Climate- and scale-dependent sex ratio in threatened Araucaria forests

Abstract

Male and female dioecious trees often differ in their sensitivity to environmental stress, resulting in unbalanced sex ratios that can affect forest regeneration. Dioecious species are thus especially vulnerable to climate change, which can intensify their sexual bias. We investigated the sex ratio in endangered pehuén (Araucaria araucana) forests to determine whether there is an imbalance in ratio at different spatial scales, and if so, whether this is climate-dependent. To assess deviations in sex ratios on local and regional scales, we quantified the proportion of males along 37 transects within 11 patches in two populations. To assess the sex–climate relationship, we used WorldClim data; for the biogeographic scale, we added published sex ratio information from other populations. We found a bias towards males, both locally and regionally. On a local scale, this bias was related to heterogeneous temperature conditions, especially in patches with particularly wet or hot summers. On a biogeographic scale, this bias increased in warmer and humid conditions. The male bias we found implies a direct reduction in the effective size of these already threatened forests. Considering climate change projections for Patagonia, predicting an increase in temperatures and their anomalies, our results suggest that males would be less affected than females; this points to an intensification of the male bias, and thus to increased vulnerability of these already endangered pehuén forests. This study provides evidence of climate-dependent sex bias in threatened dioecious forests, and discusses its importance in the context of climate change.