Increasing fire frequency may trigger eco-hydrologic divergence

Climate-induced fire regimes may change species abundance and species composition in affected forest types, potentially altering pyro-eco-hydrologic feedbacks. In some fire-prone forests across the globe, eco-hydrologic thresholds (changing points, or tipping points, in ecohydrology when vegetation shifts from one steady vegetation to another) are being exceeded due to changes in relationships between climate, fire and vegetation. Following compound disturbances, forests may fail to maintain ecological resilience. Under multiple burn conditions, Eucalyptus regnans F. Muell. forests in south east Australia are highly vulnerable to ecological tipping points. In Victoria, over 189 000 ha of obligate seeder forests have been burned two or more times within 18 years. These short return-interval fires allow Acacia dealbata to become the dominant overstorey species. Such a dramatic species replacement may result in a new evapotranspiration (ET) regime, leading to a new hydrologic state. Stand scale dynamic models were combined with field estimated ET in E. regnans and A. dealbata forests aged 10, 35 and 75/80 years. We found that long-term forest structure, ET and water yield significantly diverge between E. regnans and A. dealbata forests with increasing age. These divergences imply a non-equilibrium state after A. dealbata replaces E. regnans under high-frequency fire conditions. In senescing A. dealbata, understorey transpiration contribution of 29.8% to system ET was similar to that of overstorey transpiration (31.2%), indicating the understorey and overstorey contribute equally to total ET at the final stage of Acacia forests. In contrast, in 75-year-old E. regnans forests, understorey contribution to the total system evapotranspiration is about 16%. This suggests that, after the Acacia life cycle finishes, the ET regime will transit into a new state that will be dominated by shrubby understorey species. Our findings suggest that this climate-induced species replacement would decrease long-term ET, inferring an increase in streamflow.