Plant hydraulics provide guidance for irrigation management in mature polar plantation

Irrigation is essential for sustaining the productivity of plantation forests in arid, semi-arid, and seasonal drought regions. Current irrigation management is commonly scheduled based on time or soil water status, which often fails to capture plant physiological demand. Here we report on the variation of traits related to plant water use for 7-year-old Populus tomentosa plantations under different irrigation treatments in North China during two growth seasons with contrasting environmental dryness. Plant hydraulics were assessed by stomatal regulation strategy as well as hydraulic traits conferring water transport efficiency and safety, including branch hydraulic conductivity (Ks), thresholds for xylem embolism (Px) and percentage loss of leaf rehydration capacity (PLRC), leaf water potential at turgor loss point (Ψtlp) as well as hydraulic capacitance before and after turgor loss (Cbulk-pre and Cbulk-post respectively). Our results showed that both seasonal increases in rainfall and irrigation could modify stomatal regulation strategy, resulting in a more anisohydric stomatal behavior, but through different underlying mechanisms. Specifically, seasonal variation in stomatal regulation was primarily manipulated by the daily fluctuation of leaf water potential, Ψtlp, and possibly the water supply-demand relationship. At the same time, we speculated that irrigation-dependent alternation was presumably associated with shifted belowground traits. Seasonal variation in hydraulic traits was found for Ks and Ψtlp, with trees in the dry season displaying higher Ks and less negative Ψtlp regardless of irrigation, while other traits governing hydraulic functioning were essentially invariable across seasons or irrigation treatments. Tree growth rate was lower in the wet season and was independent of irrigation, which was likely explained by the occurrence of hydraulic impairment due to less stringent stomatal control. Overall, our results suggest that limited precipitation and high atmospheric vapor pressure deficit in the dry season would not compromise tree growth because of the tighter stomatal control, while hydraulic dysfunction could occur in the wet season due to excessive water use strategy, subsequently perhaps contributing to the reduction in growth rate. The findings of this study underscore the usefulness of physiological traits in guiding irrigation management and advocate reconsidering plantation water management strategy based on reliable indicators.