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 unbalancedsex 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 pehu´en (Araucaria araucana)forests to determine whether there is an imbalance in ratio at different spatial scales, and if so, whether this isclimate-dependent. To assess deviations in sex ratios on local and regional scales, we quantified the proportion ofmales along 37 transects within 11 patches in two populations. To assess the sex–climate relationship, we usedWorldClim 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 heterogeneoustemperature conditions, especially in patches with particularly wet or hot summers. On a biogeographicscale, this bias increased in warmer and humid conditions. The male bias we found implies a direct reduction inthe 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 affectedthan females; this points to an intensification of the male bias, and thus to increased vulnerability of thesealready endangered pehu´en forests. This study provides evidence of climate-dependent sex bias in threateneddioecious forests, and discusses its importance in the context of climate change.