Towards a tree vigor definition: Different responses of defoliation, growth and water-use efficiency to drought in pine and oak species

An improved definition of tree vigor is required to characterize forest resilience in response to drought. We developed a vigor index based on annual crown defoliation and radial-growth data. The index was applied to stone pine (Pinus pinea) and cork oak (Quercus suber) trees affected by die-off in the Doñana National Park (SW Spain), comparing a dry and a wet site. Climate data were correlated with defoliation, growth and the vigor index. Daily measures of cover greenness (gcc, green chromatic coordinate), obtained from phenocams (period 2018 −2024), and changes in stem radial growth and tree water deficit, obtained from dendrometers (2024 growing season), were also investigated. Finally, we used δ¹³C wood data to retrospectively assess intrinsic water-use efficiency (iWUE). During 2024, pines grew more than oaks on a yearly basis according to dendrometer data (2.0 vs. 0.5 mm), but annual tree rings were wider in oaks (4.34 mm) than in pines (2.50 mm). Tree water deficit was higher in oaks. In the dry site, gcc and stone pine water deficit were negatively associated, whereas in the wet site gcc and the cork oak growth rate were positively associated. Cork oak showed higher annual defoliation (26 %) than the pine (22 %). Defoliation increased in both species, whilst basal area increment decreased after the 2005 and 2012 droughts in stone pine and cork oak, respectively. The vigor index declined in the stone pine. Dry and warm winter-to-spring conditions and low soil moisture levels increased defoliation and reduced growth. Cork oak and stone pine responded more to drought in terms of defoliation and growth, respectively. The stone pine showed a sharp increase of iWUE after 2016, but the oak presented a more recent iWUE enhancement. Future research can refine the vigor index using longer time series of defoliation and growth data from multiple tree species.