Post-drought conditions and hydraulic dysfunction determine tree resilience and mortality across Mediterranean Aleppo pine (Pinus halepensis) populations after an extreme drought event

Drought-related tree mortality is a global phenomenon that currently affects a wide range of forests. Key functional variables on plant hydraulics, carbon economy, growth and allocation have been identified and play a role in tree drought responses. However, tree mortality thresholds based on such variables are difficult to identify, especially under field conditions. We studied several Aleppo pine populations differently affected by an extreme drought event in 2014, with mortality rates ranging from no mortality to 90% in the most severely affected population. We hypothesized that mortality is linked with high levels of xylem embolism, i.e., hydraulic dysfunction, which would also lead to lower tree resistance to drought in subsequent years. Despite not finding any among-populations differences in the vulnerability curves to xylem embolism, there were large differences in the hydraulic safety margin and the hydraulic dysfunction level. High mortality rates were associated with a negative hydraulic safety margin when xylem embolism reached values over 60%. We also found forest weakening and post-drought mortality related to a low hydraulic water transport capacity, reduced plant growth, low carbohydrate contents and high pest infestation rates. Our results highlight the importance of drought severity and the hydraulic dysfunction level on pine mortality, as well as post-drought conditions during recovery processes.

The study was funded by the SURVIVE-2 (CGL2015-69773-C2-2-P MINECO/FEDER) and INERTIA (PID2019-111332RB-C22 P MICINN/FEDER) Projects from the Spanish Government, and the IMAGINA project (PROMETEU/2019/110) from the Generalitat Valenciana. L.M. was supported by the Spanish MICINN (PTA2019-018094). JC.M. received a grant from the Secretaria Nacional de Educación Superior Ciencia y Tecnología de Ecuador (SENESCYT). H.M is supported by the Generalitat Valenciana and the European Social Fund (APOSTD20/2019-7956). The embolism resistance measurements were taken on the PHENOBOIS platform (Bordeaux, France) and supported by the INTERREG SUDOE Project "ForManRisk".The CEAM foundation is funded by the Generalitat Valenciana.