Woodland Treatments

     Starting in the 1940’s and continuing through the 1960’s, hundreds of thousands
of hectares of woodlands were cabled, chained, bulldozed, and treated with herbicide in
the Southwest and Great Basin, in attempts to convert these areas into more productive
rangelands for the benefit of livestock production and game animal populations. Approximately 7% percent of woodlands managed by the Bureau of Land Management on the Colorado Plateau have been treated using various techniques in the past 50 years. Treatment methods implemented by the BLM have changed over time, with chaining/buldozing being the primary methods from the 50's through the 70's. A greater diversity of methods (chemical, prescribed burns, hydroaxe, woodcut, rollerchop) have been employed from the 80's until the present (Peters and Cobb, unpublished). These treatments have met with mixed results, which is likely due to variable site-specific conditions (e.g. soil types, seed bank diversity, climate) and treatment techniques (Aro 1975). High implementation costs due to rising fuel costs in the 1970's and rapid re-establishment of trees at some sites subsequent to conversion led to this management practice being significantly curtailed and completely abandoned in some areas (Belsky 1996). However, woodland treatments continued to be implemented by the BLM for various purposes, and have been applied to approximately 25,000 hectares per year since the beginning of the 1980's through the present (Peters and Cobb, unpublished). To access a detailed inventory of woodland treatments on BLM lands of the Colorado Plateau, visit the PJ-Wood website. The long-term ecological and socio-economic implications of these treatments are crucial to understand, especially given the large aerial extent of these manipulations.

     Recent research indicates that the reduction of woodland overstory and the subsequent execution of various ground treatments with the resulting slash (including prescribed fire) can be an effective method to initiate restoration of woodland understories and grasslands in terms of ecosystem structure and composition. A paired watershed study in Bandelier National Monument in north-central New Mexico showed that slash treatments significantly reduced unsustainable erosion rates (Hastings et al. 2003), and increased total herbaceous cover three-fold over pre-treatment and control levels (Jacobs et al. 2002). This increase was correlated with significant increases in butterfly species richness and abundance (Kleintjes et al. 2004). Another study on the Cibola National Forest in New Mexico demonstrated that overstory reduction and various slash treatments (removal, clustering and scattering) significantly increased cover of native grasses and to a lesser degree, forbs and shrubs (Brockway et al. 2002). Brockway et al. (2002) note that scattering slash across the treated site to serve as mulch was the most beneficial slash treatment in terms of improving plant species diversity and conserving site resources. Research plots in western New Mexico three years post-mechanical overstory reduction showed an inverse correlation between percent ground cover (grasses, forbs and shrubs) and piñon juniper density (Albert et al. 2004). Total live biomass in treated plots increased during the first five years, before leveling off or decreasing slightly (Albert et al., 2004). While the studies cited above were all conducted in true (but degraded) woodland/ savanna, the fundamental ecological responses to treatments documented are applicable to restoration in grasslands degraded by woodland colonization. Preliminary data from treated areas sampled 2 years post-treatment on Anderson Mesa, where trees were lopped to a height of 18” maximum and slash was scattered, show significant decreases in dominant “increaser species” (species known to increase in response to heavy grazing) such as snakeweed (Gutierrezia sarothrae), blue grama (Bouteloua gracilis), and tumble mustard (Sisymbrium altissmum). There was significant increase in one native grass, squirreltail (Elymus elymoides). An increase in squirreltail is a promising treatment response, as it is a cool-season grass. Cool-season grasses “green up” in early spring, a time when pronghorn need green forage to meet their dietary requirements. While these research results have not been subject to peer review, these data do suggest positive plant community changes in response to treatments. A decrease in the dominant increaser species may open up niches for native forbs and grasses preferred by pronghorn (R. Miller, AZGFD Habitat Specialist, personal communication on 3/31/06).

     Pronghorn populations have been shown to respond positively to habitat manipulations. Aoude and Danvir (2002) physically increased forb abundance and decreased shrub cover, on 18 burned or planted treatments (totaling 4270 ha) in the sagebrush-steppe of northern Utah. They found that both fawn production and population size correlated positively with these treatments, increasing the carrying capacity of the study area. Albert et al. (2004) found that deer and elk use was dramatically higher in thinned plots as compared with the control. Increased use by wild grazers and browsers in treated areas would be consistent with a positive understory response, as the biomass production of palatable vegetation was greater than in untreated areas. These ecosystem responses to treatments are consistent with the concept that overstory reduction and slash treatments can initiate a release of resources (e.g. sunlight, water, nutrients) that become available to support increased understory vigor. Thus, treatments in areas with small trees, or semi-open canopy cover conditions will probably not show the dramatic understory response found in more developed woodland stands. Existing site conditions related to historical land use practices, soil type and condition, climate, fire, topography, current plant and seedbank diversity, as well as the presence of exotic and invasive species may dramatically influence success of overstory and slash treatments. It is clear from the scientific literature that properly sited and implemented treatments hold the potential to enhance conditions for native grassland flora and fauna, and to arrest accelerated erosion in some areas; a ubiquitous problem that further degrades the inherent potential of the land to support healthy grassland ecosystems. Researchers also caution that if possible, treatment sites should be carefully selected based upon multiple lines of corroborating historical evidence (Swetnam et al. 1999), be in flat to gently sloping areas with stable soils, with a preference for wetter north-facing slopes (Albert et al. 2004).

     While woodland encroachment is often blamed for various ecological and hydrological problems in the Interior West, some researchers dispute these claims, and warn against the blind removal of woodlands. Belsky (1996) cautioned: “Before large-scale juniper clearance or logging programs are initiated, managers must understand and carefully evaluate the consequences of their actions”. Other researchers note that in some areas such as Chaco Canyon, the documented increase in woodland extent and cover in the past century may actually represent woodlands that are recovering from
over-harvesting by the Anasazi some 800-1000 years ago (Betancourt and Van Devender
1981, Samuels and Betancourt 1982). This illustrates the importance of attempting to
distinguish between natural expansion or recolonization and woodland encroachment.

     Landis and Bailey (2005) examined the age structure and spatial arrangement of
piñon and juniper woodlands and savannas on a portion of Anderson Mesa in northern Arizona. Based upon their analysis, they urged managers to vary the intensity of thinning treatments across the landscape to account for variation in pre-settlement tree density across different soil types, aspects, and elevations and to mimic historical spatial patterns if possible. These researchers also point out that one blanket prescription for grassland restoration might reduce the range of variability present in the form of woodlands and savannas. Furthermore, the currently occurring massive die-off of piñon across the Southwest, due to extended drought and insect infestations, is an issue that merits attention with respect to woodland overstory treatments. A letter dated 12/23/03 to Gale Norton, Secretary of the Interior, signed by 13 noted southwestern ecologists, emphasized a concern that healthy piñon trees are becoming regionally scarce, and thus may need to be retained as food for wildlife and as seed sources for this declining species.

     Given that large-scale overstory reduction projects are already well-underway in
the Southwest, the question is not if, but rather where treatments will occur and how intense they will be. Therefore, a scientifically informed, precautionary approach should be employed when determining where woodland treatments should and should not occur, and whether thinning or overstory removal is appropriate for a given site. Biophysical factors such as soil type, slope, aspect, seed bank diversity, and current habitat values should all be taken into consideration prior to the execution of woodland treatments. In addition, an adaptive management framework (Johnson 1999) may be useful to identify what factors drive treatments that show successful understory and indicator species responses, and to modify treatment selection and implementation techniques accordingly.