Abstract Dryland forests face multiple climate change‐related threats, including more frequent droughts and shifts in the seasonality of precipitation. While dryland tree species tend to have adaptations for adult persistence during drought, recruitment is often episodic, occurring only in years with favourable conditions. As conditions become more arid, with fewer favourable periods for seedling establishment, variation in the presence of drought‐adapted phenotypes among populations may determine recruitment potential and lead to different outcomes. We used a 4‐year common garden experiment to evaluate seedling survival and growth in response to varying precipitation timing and amount for 23 populations of Pinus monophylla , a tree species occurring in semiarid forests of south‐western United States. Previous research on P. monophylla showed that intraspecific phenotypic variation in maternal trees persists in seedling offspring, yet fitness implications are unknown. We sowed seeds at the dry edge of the species' climatic distribution and used experimental watering treatments to simulate four seasonal precipitation regimes: ambient control (drought), spring supplemental watering, summer supplemental watering and spring + summer supplemental watering. We used a randomized block design, sowing seeds under the canopy of the native shrub Artemisia tridentata to meet known shade requirements for seedling establishment. Seed source climate was associated with differences in recruitment. Seedlings sourced from more arid climates had higher survival and above‐ground growth than seedlings from wetter climates under all simulated precipitation regimes. Climate transfer distance relationships were consistent with local adaptations to mean annual temperature and spring water availability, and supplemental summer water increased seedling survival. We found large differences in seedling establishment between sowing years, highlighting the importance of episodic recruitment under favourable conditions. Nurse shrub canopy cover was also a strong predictor of survival in the first 2 years of establishment. Synthesis . Our results show that intraspecific phenotypic variation mediates the effect of seasonal drought on dryland tree recruitment, suggesting that different populations may respond uniquely to changes in climate. The effects of precipitation may be contingent on shifts in seasonal timing and amount, and P. monophylla seedling survival may decline with increasing temperatures or decreased summer water availability.
Urza et al. (Fri,) studied this question.