Reforesting drylands under novel climates with extreme drought filters: The importance of trait-based species selection

Having regard to the substantial world-scale forest restoration needs, the efforts must be done efficiently, which necessarily forces to consider the adaptation of new forests to the extremes arising from climate change. In this context, species selection strategies should enhance long-term functional resilience in the face of novel environmental scenarios. The use of plant functional traits for selecting species under climate change might be advantageous over more traditional taxon-based criteria as an adaptive forestry management strategy. In this work, we studied which functional traits (across species) have played a relevant role on field performance and fitness in a mull-species reforestation trial in a Mediterranean dryland affected by an extreme drought event. Different traits both from the individual plant and from the species were studied in seven species both at the short and the mid-term (10 years). The relative importance (RI) or contribution of the different traits to plantation performance was assessed through boosted regression tree models. The results showed that, under favorable climatic conditions, mean survival was above 70% and individual plant functional traits held up to 60% of importance on such value. The impact of species functional traits was low in this case (less than 18%) pointing out that all the species were performing within their niche at this point. However, after the driest year on record, the role of the latter on survival rose up to 53% of RI and survival sharply decreased to 33%, with some species showing negligible survival rate (< 10%). The dynamic response of stomata and xylem resistance to cavitation, together with roofing depth, were the main traits (species traits) identified in successful performance facing the extreme environmental factors. Thus, trait-oriented approach to select species represent a key tool in the implementation of new and successful forest restoration strategies to design resistant and resilient ecosystems adapted to the climate change challenges.