Species-Specific Water-Stress Responses in Coniferous and Deciduous Trees: Insights from Sap Flow and Hysteresis Analysis

Climate change is intensifying the frequency and duration of water-limited conditions, increasing the risk of climate-induced physiological stress in trees and potentially altering their function as critical carbon sinks. Understanding species-specific responses to water stress is crucial for predicting shifts in ecosystem processes and functionality. This study investigates the photosynthetic and water-use responses of two contrasting tree species—Norway spruce (Picea abies), a conifer, in Wüstebach, and European hornbeam (Carpinus betulus), a deciduous species, in Hohes Holz—over a three-year period using sap flow measurements, in situ photosynthetically active radiation (PAR) data, and eddy covariance technique. Results show that gross primary production (GPP) declines under high vapor pressure deficit (VPD), with regulatory thresholds differing between species. Norway spruce exhibits reduced stomatal conductance and photosynthetic activity beyond a VPD threshold of 10 hPa, whereas European hornbeam maintains photosynthesis up to 16 hPa. Sap flow density measurements corroborate these thresholds, highlighting that water stress diminishes ecosystem GPP, yet conifers and deciduous trees employ distinct coping strategies. Hysteresis analysis of the relationships between sap flow and VPD, as well as sap flow and absorbed PAR (APAR), revealed significant interspecies differences. Norway spruce exhibited a directional shift in hysteresis (from counterclockwise to clockwise) in both sap flow-VPD and sap flow-APAR relationships at specific VPD thresholds, suggesting dynamic adjustments to water stress. In contrast, European hornbeam exhibited directional hysteresis changes only in sap flow-APAR relationships, implying differing physiological mechanisms underlying their water-stress responses. These findings underscore the utility of hysteresis analysis in elucidating species-specific water-stress regulation mechanisms. The study provides valuable insights into how coniferous and deciduous trees modulate stomatal conductance and sap flow under elevated atmospheric demand, shedding light on the broader implications of climate change for forest carbon dynamics.  Keywords: Water stress, GPP, Sap flow, Hysteresis analysis