PS 7-69 - Flammability and regeneration impacts of increasing longleaf pine leaf litter contribution to the fuelbed of mixed pine-oak forests of the Southeastern U.S.
Reintroducing fire to historically fire-dependent pine-oak mixed stands following decades of fire exclusion requires a flammable fuelbed. However, encroachment of shade-tolerant, fire-sensitive species may reduce flammability through increased canopy cover and moisture retention of leaf litter. We hypothesized that retained pyrophyte (pine and oak) dominance in the fuelbed would negate any flammability reduction resulting from encroaching species and favor survival and growth of pyrophytic seedlings. To test these hypotheses, we treated 4-m2 plots distributed across a gradient of canopy openness at two sites, Spirit Hill Farm, Mississippi, USA and Talladega National Forest, Alabama, USA with one of six fuelbed treatments. The treatments varied in both pyrophytic longleaf pine (Pinus palustris) and fire-sensitive sweetgum (Liquidambar styraciflua) contribution from 10-100% but maintained a fixed pyrophytic southern red oak (Quercus falcata) contribution of 30%. In these plots, we also planted seedlings of shortleaf pine (Pinus echinata), two pyrophytic oaks – white oak (Quercus alba) (MS site) and northern red (Quercus rubra) (AL site), and two encroaching species, sweetgum (L. styraciflua) and red maple (Acer rubrum). In each plot, we measured fire behavior during experimental burns in March 2021 and seedling survival and growth in July 2021.
Results/Conclusions
Our results show that increased longleaf pine litter positively correlated with fire spread rate, flame height, fuel consumption, fire maximum temperatures, and oak seedling survival rate. In contrast, increased longleaf pine litter negatively correlated with flame duration and shortleaf pine survival, but did not affect survival of encroaching species or growth of pyrophytic oaks and red maple. However, increased flammability was positively associated with sweetgum growth. Overall, our results indicate that fuelbeds containing at least 10% pine and 30% southern red oak are sufficient to maintain flammability in mixed stands and highlight the strong influence of pyrophytic species on fuelbed flammability. We also showed high resilience of pyrophytic oaks, which grew similarly across the flammability gradient, and the persistence of encroaching species, which exhibited high growth and survival even at high fire intensities. Our study suggests the existence of a flammability threshold based on fuelbed composition and indicates that, even with a flammable fuelbed, a single fire is insufficient to favor the regeneration of pyrophytic species and control the encroachment of opportunistic species after their establishment.
