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.