Two years of experimental rainfall exclusion does not promote leaf drought resistance in improved <i>Populus</i> varieties common to North China

Plantations are an important component of global forest coverage, but their performance is increasingly affected by water limitation due to climate change. Employing a rainfall exclusion facility, we report on the impacts of reduced rainfall on leaf water relations and organ morphological traits, in six Populus varieties commonly used for afforestation across North China. We exposed trees to 2 years of 50% rainfall exclusion and found that leaf hydraulic traits conferring drought resistance, including water potential thresholds triggering xylem embolism, leaf pressure-volume characteristics and metrics quantifying the risk of hydraulic dysfunction (i.e., hydraulic safety margin), were not improved, despite slightly but significantly decreased predawn leaf water potential and growth rate. Interspecific variation in response to rainfall exclusion was observed for some morphological traits, yet the adjustments were unlikely to benefit drought resistance. Overall, our results demonstrate an overall lack of physiological adaptive adjustments for leaves in response to rainfall reduction at early growth stage for these trees. If this response persists as trees age, the function of these trees will be potentially reduced due to increased risk of hydraulic failure, if the drying trend continues in their planting region.