Reservoir impoundment alters surrounding plant physiological activities revealed by stable isotopes in tree rings

Reservoir impoundment influences the growth of surrounding plants by altering the microclimate. However, the physiological mechanism of responses remains unclear. Due to the long tree-ring sequences and the distinct physiological mechanisms of stable isotopes in tree rings, we employed the technique of tree-ring carbon and oxygen isotopes to assess the physiological response to reservoir impoundment. We collected the tree rings of Pinus yunnanensis Franch. surrounding the Ertan Reservoir in southwestern China, and calculated the annual discrimination capabilities of 13C and 18O (i.e., Δ13C and Δ18O) in cellulose–α. Using moving window correlation analysis and a carbon–oxygen dual-isotope model, we identified shifts in the response patterns. The results indicate no significant difference in Δ18O across the impoundment timeline. Compared with before impoundment period (1980–1997), Δ13C increases during initial impoundment period (1998–2004), followed by a rapid decline and stabilization during stable impoundment period (2005–2020). The 18O enrichment in P. yunnanensis is mainly driven by the uptake of surface soil water in late-growing season (notably Sept.). Reservoir impoundment attenuates Sept. humidity declines, stabilizing surface soil moisture and stomatal conductance. In contrast, 13C discrimination correlates closely with environmental conditions in early growing season (especially Jun.). During the initial impoundment period, high humidity limits the stomatal conductance, while lower air temperature reduces photosynthetic activity. At the onset of the stable impoundment period, the regional climate changes reversely with reservoir microclimate unable to counteract the regional shifts. Under the interactive effects of temperature and humidity, the photosynthetic capacity increases markedly, leading to a decline in Δ13C. As environmental conditions stabilize in the stable impoundment period, Δ13C also remains unchanged. These findings will promote our ability to predict future impacts of large-scale engineering on terrestrial ecosystems under global climate change.