Effects of Rhododendron maximum L. on Acer rubrum L. Seedling Establishment

ABSTRACT
Rhododendron maximum L. restricts regeneration of overstory species; however, the mechanisms are poorly understood. Three treatments were used to examine the effects of R. maximum germination success and survival of Acer rubrum L. under a closed overstory canopy: (1) R. maximum understory, (2) open understory, and (3) open understory with shadecloth. Shadecloth treatments mimicked the low light conditions beneath R. maximum (<5 % full sun) while controlling for R. maximum forest floor influences. Soil moisture was significantly lower under R. maximum than under open or shadecloth treatments. Initially, Acer rubrum survival was similar for open understory and shadecloth treatments, while germination and survival in R. maximum plots were low. After approximately 20 d, mortality in the shadecloth treatment increased and survival rates were less than 5% by the end of the sampling period. Survival in the open understory treatment stabilized at >2✕ that observed under shadecloth, while percent germination and survival in R. maximum plots were lower than in either of the other treatments throughout the sampling period. These results suggest that in addition to the light limitation associated with R. maximum, edaphic effects such as low soil moisture or allelopathic compounds may inhibit Acer rubrum success.