A global synthesis of hydrological sensitivities to deforestation and forestation

Hydrological sensitivity to forest change, defined as hydrological response intensity (%) per unit of forest cover change (%), is essential for understanding the magnitude of possible hydrological consequences caused by forest disturbance (e.g., deforestation, wildfire, and insect infestation) or forestation (e.g., reforestation and affores-tation). This synthesis estimated and compared hydrological sensitivities (HSf) of annual streamflow to defor-estation and forestation based on quantitative analyses of 311 watersheds across the globe. The roles of climate (both inter-annual and intra-annual) and watershed properties (e.g., topography-related water retention ca-pacity, site condition, watershed size, forest type, and soil type) in HSf were assessed in deforestation and forestation groups, respectively. The key findings are: (1) hydrological sensitivities to forestation are significantly larger than those to deforestation, with an average value of 1.24% and 0.91% change in annual streamflow following 1% forestation and deforestation, respectively; (2) annual climate dryness (defined by PET/P at the annual scale) is the primary contributor to HSf to deforestation and forestation, with a relative importance of 75.5% and 60.6%, respectively, but intra-annual synchronicity of water and energy (i.e., greater matching in the timing of maximum P and maximum PET at the monthly scale) produces a significant impact on HSf to fores-tation; (3) leaf area index (LAI) has a contrasting effect on HSf to deforestation (negative response) versus forestation (positive response); (4) water retention index (IR) has a negative role in HSf, demonstrating that watersheds with larger water retention capacities are less hydrologically sensitive, particularly in the forestation group; (5) contrast to our general expectation, hydrological sensitivities to forestation are significantly greater in larger watersheds; and (6) hydrological responses are more sensitive to deforestation in watersheds with pure forest types and are more sensitive to forest cover change in Lithosols-dominated watersheds. Our findings suggest that hydrological effects between deforestation and forestation are not simply reversed and demonstrate that hydrological sensitivities are significantly influenced by climate and watershed properties. Hydrological sensitivities and their contributing drivers must be considered in protecting water and other aquatic properties.

Automated summary

This synthesis estimated and compared the hydrological sensitivities (HSf) of annual streamflow to deforestation and forestation based on quantitative analyses of 311 watersheds worldwide. The study found that the hydrological sensitivities to forestation are significantly larger than those to deforestation, with an average change in annual streamflow of 1.24% following 1% forestation and 0.91% following 1% deforestation. Climate and watershed properties were identified as important factors influencing hydrological sensitivities.