Abstract
Soil moisture depletion during the growing season can induce plant water stress, thereby driving declines in grassland fuel moisture and accelerating curing. These drying and curing dynamics and their dependencies on soil moisture are inadequately represented in fire danger models. To elucidate these relationships, grassland fuelbed characteristics and soil moisture were monitored in nine patches of tallgrass prairie under patch-burn management in Oklahoma, USA, during two growing seasons. This study period included a severe drought (in 2012), which resulted in a large wildfire outbreak near the study site. Fuel moisture of the mixed live and dead herbaceous fuels (MFM) clearly tracked soil moisture, expressed as fraction of available water capacity (FAW). MFM decreased with decreasing soil moisture below an FAW threshold of 0.59 and fell below 30% only when FAW fell below 0.30. Likewise, the curing rate increased linearly as FAW declined below 0.30, while Normalized Difference Vegetation Index (NDVI) readings failed to adequately respond to rapid drying and curing of the fuelbed. Incorporating soil moisture observations into grassland fuelbed models could result in more accurate fuel moisture and curing estimates, contributing to improved wildfire danger assessments and reduced losses of life and property due to wildfire outbreaks.
Highlights
Fire is an integral part of grassland ecosystems worldwide and can, in certain contexts, increase biodiversity (Deak et al 2014) and improve the performance of grazing livestock (Limb et al 2011)
We focused on the moisture of the mixed live and dead herbaceous fuels (MFM), which can include all four categories of Kidnie et al (2015), as well as on dead fuel moisture (DFM, old dead and new dead together) and green fuel moisture (GFM), because only these categories can be objectively measured without manual separation of bulk samples containing both live and dead fuels
This study quantifies the relationships between soil moisture, as represented by fraction of available water capacity (FAW), and grassland fuel moisture, CR and greenness during the growing season
Summary
Fire is an integral part of grassland ecosystems worldwide and can, in certain contexts, increase biodiversity (Deak et al 2014) and improve the performance of grazing livestock (Limb et al 2011). Wildfires increase greenhouse gas emissions and cause major economic losses to society worldwide (Yebra et al 2008). The National Interagency Fire Center (NIFC) reported that federal firefighting expenditures in the US exceeded US$2.9 billion in 2017 (NIFC 2017). Savanna and grassland wildfires are widespread, accounting for approximately 90% of the global area burned over the last century (Mouillot and Field 2005). These grassland wildfires can be large and devastating.
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