Soil-climate interactions explain variation in foliar, stem, root and reproductive traits across temperate forests

Aim The individual effects of climate and soil properties on functional trait distributions have become increasingly clear with recent syntheses of large datasets. However, the distribution of traits in a given climate may depend on the fertility of the soil. Our aim was to quantify how soil-climate interactions explain community-level variation in functional traits from every plant organ to improve predictions of plant community responses to environmental change. Location Temperate forests throughout New Zealand. Methods We measured traits of foliar, stem, root and reproductive tissue for 64 species and calculated abundance-weighted mean trait values on 324 forest plots. Multiple linear regression was used to model the variation in each of the traits as functions of mean annual temperature (MAT), vapour pressure deficit (VPD), soil pH, soil total phosphorus (P) and their interactions. Results Soil-climate interactions explained significant variation in functional traits. For example, specific leaf area (SLA) was highest in high-P soil within a wet and warm climate; however, strong interactions indicate that SLA was lowest in wet and warm climates in low-P soil. Root tissue density was lowest in warm climates and high-P soil, but it was high in warm climates and low-P soil and in cold climates and high-P soil. According to model predictions, the largest potential responses of vegetation to warming may occur in fertile and wet environments. Main conclusions Pervasive soil-climate interactions demonstrate that interpreting simple bivariate relationships between traits and climate must be done with caution because the adaptive value of traits in a given climate depends on the fertility of the soil. Predictions of vegetation responses to climate change will improve significantly by incorporating local-scale soil properties into modelling frameworks. Rising global temperatures may shift community-level trait values in opposite directions depending on whether the soil is fertile or infertile.