Introduction

Land degradation is a widespread problem that affects nearly a quarter of the global land area, with a disproportionate greater extent in drylands (Stavi and Lal 2015). Land degradation decreases ecosystem goods and services, and plant communities may cross irreversible thresholds. Widespread degradation of native plant communities can result in declines in wildlife dependent on them, reduce biodiversity, and threaten the economic livelihoods of people who use them. This is particularly evident in rangelands, where the vast expanse of these uncultivated lands provides essential habitat for many wildlife species, a reservoir of biodiversity, and support for rural economies. Therefore, restoring degraded plant communities is a global priority and a major challenge (Suding 2011).

In western North America, degraded Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & A. Young] S.L. Welsh) communities are a priority for restoration. Wyoming big sagebrush communities occupy vast areas of the western United States (Miller et al. 1994; West and Young 2000), providing critical wildlife habitat for sagebrush-associated wildlife and an important forage base for livestock production (Connelly et al. 2000; Crawford et al. 2004; Davies et al. 2006). Historical overuse by sheep, cattle, and horses decreased native large perennial bunchgrasses (bunchgrasses excluding Sandberg bluegrass [Poa secunda J. Presl]) and increased sagebrush dominance in many big sagebrush communities (Davies et al. 2011a). These degraded communities were estimated to comprise 25% of the big sagebrush ecosystem approximately 2 decades ago (West 2000). Wyoming big sagebrush communities make up a disproportionate amount of the degraded sagebrush communities because they are generally less resilient than other common big sagebrush types (Miller and Eddleman 2000; Davies et al. 2011a). Because Wyoming big sagebrush communities are important to wildlife and livestock production, their restoration is paramount.

Restoring degraded Wyoming big sagebrush communities decreases their potential for transition to exotic annual grasslands. Degraded Wyoming big sagebrush communities are likely to transition to exotic annual grasslands with fire, a periodic disturbance in this ecosystem, because resilience and resistance to exotic annual grass invasion decrease when large perennial bunchgrasses decline (Chambers et al. 2007, 2014). Mature large perennial bunchgrasses are critical to limiting exotic annual grasses, in part because their resource use overlaps substantially with exotic annual grasses (Davies 2008; James et al. 2008).

Substantial invasion of Wyoming big sagebrush communities by exotic annual grasses, largely cheatgrass (Bromus tectorum L.), is generally irreversible. Exotic annual grass invasion is especially problematic because these species increase fine fuel continuity and fire frequency (Stewart and Hull 1949; Balch et al. 2013), which reduces native perennial vegetation, and leads to a positive feedback that favors a short exotic annual grass-fire cycle (D'Antonio and Vitousek 1992; Brooks et al. 2004). Exotic annual grasses are also highly competitive with seedlings of native vegetation and often preempt resources, resulting in the exclusion of native species (Melgoza et al. 1990; Nasri and Doescher 1995; Rafferty and Young 2002; Humphrey and Schupp 2004). To date, no large-scale cost-effective strategies exist to control exotic annual grasses across the vast areas they have invaded (Stohlgren and Schnase 2006). Therefore, it is critical to restore community resilience in depleted Wyoming big sagebrush communities before they transition to exotic annual grasslands.

Restoring the large perennial bunchgrass component in degraded Wyoming big sagebrush communities is particularly important because they dominate the understory in resilient communities (Davies et al. 2006), are the plant lifeform best able to limit exotic annual grass invasion and dominance (Chambers et al. 2007; Davies 2008; James et al. 2008), and are crucial to providing resilience to fire in this ecosystem (Chambers et al. 2014). However, restoration of the large native perennial bunchgrass component of a sagebrush-bunchgrass community is difficult. Excluding livestock to restore large perennial bunchgrasses in sagebrush communities has generally been unsuccessful, especially when sagebrush cover is high (Sneva et al. 1980; West et al. 1984; Davies et al. 2016). Competition from sagebrush is likely limiting the recovery of understory species in these degraded communities. Management may need to reduce sagebrush to promote recovery of the understory (Sneva et al. 1980; Boyd and Svejcar 2011). Reducing sagebrush as a stand-alone treatment in degraded Wyoming big sagebrush communities often does not promote the native herbaceous understory and can result in an increase in exotic annuals (Davies et al. 2012; Davies and Bates 2014; Swanson et al. 2016). The native herbaceous understory may not increase because these species may be largely lacking from degraded sagebrush communities and their seedbank may be limited (Young and Evans 1975; Chamber 2000). Therefore, in degraded Wyoming big sagebrush communities, sagebrush reduction followed by seeding may be necessary to facilitate increases in the native understory.

In Oregon, mowing to reduce sagebrush, followed by seeding native large perennial bunchgrasses in degraded Wyoming big sagebrush communities, approximately doubled the density of large bunchgrasses in a short-term study (Davies and Bates 2014). However, large bunchgrass cover was not different than the untreated control and exotic annual grasses increased substantially (Davies et al. 2014). The short-term nature of this study precluded knowing if large perennial bunchgrass cover may increase over time in this treatment and potentially reduce exotic annual grasses. Native perennial grasses in semiarid and arid ecosystems are slow growing (Holmes and Rice 1996; James et al. 2009); thus, a longer-term evaluation is needed to determine if this treatment strategy (mowing and seeding) will result in desired outcomes, especially further recruitment of large bunchgrasses and increases in their cover.

The purpose of this study was to investigate if mowing and seeding large native perennial bunchgrasses in degraded Wyoming big sagebrush communities would promote the recovery of large native bunchgrasses and limit exotic annual grasses over extended time frames (10+ yr). To accomplish this, we annually sampled experimental plots from Davies et al. (2014) for 11 yr post treatment. We hypothesized that 1) large perennial bunchgrass cover and density would be greater with mowing followed by seeding, 2) exotic annual grass cover and density would increase with mowing and seeding but decrease over time as large perennial bunchgrasses increase, and 3) exotic annual grass cover and density would be greatest in areas mowed but not seeded.

Methods

Study Area

The study was conducted in degraded Wyoming big sagebrush plant communities in southeastern Oregon 40–50 km southwest of Burns, Oregon. The overstory was primarily Wyoming big sagebrush. Bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve), Thurber's needlegrass (Achnatherum thurberianum [Piper] Barkworth), squirreltail (Elymus elymoides [Raf.] Swezey), and Sandberg bluegrass occurred at the study sites. Bluebunch wheatgrass and Thurber's needlegrass would have been dominant perennial bunchgrasses in these plant communities if they were not degraded (NRCS 2013). Study sites were on the Loamy 10-12PZ (R023XY212OR) ecological site (NRCS 2013). The exotic annual grass, cheatgrass, was common across study sites, but its cover was low (< 1%) before treatment (Davies et al. 2014). We considered the herbaceous understory as “depleted” at study sites because the density and cover of native large perennial bunchgrasses and native perennial forbs were insufficient to prevent transition to exotic annual grass community following disturbance. Foliar cover of sagebrush, Sandberg bluegrass, large perennial bunchgrass, and perennial forb averaged 14.9%, 1.7%, 1.5%, and 0.5% across study sites before treatment, respectively (Davies et al. 2014). Cover of native herbaceous perennial plant groups was lower at the study sites than in relatively intact Wyoming big sagebrush communities (Davies et al. 2006; Davies and Bates 2010). The most substantial deviation was that native large perennial bunchgrass cover was 5.9- to 6.7-fold less at the study sites than in relatively intact Wyoming big sagebrush communities (Davies et al. 2006; Davies and Bates 2010). Sagebrush cover was 20–50% greater at the study sites than the average for relatively intact Wyoming big sagebrush steppe in this region (Davies et al. 2006; Davies and Bates 2010), suggesting that sagebrush may be limiting herbaceous vegetation. Elevations at study sites were 1 263–1 350 m, and slopes were flat (0–4%). Soils at study sites were loamy, well drained, and 50–100 cm deep. Soil surfaces had a physical crust with vesicular pores. Climate across the study area is characteristic of the northern Great Basin. Most precipitation occurs during the cool, wet winter and spring months, and summers are hot and dry. Long-term (1981–2010) average annual precipitation was 244 mm (PRISM 2020). Annual precipitation was variable over the duration of the study (Fig. 1). Study sites were fenced to exclude livestock from experiment plots.

Fig. 1.

Annual (A) and April through June (B) precipitation in the study area from 2008 to 2019. Dashed line in panel A represents the long-term (1981–2010) average annual precipitation for the study area (PRISM 2020).

Results

Discussion

Restoring large native bunchgrasses in degraded Wyoming big sagebrush communities will increase resilience and resistance to inevitable disturbance, improve wildlife habitat, and provide a better forage base for livestock production. Mowing followed by seeding native bunchgrasses partially achieved these objectives but also had some undesired effects. Negative effects included substantial increases in exotic annual grasses and forbs and decreases in biological soil crusts. However, mowing and seeding approximately doubled large perennial bunchgrass cover and density. This was a substantial improvement but did not approach the levels found in intact Wyoming big sagebrush communities. Intact Wyoming big sagebrush community with the same dominant perennial bunchgrasses as our study sites in this part of Oregon average 9–13% large perennial bunchgrass cover (Davies et al. 2006; Bates and Davies 2019), but at the conclusion of our current study, large perennial bunchgrass cover was 4.6% in the MOW-SEED treatment. Density of large perennial bunchgrasses at the end of our study in the MOW-SEED treatment was approximately half of the density found in intact Wyoming big sagebrush community types in this region (Bates and Davies 2019).

Fig. 2.

Cover (mean ± standard of error) of major plant groups in the MOW, MOW-SEED, and untreated CONTROL treatments in degraded Wyoming big sagebrush communities in southeastern Oregon for 11 yr post treatment.

Fig. 3.

Cover (mean ± standard of error) of ground cover groups in the MOW, MOW-SEED, and untreated CONTROL treatments in degraded Wyoming big sagebrush communities in southeastern Oregon for 11 yr post treatment.

Fig. 4.

Density (mean ± standard of error) of major plant groups in the MOW, MOW-SEED, and untreated CONTROL treatments in degraded Wyoming big sagebrush communities in southeastern Oregon for 11 yr post treatment.

Our results suggest that large native bunchgrasses were seed limited in these degraded Wyoming big sagebrush communities. Evidence for this conclusion was the lack of increase in large bunchgrass density in areas mowed but not seeded, as well as short-term (3–4 yr) studies in degraded Wyoming big sagebrush communities (Davies et al. 2012; Davies and Bates 2014). Similarly, native perennial herbaceous vegetation was seed limited in conifer-encroached mountain big sagebrush (Artemisia tridentate subsp. vaseyana) communities (Allen et al. 2008). In contrast, when seed limitation was overcome with seeding and mowing reduced sagebrush competition, the density of large perennial bunchgrasses doubled. In Utah, seeding after sagebrush reduction with two-way chain harrowing also increased perennial grasses in degraded Wyoming big sagebrush communities (Monaco et al. 2018). Clearly, in our study, to increase the density of large perennial bunchgrasses seeding these species was necessary.

Exotic annual grasses, predominantly cheatgrass, increased substantially with mowing, likely because of reduced competition and the physical disturbance of mowing. Reduction of perennial vegetation (i.e., sagebrush) may result in increased exotic annual grasses because competition from perennial vegetation for resources is critical to limiting exotic annual grasses (Chambers et al. 2007, 2014; Davies 2008; Davies and Johnson 2017). The soil disturbance associated with mowing and the decrease in sagebrush can increase soil nutrient concentrations (Davies et al. 2011b), favoring exotic annual grasses as they respond more rapidly to elevated soil resources than do native plants (Young & Allen 1997; Vasquez et al. 2008). Exotic annual grasses are generally more successful with elevated water and soil nutrient availability (Huenneke et al. 1990; Burke & Grime 1996; Davis et al. 2000). Similarly, greater soil surface litter after mowing probably favored exotic annual grasses because it improves microsite characteristics for establishment of exotic annual grasses (Evans & Young 1970, 1972; Whisenant 1990; Newingham et al. 2007). Declines in biological soil crusts may have also contributed to increases in exotic annual grasses. Exotic annual grass increases are often correlated with declines in biological soil crusts (Ponzetti et al. 2007; Dettweiler-Robison et al. 2013a). The combined effects of mowing created an environment that was likely favorable to exotic annual grasses.

The increase in exotic annual grasses likely limited further increases in large native bunchgrasses. After the initial establishment, the lack of increases in large bunchgrass abundance in the mowed and seeded treatment for over a decade is compelling evidence that their establishment was obstructed. Exotic annual grasses are highly competitive with native vegetation, especially at the seedling life stage, and may prevent their recruitment (Melgoza et al. 1990; Nasri and Doescher 1995; Clausnitzer et al. 1999; Rafferty and Young 2002). Once exotic annual grasses become abundant, control is often necessary to allow native species to increase (Marushia and Allen 2011; Nafus and Davies 2014). We may have been able to achieve further increases in native bunchgrasses if exotic annual grasses were controlled. Preemergent herbicide control of exotic annual grasses can facilitate increases in bunchgrasses (Davies and Sheley 2011). Similarly, targeted grazing can be used to decrease exotic annuals and increase perennial grasses (Schmelzer et al. 2014; Porensky et al. 2020). At present, the high density of exotic annual grasses in mowed treatments suggests that it is unlikely that the abundance of native large perennial bunchgrasses will increase without further intervention.

Sagebrush was recovering from the mowing treatment. By the end of the study, sagebrush cover had increased from 1.6% and 3.3% to 6.9% and 6.5% in the MOW-SEED and MOW treatment, respectively. Thus, the effect of mowing on sagebrush cover was transient. Other studies have also found transient effects from mechanical treatment on sagebrush cover (Watts and Wambolt 1996; Summers and Roundy 2018). The opportunity to increase native perennial bunchgrasses after mowing dense sagebrush stands may be temporally limited because competition from sagebrush likely increases over time. As big sagebrush cover increases, native perennial grasses decrease because of greater competition (McDaniel et al. 2005; Davies and Bates 2019). However, sagebrush cover at the end of our study in mowed treatments was likely not great enough to have a substantial effect on bunchgrasses. Thus, we suspect that at the conclusion of the current study, exotic annual grasses were the primary factor limiting increases in large perennial bunchgrasses.

Biological soil crusts were negatively impacted by mowing for the duration of our study. The disturbance associated with drill seeding appeared to result in further declines in cover of biological soil crusts. Other studies have found that biological soil crusts were reduced with mowing treatments (Davies et al. 2011b, 2012; Davies and Bates 2014; Condon and Gray 2020) and other soil surface disturbances (Ponzetti & McCune 2001; Ponzetti et al. 2007; Root & McCune 2012; Dettweiler-Robinson et al. 2013b). The lack of any evidence of recovery of biological soil crusts in the current study is concerning as it suggests that this may be a long-term or possibly a permanent reduction. This agrees with prior research that found biological soil crusts were generally slow to recover (Hilty et al. 2004). After mowing reduced biological soil crusts, increases in exotic annual grasses likely hindered their recovery. Increases in exotic annual grasses are one of the major factors resulting in declines in biological soil crusts (Belnap et al. 2006; Dettweiler-Robison et al. 2013a). Thus, we expect that the reductions in biological soil crusts are likely to be permanent without control of exotic annual grasses. These long-term declines in biological soils crust are of concern because they provide some ecosystem services and contribute to community stability in arid rangelands (Belnap et al. 2001; Harper & Belnap 2001; Belnap 2006). However, in the current study, biological soils crusts are quite low in communities without treatment and, thereby, likely contribute little to their resilience and resistance to exotic annual grass invasion. Thus, long-term reductions in already low biological soil crusts may be an acceptable trade-off for increases in native bunchgrasses.

Multiple barriers appear to limit restoration of degraded Wyoming big sagebrush communities. For the current study, these limitations include 1) insufficient seed of large native bunchgrasses, and likely perennial forbs, in the seed bank; 2) overabundance of sagebrush is also limiting the understory; 3) exotic annual grasses probably suppress the native understory once sagebrush is reduced; and 4) environmental variables limiting establishment of perennial vegetation. Thus, for successful restoration of degraded Wyoming big sagebrush communities, multiple barriers must be overcome. This will likely be expensive because multiple treatments often will be necessary and may need to be sequential, not simultaneous. Alternatively, improvements in initial establishment of native perennial species, before substantial increases in exotic annual grasses, could preclude the need for additional treatments. Advancements in seed delivery, timing of seed delivery, and seed enhancement technologies may meet this need in the future (Madsen et al. 2016; Copeland et al. IN PRESS). Degraded Wyoming big sagebrush communities are difficult to restore because these communities have low resistance to exotic annual grass invasion (Davies et al. 2011a; Chambers et al. 2014), and native perennial species only establish sporadically from seed and are slow growing. Thus, even when seed limitations and competition from sagebrush are mediated, restoration of the perennial herbaceous understory remains uncertain because other factors may prevent success.

Management Implications

Wyoming big sagebrush plant communities with depleted understories are a management challenge. Wildfire is inevitable in most of these degraded sagebrush communities, and without restoration of large bunchgrasses these communities are likely one fire away from transitioning to exotic annual grasslands. Restoration attempts in degraded Wyoming big sagebrush communities are generally unsuccessful and often increase exotic annual grasses. Our current effort increased large native bunchgrasses but also substantially increased exotic annual grasses. Thus, the use of this treatment to restore the understory in degraded Wyoming big sagebrush communities is inconclusive based on our mixed results. Refinement of this strategy and development of other strategies to restore degraded sagebrush communities are clearly needed. We suggest that researchers develop tools and strategies to improve the establishment of seeded species, investigate methods to seed into sagebrush communities with minimal disturbance, and evaluate integrating treatments to seed desired species and control exotic annual grasses. The results of this study question the appropriateness of applying mowing followed by seeding large native bunchgrasses to restore the understory in degraded Wyoming big sagebrush communities, as this increased exotic annual grasses, which increases the likelihood of more frequent fire (Stewart and Hull 1949; Balch et al. 2013). However, the increase in large native bunchgrasses and decrease in sagebrush cover with mowing and seeding bunchgrasses probably slightly improved the resilience of the plant community to fire. Though mowing followed by seeding achieved some desired plant community effects, these treatments should probably not be applied in degraded Wyoming big sagebrush communities without having a plan and the resources to control exotic annual grasses as needed. The partial success of mowing followed by seeding and cost of additional treatments likely necessary to improve success suggests preventing degradation of these communities and restoring these communities when they still have a greater abundance of large perennial bunchgrasses should be a management priority.

Declaration of Competing Interest

None.