The expansion of juniper woodlands has altered the vegetation composition across the Intermountain West (Bunting et al. 1999, Miller et al. 2005, Miller et al. 2008). The transition from sagebrush steppe to woodland reduces forage quantity and quality for wildlife and domestic animals (Wisdom et al. 2000), negatively impacts wildlife habitat for sagebrush obligate species such as the greater sage-grouse (Centrocercus urophasianus [Bonaparte, 1827]; Baruch-Mordo et al. 2013), disrupts nutrient cycling, increases erosion, and changes the fire frequency of the system (Blackburn and Tueller 1970, Miller and Tausch 2001, Bates et al. 2007). To mitigate problems associated with this encroachment, land managers have utilized a wide range of strategies on the landscape, among which are prescribed fire and mechanical treatments. These treatments change the fuel structure of these landscapes, influencing the abundance and continuity of herbaceous biomass, shrub biomass, and downed woody debris, leading to altered expectations for fire behavior and effects associated with a potential future wildfire.
The response to prescribed fire was similar across the three juniper woodland regions. Prescribed fire resulted in an increase in live herbaceous biomass in Phases 1, 2, and 3 in western juniper, pinyon −juniper, and Utah juniper sites. An increase in live herbaceous biomass is expected post fire. The removal of competition from shrubs and trees combined with the rapid release of nutrients into the system facilitates regeneration and growth (Everett and Ward 1984, Agee 1993, Rau et al. 2008, Miller et al. 2014). An increase in herbaceous biomass is expected to continue until available space and resources are expended (Tausch and Tueller 1977, Everett and Ward 1984, Bates et al. 2005).
Prescribed fire decreased shrub biomass by about 90% in Phase 1 and Phase 2 woodlands; however, the reduction of shrub biomass in Phase 3 was variable. This variability was unexpected. Big sagebrush is particularly sensitive to fire and experiences stand replacement when consumed (Wambolt and Payne 1986, Bunting et al. 1987). Sagebrush biomass is expected to increase given time, but the recruitment process is slow and it may take 35 to 100 years to fully recover to pre-fire conditions (Pieper and Wittie 1990, Wambolt et al. 2001). The prescribed fire treatment was designed for 100% of the plots to be blackened; thus, a surviving shrub component indicates an incomplete prescribed fire. This is most likely due to the limited availability of fine fuels to support the flaming front in a Phase 3 woodland.
Downed woody debris had the highest variability in consumption of any fuel variable measured. Generally across sites, DWD decreased in Phases 1 and 2, with the largest reduction in Phase 1. In Phase 3, we generally observed an increase in DWD, particularly in the larger size class (100-h fuels), but the variability was high across sites. This variability was probably due to continuity of fine fuels and their ability to carry fire, and also to the wide variety of fuel moisture and weather conditions for which these treatments were implemented across the woodland sites.
Fuel consumption decreased along the successional gradient from young to older woodlands. Phases 1 and 2 had the highest herbaceous fuel load, which more likely resulted in a continuous flaming front, as was reflected in the greater shrub biomass consumption. Thus, when DWD was consumed, it occurred in those phases. Phase 3 was known for a lack of fuel continuity, making it difficult to burn (Blackburn and Tueller 1970, Pieper and Wittie 1990, Miller and Tausch 2001). Fire treatments are therefore often not recommended for Phase 3 (Bates et al. 2000, Miller et al. 2005) because it requires more extreme fire conditions that are conducive to a crown fire (Huffman et al. 2009), which is generally not desired.
The total fuel load on site decreased in Phases 1 and 2 after prescribed fire treatment. Although there was a sizable increase in the herbaceous biomass, it had not yet compensated for the amount of sagebrush biomass consumed two years post treatment. This difference will likely decrease in the future as herbaceous biomass continues to increase into the open spaces and as shrubs recover from the treatment (Tausch and Tueller 1977, Everett and Ward 1984, Bates et al. 2005). Fire severity in these two phases is also expected to decrease, with the exception of that part of Phase 2 that experienced an increase in 1000-h DWD solid fuel. In Phase 3, pinyon −juniper had a herbaceous biomass increase greater than the amount of shrub biomass consumed. This suggests that there is an increase in fuel and fuel continuity across the system, increasing the probability of fire ignition and spread.
Because the young juniper woodlands did not have an abundance of 1000-h DWD and exhibited high variability, the observations of these fuel categories were not normally distributed and were therefore excluded from statistical analysis. These fuels were estimated along five 30 m transects, (i.e., 150 m total transect length per plot) since larger fuels have been shown to vary at broader scales than the fine fuels (Keane et al. 2012). For future studies, we recommend longer transects or a different sampling methodology for the 1000-h fuel categories for fuels assessments in sagebrush steppe and juniper woodlands.
Herbaceous biomass response varied by successional phase following mechanical treatment. Mechanical treatments did not significantly increase herbaceous biomass in Phase 1, but increased two-fold to three-fold in Phase 2, and three-fold to six-fold in Phase 3 in western juniper (Table 1) and Utah (Table 3) juniper. Herbaceous biomass in Phase 1 woodland would be expected to be the least effected by juniper woodland encroachment, thus it was not surprising that treatment results were not significant. However, an increase in herbaceous biomass in Phase 2 was found in two of the woodland regions, supporting the notion that, even at lower juniper densities, removal of juniper releases enough resources for a herbaceous vegetation response to be measurable (Bates et al. 2005, Miller et al. 2005). Other studies were primarily conducted in Phase 2 and Phase 3 juniper woodlands and found that mechanical treatments increased soil nitrogen and water availability, leading to an initial flush of herbaceous biomass in the first two years post treatment (Tausch and Tueller 1977; Bates et al. 1998, 2000, 2005). Generally, herbaceous biomass peaked within the first five to ten years, and shrubs eventually increased in abundance (Tausch and Tueller 1977, Skousen et al. 1989, Bates et al. 2005, Miller et al. 2014). Increased herbaceous fuel connectivity may lead to increased probability for a fire to carry across the landscape.
Shrub biomass was generally not affected by mechanical treatment. It was expected that shrub biomass would increase as sagebrush would have benefited from the increase in soil nitrogen and water availability. Previous studies showed that chaining treatments (a type of mechanical treatment that has been used for brush control) caused a vigorous shrub response within the first two years post treatment (Tausch and Tueller 1977, Skousen et al. 1989). However, Bates et al. (2005) found minimal shrub response 13 years after a mechanical treatment. He cited a lower initial shrub density within his plots as a possible cause of this slower response. This would not be accurate in our study as Phase 2 still had a relatively intact shrub component. Continued long-term study is needed to determine if the shrub layer will respond to the cut-and-leave mechanical treatment.
Changes in DWD varied by successional phase. In Phase 1, we did not observe any increase in 10-h DWD fuels in two of the regions, but a significant increase was recorded in the pinyon −juniper region (Table 2). On Utah juniper and pinyon −juniper sites in Phase 1, we observed an increase in 100-h DWD. The increase in larger fuels indicates that conversion of ≤10% juniper tree cover to surface fuel may be defined by tree trunks and has a minimal influence on the smaller fuels in the fuel bed. Mechanical treatment influences on DWD in Phase 2 and Phase 3 were more pronounced (Tables 1, 2 and 3). The fuel increase was expected and is a function of converting live tree biomass to downed woody debris, demonstrating that juniper canopy cover will remain in the fuel bed two years post treatment.
Mechanical treatments used chainsaws to remove all trees taller than 0.5 m, clearly reducing the probability of a future crown fire. While the potential of a canopy fire has been dramatically reduced by the mechanical treatment, there is a corresponding increase to DWD surface fuels, which can increase the potential for a high-severity surface fire. In these surface DWD fuels, fire-season moisture content is less than in live trees and the fuel is now layered on the surface, which can increase soil heating in the event of a fire, leading to increased mortality of herbaceous vegetation and opening up the landscape for invasion by exotic annual grasses.
The heavier woody fuels (100-h DWD) added to the fuel bed were substantial in our study. For example, pinyon −juniper pre treatment had 1620 kg ha−1 100-h DWD, but post treatment it had over 7200 kg ha−1 100-h DWD. The 100-h DWD and 1000-h fuels can remain in the ecosystem for decades. Decay rates in the sagebrush steppe are variable and slow (Harmon et al. 1986) and may be influenced more through abiotic factors than biotic factors (Waichler et al. 2001). As the 1000-h fuels decompose and become rotten, they have an increased risk of smoldering and soil heating when burned (Passovoy and Fulé 2006), which may increase fire’s effects on soil and vegetation.
We focus on three fuel components, including live herbaceous, total shrub, and 10-h DWD, due to their importance in influencing fire intensity and spread as well as fire effects (Rothermel 1983, Ottmar et al. 2007). Both prescribed fire and mechanical treatments increased live herbaceous biomass on juniper woodland sites (Fig. 2). The percent increase was greater for fire treatment compared to mechanical treatment. Mechanical treatments understandably increase 10-h DWD the greatest, given that trees were cut and left on the sites. This increase was greatest where woody plant cover was highest —in the Phase 3 woodland. The greater the pre-treatment pinyon and juniper cover, the greater the increase in 10-h DWD post-treatment.
The response of live herbaceous biomass was most variable for Phase 3 woodlands as compared to Phases 1 and 2 (Fig. 3; Tables 1, 2 and 3). Percent increases in live herbaceous biomass were greatest for Phase 3 woodlands for both fire and mechanical treatments, but those sites had low herbaceous biomass prior to treatment (Tables 1, 2 and 3); thus, small absolute increases resulted in large relative increases. Small residual amounts of herbaceous plant populations resulted in erratic responses of those species.