Age and distribution of an evergreen clonal shrub in the Coweeta Basin: Rhododendron maximum L

Background/Question/Methods

In many areas of the Appalachians, Rhododendron maximum (Rmax) forms a dominant evergreen sub-canopy layer. Rmax is a key species in southern Appalachian forests for several reasons: 1) it contributes to landslide initiation; 2) it inhibits regeneration of herbaceous and woody species, particularly tree species; and 3) it can alter ecosystem processes, such as water and carbon balance, and biogeochemistry.  Much of the literature focusing on Rmax’s role in Appalachian forests, has stated that Rmax has increased in coverage over the last 70+ years; however, the empirical evidence to validate this statement is scant and conflicting.  Thus, the extent of establishment and spread of Rmax in Appalachian forests during the past century is an unsettled question.  To investigate the age and distribution of Rmax across the Coweeta Basin, a 1626 ha watershed in western North Carolina, we selected 16 perennial, second-order streams.  For each location, we established a 100-m stream reach and transposed this 100-m length to a parallel line on the ridge. We used a Global Positioning System to establish points around the site boundaries and around Rmax patches to map the distribution of Rmax across the hillslopes. In each site, three transects were established perpendicular to the streams and transects extended from the stream edge to the ridge. For each transect, diameters of overstory trees (≥2.5-cm dbh) were measured in 10-meter width belts and tree saplings (<2.5-cm dbh) and shrubs including Rmax stems were measured in 1.0-meter width belts. We cut a cross-section of Rmax at 0 m, 5 m, 10 m, 15 m, 20 m, and then every 10 m along each transect to determine the age of individual ramets and extracted increment cores from the nearest neighbor trees to determine age and radial growth.

Results/Conclusions

The 16 hillslope sites were different sizes (ranging from 0.3 to 1.9 ha) depending on the distance from stream to ridge (ranging from 21 to 288 m). Rmax cover ranged from 25 to 100 %. In total, we aged 430 Rmax cross-sections and ramets ranged from 25 to 120 years old. We found that distance from stream explained a significant (p=0.011), but small percentage (<2 %) of the variation in Rmax age. It appears that Rmax has not expanded upslope over the last 100 years; rather the ranges in sizes and ages suggest that ramets are recruiting under established Rmax canopies by vegetative reproduction.