Unexpected drought resistance strategies in seedlings of four Brachychiton species

Functional traits associated with drought resistance can be useful for predicting tree responses to a drying climate. Yet drought resistance is likely achieved through a complex combination of constitutive traits (traits expressed even in benign environments) and plastic traits (traits expressed only in response to drought). Because few studies measure multiple traits for multiple species under both well-watered and drought conditions, we often struggle to identify suites of constitutive and plastic traits indicative of drought resistance strategies. Using a greenhouse experiment, we examined nine drought resistance traits (six morphological/allocation traits plus assimilation, stomatal conductance and water-use efficiency) in well-watered and water-stressed seedlings of four Brachychiton (Malvaceae Juss.) species with ranges spanning a strong aridity gradient in east-central Australia. In benign conditions, constitutive biomass allocation was consistent with expectations, with xeric species investing more heavily in roots and stem tissue and less in leaf tissue than mesic species (P = 0.004). Under drought conditions, xeric species decreased relative biomass allocation below-ground while mesic species increased relative below-ground allocation (treatment × species interaction P = 0.0015). Relative water content of the stems was slightly higher in xeric species (P = 0.055), and remained stable during drought while decreasing in mesic species (treatment × species P = 0.001). Specific leaf area (SLA) and leaf dry matter content (LDMC) did not fit with expectations under either benign or water-limited conditions. Moreover, stomatal conductance and carbon assimilation were unexpectedly highest and intrinsic water-use efficiency (WUEi) lowest in the xeric species in benign conditions. Only under drought did the xeric species manifest higher WUEi than the mesic species (treatment × species P < 0.0001). We found that even closely related species exhibited diverse combinations of drought resistance traits. Notably, traits commonly used as proxies for drought tolerance (e.g., SLA, LDMC, well-watered WUEi) performed more poorly than constitutive allocation traits. This study highlights the need to consider multiple traits and phenotypic plasticity when assessing species' drought resistance for forest management in the face of climate change.