Can soil moisture signal when prairie vegetation is drying out?

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During the growing season, depleted soil moisture can cause plant water stress, lowering grassland fuel moisture and accelerating curing. Fire danger models do not adequately represent these drying and curing dynamics or their dependence on soil moisture.

To examine these relationships, researchers monitored grassland fuelbed characteristics and soil moisture in nine patches of tallgrass prairie managed using patch burning in Oklahoma, USA, over two growing seasons. The period included a severe drought in 2012 that resulted in a large wildfire outbreak near the study site.

The fuel moisture of mixed live and dead herbaceous fuels (MFM) clearly tracked soil moisture, expressed as the fraction of available water capacity (FAW). MFM decreased as soil moisture declined below an FAW threshold of 0.59, and fell below 30% only when FAW fell below 0.30. The curing rate likewise increased linearly as FAW declined below 0.30. In contrast, Normalized Difference Vegetation Index (NDVI) readings did not adequately respond to rapid fuelbed drying and curing.

The researchers suggest that incorporating soil moisture observations into grassland fuelbed models could improve estimates of fuel moisture and curing, contributing to better wildfire danger assessments and reduced loss of life and property from wildfire outbreaks.