Rangelands occupy c. 40% of the Earth's land area, primarily in arid and semi-arid regions, and provide diverse ecosystem services to large human populations (Scurlock & Hall 1998; Reynolds et al. 2007). Effective rangeland stewardship and restoration, amidst numerous environmental and socio-economic challenges, require a comprehensive understanding of ecosystem dynamics and their response to episodic and chronic disturbances. Initial studies on North American rangelands, including the sagebrush-steppe that cover 6 × 105 km2 of the Intermountain West and Great Basin eco-regions, were conducted within the Clementsian model (Dyksterhuis 1949) that emphasized predictable and reversible changes in vegetation composition, either towards or away from a reference climax plant community (e.g. Pechanec, Pickford & Stewart 1937; Mueggler 1950; Harniss & West 1973). However, subsequent investigations found that vegetation dynamics, particularly large fluctuations in the abundance of perennial grasses, were often inconsistent with this model (Anderson & Holte 1981; Anderson & Inouye 2001). This highlighted the need to consider resilience-based frameworks that accommodate multiple alternative states and thresholds (Westoby, Walker & Noy-Meir 1989) to inform ecosystem management (Briske, Fuhlendorf & Smeins 2003).
Over the past century, invasion by cheatgrass Bromus tectorum has posed an additional challenge for management and restoration of the sagebrush-steppe (Mack 1981; Reisner et al. 2013). This invasion has had a number of direct and indirect effects, including a shortening of fire return intervals from 60–110 years to 3–5 years (Brooks et al. 2004; Chambers et al. 2007; Baker 2011), and modification of soil structure and biogeochemical cycling (Kulmatiski, Beard & Stark 2006; Shinneman & Baker 2009) that can have important consequences for ecosystem services and human livelihoods. Cheatgrass is widely perceived to have influenced the resilience of the sagebrush-steppe, and create threshold conditions that yield an alternative state (Stringham, Krueger & Shaver 2003; Davies et al. 2012). Current management models, developed through expert opinion, consider cheatgrass as an alternative stable state that is irreversible without management prescriptions (Fig. 1a). However, the presence of thresholds is challenged by scattered evidence suggesting cheatgrass populations to be less persistent than widely assumed (West & Yorks 2002; Mata-González et al. 2007; Bradley & Wilcove 2009). This establishes a need to investigate the influence of cheatgrass on resilience of the sagebrush-steppe with empirical patterns that exist in historical vegetation records.
Conceptually, state-and-transition models (STM) are founded upon the theory of multiple equilibria and they accommodate alternative states in order to operationalize resilience concepts for ecosystem management (Westoby, Walker & Noy-Meir 1989; Briske et al. 2008). STMs account for vegetation dynamics as transitions within and between alternative states (Stringham, Krueger & Shaver 2003; Bestelmeyer et al. 2004). States consist of one or more phases, or closely related vegetation communities, that may show frequent and reversible transitions. States, however, are assumed to be separated by biophysical thresholds originating from the relative strength of negative and positive feedback mechanisms. Transitions and thresholds are linked with causal anthropogenic and natural events and drivers to interpret and anticipate outcomes of management actions qualitatively, but the incorporation of additional empirical information, including trends seen in long-term historical records, would further strengthen the STM framework (Bestelmeyer 2006; Knapp et al. 2011; Bagchi et al. 2012).
Here, we quantify the historical response of resident species to cheatgrass invasion, along with other plausible drivers of vegetation change, and identify the temporal scale at which these dynamics occurred. Limited knowledge of temporal aspects is recognized as a serious omission in STMs by researchers and managers alike (Knapp et al. 2011). We use long-term records of vegetation dynamics at two sites from the sagebrush-steppe in Idaho, USA (Table 1). Collectively, these data sets span nine decades and cover the entire history of cheatgrass invasion in these regions. The first data set (1923–1973) is from the US Sheep Experiment Station near Dubois, Idaho (USSES, 113 km2 area) and the second data set (1950–2006) is from the Idaho National Engineering and Environmental Laboratory near Idaho Falls, Idaho (INEEL, 2315 km2 area). Specifically, we (i) identify vegetation communities and represent long-term dynamics as transitions between communities through time, (ii) derive quantitative information regarding the frequency, magnitude and directionality of transitions, as evidence of thresholds and alternative stable states (iii) investigate the incidence of community transitions in relation to precipitation patterns and the extent of cheatgrass invasion, and (iv) compare empirical patterns of community transitions with those identified in representative expert STMs for the sagebrush-steppe ecosystem.
|Location||44°N, 112°W||43°N, 112°W|
|Average elevation||1650 m (1465–3084 m)||1500 m (1460–1620 m)|
|Average temperature||6·1 °C (−31–38 °C)||5·6 °C (−30–37 °C)|
|Frost-free days||120 yr−1||90 yr−1 (68–123 yr−1)|
|Average precipitation||282 mm yr−1 (183–417 mm)||199 mm yr−1 (83–360 mm)|
|Soils||Aeolian loess, Calcic agrixeroll, loamy and sandy-loam formations||Aeolian deposits, Calciorthids, loamy and sandy formations|
|Major native species|| |
Balsamorhiza sagittata, Chrysothamnus viscidiflorus,
Crepis acuminata, Phlox longifolia, Pseudoroegnaria spicata, Elymus albicans
|Artemisia tridentata (var. wyomingensis and tridentata), Chrysothamnus viscidiflorus, Phlox hoodi, Elymus lanceolatus, Elymus elymoides, Pseudoroegnaria spicata|
|Invasive species||Bromus tectorum||Bromus tectorum|
|Data coverage||1923–1973 (29 samples)||1950–2006 (10 samples)|
|Sampling||13–26 plots yr−1||34 plots yr−1|
|Key references||Pechanec, Pickford & Stewart (1937), Mueggler (1950), Zachmann, Moffet & Adler (2010)||Harniss & West (1973), Anderson & Holte (1981), Anderson & Inouye (2001)|