Comparing mechanical mastication, herbicide application, and prescribed fire within an established longleaf pine (Pinus palustris Mill.) ecosystem

Longleaf pine (Pinus palustris Mill.) forests once dominated the landscape throughout the Southeast and much of its success could be attributed to ecological disturbances such as fire. However, the use of fire as a management tool may be at risk due to a growing human population, negative impacts resulting from smoke production, and the imposition of restrictive federal and state laws, policies, and standards. This study was designed to determine whether alternative silviculture treatments such as herbicide or mechanical mastication can be used as surrogates to prescribed fire. We applied three commonly used silviculture treatments (prescribed burning, mechanical mastication, and herbicide) one time in May 2008 to eighteen approximately equal sized treatment units (0.405 ha) at the Aiken Gopher Tortoise Heritage Preserve, which is located in Aiken County, South Carolina. The firing techniques used during the prescribed fire consisted of a mix of backing, flanking, and head fires. The herbicide used was the granular form of hexazinone [3-cyclohexyl=6-(dimethylamino)-1-methy-1,5-triazine-2,4(1H,3H)-dione] also known as DupontTM Velpar ULW®, which was broadcast evenly at a rate of 1.26 kg a.i./ha. A Bobcat T-300 with a forestry cutter head and hand tools were used for mechanical mastication; these tools were used to masticate any midstory vegetation (i.e. Quercus spp.). Additional treatments were applied in a split-plot design, including rake and non-rake subplots within each of the herbicide and mechanical mastication treatment units. We monitored the response of the understory herbaceous layer (P. palustris Mill.). We also measured the litter depth of the forest floor, monitored the foliar cover of Aristida stricta, tracked the recruitment of Aristida stricta seedlings, and evaluated which treatment provided the maximum usage forage (medium = M, high = H, and very high = VH) for gopher tortoises pre- and post-treatment. No significant differences were determined between the species richness, species diversity, and evenness following treatments for two consecutive growing seasons. Both prescribed fire and mechanical mastication promoted species richness and diversity values that exceeded pre-treatment levels by the end of the second growing season. Prescribed fire treatments generated the highest relative increases in the evenness values, followed by mechanical mastication, and then herbicide. Mechanical mastication and herbicide treatments generated higher longleaf pine seedling survivorship while prescribed fire negatively affected the longleaf pine seedling survivorship. While the broadcast application of hexazinone caused initial decreases in species richness and diversity, the understory plants gradually began to recover the ensuing year. Prescribed fire positively influenced the Aristida stricta foliar cover throughout the study. Initial Aristida stricta foliar cover declines were observed following both the herbicide and mechanical mastication treatments; however, it began to recover the following year. Litter depths were not significantly influenced by any of the study treatments. Prescribed fire generated the greatest initial litter depth reduction (54%) and maintained the slowest litter recovery throughout the study. However, initial (2010) litter depth reductions were also observed each post-treatment year within the herbicide (38%) and mechanical mastication (39%) units. Aristida stricta seedling counts were not significantly different across the herbicide and mechanical mastication treatment units. However, the rake subplots promoted non-significantly higher A. stricta seedling counts and relative differences following initial treatments versus non-rake subplots. The rake subplots yielded the highest initial increases and maintained the highest relative difference each post-treatment year. No significant differences were determined between treatment types for the VH or M ranking gopher tortoise forage values. Significant treatment differences were determined for the H value forage in both post-treatment years. While there were mixed results across each treatment, no significant differences were observed for the prescribed fire treatment units throughout the study. The prescribed fire units yielded positive increases across all preferred gopher tortoise forage initially following treatment and maintained positive gains for the VH and M usage flora species throughout the study. Mechanical mastication produced some gains for the VH and M species initially following treatment; however, these were short-lived and quickly fell below pre-treatment levels by the end of the second post-treatment growing season. The herbicide treatment caused significant decreases for the VH and H gopher tortoise forage species during both post-treatment years. Based on results from this study, prescribed fire is the preferred silviculture tool that provides the maximum benefit to a xeric sandhills mature longleaf pine community by suppressing woody species, encouraging a diverse herbaceous understory, promoting an overall higher usage forage for gopher tortoises, and reducing litter layer accumulation. However, in areas that the use of fire may be limited or restricted, our study suggests that the use of herbicide and/or mechanical mastication treatments can be used to gain the desired structure and appearance and allow for regeneration of longleaf pine, but these alternative silviculture tools may not promote the desired understory herbaceous layer for target species such as the gopher tortoise. Caution should be made when applying these modern silviculture treatments, since impacts to the ecosystem resilience has not been documented long-term. These modern tools may be the next perturbation that will mimic stochastic events like fire and hurricanes. However, the longleaf pine ecosystem evolved under a fire regime and shifts may result from the new disturbance; consequently, close monitoring should occur following their use.