When to hold 'em and when to fold 'em: divergent plant water use by woody species in the Caatinga dry forest of Brazil

Tropical dry forests are amongst the most biodiverse semi-arid ecosystems, but also amongst the most threatened forests of the world. In these ecosystems, water availability is a strong control of ecological processes, like plant production, growth, and phenology. For example, in the Caatinga dry forests and shrublands of northeast Brazil, woody species are particularly adapted to unpredictable rain pattern, and hence plant green up is often tightly coupled to rainfall pulses. Still, the seemingly tight coupling of plant responses to water availability is not well-studied in dry forests. The objective of this research is to understand how woody species of the Caatinga respond to water availability over time. For a relatively old and intact forest stand (+50 years), we considered 16 species representing four functional groups: early deciduous high woody density (EDHWD), late deciduous high wood density (LDHWD), deciduous low wood density (DLWD), and evergreen (EV). From April to August 2018 (wet to dry season), pre-dawn and mid-day leaf water potentials were measured, and soil moisture and precipitation monitored. Other measurements included specific leaf area, leaf water content, specific leaf mass, and chlorophyll content. We hypothesized that 1) the difference in leaf water potentials, and the divergence in leaf water potentials from pre-dawn mid-day, would indicate different water-use strategies employed by each species; 2) these divergences align with functional groups; 3) differences within functional groups could be explained by differences in leaf traits. Results support that EDHWD species display greater differences in pre-dawn vs. mid-day potentials than LDHWD and DLWD species. At the same time, DLWD species with larger leaf areas did not maintain leaves into the dry season and had less negative leaf water potentials than other DLWD species with smaller leaf areas. These difference in water use indicate that plant functional groups in the Caatinga partition water resources temporally, according to hydrological niches. Such understanding is important for understanding ecosystem resilience and predicting how Caatinga species might respond to changes in climate, such as more intense rains distributed over a short time frame.