Contrasting species decline but high sensitivity to increasing water stress on a mixed pine-oak ecotone

1. Forest decline under environmental stress is expressed by regeneration failure and accelerated mortality in all ontogenic stages at the population level. Characterizing functional traits and mechanisms that best capture species decline and mortality is essential to assess forest dynamics. 2. We analyzed sensitivity to increasing water stress in two species with different water-use strategies on a mixed Quercus pyrenaica - Pinus sylvestris forest where adult pines express vulnerability to climate change but oaks do not. We compared dynamics of radial growth, wood δ13C and sapwood nonstructural carbohydrates (NSC) in response to drought at different time scales in both species and two age-cohorts in pine. 3. Both species were very sensitive to water stress, which influenced trait phenotypic plasticity at short- and long-time scales. Water-use strategy in pines of both ages was more conservative than in the more drought-tolerant oak. Both species showed negative growth trends despite in...