Effects of soil and light on post-wildfire competition between dominant woody species in a pine-oak woodland

Background/Question/Methods

The Lost Pines of central Texas are the westernmost stands of loblolly pine (Pinus taeda) in the United States. They support a diverse plant community that includes several species of native oak (Quercus margaretta, Q. marilandica, and Q. stellata). In September 2011, the Bastrop County Complex wildfire burned much of the Lost Pines, including almost all of Bastrop State Park. Immediately post-wildfire, severely burned areas of the park were treated with hydromulch seeded with sterile native grasses (× Triticosecale and Leptochloa dubia). The fire, combined with a record drought, killed most of the trees in the Park. Pines are only able to regenerate through germination of seedlings, but oaks can produce basal re-sprouts that draw on resources from the parent tree, allowing higher growth rates immediately post-fire. Post-fire competition between pines and oaks may shift vegetation recovery trajectories towards either oak- or pine-dominance. We ask how soil types and light availability affect post-fire recovery of loblolly pines and oaks. Fifty-six permanent plots within the Park were surveyed for plant species composition, plant height, and canopy cover between 1999 and 2014. Soils data used in this study were obtained from the Soil Survey Geographic (SSURGO) Database.

Results/Conclusions

Pre-fire data suggest that mature pines dominated lower-elevation areas with sandy soils and mature oaks dominated higher-elevation areas with gravelly soils. Post-fire hydromulch application to burned plots suppressed loblolly regeneration while having little inhibitory effect on oak re-sprouting, and unburned plots were too shady for loblolly seedlings; initial post-fire analyses suggested that pine seedlings were being outcompeted by oak re-sprouts. In 2013 and 2014, however, oak re-sprout die-back has reduced light competition and pines have become established in more severely burned areas, suggesting that future trajectories will not be towards complete oak dominance. Preliminary analyses of soils data and canopy cover indicate that, because oak re-sprout numbers and immature pine numbers are affected by both soil type and canopy cover, soil type and light availability may affect future vegetation recovery trajectories. Better predictions will help plan further recovery efforts.