Experimental manipulation of fuel structure to evaluate the potential ecological effects of fire.
A better understanding of how dominant fuels affect fire behavior can improve predictions and comparisons of the ecological effects of fires in forests and other ecosystems. Current methods for evaluating effects of fuel characteristics on fire behavior, including maximum temperature and heating duration, range from small-scale laboratory to large-scale field experiments. Small-scale experiments often have many replicates and high measurement precision but can lack realism, while field experiments may have few replicates and lower measurement precision, thereby making comparisons across ecosystems difficult. Here, we present a method to experimentally evaluate ecological effects of fire while maintaining realism in fuel structure. We applied the method to investigate fire behavior effects of cogongrass (Imperata cylindrica), an invasive grass with a vertical growth form that is widespread across Southeast US forests. Examining the effect of fuel structure (piled vs. standing) on fire behavior for a range of fuel loads illustrated how more realistic standing fuels produced shorter heating duration (s above 100°C), taller flame heights, and faster spread rates compared to piled fuels. Average heating duration was ~2-4 times longer and ranged more widely when fuels were piled (80-277 s) compared to standing (41-57 s). Flame heights were ~1.4 times taller when fuels were standing than piled. These differences highlight that maintaining natural fuel structure in experimental fires produces more realistic estimates of fire behavior and effects. Consequently, not maintaining realistic vertical fuel structure could lead to overestimation of potential fire impacts related to temperature (e.g., tissue damage) but underestimate potential impacts related to flame heights, such as total engulfment of tree seedlings and saplings by fire. Altogether, our method effectively maintained fuel structure, enabling assessment of more probable fire behavior and impacts of the invasive grass than if fuels were simply piled. This approach may help further bridge the gap in realism between small-scale experiments and large-scale fires, enabling comparisons of the ecological effects of fires and fire-invasion interactions across forest ecosystems.