Indicators of ecosystem function identify alternate states in the sagebrush steppe

Abstract

Models of ecosystem change that incorporate nonlinear dynamics and thresholds, such as state-and-transition models (STMs), are increasingly popular tools for land management decision-making. However, few models are based on systematic collection and documentation of ecological data, and of these, most rely solely on structural indicators (species composition) to identify states and transitions. As STMs are adopted as an assessment framework throughout the United States, finding effective and efficient ways to create data-driven models that integrate ecosystem function and structure is vital. This study aims to (1) evaluate the utility of functional indicators (indicators of rangeland health, IRH) as proxies for more difficult ecosystem function measurements and (2) create a data-driven STM for the sagebrush steppe of Colorado, USA, that incorporates both ecosystem structure and function. We sampled soils, plant communities, and IRH at 41 plots with similar clayey soils but different site histories to identify potential states and infer the effects of management practices and disturbances on transitions. We found that many IRH were correlated with quantitative measures of functional indicators, suggesting that the IRH can be used to approximate ecosystem function. In addition to a reference state that functions as expected for this soil type, we identified four biotically and functionally distinct potential states, consistent with the theoretical concept of alternate states. Three potential states were related to management practices (chemical and mechanical shrub treatments and seeding history) while one was related only to ecosystem processes (erosion). IRH and potential states were also related to environmental variation (slope, soil texture), suggesting that there are environmental factors within areas with similar soils that affect ecosystem dynamics and should be noted within STMs. Our approach generated an objective, data-driven model of ecosystem dynamics for rangeland management. Our findings suggest that the IRH approximate ecosystem processes and can distinguish between alternate states and communities and identify transitions when building data-driven STMs. Functional indicators are a simple, efficient way to create data-driven models that are consistent with alternate state theory. Managers can use them to improve current model-building methods and thus apply state-and-transition models more broadly for land management decision-making.

Can wildfire serve as an ecohydrologic threshold‐reversal mechanism on juniper‐encroached shrublands

“…Millions of hectares of diverse sagebrush steppe in the western USA remain at risk to displacement by woodland encroachment (Suring et al ., ), and researchers, government agencies, and land managers are actively seeking identification of early warning signs for threshold exceedance and restoration pathways to reverse woodland encroachment resilience and trajectories (Scheffer et al ., ; Suding et al ., ; Briske et al ., , , ; McIver et al ., ). Early warning signs of ecohydrologic thresholds for resource‐degrading sagebrush‐to‐woodland conversions likely vary substantially across the diverse domain in which pinyon and juniper species have encroached (Davenport et al ., ; Miller et al ., , ; Romme et al ., ) and are not well established (although see Pyke et al ., ; Kachergis et al ., ; Sheley et al ., ). Restoration pathways are trajectories toward re‐establishment of pre‐threshold states triggered by disturbance or management actions and are assessed through indicators of re‐emerging structure–functional attributes of the pre‐threshold state (Briske et al ., ).…”

Comparison of species and trait‐based approaches for describing sagebrush steppe response to range management

“…Detailed methods are reported in Kachergis et al. (), but they are summarized here. We sampled seventy‐six 20 × 50 m plots for vegetation in 2007 and 2008 and for soils in 2009.…”

State and Transition Models: Theory, Applications, and Challenges

“…Descriptions of alternative states tend to focus on the relationships of vegetation structure to the processes maintaining that structure, such as erosion, fire frequency, or nitrogen fixation (Petersen et al 2009;Kachergis et al 2011). Some STMs depict both alternative states and transient dynamics within states by using boxes for plant communities and separating certain communities using irreversible transitions across a threshold boundary, signifying a state transition (Oliva et al 1998).…”