PS 28-143 - The impact of Chinese tallow (Triadica sebifera) leaf litter on forest flammability and fire behavior in endangered pyric pine (Pinus spp.) forests

Abstract: Endangered longleaf and slash pine (Pinus palustris, P. elliottii) savannas and flatwoods are found along the Gulf of Mexico coast of the southeast USA and are threatened by the invasive tree species Chinese tallow (Triadica sebifera; hereafter: tallow). Pyric pine forests rely on frequent, low intensity, surface fires to maintain their structure and health, and, as a result, fire is a commonly used management tool in these ecosystems. However, there is a concern that encroaching tallow may interfere with fire regimes as the effect tallow litter has on forest flammability and fire behavior has not been thoroughly explored. This study aimed was to understand how increasing proportions of tallow leaf litter impacts fire behavior and, subsequently, the viability of tallow seeds found on the forest floor.

We established 20 4-m² plots in January 2023 at Mary Olive-Thomas Demonstration Forest in Auburn, AL, USA with five fuel treatments at a constant fuel loading (0.75 kg/m²) to simulate conditions associated with increasing invasion by tallow. These treatments included a gradient of tallow litter contribution from 0% tallow and 100% pine to 100% tallow and 0% pine. Within these plots, we placed mesh bags containing 20 tallow seeds each to determine the effect of fire on seed viability under different fuelbed conditions. We determined seed viability using the tetrazolium (TZ) test by allowing seed embryos to imbibe in a 1% TZ solution for 72 hours, and seeds stained red were presumed to be viable.

We found that fuel consumption (%), rate of spread (ROS, m/min), and flame height (FH, cm) were significantly lower in plots with 100% tallow litter compared to plots with 0% tallow (91.3% v. 6.5% consumption, p< 0.001; 0.77 m/min v. 0.16 m/min ROS, p=0.001; 18.8 cm v. 4.6 cm FH, p< 0.001; 0% tallow v. 100% tallow, p-value). Fire temperature was similar across all plots that burned; however, plots with more tallow litter were less likely to burn. Since fire was suppressed in 100% tallow litter, seeds did not burn in those plots. Seed viability was higher in unburned plots, including 100% tallow plots, compared to burned plots at 72.5% and 65% viability (± 0.085, ± 95% CI; p=0.0416), respectively. These results demonstrate the impact tallow litter has on native fire-dependent ecosystem’s flammability. This fire suppressing characteristic of tallow litter allows it to persist and continue to spread across the landscape, especially in areas not frequently managed with fire.