. maximum versus non-R. maximum understories. During the growing season, light levels beneath Rhododendron were observed to be <2% of full sun. Low-tight environments associated with R. maximum understories are extremely limiting with respect to regeneration of important hardwood species.