Vegetation responses to fire and herbicide in intensively managed, mid-rotation pine

• Conservation of biodiversity is a primary goal of sustainable forestry.
• Fire and herbicide in southeastern plantation pine may support sustainability.
• Herbicide was more effective at hardwood competition control than prescribed fire.
• Dormant season fire perpetuated greater understory vegetation species diversity.
• Fire and herbicide in southeastern plantation pine support sustainable forestry.

Intensively managed pine forests in the southeastern U.S. now cover approximately 15.8 million ha. Forest managers are increasingly expected to consider biodiversity in land management decisions. Habitat management, particularly dormant-season prescribed fire with or without prior herbicide application may provide an opportunity for achieving sustainability goals relative to biodiversity in intensively managed pine stands. Therefore, we investigated vegetation structure, coverage, and diversity responses to combinations of prescribed fire and herbicide (imazapyr) treatments within mid-rotation, thinned, intensively managed pine stands within a managed forest matrix in east-central Mississippi, USA during 1999–2008. We used a randomized complete block design of 6 mid-rotation, thinned pine stands (blocks) each with 4 treatments [control, burn only (dormant-season 3 year fire return interval), herbicide only (Arsenal AC® at 12 oz./acre), burn + herbicide) assigned to 10-ha experimental units. We measured vegetation structure (understory, midstory, and overstory components) and understory plant coverage using standard techniques from one year pre-treatment (1999; our baseline) through 9 years post-treatment (2008). To evaluate treatment effects, we used repeated measures mixed models analysis of covariance with baseline data as our covariate. We detected 338 plant species including 140 forbs, 5 ferns, 35 grasses, 11 herbaceous vines, 21 legumes, 26 sedges and rushes, 9 semi-woody vines, 76 woody plants, and 15 woody vine species. Treatment effects included concomitant reductions in upper level visual obstruction (m), increased herbaceous understory coverage, and hardwood midstory competition reduction. Herbicide was more effective at reducing hardwood midstory basal area, but repeated disturbances by prescribed fire maintained favorable vegetation structure conditions and improved understory plant species diversity throughout the latter years of study. Overstory pine basal area demonstrated positive responses to treatments. Prescribed fire and herbicide, within thinned, intensively managed pine stands at levels investigated here, appears to promote a stand structure consisting of a pine overstory and a diverse understory, reflective of historic conditions in the southeastern U.S. Implementation of these treatments can help forest managers achieve sustainable forestry goals in intensively managed pine stands of the southeastern U.S.