Recovery of groundcover bunchgrasses during pine savanna restoration: Roles of prescribed fire and mammalian herbivores

Background/Question/Methods

Prescribed fires and mammalian herbivores potentially influence groundcover vegetation in longleaf pine savannas. Prescribed fires during the lightning season favor grasses over shrubs. White-tailed deer (Odocoileus virginianus), cottontail rabbits (Sylvilagus floridanus), and cotton rats (Sigmodon hispidus) forage in pine savanna groundcover. We hypothesized that lightning-season prescribed fire and mammalian herbivory during the winter (when warm season grasses are the most abundant green vegetation) should influence recovery of dominant C4 bunchgrasses (primarily Schizachyrium scoparium, Schizachyrium tenerum, Andropogon gyrans, Aristida purpurascens) at Girl Scout Camp Whispering Pines in eastern Louisiana. We initiated a still ongoing study of effects of lightning-season prescribed fire and winter mammalian herbivory on bunchgrasses in 2004. We established three 1 meter by 1 meter plots in each of 20 randomly selected sites, 10 in open pine savanna, and 10 next to shrub thickets still present after 10 years of prescribed fire. One plot was an undisturbed control; the other two plots were covered by exclosures during winter. In 2010, we elevated one exclosure plot at each site 30 cm above the ground to allow smaller mammalian herbivores access while still excluding large herbivores. We counted stems of bunchgrasses annually in late summer from 2004 to 2011.

Results/Conclusions

Reintroduction of lightning-season prescribed fire produced conditions conducive for warm season C4 grasses in the pine savanna groundcover at Camp Whispering Pines. Numbers of stems of bunchgrasses increased linearly from 2004 to 2008, both in shrub thickets and open pine savanna. Mean densities of all species combined increased over the four-year period, from 50 stems/m² to 147 stems/m² in shrub thickets and from 181 stems/m² to 718 stems/m² in open pine savanna. Thus restoration of prescribed fire regimes mimicking natural lightning fires enhanced warm season C4 grasses, which on average increased 3-4 fold over the period of study. In addition, rates of increase were considerably and significantly greater for bunchgrasses inside than outside exclosures, resulting in almost twice as many stems inside exclosures compared to outside exclosures. Thus, we conclude that winter herbivory by mammals slowed increases in stem numbers of bunchgrasses during restoration. Data from 2010 further indicate a continuation of effects observed in earlier years. Data indicate that numbers of stems of bunchgrasses increase differently when different sized herbivores are denied access to plots. Our study thus indicated that lightning-season prescribed fires favor increases in bunchgrasses, but those increases may be slowed considerably by winter herbivory.