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

January 22, 2021

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, result ing 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 meteoro logical variables) across 21 representative ecotones. Following prescribed fire, we measured fire spread along each transect. Vegetation structure and micro climate significantly predicted fire spread along the savanna-wetland ecotone. Fire spread was most influ enced 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 vegeta tion 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 bound aries, or even ecosystem states