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Hardwood reduction in sandhills

man in front of overgrown plants

Hardwood shrub and tree encroachment in longleaf systems is correlated with decreased herbaceous cover and species richness, due to a decrease in light penetration and an increase in suppressive leaf litter accumulation. Mechanical and chemical woody reduction treatments can help to set a longleaf savanna on a trajectory toward the re-establishment of a herbaceous-dominated understory, especially on sites with remnant groundcover. A number of studies have shown mechanical and chemical treatments to be safe and effective methods for re-creating fine fuel structures that facilitate the application of frequent prescribed fire in longleaf pine systems.

Though widely used, mechanical and chemical treatments are not always successful. One of the most difficult obstacles to the restoration of longleaf pine savannas is the re-sprouting of woody species after removal of aboveground biomass. Resprouting ability is a widespread adaptation among woody plants, which facilitates persistence in frequently-disturbed ecosystems.  Resprouting responses tend to be particularly aggressive in severely fire-suppressed longleaf pine sandhills, wherein hardwoods have had many years (or even decades) to establish extensive underground carbohydrate reserves. In these cases, mechanical techniques alone (e.g. mowing or thinning) are typically ineffective, and herbicidal treatments with systemic effects are required. Relatively few randomized, controlled herbicidal woody control studies have been conducted in longleaf pine savannas, and it can be difficult for managers to find specific guidance on rates, application methods, and natural community impacts of common herbicides. 

In this study, we compared four different herbicide formulations for use in woody-encroached longleaf pine sandhills, applied using a semi-directed foliar spray method. The treatments included 1) Imazapyr (low rate), 2) Imazapyr (high rate), 3) Fosamine/Triclopyr/Aminopyralid mix, 4) Imazapyr/Triclopyr mix, and 5) Control (no treatment). We applied these treatments to nine study blocks at three different sandhill preserves, each with pre-treatment conditions characterized by moderate to severe oak encroachment as well as remnant native groundcover. The purpose of the study was to evaluate the effectiveness of the different herbicide treatments for controlling target oak species, as well as to evaluate direct and indirect effects on nontarget native groundlayer species.