The joint effects of fire and herbivory on hardwood regeneration in central Texas woodlands

Abstract

Oaks (Quercus spp.) are not regenerating in forests and woodlands in central Texas and elsewhere. This has been attributed to fire suppression. However, overabundant white-tailed deer (Odocoileus virginianus) can also limit oak regeneration. We hypothesized that both fire re-introduction and protection from deer would increase the number and growth of hardwood seedlings, saplings, and sprouts in a central Texas woodland co-dominated by Texas red oak (Quercus buckleyi) and Ashe juniper (Juniperus ashei). We measured the separate and joint effects of prescribed fire and deer herbivory on the number, size, and growth of Ashe juniper and hardwoods, tree mortality, and canopy cover. We collected data one year before and three years after deer-fence construction and a prescribed fire. Fire stimulated re-sprouting in oak and other hardwoods, but had no detectable effect on their seedlings or saplings, even three years later. Deer exclusion increased the number of seedlings transitioning to the sapling size class. Both fire and deer exclusion together were required to increase average sprout height above the browseline. The apparent adaptations of native hardwoods to fire are the strongest evidence we have for an important role for fire pre-settlement. Our results also indicate that fire suppression in central Texas and other parts of the south-central US is causing a shift, not to more mesic-adapted species as observed in the eastern US, but to juniper (Juniperus spp.), which is at least as xeric-adapted as oak. Therefore, thinking about the “oak regeneration problem” needs to be expanded beyond “mesophication” to incorporate the shift to juniper in drier regions. It is likely that deer control is necessary to allow fire to have positive effects on the regeneration of oaks and other hardwoods in this region and wherever deer are over-abundant. Moreover, the negative effects of deer herbivory on oak growth may partially account for reported failures of single fires alone to promote hardwood regeneration elsewhere.