For pioneer riparian trees, the successful establishment of juveniles is the major demographic bottleneck limiting population growth and persistence. First-year seedlings growing in arid climate zones face high vapor pressure deficits and highly variable water tables, among other stressors. In this study, we investigated the survival and physiological stress response of riparian tree seedlings subjected to a range of water table decline rates, focusing on the native species that dominate the near-channel ecosystem along Central Valley, CA rivers: Fremont cottonwood (Populus fremontii), Goodding's willow (Salix gooddingii), and narrow-leaved willow (S. exigua). Seedling properties measured include survival, growth, xylem water potential, and water use efficiency using stable carbon isotopes. Seedling survival probability was modeled as a function of root growth and root-to-shoot ratio using logistic regression. Water table decline rates ≥6 cm/day resulted in full mortality, whereas survival was >63% at rates 0-1 cm/day, and ranged 12-38% at an intermediate 3 cm/day rate. For all species, survival was positively correlated with root growth rate and negatively correlated with shoot-to-root mass ratio, though there was no evidence for seedlings' active adjustment of those properties in response to increased drought stress. Goodding's willow maintained the highest survival and growth across the treatment gradient and showed the largest increase in seasonal water use efficiency in response to water stress. Instantaneous measures of xylem water potential did not show any trend with treatment or species, suggesting that this metric is a poor predictor of relative performance for young riparian seedlings. The willow species' better drought tolerance relative to cottonwood is consistent with the different abiotic conditions experienced during establishment; river flows are generally stable and high in the early spring, when cottonwoods release seeds, and decline more steeply during the later willow dispersal period.
