Is fire enough? The joint effects of fire and deer herbivory on hardwood regeneration and species composition in central Texas woodlands

Background/Question/Methods

The failure of Quercus (oak) and other tree species to regenerate is a serious problem in central Texas, in the northeastern and north-central US, and elsewhere.  Fire suppression and high ungulate densities are suspected causes. Under current conditions of fire suppression and high white-tailed deer (Odocoileus virginianus) densities, the failure of hardwood regeneration in central Texas is shifting woodlands from diverse communities co-dominated by Quercus buckleyi (Texas red oak) and Juniperus ashei (Ashe juniper) to pure stands of J. ashei. The resulting loss of habitat for the federally endangered golden-cheeked warbler (Dendroica chrysoparia) is a conservation concern. We measured the separate and joint effects of prescribed fire and protection from deer on hardwood recruitment in a factorial experiment in two vegetation types at Balcones Canyonlands National Wildlife Refuge. We measured (a) the number, size, and growth rates of J. ashei and each hardwood species, including Q. buckleyi, (b) mortality rates of all tree species, and (c) abiotic variables such as light intensity and fuel load. We collected data before and after deer-fence construction, pre-fire thinning of J. ashei, and a prescribed fire.

Results/Conclusions

A prescribed fire increased the number of Q. buckleyi sprouts per tree. Tree size (diameter-breast-height, DBH), did not affect the number of sprouts/tree post-fire. Protection from deer significantly increased the height of Q. buckleyi sprouts, but the average height of protected sprouts has not yet not increased above the probable browseline. Protection from deer also increased the median number of Ilex decidua stems present after fire; I. decidua is a preferred browse species. The fire killed only 16.52% of mature J. ashei (individuals with DBH > 10.16cm) but higher proportions of smaller individuals. Canopy openness was significantly higher in burned areas, but was not increased above the 30% maximum tolerated by golden-cheeked warblers. Post-fire hardwood re-sprouting was most likely due to basal heating and increased light availability. These results support the hypothesis that both fire suppression and high deer densities contribute to the lack of recruitment of Q. buckleyi and other hardwood species in central Texas. It remains to be seen whether fire alone is sufficient to allow recruitment into adult size classes. This study also demonstrates that low-intensity fire can be an effective restoration strategy in golden-cheeked warbler habitat, without sacrificing habitat quality in the short-term.