Inter-specific tolerance to recurrent droughts of pine species revealed in saplings rather than adult trees

Understanding species' tolerance to recurrent extreme droughts is key to predict species' performance and forest dynamics under ongoing climate change. Inter-specific differences in juvenile responses can largely shape forest composition and structure. However, the vulnerability of tree species is typically evaluated in adult canopy dominant individuals. The question of whether inter-specific differences in drought tolerance vary between adult and young trees remains largely unexplored. We sampled canopy dominant trees and saplings of three co-occurring pine species with different drought tolerance (Pinus pinaster, P. nigra and P. sylvestris) in a relict forest in the Northern Iberian plateau. Using basal area increment in canopy dominant trees and shoot elongation in saplings, we reconstructed annual growth for the period 2000-2015 and calculated resilience and resistance indices to two consecutive extreme droughts (2005 and 2012). We used generalized least square models to analyze species-specific differences in resistance and resilience in adults and saplings, respectively after accounting for tree size and stand basal area effects. In addition, we recorded living and dead saplings to assess inter-specific dissimilarities after the drought events. Drought events strongly reduced the growth of both adults and saplings (40% on average), but they were more resilient to the 2005 than to the 2012 drought. Increased magnitude and frequency of drought events led to inter-specific differences in drought responses, which were revealed in saplings rather than adults after the 2012 drought. P. pinaster saplings were more resilient than P. nigra and P. sylvestris, consistently to the drought stress experienced by each species in the dry-edge of their natural ranges. Moreover, a greater recruitment and similar sapling mortality confirm P. pinaster as the most successful pine species from a demographic perspective. In addition, tree size reduced resilience whereas basal area in the tree neighborhood did not show any significant effect on pine tolerance to drought. In conclusion, we showed that increased the magnitude and frequency of drought events uncovers species-specific differences in drought resilience in saplings rather than adults. Our results suggest potential shifts in the dominance of P. pinaster as the expense of P. nigra and P. sylvestris under climate change in this relict forest. This highlights the need to include juveniles and demographic rates in models aiming to describe and predict species responses to climate change in dry-edge forests.