Early Differentiation of the Phenotypic Space and Performance of Juniperus thurifera Across Woodland-Expanding Areas

Forest expansion after rural abandonment is changing landscapes, bringing new challenges and opportunities. Juvenile performance has an effect in the establishment of new forests and the subsequent acclimation of adult trees. However, most studies have focused on the performance of adult trees. Here, we analysed differences in the phenotypic trait space of juveniles across gradients of Juniperus thurifera woodland expansion and explored which traits shape juveniles’ performance, seeking specific drivers of such differentiation and the role of the ontogeny. Thus, we surveyed functional leaf and plant-size traits in 180 juveniles growing in 18 plots at three different tree density stages (mature woodland, recently established area and a transition zone between both) and recorded local environmental conditions around each individual. We also estimated their radial growth and resilience to drought (in terms of resistance, recovery and resilience) as performance indicators. We found no relevant effects of the study environmental variables, but we did find a shift in the phenotypic space among juveniles across the gradient, with taller crowns and higher amount of leaf N and δ 13 C at the expanding edge. This trait differentiation across the gradient implied higher performance of young trees growing in old agricultural fields, both in growth rate and drought resilience terms. Our results showed a relevant land-use legacy on early trait differentiation that shapes more efficient phenotypes in the expanding edge compared to those in mature woodlands, suggesting that the future of these new forests is not compromised under current conditions.