A Standardized Framework to Estimate Drought-Induced Vulnerability and Its Temporal Variation in Woody Plants Based on Growth

Forests and scrubland comprise a large proportion of terrestrial ecosystems and, due to the long lifespan of trees and shrubs, their capacity to grow and store carbon as lasting woody tissues is particularly sensitive to warming-enhanced drought occurrence. Climate change may trigger a transition from forests to scrubland in many drylands during the coming decades due to the higher resilience of shrubs. However, we lack standardized frameworks to compare the response to drought of woody plants. We present a framework and develop an index to estimate the drought-induced vulnerability (DrVi) of trees and shrubs based on the radial growth trajectory and the response of growth variability to a drought index. We used tree-ring width series of three tree (Pinus halepensis Mill., Juniperus thurifera L., and Acer monspessulanum L.) and three shrub (Juniperus oxycedrus L., Pistacia lentiscus L., and Ephedra nebrodensis Tineo ex Guss.) species from semi-arid areas to test this framework. We compared the DrVi values between species and populations and explored their temporal changes. Across species, the strongest DrVi values were found in declining P. halepensis stands and J. oxycedrus from the same site, while the lowest DrVi values were found in A. monspessulanum, P. lentiscus, and E. nebrodensis. Across populations, J. oxycedrus presented higher vulnerability in one of the dry sites. The P. halepensis declining stand showed a steady increase in DrVi value after the 1980s as the climate shifted toward warmer and drier conditions. We conclude that the DrVi allows comparing species and populations using a standardized general framework.