Where fire stops: vegetation structure and microclimate influence fire spread along an ecotonal gradient

Positive feedbacks influenced by direct and indirect interactions between fire, vegetation, and microclimate can allow pyrophilic and pyrophobic ecosystems to co-occur in the same landscape, resulting in the juxtaposition of flammable and non-flammable vegetation. To quantify the drivers of these feedbacks, we combined measurements of vegetation, fuels, and microclimate with observations of fire spread along ecotonal gradients. We established 113 permanent transects (consisting of 532 plots), each traversing an ecotone between savanna and wetland in the Sandhills of North Carolina, USA. In each plot, we recorded cover of ten plant functional types. We collected surface fuels at a subset of our transects. We continuously monitored microclimate (nine meteorological variables) across 21 representative ecotones. Following prescribed fire, we measured fire spread along each transect. Vegetation structure and microclimate significantly predicted fire spread along the savanna-wetland ecotone. Fire spread was most influenced by vegetation structure, specifically C₄ grass cover, which accounted for 67 % of the variance explained by our model. We have identified the components of the fire, vegetation, and microclimate feedback that control where fires stop under current conditions, but their control should not be considered absolute. For example, when ignited in savanna, prescribed burns continued through wetland vegetation 43 % of the time. The feedback operating within these systems may be relatively weak as compared to other savanna systems. Environmental changes may alter fire spread extent, and with it ecosystem boundaries, or even ecosystem states.