Although there is compelling evidence that tree diversity has an overall positive effect on forest productivity, there are important divergences among studies on the nature and strength of these diversity effects and their timing during forest stand development. To clarify conflicting results related to stand developmental stage, we explored how diversity effects on productivity change through time in a diversity experiment spanning 11 years. We show that the strength of diversity effects on productivity progressively increases through time, becoming significantly positive after 9 years. Moreover, we demonstrate that the strengthening of diversity effects is driven primarily by gradual increases in complementarity. We also show that mixing species with contrasting resource‐acquisition strategies, and the dominance of deciduous, fast‐developing species, promote positive diversity effects on productivity. Our results suggest that the canopy closure and subsequent stem exclusion phase are key for promoting niche complementarity in diverse tree communities.

The function and conservation of many forest ecosystems depend on the distribution and diversity of the community of rodents that consume and disperse seeds. The habitat preferences and interactions are especially relevant in alpine systems where such granivorous rodents reach the southernmost limit of their distribution and are especially sensitive to global warming. We analyzed the community of granivorous rodents in the Pyrenees, one of the southernmost mountain ranges of Europe. Rodent species were identified by DNA with particular attention to the Apodemus species, which are prominent seed-dispersing rodents in Europe. We confirmed for the first time the presence of the yellow-necked mouse, Apodemus flavicollis, in central Pyrenees, a typical Eurosiberian species that reaches its southernmost distribution limit in this area. We also found the wood mouse, Apodemus sylvaticus, a related species more tolerant to Mediterranean environments. Both rodents were spatially segregated by altitude. A. sylvaticus was rare at high altitudes, which might cause the genetic differentiation between populations of the different valleys reported here. We also found other seed consumers like dormice, Elyomis quercinus, and voles, Myodes glareolus, with marked habitat preferences. We suggest that population isolation among valleys may increase the genetic diversity of rodents, like A. sylvaticus. We also highlight the potential threat that global warming may represent for species linked to high-altitude refuges at the southern edge of its distribution, like Apodemus flavicollis. Finally, we discuss how this threat may have a dimension in the conservation of alpine forests dispersed by these rodent populations.

There is increasing evidence that the strength of tree diversity effects on productivity varies considerably over the course of forest development. Evidence points to canopy closure and the subsequent self‐thinning as key phases of forest development during which positive diversity effects emerge. A number of studies have shown that self‐thinning can differ among species, and also in mixtures compared with monocultures. Yet, how diversity influences the process of canopy closure and self‐thinning remains poorly understood. In this study, using 11 years of growth and mortality records from a large diversity experiment, we fitted self‐thinning trajectories for 37 tree communities with equal initial densities and explored whether and how functional diversity and identity may affect these trajectories. We then examined whether the diversity effects on self‐thinning were influenced by differences in growth or in mortality. We found that tree communities' functional diversity and identity strongly influence the self‐thinning process. First, we observed that tree communities dominated by early successional species, and slow‐growing evergreens begin self‐thinning at a larger mean tree size. Second, we found that mixing species with contrasting resource‐use strategies, and the dominance of deciduous, fast‐growing species, reduce tree mortality rate in relation to mean tree size during self‐thinning (i.e. shallower self‐thinning slope). The lower rates of self‐thinning in these functionally diverse communities seem to be explained by both an increase in tree growth and a reduction in density‐related mortality simultaneously over time. Synthesis. Overall, this study highlights that increasing tree diversity has the potential to enhance forest productivity in the long term through a better performance during the self‐thinning process when competition for resources is most intense.