Factors driving Pinus jeffreyi regeneration at a forest-shrubland ecotone in eastern California, USA

Global climate change will affect distribution of plant species, community species composition, and ecosystem functioning. This research focuses on large-scale precipitation and small-scale microhabitat drivers of Pinus jeffreyi regeneration at a forest-shrubland ecotone in eastern California. Germination and establishment were examined by planting Pinus jeffreyi seeds and seedlings into areas of altered snow depth (increased, decreased, and ambient snow depth manipulated by snowfences) and microhabitat (under the potential nurse species Artemisia tridentata and Purshia tridentata and in open intershrub spaces) in the spring of 2006, and monitoring survival and growth throughout the growing season. Soil water availability, photosynthetically-active radiation, air temperature, and soil temperature were measured in snow depth and microhabitat treatments. Snow melted earlier on decreased-depth compared to increased-depth plots. Soil water content at 15 cm depth was higher for increased snow depth plots during the beginning of the snowmelt period. There were differences in PAR between nurse shrub canopies and open intershrub spaces, but there were no differences in soil or air temperature. Overall seedling mortality was very high (ca. 80%) by the end of October. No differences in seedling growth or mortality due to snow depth were observed. Seedling mortality due to drought was higher in intershrub spaces as compared to under nurse shrubs, but mortality due to herbivory was higher under nurse shrubs. No differences in seedling growth among microhabitats were observed. Results suggest that microhabitat niches may be equally if not more important than changes in precipitation patterns for Pinus jeffreyi establishment at this ecotone.