Stable isotopes reveal differences in climate sensitivity and physiological responses between dieback and healthy trees in a shelter forest

Tree-ring stable isotopes provide insights into the drought-induced eco-physiological mechanisms of forests. We investigated the growth of poplar and Mongolian pine trees decline in the Three-North Shelter Forest, and analyzed the differences in tree-ring width (TW), delta C-13, delta O-18, and intrinsic water-use efficiency (iWUE) between dieback and healthy trees. An R package was used to quantify the relative contributions of climatic and physi-ological factors to growth. Both TW and basal area increment were lower for the dieback poplar and Mongolian pine than for the healthy trees.delta C-13, delta O-18, and iWUE were higher for the dieback poplar than for the healthy individuals; there were no significant differences in Mongolian pine. The measured iWUE for healthy poplar, healthy and dieback Mongolian pine were consistent with the modeled iWUE for C-i/C-a scenario, indicating the stomata were open and C-i increased along with C-a. In contrast, the measured iWUE was higher for the dieback poplar than for the healthy individuals, which was consistent with the constant C-i scenario and partial stomatal closure. A significant decreasing C-i/C-a was detected only for the dieback poplar, reflecting the diversity in the leaf gas-exchange strategies of the two species. Pearson coefficients showed that on the annual timescale, tree growth was significantly negatively correlated with temperature, VPD, and iWUE, but positively correlated with SPEI. Moreover, the relationships between TW and stable isotopes and stomatal regulation, which differed in the two species, were affected by the dieback process. Furthermore, iWUE and delta O-18 had the highest and lowest relative contributions to TW, respectively. The relative contribution of meteorological factors (Tem, VPD and SPEI) to the growth was significantly lower than that of physiological factors (iWUE and delta O-18). Compared with the healthy controls of both species, the dieback trees were less sensitive to climate conditions, but the effects of physiological activities on growth were greater. These results provide a better understanding of the climate sensitivity and physiological responses of trees under long-term drought conditions.

Automated summary

Tree-ring stable isotopes play a crucial role in understanding the eco-physiological mechanisms of drought-induced forest decline. The study showed that the dieback trees had lower tree-ring width, higher stable isotopes values, and lower intrinsic water-use efficiency compared to the healthy trees. The dieback trees were less sensitive to climate conditions but more affected by physiological activities.