Effects of prescribed fire on soil hydrology on a global scale: a systematic review and a meta-analysis

Wildfires are becoming more intense and threatening increasingly larger areas, with loss of biodiversity and increased hydrogeological risks. Prescribed fires are one of the most effective tools to control and limit the risk of catastrophic wildfires, preserving the natural state of forests. However, indications about the correct use of prescribed fire are needed by forest managers, and the analysis of published data reported in the relevant literature may be essential. This paper presents a review of 41 papers published in international scientific journals in the last 20 years about the hydrological and erosive effects of prescribed fire on forest soils. A quantitative database of observations about water infiltration, soil water repellency, surface runoff and soil erosion has been set up, based on measurements in 85 case studies reported in those articles. The effects of annual precipitation, soil slope, burn severity, fire application season, post-fire ground cover, and vegetation type on post-fire hydrology have been statistically explored using meta-analysis techniques. The bibliographic research has revealed that the case studies are not homogeneously distributed on the global scale but concentrated in few countries. The meta-analysis has shown that water infiltration decreases and soil water repellency appears in the short-term after prescribed fire. Noticeable increases in surface runoff (up to 20-fold the values measured in the unburned soils) and mainly in soil erosion (with peaks of 700-fold the pre-fire conditions) are common in the few months after the prescribed fire. Water infiltration is significantly influenced by precipitation, soil slope, soil burn severity, and vegetation type. All these variables together with burn season and ground cover after fire application are significant factors of variability of surface runoff, while only soil slope, vegetation type and burn season are significant drivers of soil erosion. The period of soil disturbance due to fire on soils commonly last few months, but some studies show that the pre-fire hydrological and erosive response to prescribed fire does not restore after two years. The post-fire increase in soil erosion is higher compared to surface runoff, and the highest soil loss is observed when fire is applied in summer in forest covered by trees, at soil slopes higher than 40%, moderate to high severities. On a practical approach, indications about the control and mitigation of the hydrogeological hazard after prescribed fire are given to land managers as follows: (i) need for post-fire management actions and control of soil burn severity and level of post-fire ground cover burning; (ii) temporal and spatial extensions of experimental activities to multi-year monitoring, catchment-scale investigations and observations also in case of repeated applications of prescribed fire; (iii) integration of the experimental observations with measurements of soil properties, vegetation characteristics and water quality; (vii) guidelines for standardized and appropriate measurement and analytical methods in experimental activities, in order to ensure the comparability of data and consistent interpretation of results.