Data From: Seed source climate and precipitation timing determine dryland tree recruitment in hot and dry range margins

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 favorable conditions. As conditions become more arid, with fewer
favorable 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 four-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 southwestern
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
aboveground 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
favorable conditions. Nurse shrub canopy cover was also a strong predictor
of survival in the first two years of establishment. 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.