Factors determining post-wildfire plant community recovery trajectories in Central Texas

Abstract

Wildland fires are becoming more frequent and more severe in the United States, due in part to climate change and in part to long-term fire suppression and the subsequent build-up of fuels. Following wildfires of greater severity than what were historically present in an area, plant community recovery trajectories may diverge from the pre-disturbance plant community.

The Lost Pines region of central Texas supported the westernmost stands of loblolly pine (Pinus taeda) in the United States. In 2011, a wildfire burned most of Bastrop State Park (BSP), located in the Lost Pines. Pre-fire, BSP was a mostly closed-canopy forest dominated by loblolly pine and several species of oak (Quercus spp.), with sparse herbaceous vegetation and a dense mid-canopy of yaupon (Ilex vomitoria). Most plants in BSP were either killed or top-killed in the wildfire. We studied pre- and post-fire plant community dynamics to understand and predict post-fire plant community recovery trajectories.

Top-killed oak species sprouted vigorously in more severely-burned plots (Chapter 1, Chapter 2); yaupon sprouted in all burn severity classes (Chapter 3). Loblolly pine, which can only recruit from seed, established more slowly than sprouting species, in part due to the transitory inhibitory effect of an erosion control product (Chapter 3). In the first year after the fire, it appeared that oak sprouts might out-compete loblolly pine seedling recruitment. However, in 2015, a large loblolly pine recruitment event occurred following a year of unusually high precipitation (Chapter 1, Chapter 2). These results indicate recovery trajectories towards continued survival of the loblolly pine population in BSP, although with a potentially greater abundance of oak species than what was present pre-fire. Furthermore, yaupon is likely to re-form dense thickets such as those present pre-fire without measures to prevent woody plant encroachment.

Immediately post-fire, the herbaceous plant community increased in abundance, richness and diversity, likely due to greater canopy openness (Chapter 4). Very few invasive species were present either pre- or post-fire (Chapter 5). Alternate trajectories towards open-canopy savanna with a diverse understory community and lower mid-story tree abundance could be maintained by management actions such as prescribed fire or mechanical thinning.