Substrate effects on morphology and biomass allocation of forest understory plants

Considerable research has been devoted to the role of coarse woody debris (CWD) in forest ecosystems. However, few studies have examined how plant performance is affected by rooting substrate. We examined morphology and biomass allocation in three common understory plants in young coniferous forests of western Washington. We asked whether plant traits differed between substrates (CWD vs. forest floor), and whether variation could be attributed to differences in resource availability. Replicate samples of CWD and forest floor (fine litter/mineral soil) were collected and analyzed for volumetric moisture, pH, total carbon (C), total nitrogen (N), and available N. Thirty replicates of three species, Maianthemum dilatatum (clonal herb), Tiarella trifoliata (non-clonal herb), and Vaccinium parvifolium (shrub), were excavated from paired substrates (CWD and forest floor). Plant morphology and biomass were measured for above- and belowground portions of each plant. CWD had greater moisture, greater total C, less total N, and lower pH than did forest floor; available N was similar between substrates. Species did not differ in aboveground traits between substrates, but did differ in belowground traits. Maianthemum root systems were more linear and extended deeper in CWD; Tiarella and Vaccinium roots covered greater area in CWD (and were shallower for Tiarella). Ratio of above- to belowground biomass was consistently greater on CWD for all species (but significant only for Maianthemum). Trends in biomass allocation are consistent with the higher moisture content of CWD, but the diverse morphological responses of species suggest complex responses to resource availability and other factors.