How do thinning and burning treatments in southwestern conifer forests in the United States affect wildlife distribution, abundance, and population performance? (systematic review)
After a century of fire suppression, logging, and grazing, conifer forests in the southwestern United States have undergone a dramatic departure from conditions that existed prior to Euro-American settlement. Today’s ponderosa pine and mixed conifer forests are characterized by homogenous, dense, small-diameter stands that are susceptible to stand-replacing crown fires. There is now an emphasis on ecological restoration in the Southwest, whereby forests are thinned, burned, or both to approximate presettlement structural conditions. Ecological restoration treatments expose wildlife species to short- and long-term alterations to their habitat. Treatments are an effort to return forest structure and composition to within the range of natural variability, which should benefit native wildlife species. However, both thinning and burning treatments are being implemented across thousands of acres of forest in the southwestern United States, with limited quantitative data regarding wildlife responses. Individual species have been studied, but no review exists that quantitatively examine the effects of thinning and burning treatments on multiple wildlife species in a systematic review framework.
Primary objective: How do thinning and burning treatments in southwestern conifer forests in the United States affect wildlife density and population performance?
Secondary objective: Which wildlife species are most vulnerable to habitat alteration? How do the impacts of thinning and burning treatments compare to those of selective harvesting, wildfire, and overstory removal?
To identify studies relevant to our review, we searched databases supported by Northern Arizona University during September-December 2008, using a defined combination of search terms. We then eliminated papers, first based on title, then abstract, then full text, based on a set of criteria that specified the review subject (wildlife species in southwestern conifer forests), intervention (small-diameter tree removal, burning, thin and burn, selective harvest, wildfire, or overstory removal), comparator (untreated control), and outcome (density, abundance, or reproductive response variable, including recruitment, number of offspring, percent offspring survival, etc.). We assessed study quality based on whether the study was replicated and/or peer-reviewed, and applied a weighting factor (sampling area) to data used in the quantitative analysis. Other covariates included treatment, forest type, time since treatment, species, study type, density estimation method, replication, quality of study, and study (identifying the origin of the data). We identified data that met the requirements of meta-analysis, calculated effect sizes using the response ratio metric, built generalized linear models to predict effect size based on covariates, and identified the most parsimonious model using a model selection approach. Each covariate in the best-fitting model was examined via forest plots by calculating mean effect sizes with bootstrapped confidence intervals. Data that were not appropriate for meta-analysis were analyzed using vote-counting techniques.
Our review identified 56 relevant studies, which were dominated by avian studies and generally occurred less than 10 years post-treatment. Although the qualitative analysis resulted in broadly neutral or positive responses to treatments in terms of species abundances, the meta-analysis revealed a pattern of generally positive density responses to the restoration-like treatments (small-diameter removal, burning, and thin/burn) and negative responses to the high-severity treatments (wildfire and overstorey removal). We recorded more positive responses by individual species to the high-severity treatments using the qualitative analysis compared to the meta- analytic approach. Reproductive responses were generally positive in the restoration treatments and negative in the high-severity treatments, but were compromised by low numbers of observations. Overall, thinning and/or burning did not negatively affect species’ abundances or densities compared to unmanaged forest stands, and were less detrimental than overstorey removal or wildfire.
This review suggests that thinning and prescribed burning of southwestern ponderosa pine and dry mixed conifer forests will benefit passerine birds and small mammals. Based on the existing literature, small-diameter removal and/or burning does not negatively affect species’ densities compared to unmanaged forest stands, and is less detrimental than overstorey removal or wildfire. However, no one treatment benefitted all species, at least in the short term. Thus, a combination of various treatments in a patchy arrangement in time and space across the landscape is likely to result in higher diversity than any one treatment.
The majority of studies in the analysis examined responses of birds to treatment, and we suggest that existing studies be carefully consulted before initiating similar research in order to eliminate duplication of effort. Other under- or unrepresented taxa include reptiles and amphibians, rare birds and small mammals, medium and large mammals, including both predators and ungulates, and birds of prey. Furthermore, the lack of studies that assess reproductive responses across all species indicates a paucity of research on this important fitness parameter. Finally, studies need to be conducted at larger temporal and spatial scales in order to understand both short- and long-term implications of treatments at the landscape level.
After a century of fire suppression, grazing, and logging, ponderosa pine forests in the western United States have undergone a dramatic departure from conditions that existed prior to Euro American settlement (Covington and Moore 1994, Taylor and Skinner 1998, Fry and Stephens 2006). Many areas within this ecosystem are currently dominated by homogenous, dense stands of small-diameter trees characterized by low plant and wildlife diversity and susceptibility to stand-replacing crown fires (Cooper 1960, Fulé et al. 1997). These fires are different from the frequent, low-severity ones that used to occur, which would maintain forests by removing the understory and small diameter trees, resulting in a patchy structure of mostly mature trees and a meadow-like ground cover (Everett et al. 2000, Covington 2003). There is now an emphasis on implementing fuels reduction treatments in ponderosa pine forests, whereby fuel loads are reduced through both silvicultural treatments and controlled burns, decreasing the likelihood of crown fire.
Thinning and burning treatments expose wildlife species to short- and long- term alterations to their habitat. In the short-term, both mechanical harvesting of trees and prescribed fire are disturbance events that have immediate effects on the environment: removing or killing live trees, reducing shrub and herbaceous ground cover, altering structural components such as snags and downed woody material, and creating sites susceptible to colonization by invasive plant species (Chambers and Germaine 2003, Collins et al. 2007). In the long term, successful restoration treatments should create a forest with a decreased density of trees compared to today’s conditions, but increased basal area due to the prevalence and growth of large, mature trees with a fairly open canopy; in addition, such treatments should increase understory plant cover and species diversity (Waltz et al. 2003, Metlen and Fiedler 2006). This increased spatial and temporal heterogeneity will diversify habitat available for wildlife. Thus, these “natural conditions” should, in turn, restore the native, diverse assemblage of animal species (Covington 2000, Allen et al. 2002).
Both thinning and burning treatments are being implemented across thousands of acres in the ponderosa pine forests of the western United States, with limited understanding of the implications to wildlife. Fuels reduction treatments have only been implemented in the last 10-20 years, and thus the corresponding studies on wildlife are relatively recent and limited in temporal and spatial scale. Individual species have been studied, but no review exists that quantitatively analyzes the existing literature across taxa. Existing reviews include summaries of the impacts of thinning and burning treatments on birds (Block and Finch 1997, Finch et al. 1997, Bock and Block 2005b;a), and qualitative reviews describe the effects of thinning and/or fire on all wildlife species (Lyon et al. 2000, Chambers and Germaine 2003, Pilliod and Bull 2006). However, none of these reviews quantitatively examine the effects of thinning and burning treatments on all wildlife species in a systematic review framework.
The objective of this review is to systematically review and evaluate the impacts of density-reducing treatments, including thinning and burning, on wildlife vertebrate species in ponderosa pine forests in the western United States. We will compare the treatments to controls, as well as to more severe forest treatments including highgrading, clearcutting, and high severity wildfire. Our objectives are to (1) determine which treatments had the greatest effect on wildlife, (2) determine which species were most and least sensitive to habitat manipulation, and (3) identify species or groups of species for which there was a paucity of field experimentation and data. This review will serve as a starting point for researchers and managers in understanding the comprehensive impacts on wildlife of fuels reduction treatments and determining future monitoring and research needs.