Effects of Rhododendron maximum on the understory herb Cardamine diphylla (Brassicaceae) and associated soils in winter1

Cardamine diphylla is an understory herb of rich forests that exhibits a wintergreen life history strategy in western North Carolina. Unlike many forest floor plants, C. diphylla is able to persist under the evergreen shrub Rhododendron maximum, which is known to inhibit the growth of woody and herbaceous plants. We compared morphological and chemical characteristics of C. diphylla leaves in areas of a mesic cove forest with and without R. maximum during the winter. We also compared soil properties and the taxonomic richness of co-occurring herbaceous and woody plants. Results showed that C. diphylla growing under R. maximum produced significantly less foliar biomass but had significantly higher levels of foliar nitrogen and sulfur. Plants under R. maximum also had a higher proportion of green (rather than purple) abaxial leaf surfaces. Soils under R. maximum contained significantly less total nitrogen and humic matter, but cation exchange capacity and pH did not differ between the two areas. Compared to the surrounding forest, plots in R. maximum thickets supported 35% fewer woody taxa and 41% fewer herbaceous taxa, demonstrating the challenges of this microhabitat to understory plants. The ability of C. diphylla to persist under R. maximum may be advantageous if R. maximum continues its opportunistic expansion in southern Appalachian forests.