Seasonal water availability drives trait variation in isolated Basin and Range Pinus ponderosa

Combined effects of rising temperatures and drought are threatening forests globally. These unprecedented conditions are likely to decrease forest resilience, leading to widespread tree mortality and loss of associated forest ecosystem services. Drought adaptations may, however, confer success under these projected extreme conditions and be key to the perpetuation of long-lived tree species. The timing, amount, and type of moisture strongly influence the degree of drought adaptation in a given population, and local topographic heterogeneity may exacerbate or mitigate these effects, driving variation in trait response both within and among populations. Although drought adaptations are well-studied in widespread tree species, knowledge is limited on the extent of drought-responsive traits in disjunct conifer populations.In the northern portion of the arid Basin and Range province of the western United States, Pinus ponderosa var. scopulorum is isolated to montane sky islands, making it a model system for testing the effects of climate and topography on conifer species trait variation. We sampled 55 populations across six ranges in Nevada and Utah to quantify trait variation in cone volume, wood density, specific leaf area, and needle lifespan, and asked how these characters were related to conditions at collection sites. To investigate the relationship between interacting climatic and topographic conditions with drought adaptations we explored whether seasonal climatic moisture deficit (CMD), monsoonality, and aspect were associated with trait variation using linear mixed models. Traits varied widely both within and among populations, with seasonal water availability most strongly linked to trait responses. Cone volumes increased with increasing summer CMD but decreased in areas with a heavy monsoon influence. The seasonality of moisture similarly influenced wood density, with densities increasing with increasing winter CMD, signaling the importance of winter moisture for tree growth. Needle lifespan was also influenced by monsoonality, indicating that late summer precipitation leads to earlier needle shed and reduced retention. Local topographic variation had a minor influence on trait variation for the populations tested here, mediating climate effects on northern slopes and acting as a compounding stressor on southern aspects for wood density response alone. These findings suggest the importance of seasonal moisture stress on drought-adapted conifer traits in semi-arid environments and highlight the potential of these conifers to alter traits to conserve growth under water limitations. Both responses are likely to be important given the threat of altered climatic conditions for these systems