Hydrastis canadensis L. (Ranunculaceae) distribution does not reflect response to microclimate gradients across a mesophytic forest cove

Spatial patterns of understory plant distribution can reflect availability of suitable abiotic microsites. Hydrastis canadensis is a native, herbaceous perennial whose distribution may be constrained by microsite availability. We planted 5 transects each on south- and north-facing cove hillsides with clonally derived rhizomes of H. canadensis. Transects were spaced 20, 40, 60, 80, and 100 m from a third-order stream. Because the transect 20 m from the stream on the south-facing hillside was adjacent to a natural H. canadensis patch, this transect was postulated to represent suitable habitat. We tested the effects of aspect and distance from stream on phytometer growth measures (survival, leaf area, and both rhizome and leaf area relative growth rates). We also monitored temperature, humidity, and light, then quantified environmental distances for these measures between each transect location and the transect in suitable habitat. Plant growth measures were then regressed on these distances to test hypotheses about factor effects. Neither survival nor relative growth rates depended on aspect or distance from the stream, although leaf area was greater on the north-facing aspect in both years and increased with proximity to the stream in 2003. Rhizome relative growth rate did not depend on any of the environmental distance measures, although leaf area change depended on cumulative light, increasing as the environmental distance from the suitable site increased. The relatively weak association between environmental variation across the forested cove reinforces other studies suggesting that H. canadensis has a relatively broad ecological niche, and its rarity is unlikely due to lack of availability of suitable habitat.