Limiting resource and leaf functional traits jointly determine distribution patterns of leaf intrinsic water use efficiency along aridity gradients

Intrinsic water use efficiency (iWUE) is a critical eco-physiological function allowing plants to adapt to water- and nutrient-limited habitats in arid and semi-arid regions. However, the distribution of iWUE in coexisting species along aridity gradients and its controlling factors are unknown. We established two transects along an aridity gradient in the grasslands of Losses Plateau (LP) and Inner Mongolia Plateau (MP) to elucidate the patterns and underlying mechanisms of iWUE distribution in coexisting species along aridity gradient. We determined leaf carbon (delta C-13) and oxygen (delta O-18) stable isotopes, functional traits related to carbon fixation, and limiting resources. Bulk leaf delta C-13 and delta O-18 were used as proxies for time-integrated iWUE and stomatal conductance (gs) during the growing season. Our results showed that variability in iWUE within transect was primarily controlled by species, sampling sites and an interactive effect between species and sampling sites. Mean values of iWUE (iWUE(Mean)) increased and coefficient of variation (CV) in iWUE (iWUE(CV)) decreased with an increase in aridity, demonstrating that increases in aridity lead to conservative and convergent water use strategies. Patterns of iWUE(Mean) and iWUE(CV) were controlled primarily by the ratio of soil organic carbon to total nitrogen in LP and soil moisture in MP. This revealed that the most limited resource drove the distribution patterns of iWUE along aridity gradients. Interspecific variation in iWUE within transect was positively correlated with Delta O-18, indicating that interspecific variation in iWUE was primarily regulated by gs. Furthermore, relationship between iWUE and multi-dimensional functional trait spectrum indicated that species evolved species-specific strategies to adapt to a harsh habitat by partitioning limiting resources. Overall, these findings highlighted the interactive effects of limiting resources and leaf functional traits on plant adaptation strategies for iWUE, and emphasized the importance of considering biological processes in dissecting the underlying mechanisms of plant adaptation strategies at large regional scales.

Automated summary

The distribution of intrinsic water use efficiency (iWUE) in coexisting species along aridity gradients and its controlling factors were investigated in this study. It was found that iWUE variability was primarily controlled by species, sampling sites, and their interaction. The mean iWUE increased and the coefficient of variation decreased with increasing aridity, indicating conservative and convergent water use strategies. The distribution patterns of iWUE were primarily driven by the ratio of soil organic carbon to total nitrogen in the Losses Plateau and soil moisture in the Inner Mongolia Plateau. The interspecific variation in iWUE was positively correlated with stomatal conductance, suggesting that interspecific variation in iWUE was primarily regulated by gs. The study highlighted the importance of considering limiting resources and leaf functional traits in understanding plant adaptation strategies for iWUE at large regional scales.