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Evaluating the Effects of Pinyon Thinning Treatments at a Wildland Urban Interface

 
Equipment used to prepare the soil.
This study evaluated the short-term effects of thinning methods for pinyon pine woodlands at two sites in the southwestern Great Basin. Both cut/pile/burn and mastication treatments were equally effective at reducing the target fuels which were mature, live pinyon trees. Application costs though differed substantially, with the cut/pile/burn technique being less expensive. Thinning treatments increased the abundance of herbaceous vegetation, although in some cases the strength of the increase was constrained by the level of pre-treatment tree dominance. Increases in perennial grass cover and density in response to thinning were usually greatest at lower levels of pre-treatment pinyon dominance, whereas native annual forb density and cover responded fairly equally along the tree-dominance gradient. Shrub abundance declined in response to pre-treatment tree dominance and the response to thinning treatments appeared more subtle than for herbaceous vegetation.  Shrub cover within the control decreased slightly during the 3 post-treatment years, while it increased slightly within both thinning treatments.  The response of slower-growing plants such as shrubs will need to be evaluated during future years to determine differences between thinning treatments. Species richness within the two thinning treatments steadily increased relative to the control over the course of the 3 years following treatment.  Species richness was also consistent across the pinyon-dominance gradient, which suggests that the ability for vegetation to recover at this location may not be significantly limited by a lack of seed availability or lack of species diversity within highly tree-encroached shrublands. Cover of herbaceous live fuels (perennial and annual forbs and grasses) and live woody fuels (live shrubs and trees) were also similar between the two thinning treatments and higher than the control. The greatest difference in fuel structure between the two fuel treatments appeared to be dead woody fuel loads—mastication clearly resulted in greater fine fuel loads (1–100 hour fuels) as well as the shredded woody particles.   The stimulation of live herbaceous cover—especially at lower pre-treatment pinyon dominance—may have important implications for fire spread by enhancing the continuity of surface fuels, especially during dry years.

Project Details

Woodlands dominated by pinyon pine (Pinus spp.) and/or juniper (Juniperus spp.) occupy over 30 million ha of the western United States (West 1999), while only 3 million ha were occupied prior to the late 1800s (Miller et al. 1999). This 10-fold expansion of pinyon-juniper range may have only just begun since it currently occupies less area than climatic conditions seem to allow (Miller et al. 2000). The historic range and density of pinyon-juniper never reached its climatic potential because natural disturbances, especially wildfire, prevented trees from encroaching into adjacent shrublands. Historic fire regimes throughout the western US were substantially altered following European-American settlement during the 1900s, and pinyon-juniper/sagebrush fire regimes were changed because of direct fire suppression and reductions in surface fuels caused by livestock grazing (Miller and Rose 1999). A shift from sagebrush-dominated to pinyon-juniper–dominated vegetation caused by fire exclusion can result in loss of habitat for sagebrush-dependent wildlife, decreased species diversity, depleted soil seed banks, decreased aquifer recharge, and increased soil erosion rates (Koniak and Everett 1982, Reid et al. 1999, Davenport et al. 1998, West 1999, Miller et al. 2000).

As sagebrush-steppe has converted to pinyon-juniper woodlands, fire regimes have shifted from moderate intensity surface fires with moderate return intervals (~50 years) to high intensity crown fires with longer return intervals (>100 years). Changes in vegetation composition, fuel structure, and fire regime are generally characterized as shifts in fire regime condition class (FRCC). Historical, pre-settlement or otherwise “natural” conditions are classified as FRCC1, while moderate and high departures from historical conditions are classified as FRCC2 and FRCC3, respectively (Hann and Bunnell 2001).

In stands where encroaching woodlands are relatively young—having established since the mid-1900s—tree cover is low and surface fuels may still carry low to moderate intensity surface to passive crown fires. If early-succession woodland stands begin to dominate a sagebrush-steppe community, it is categorized as FRCC2—the community moderately deviates from historic natural fuel and fire regime characteristics. The potential is often relatively high for restoring FRCC2 areas back to their pre-invasion state by using tree thinning and without the need to actively re-vegetate sagebrush-steppe species. Where invading woodlands are relatively old—having established before or soon after the beginning of the 1900s—tree cover is high, whereas cover and seed bank densities of shrubs, grasses, and herbs are low. In these closed-canopy woodlands, fire does not propagate easily, except under extreme fire weather conditions, which typically results in intense crown fires that endanger rural communities and have undesirable effects on soils and plants (Miller et al. 2000). The potential may be relatively low for FRCC3 landscapes to return to pre-invasion conditions following tree thinning, especially without actively re-vegetating sagebrush-steppe species.

Various thinning treatments have been used to reduce density and cover of pinyon and juniper, and ultimately shift FRCC2 and FRCC3 sagebrush ecosystems to historical FRCC1 conditions, but their effects have been poorly documented and are difficult to predict. This lack of predictability makes many land managers wary of embarking on expensive thinning projects that could potentially have undesirable side effects. The existing information void also complicates the environmental review and approval process and can stall fuels reduction projects in the planning phase. Prudent land management requires that expensive, broad-scale landscape manipulations should be studied and evaluated first to identify the best prescription to correct the problem before obligating significant resources to treatments that may do more environmental harm than good. Thus, there is a significant management need across the United States for fuel management prescriptions that can effectively restore FRCC1 fuel and fire regime characteristics while minimizing negative ecological side-effects.

One of the primary concerns about thinning treatments is that they cause significant amounts of disturbance, which may promote the dominance of non-native plants such as cheatgrass (Bromus tectorum) (Brooks and Pyke 2001). In some cases, invasive plants create new fuel conditions and alter fire regimes (D’Antonio and Vitousek 1992, Brooks et al. 2004). Cheatgrass is prevalent in the pinyon-juniper/sagebrush-steppe ecotone, especially in disturbed areas. There is a very real concern that efforts to restore shrub-steppe to FRCC1 conditions may increase cheatgrass dominance, promote recurrent fire, and push landscapes into non-native annual grasslands.

Some thinning methods such as mastication reduce woody plant material into small wood chips leaving them as mulch on the ground, and this mulch has been shown to reduce the dominance of cheatgrass (Wolk and Rocca 2009). This reduced dominance of cheatgrass may be the result of increased soil carbon from the mulch which leads to reduced soil nitrogen levels that inhibits growth of invasive plants. Alternatively, the mulch may significantly shade the soil surface and inhibit seed germination and seedling growth. Regardless of the mechanism, reduced dominance of invasive plants such as cheatgrass can lead to increased density, cover, and diversity of native species, which could provide an additional benefit of tree thinning treatments.

Land managers at the BLM Bishop Field Office have a significant need to thin woodlands dominated by pinyon to reduce fire hazards at wildland urban interface areas, restore historical fuelbed conditions, and promote native plant species diversity while minimizing positive effects on cheatgrass.  However, before they implement large-scale thinning treatments they need information to reliably determine the best methods to achieve their goals. Specifically, recommendations are needed on management approaches to effectively restore tree-dominated FRCC2 and FRCC3 sagebrush-steppe landscapes (high intensity, long return interval crown fires) to FRCC1 conditions (moderate intensity and return interval surface fires). Approaches should also minimize the subsequent dominance of cheatgrass, which has the potential to replace one fuel hazard (dense trees) with another (continuous fine fuels), and potentially shift the landscape into another FRCC2 or FRCC3 situation (mixed intensity, short return interval, fast-moving surface fires). In addition, ideal treatments should also promote native plant dominance and diversity.

Objectives

The purpose of this project was to establish a fire management experiment and demonstration site that addressed significant local knowledge gaps hindering the management of pinyon woodland fuels within lands managed by the BLM Bishop Field Office. The need for this information is not unique to the Bishop Field Office, or even adjacent Inyo National Forest lands, but rather is typical of information needed by federal, state, and other land managers throughout the Intermountain West. The specific objectives of the project were to:

1. Establish demonstration sites to illustrate the effects of cut/pile/burn, mastication, prescribed fire treatments on the vegetation community and fuelbed structure.

2. Compare the implementation costs of cut/pile/burn and mastication treatments.

3. Compare the immediate effects of pinyon thinning treatments on their target fuel types, standing live pinyon fuels.

4. Compare the short-term effects of pinyon thinning treatments on cheatgrass abundance during the first 3 post-treatment years.

5. Compare the short-term effects of pinyon thinning treatments on native plant communities during the first 3 post-treatment years.

6. Compare the short-term effects of thinning treatments across a range of initial pinyon cover (where trees are just beginning to invade sagebrush-bitterbrush steppe to where trees have reached canopy closure).

7. Compare the short-term effects of pinyon thinning treatments on live herbaceous fuel characteristics.

8. Compare the short-term effects of pinyon thinning treatments on litter and duff fuel characteristics.

9. Compare the short-term effects of pinyon thinning treatments on woody fuel characteristics.



USGS Contact For This Project
Matthew Brooks
mlbrooks@usgs.gov
(559) 240-7622
Yosemite Field Station
40298 Junction Dr., Suite A
Oakhurst, CA 93644
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