Aridity threshold triggers abrupt increase in shrub biomass through changes in leaf functional traits

Increases in aridity due to climate change in global drylands have led to changes in ecosystem structural and functional attributes, including shrub biomass. However, little is known about climate triggers of change in shrub biomass and the potential mechanism driving this process. Herein, we measured variations in shrub biomass and leaf functional traits in a dominant leguminous shrub species, Caragana versicolor Benth., along an aridity gradient in high-elevation regions of the southwestern Tibetan Plateau. We found an abrupt increase in C. versicolor cover and biomass beyond an aridity point of 0.35, at which C. versicolor leaf functional traits shifted from allocating N to the leaf (increasing N_mass) to modifying leaf anatomical structure (decreasing SLA) in response to increasing aridity. Leaf P_mass paralleled the changes of N_mass, while leaf N:P ratio maintained a constant value along the aridity gradient. Increased N_mass and decreased SLA consequently led to an increase in leaf δ¹³C (i.e., an indicator of water-use efficiency). Furthermore, aridity showed direct and indirect effects through interactions with leaf functional traits on C. versicolor biomass across the aridity gradient. Overall, our data show that shrub species in drylands cope with increasing aridity through changes in leaf functional traits, thereby formulating a leaf traits-biomass linkage. This process is crucial to understand plant dynamics under increases in aridity expected with climate change in many drylands.