Analysis of Vegetation Changes in Rock Creek Park, 1991-2015

Krafft, Cairn; Hatfield, Jeff

Rock Creek Park is a 702 ha natural area that lies within the boundaries of Washington, D.C. (Ferebee 2003). Long-term vegetation plots were established by the National Park Service (NPS) in 1991, as part of a region-wide effort to develop a vegetation monitoring plan. One of the goals of the monitoring was to determine the effect of white-tailed deer (Odocoileus virginianus) populations on the native vegetation, since white-tailed deer are known to have a major impact on other native plant communities (Russell et al. 2001, Horsley et al. 2003). Vegetation data collected at Rock Creek Park every 4 years during 1991-2015 were analyzed for differences among 3 regions within the park and among years. The variables measured and analyzed were percentage of twigs browsed, percentage of canopy cover, species richness of herbaceous plants, number of tree seedlings in each of 7 height classes, tree seedling stocking rate for low deer density and high deer density areas, percentage of cover and species richness for tree, shrub and vine cover <2 m in height, mean diameter at breast height (DBH) of trees greater than or equal to 1 cm DBH, and number and species richness of tree stems greater than or equal to 1 cm DBH. Repeated measures analysis of variance (ANOVA) was used to test for differences (P<0.01) among years, and causative factors should be investigated further. In addition, importance values were calculated for the 10 most important tree species in each region and changes over time were reported. Where warranted by the normality and lack of sparseness of the data, native and non-native species were analyzed separately for the variables listed above. Results for non-native shrub and vine percentage cover and species richness did not exhibit the same significant declines over time as observed for native species, although percentage cover for both of these categories remained extremely low. Future sampling recommendations are also discussed.