Ecological responses to herbicides and fire in pine plantations

Planted pine forests are an important component of landscapes in the southeastern United States and provide a significant portion of the nation’s wood supply. Herbicides are an important tool for managing competing vegetation in a wide variety of forest types, but particularly in southern pine plantations. NCASI has supported a variety of research projects to enhance understanding of how herbicides can be used to safely achieve forestry and wildlife objectives.

Since 1999, NCASI and Weyerhaeuser Company have been supporting a study in east-central Mississippi to investigate how animal and plant communities respond to use of herbicides and prescribed fire in mid-rotation pine plantations. Results are presented in two recent publications authored by Raymond B. Iglay and others associated with Mississippi State University and Weyerhaeuser Company.

The first paper, “Effect of Plant Community Composition on Plant Response to Fire and Herbicide Treatments,” appeared in Forest Ecology and Management (Vol. 260, pp. 543-548).The abstract follows.

Vegetation management, using prescribed fire and herbicides, is used in forestry applications to reduce competition with desired species, improve wildlife habitat, and meet other silvicultural objectives. Although plant communities resulting from such treatments are generally known, it is unclear how pre-treatment plant community structure may influence specific plant community responses. Therefore, to examine how species dominance may impact response of plant communities to vegetation management, we compared the top contributors to plant biomass (kg ha1) among prescribed fire and herbicide (imazapyr) treatments within intensively managed pine stands in east-central Mississippi, USA. Ninety-two species of 390 collected comprised 95% of plant biomass and six species comprised 55% of total biomass. Dominant species may have restricted plant diversity. Prescribed fire with and without imazapyr improved species richness but did not control some highly competitive species. None of the treatments tested is necessarily an optimal solution to control well-established understory plant species. Although management prescriptions consider exotic and invasive plant species, control of well-established native species should also be considered to tailor vegetation management to meet forestry and wildlife habitat objectives. More research is needed concerning plant response to multiple herbicide tank mixtures with and without prescribed fire to optimize future vegetation management for multiple objectives.

The second paper, “White-Tailed Deer Carrying Capacity in Mid-Rotation Pine Plantations of Mississippi,” recently appeared in the Journal of Wildlife Management (Vol. 74, pp. 1003-1012). The abstract follows.

Herbicides, commonly used for vegetation management in intensively managed pine (Pinus spp.) forests of the southeastern United States, with and without fire, may alter availability of quality forage for white-tailed deer (Odocoileus virginianus; deer), an economically and socially important game species in North America. Because greater forage quality yields greater deer growth and productivity and intensively managed pine forests are common in the southeastern United States, forest managers would benefit from an understanding of fire and herbicide effects on forage availability to improve habitat conditions for deer. Therefore, we evaluated independent and combined effects of fire and herbicide (i.e., imazapyr) on forage biomass and deer nutritional carrying capacity (CC) on land owned and managed by Weyerhaeuser NR Company in east-central Mississippi, USA. We used a randomized complete block design of 6 pine plantations (blocks) divided into 4 10-ha treatment plots to each of which we randomly assigned a treatment (burn-only, herbicide-only, burn + herbicide, and control). We estimated biomass (kg/ha) of moderate- and high-use deer forage plants during July of 1999–2008, then estimated CC for diets to support either body maintenance (6% crude protein) or lactation (14% crude protein) with a nutritional constraints model. Herbaceous forages responded positively to fire and herbicide application. In most years, CC estimates for maintenance and lactation were greater in burn + herbicide than in controls. Maintenance-level CC was always greater in burn + herbicide than in controls, except at 1 year posttreatment. Burn + herbicide was 2.6–8.3 times greater ( = 4.0) than control for lactation-level CC in 8 of 9 years posttreatment. We recommend fire and selective herbicides to increase high-quality deer forage in mid-rotation, intensively managed pine plantations.

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