LAUSR

BACKGROUND AND AIMS In dryland ecosystems, conifer species are threatened by more frequent and severe droughts, which can push species beyond their physiological limits. Adequate seedling establishment will be critical for future resilience to global change. We used a common garden greenhouse experiment to determine how seedling functional trait expression and plasticity varied among seed sources in response to a gradient of water availability, focusing on a foundational dryland tree species of the western United States, Pinus monophylla. We hypothesized that the expression of growth-related seedling traits would show patterns consistent with local adaptation, given clinal variation among seed source environments. METHODS We collected P. monophylla seeds from 23 sites distributed across rangewide gradients of aridity and seasonal moisture availability. A total of 3,320 seedlings were propagated with 4 watering treatments representing progressively decreasing water availability. Aboveground and belowground growth-related traits of first-year seedlings were measured. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modeled as a function of watering treatment and environmental conditions at the seed source locations (i.e., water availability, precipitation seasonality). KEY RESULTS We found that, under all treatments, seedlings from more arid climates had larger aboveground and belowground biomass compared to seedlings from sites experiencing lower growing-season water limitation, even after accounting for differences in seed size. Additionally, trait plasticity in response to watering treatments was greatest for seedlings from summer-wet sites that experience periodic monsoonal rain events. CONCLUSIONS Our results show that P. monophylla seedlings respond to drought through plasticity in multiple traits, but variation in trait responses suggests that different populations are likely to respond uniquely to changes in local climate. Such trait diversity will likely influence the potential for future seedling recruitment in woodlands that are projected to experience extensive drought-related tree mortality.