Bird Response to Prescribed Burning and Herbicide in Loblolly Pine Plantations
Commercial pine forests occupy approximately 25 million ha of the Southeast. Many of these systems are under intensive, short-rotation (30-35 yrs) management and will undergo commercial thinning, pruning, and understory competition control during the rotation. Prescribed fire and herbicide are frequently used in southeastern pine forests to control understory hardwood competition. These management practices are also used to enhance wildlife habitat and prepare seedbeds. Fire is a natural process that has been an integral part of southeastern pine ecosystems for thousands of years. Historically, prescribed burning has been the management tool of choice for site preparation and understory hardwood competition control. Increasingly, prescribed fire use is being replaced by herbicides because of factors such as smoke management, liability concerns, and limited number of allowable burning days.
Although effects of fire on
wildlife and plants are well documented, few studies have examined effects of
herbicide treatments on bird communities or combined effects of fire and
herbicide, especially in mid-rotation loblolly pine stands. A team of scientists from the Department of
Wildlife and Fisheries,
The study is being conducted on
industrial forest lands, owned and managed by Weyerhaeuser Company in
Breeding bird communities were surveyed using 10 min, 50m radius circular point counts during May and June 1999-2001. Point counts index relative abundance (mean number of individuals/species) and species richness (number of species; Figure 1). Researchers also generated a measure of the “avian conservation value” of the stand by weighting individual species abundance by the Partners in Flight Conservation Priority Score (Fig. 2). Partners in Flight Conservation Scores are based on the relative scarcity and population trends of bird species. Species with restricted distribution, narrow habitat requirements, and regionally declining population trends are scored as having higher conservation value than widely distributed, abundant, stable species. Weighting abundance values by conservation scores provides a measure of the contribution of local bird communities to regional avian conservation goals.
In 1999, prior to treatments, bird communities were similar among stands with regard to mean species richness (Fig. 1), mean conservation value (Fig. 2) and mean total abundance. Pre-treatment relative abundances of most species were similar among treatments. In September 1999, Arsenal® herbicide was applied to plots assigned herbicide or herbicide/burn using a skidder sprayer, at a rate of 12oz/acre. In January 2000, plots assigned to burn or herbicide/burn treatments were prescribed burned.
All competition control practices altered the plant community and initially resulted in reductions in total bird abundance and species richness. During the first post-treatment growing season (2000), mean species richness (Fig.1), mean conservation value (Fig. 2) and mean total abundance did not differ among treatment plots. However, not all species responded the same. Treatments reduced abundance of some species, but increased abundance of others. Seven of the 42 species observed: white-eyed vireo, Kentucky warbler, hooded warbler, yellow-breasted chat, Northern cardinal, tufted titmouse and blue-gray gnatcatcher were more abundant in control plots than treated plots, whereas 5 of 42, including mourning dove, indigo bunting, hairy woodpecker, great-crested flycatcher, and eastern wood-pewee were more abundant in treated plots than control plots.
The second year post-treatment
(2001), mean species richness, (Fig. 1) mean conservation value (Fig. 2) and
mean total abundance were similar among all treatments. Again, some species were less abundant in
treated plots but others were most abundant in treated plots. Four of 42 species: white-eyed vireo,
All management regimes resulted in a
short-term (1-year) reduction in species richness, total abundance, and
conservation value. Midstory species
such as hooded warblers and white-eyed vireos declined whereas, open canopy
species such as great-crested flycatcher and Eastern wood pewee increased. As the plant community recovered and the
herbaceous layer developed, birds like northern bobwhite, common yellow throat,
and indigo bunting that are associated with shrubs and herbaceous ground became
more abundant in treated plots.
Although species richness was similar among treatments, community
composition differed with high priority species such as northern bobwhite
occupying treated areas and others such as
Fire is a natural process to which many plant and animal species in southern pine forests are adapted. In the absence of fire, fire dependent systems succeed along un-natural trajectories due to encroachment of hardwood species. On true pine sites, hardwood encroachment shades out understory plant species important to many wildlife species as food and cover. In managed southern pine systems, prescribed fire has historically been used for hardwood competition control and wildlife habitat enhancement. However, the forest industry is increasingly replacing fire with herbicide applications. Herbicides may be equally or more effective for hardwood competition control, but may not have the same ecological effects as fire. Herbicide, herbicide in combination with fire, and to a lesser extent fire, when used for hardwood competition control, result in immediate and dramatic changes in the understory and herbaceous plant community. Fire changes the composition of the understory vegetation, whereas herbicide has a strong effect on shrub and midstory woody components. Herbicidal control of hardwood encroachment may increase the efficacy of prescribed fire. Herbicide, fire, and herbicide in combination with fire to varying degrees release a herbaceous ground cover. These changes in vegetation structure affect the bird community.
Although fire is an important natural process in Southern pine systems, the reality is that it will never occur to the extent and frequency that it did in the past. Selective herbicides like Arsenal can be used to recapture fire excluded pine systems and facilitate reintroduction of prescribed fire regimes. Application of selective herbicide under the conditions of our study did not have negative impacts on bird communities after the first growing season. In fact, herbicide used in combination with fire produced conservation benefits by creating regionally scarce pine/herbaceous communities colonized by regionally declining, high priority early successional species. The take home message is this: vegetation management affects bird communities. Midstory and shrub species will flourish in dense, fire-excluded stands, whereas early successional and pine/grassland species will benefit from midstory management. The management regime that is best for an area depends upon the suite of species for which one wants to manage. Further research is needed to measure the continued response of the bird communities and to study the effects of a fire regime to determine the long-term conservation benefits of these silvicultural treatments for bird communities.
Figure 1. Mean species richness averaged over 4 points and 4 visits, between
treatments within years.
Figure 2. Mean conservation value averaged over 4 points and 4 visits, between
treatments within years.