Let it snow? Spring snowpack and microsite characterize the regeneration niche of high-elevation pines

Aim: The persistence potential of forests under rapid climate change will depend on species-specific tolerances to increasing growing season soil moisture stress as snowpack declines. High-elevation tree species may be particularly vulnerable to increasing water stress and associated changes to disturbance regimes because they occur at the environmental margins of tree distributions and are considered snowpack dependent. Here, we evaluate the interacting effects of climate, disturbance, and microsite conditions on tree regeneration in high-elevation, migration-limited pines that have experienced recent disturbance-induced tree mortality.

Location: Great Basin (California & Nevada), USA

Taxon: Gymnosperms; Pinaceae

Methods: We used field observations from 70 sites that varied in climate, disturbance, and local site conditions across semi-arid, high-elevation forests of the Great Basin. We employed structural equation models to evaluate how climate and disturbance interact with microsite conditions to influence regeneration.

Results: We found a broad range of establishment conditions of high-elevation conifers - whitebark, limber, and bristlecone pines – across climatic and disturbance gradients in the Great Basin, but our research detected clear differences in the regeneration niche for each species that may lead to differential survival as climate and disturbance conditions continue to change. Regeneration of whitebark and bristlecone pines diverged in their responses to spring snowpack conditions, with whitebark pine increasing and bristlecone pine decreasing with greater spring snowpack. Limber pine regenerated across a range of climatic and landscape conditions, and this generalist strategy may be advantageous if future climate and disturbance conditions exceed tolerances of more specialized species.

Main Conclusions: Our findings highlight the critical role that spring snowpack, and consequently growing season soil moisture, plays in determining the persistence potential of high-elevation conifers. However, this role varies among species and thus may drive compositional changes as earlier snowmelt drives soil moisture declines across mountainous landscapes of the western United States.