The phenotypic response of co-occurring Banksia species to warming and drying

Projected warmer and drier climates are expected to impact heavily on plant diversity in Mediterranean-climate ecosystems, but experimental investigations of sensitivity and adaptive capacity are needed to better understand species responses. Here, we examine the effects of warming and drying on growth and allocation in seedlings from populations of two co-occurring Banksia shrub species from south-west Western Australia. We hypothesised that the species would show ecological divergence in functional traits reflecting niche differentiation. We expected to see tolerance to warming and drought correlated with position of the population on a climate gradient. We predicted that populations at the warmer/drier end of the gradient would show greater homeostasis of growth and allocation patterns in response to experimental treatment. Seedlings of the two species differed in leaf and allocation traits and in responses to experimental warming and drying. B. coccinea had smaller leaves with higher specific leaf area, and accumulated less overall biomass compared to B. baxteri when grown under cooler conditions. Under warmer conditions, B. coccinea could maintain growth, whereas B. baxteri suffered significant decline in biomass accumulation. Under water deficit conditions, both species showed significant reductions in leaf mass and area. Under combined warmer/drier conditions B. baxteri forfeited height growth and biomass and increased leaf allocation. The results support our hypothesis that seedlings of B. baxteri and B. coccinea are divergent in key functional traits and their sensitivity to warming and drying. However, we found no evidence for inter-population variation in traits being associated with position on a climate gradient.