Tree species resistant to drought stress are likely to be more successful under the moisture-limited conditions predicted with climate change, and variation in drought-resistant traits can influence a species’ sensitivity to change. Trait variation is often tied to the timing, amount, and type of precipitation a given forest stand is exposed to, and local topographic heterogeneity may exacerbate or mitigate soil moisture stress. Although drought resistance is well-studied in widespread tree species, knowledge is limited on trait variation in droughtresistant conifers with more narrow distributions or isolated populations.
Pinus ponderosa is amongst the most widespread conifers in semi-arid forests of western North America, and var. scopulorum occurs in some of the most arid portions of the species’ range. In the Northern Basin and Range physiographic province of western North America, scopulorum is isolated to montane sky islands, making it a model system for testing the effects of climate and topography on trait variation in drought-resistant conifers. To investigate the relationship between interacting climatic and topographic conditions and drought-resistant traits, we sampled 55 stands across six ranges in Nevada and Utah to quantify trait variation in cone volume, wood density, latewood to earlywood ratio (LER), specific leaf area (SLA), and needle lifespan and asked how these traits vary across the regional climate gradient. We then used linear mixed models to explore whether seasonal climatic moisture deficit (CMD), monsoonality, aspect, elevation, or tree-level attributes are associated with trait variation.
Traits varied widely both within and among ranges, with trait responses strongly linked to seasonal water availability. Cone volumes and SLA increased with increasing summer (July-Aug) CMD, while cone volume decreased with increasing winter (Nov-Jan) CMD. Winter moisture stress was also tied to wood formation, as wood densities increased under greater cold season (Oct-Feb) CMD. Needle lifespan and LER were not shaped by seasonal CMD but rather by the degree of monsoonality, indicating that drought-resistant conifers may be able to take advantage of late-summer precipitation. Contrary to our expectations, local topographic variation had no influence on trait variation in the stands sampled here, while elevation and tree diameters were significant for wood traits alone. These findings suggest that both seasonal moisture stress and the timing of precipitation influence drought-resistant conifer trait variation in semi-arid environments and highlight the potential for these conifers to withstand extreme fluctuations in seasonal water availability, which is predicted to become more common as the climate continues to change.
