Shifts in trait diversity across the range of an endemic treeline species in central Chile.

Background and aims: Climate change is altering biodiversity by shifting species' ranges. Yet, the mechanisms underlying these shifts are not well understood for plants, whose responses to changes in precipitation or temperature are often idiosyncratic. Here, we evaluated if ecological strategies mediate tree growth and whether intraspecific trait diversity shifts across the range of an endemic tree species in a region experiencing a extreme drought.

Methods: We measured functional traits of 45 individuals of Kageneckia angustifolia from populations at the center and trailing and leading edges of its distribution, i.e. where abiotic stress is higher and lower, respectively, in central Chile. We identified individual-level ecological strategies using leaf and xylem traits and tested their association with tree growth, and estimated statistical moments of intraspecific trait variation and multidimensional trait diversity across populations with bootstrapping.

Key results: We found that intraspecific trait variation of K. angustifolia was dominated by three ecological strategies: hydraulic safety and efficiency, resource use, and water use. Across populations, tree growth was positively correlated with the hydraulic safety and efficiency ecological strategy, xylem traits, and leaf thickness. Our results show that mean leaf traits shifted towards values associated with conservative resource use and xylem traits shifted towards values associated with hydraulic safety in the trailing edge. Similarly, variance decreased towards the trailing edge, skewness was positive for most traits, and kurtosis was largely negative. Consequently, multidimensional trait diversity decreased non-linearly towards the trailing edge.

Conclusions: Our results revealed that asymmetric environmental filtering shifted trait distributions and, therefore, lowered trait diversity of K. angustifolia, likely due to increasing drought stress. Our results suggest that these shifts enable greater persistence of populations at the trailing edge, yet the species' long-term viability likely depends on its capacity to disperse to sites with suitable abiotic conditions beyond its current distribution.