Abstract
Global change is altering fire frequency and severity in many regions across the world. In this work, we studied the impact of different frequency and severity regimes on the soil biochemical properties in burned areas with different environmental conditions. We selected three sites dominated by pine ecosystems along a Mediterranean-Transition-Oceanic climatic gradient, where we determined the fire frequency, and severity of the last wildfire. Four years after the last wildfire, we established 184 4 m2 plots. In each plot, we collected a composed soil sample from a 3 cm depth, and measured several ecological variables potentially affected by the fire frequency and severity (cover of bare soil, cover of fine and coarse plant debris, cover of vegetation, and vegetation height). From each soil sample, we analyzed the enzymatic activities corresponding to the biogeochemical cycles of carbon, nitrogen, and phosphorus (β-glucosidase, urease, and acid-phosphatase, respectively), and the microbial biomass carbon. The results indicated that fire frequency only played a significant role in soil biochemical properties at the Mediterranean and Transition sites. Specifically, we found that increases in frequency contributed to increased urease and phosphatase activities (at the Transition site), as well as microbial biomass carbon (at the Mediterranean and Transition sites). In relation to burn severity, we found opposite patterns when comparing the Mediterranean and Oceanic sites. Specifically, increased severity significantly decreased β-glucosidase, urease, and microbial biomass carbon at the Mediterranean site, whereas at the Oceanic one, severity significantly increased them. Burn severity also decreased microbial biomass carbon at the Transition site. Our results also indicated that, overall, fire frequency determined the studied ecological variables at the Mediterranean and Transition sites, but clear indirect effects on biochemical properties due to changes in ecological variables were not found. This study adds to the knowledge on the impact of shifts in fire regimes on soils in the current context of change.
Highlights
More than half of the Earth’s surface is vulnerable to forest fires [1,2]
Burn severity is a magnitude fire regime attribute defined as the loss of or change in ecosystem biomass that can be measured by remote sensing methods [20]
At the Mediterranean and Transition sites, we focused on a study frame of 3000 ha, and at the Oceanic site, we selected the entire wildfire (2500 ha)
Summary
More than half of the Earth’s surface is vulnerable to forest fires [1,2]. In general, ecosystems are adapted to natural fire regimes, recovering under a relatively wide range of fire frequencies and severities [3]. Burn severity is a magnitude fire regime attribute defined as the loss of or change in ecosystem biomass that can be measured by remote sensing methods [20]. Both attributes have been shown to shape the ecosystem structure, composition, and resilience [12,20] and determine the fire impacts in relation to soil properties [16,17]. It is not clear whether the fire frequency and burn severity effects on soils are consistent across ecosystems with different environmental conditions, such as different climates and soil types [17]
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