Microsite conditions in a low-elevation Engelmann spruce forest favor ponderosa pine establishment during drought conditions

Abstract Warming climate in U.S. forests may limit future viability of dominant high-elevation species at their lower elevation range limits but favor more drought-adapted sympatric species, potentially contributing to species distribution changes. Adaptive management may address future uncertainty in species distributions by facilitating regeneration of multiple species with contrasting silvics. However, the success of this approach will depend on suitable microsite conditions, the effects of which are not well understood across species and forest types. These concerns are present in the southern Rocky Mountain region, where current and future climate may restrict Engelmann spruce regeneration but benefit sympatric species like ponderosa pine. We used a planting experiment to systematically evaluate microsite conditions which support critical first-season survival and root growth of these contrasting species in a low-elevation spruce forest during a droughty 2018 growing season. Our assessment targeted species responses to variation in canopy cover generated by different silvicultural regeneration treatments, with and without additional shelter from coarse woody debris. We also considered microsite variables of seedling size, vegetation and litter cover, soil moisture and depth, and competition from established tree regeneration. Survival of ponderosa pine was nearly twice that of Engelmann spruce, but the relative effects of microsite variables were similar. Coarse woody debris shelter improved the probability of survival by approximately 357%, though the effect was stronger for spruce. Influences of canopy cover were minimal and opposite our expectations; spruce was marginally inhibited by canopy cover, while pine was positively influenced by more dense canopy. Smaller seedling heights improved the probability of survival for both species by 179% compared to mean seedling heights. Pine root growth was 150% greater in open versus dense canopy microsites and limited in coarse woody debris shelter. We discuss likely site and physiological factors underlying these results. This study provides insight into the factors which define suitable microsites for first-season establishment for these species in rapidly changing low-elevation spruce forests, especially during drought conditions. We discuss implications for adaptive management and silviculture, concluding that regeneration efforts which seek to improve forest resilience to climate-related changes in species viability could consider more drought-adapted, sympatric species as viable supplements to moisture-dependent species at their current lower range limits.