Drier, more seasonal climates strengthen coordination of hydraulic, leaf economic and reproductive strategies in subtropical forest tree communities

Abstract

Trade-offs relating to resource acquisition and conservation, reproduction and longevity generate considerable trait variation among co-occurring tree species, yet little is known about how the nature and strength of these trade-offs shift along gradients of resource availability. For rainforest and dry forest tree communities distributed across a continuous gradient of moisture availability in Australia's subtropics (600 to 2500 mm rainfall yr⁻¹), we examined the coordination of hydraulic, leaf economic and reproductive traits using a metric of multi-trait covariance and pairwise trait correlations. We hypothesized that trait coordination would be strongest at the dry and wet extremes of the moisture gradient, capturing strong moisture- and light-driven trade-offs, respectively. Overall trait covariance peaked in the driest sites and declined linearly along the regional moisture gradient, a trend also seen in the correlation of many trait pairs. In dry forests, stronger coordination of hydraulic and leaf economic traits captured a drought tolerance-drought avoidance spectrum, while correlations involving seed size reflected a ‘stature–recruitment' trade-off. Albeit far weaker, we found that shade-tolerance and height-driven constraints on hydraulic function drove some trait coordination in wetter, light-limited sites. Our findings underscore the importance of interpreting trait-based trade-offs within the context of each site's resource availability. We established that moisture limitation places much stronger constraints on trait combinations than light limitation and identified key trade-off axes that will likely become increasingly important under ongoing climate change.