WHERE DO THEY COME FROM, WHERE DO THEY GROW? POSTFIRE CONIFER SEED DISPERSAL AND ESTABLISHMENT IN RUGGED TERRAIN

Abstract

Changing disturbance regimes and climate can disrupt regeneration mechanisms in obligate seeders by increasing distances to seed sources and creating unsuitable establishment conditions. However, complex topography may buffer these effects by generating dynamic wind patterns that support seed dispersal or harboring suitable microclimates for seedling growth despite a warming, drying climate. We conducted complementary field experiments on the Bridger-Teton National Forest (Wyoming, USA) to assess the effect of complex topography on: (1) conifer seed dispersal, (2) lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii) germination and seedling establishment, and (3) microclimate. We trapped seeds in areas of stand-replacing fire that varied along a ruggedness gradient and planted seeds in areas that varied in terrain shape (concave or convex) and ruggedness (rugged or gentle) outfitted with a microclimate datalogger. Rugged terrain enhanced lodgepole pine seed dispersal by increasing seed delivery frequency and density farther from seed sources. However, we found no difference in wind-dispersed seed delivery upslope versus downslope of mature forest. Ruggedness impeded germination, and concave terrain fostered greater germination in lodgepole pine but not Douglas-fir. Rugged sites lost soil moisture rapidly and were drier across the growing season than gentle sites. Soils in concave rugged sites were also warmer across the growing season than gentle or convex rugged sites. These strong but contrasting effects suggest that rugged terrain may produce substantial postfire spatial heterogeneity in conifer regeneration.