Within-stand variation in understorey vegetation affects fire behaviour in longleaf pine xeric sandhills

The frequent fires typical of the longleaf pine ecosystem in the south-eastern USA are carried by live understorey vegetation and pine litter. Mature longleaf pine stands in the xeric sandhills region have a variable understorey vegetation layer, creating several fuel complexes at the within-stand scale (20 m(2)). We identified three fuel complexes found in frequently burned stands on the Carolina Sandhills National Wildlife Refuge, and used prescribed fire to test whether distinct sets of fire conditions were associated with each fuel complex. Study plots were dominated by either turkey oak or wiregrass in the understorey, or lacked understorey vegetation and contained only longleaf pine litter. Turkey oak-dominated plots had the highest fuel loads, and during burns they had higher total net heat flux than wiregrass- or longleaf pine litter-dominated plots, and longer burn durations than wiregrass-dominated plots. Across all plots, the quantity of litter fragments had the greatest effect on fire temperature and duration of burn. These results show that the patchy understorey vegetation within longleaf pine stands will create heterogeneous fires, and areas dominated by turkey oak may have increased fire intensity and soil heating compared with the other two fuel complexes.