Disentangling the role of sex dimorphism and forest structure as drivers of growth and wood density in expanding Juniperus thurifera L. woodlands

Abstract

Key message: The dioecious tree species Juniperus thurifera L. is undergoing a spontaneous process of forest expansion in southwest Europe. We investigated how growth, climate sensitivity, and wood density varied simultaneously between sexes and among stages of expansion while accounting for the variability of forest structure. We found few sex-based differences but detected lower wood density, greater growth rates, and higher sensitivity to drought in expanding fronts compared to long-existing forests. Context: Juniperus thurifera L. (Spanish juniper) is a dioecious tree species undergoing a natural process of forest expansion in southwest Europe. Aims: To assess how radial growth and wood density are simultaneously shaped by sex-based differences, the stage of forest expansion (long-existing forests, transition zones, and expanding fronts), variability in forest structure, and climate (in the case of radial growth). Methods: We measured forest structure characteristics, tree rings, and wood density in 17 plots dominated by Spanish juniper in three stages of forest expansion in central Spain. We used linear mixed models (LMMs) to explore the main drivers of variability in radial growth and wood density and sex- and stage-based differences in climate-growth sensitivity. Results: Rather than by sex, growth and wood density were mainly shaped by the stage of forest expansion, forest structure variables that characterize these stages, and climate variables (in the case of growth). Conclusion: Sexual dimorphism had a minimal effect in growth and wood density in expanding Spanish juniper woodlands. Expanding fronts could be benefiting from land-use legacies in the abandoned fields they are colonizing, as reflected in higher growth rates and lower wood density, especially during years with less summer drought stress. However, this pattern could be reversed in the event of an increase in drought episodes.