PS 28-133 - Impacts of seasonal fire regimes on physical and chemical properties of soil

Abstract: Increasing temperatures and extreme heat events have become more common in recent years in many regions of the world, including the southwestern United States, often leading to naturally occurring wildfires. Prescribed fire is a valuable land management and restoration technique that can be used to consume fuels to reduce wildfire risk, as well as maintain plant communities, suppress biological invasion, and improve wildlife habitats. However, the effects of these burns may depend on seasonality – the time of year the burns are conducted – and whether the timing matches that of historical fire regimes. Previous work has found that burns conducted at different times of the year may have vastly different effects on plant community composition, possibly through impacts on key soil properties. However, very few replicated studies on fire disturbance have rigorously examined the impacts of fire season on these properties. Understanding soil properties with respect to burn seasonality could provide a more comprehensive assessment of this restoration technique, thus allowing land managers to optimize burns for different restoration goals.

We conducted our study in 12 sites representing two distinct burn seasons (six winter and six summer). Sites are approximately 1.5 acres in size and have been burned every 2-3 years as part of an ongoing ~20-year study at the Lady Bird Johnson Wildflower Center in southwest Austin. Within each site, we collected soil from six points along a 50 m transect, parallel to an existing transect where long-term vegetation data has been collected since 2001. At each point, we characterized current vegetation cover (species identity and proportion cover) and quantified the following soil properties using the National Resource Conservation Service’s standardized methods: bulk density, porosity, water repellency, water content, pH, and electrical conductivity. We found significant differences in porosity, bulk density, and water content between the soils of winter and summer-burn sites. Specifically, porosity and water content were found to be higher in winter-burn sites, while bulk density was found to be higher in summer-burn sites. We attribute these differences to the greater presence of King Ranch bluestem at winter-burn sites in contrast to the more exposed soil at summer-burn sites, suggesting the presence of long-term feedback loops between soil features and plant communities.