Effects of microhabitat on the distribution and abundance of a rare understory plant, Actaea elata

The abundances and distributions of plants are greatly affected by a species’ ecological requirements.  Thus, narrow environmental constraints may contribute to a species’ rarity.  Actaea elata (tall bugbane) is a rare, perennial herb found in the understory of mixed western red cedar-hemlock forests from Oregon to British Columbia. Results from initial monitoring of B.C. populations of A. elata indicated that there was variation in population density and structure between different forest types (coniferous vs. deciduous). To assess the importance of the microhabitat variation within these forest types on both plant distribution and the growth and reproduction of A. elata individuals, we measured several environmental variables at vegetative and reproductive plants, as well as at random points, at four sites encompassing a variety of forest types. We observed significant differences between sites in these variables; specifically, the lowest soil moisture, highest herbaceous cover, and lowest canopy closure were found at the primarily deciduous site with the densest and most fecund A. elata population.  This indicates that overall site characteristics may be critical in controlling the plant’s abundance and distribution.   In addition, reproductive individuals were more likely to be located in microhabitats of lower canopy closure than both vegetative individuals and control plots, highlighting the importance of light in the reproductive potential of these plants.  Knowing what environmental characteristics constrain A. elata can be used to help refine practices for locating new populations and inform conservation decisions about habitat management.