Written by Brad Schultz, Extension Educator, Winnemucca, Nevada
State and transition models were developed to help land managers make better decisions when managing vegetation for a suite of potential land uses — for example, livestock forage and wildlife habitat — and/or ecosystem services such as erosion control, water infiltration, and wildfire risk reduction. They provide a framework (Figure 8) for describing, understanding, predicting, and controlling ecosystem dynamics; the integrated goal of science and management. State and transition models help understand complex systems, with multiple interactive drivers for changes — competition, precipitation, and response to management actions — that operate at different intensities, frequencies, and durations across different scales of time and space.
Figure 9 is a state and transition model for a Loamy 8-10 inch ecological site from central Nevada. The reference community (PNC) is a Wyoming sagebrush-bunchgrass community. Indian ricegrass (Achnatherum hymenoides) and Thurber’s needlegrass (Achnatherum thurberianum) are the predominant bunchgrasses, representing 30% to 50% of the total annual biomass production. Bottlebrush squirreltail (Elymus elymoides) and Sandberg bluegrass (Poa sandbergii) are minor but constant perennial bunchgrasses in the community. Cheatgrass (Bromus tectorum), an invasive annual grass can establish on sites that are disturbed or mismanaged. Wyoming big sagebrush (Artemisia tridentata wyomingensis, a non-sprouting shrub, is the most common shrub, and at a mid- to late-seral stage has an annual biomass production of 25% to 35% of the total community production.
Building on what has been learned, the green box in Figure 9 is a resilient sagebrush-bunchgrass state, with seven community phases within the resilient state. Depending on the specific management goals for the site there could be more or fewer phases. The arrows connecting the green and yellow boxes are pathways between community vegetative phases within the resilient state of the Wyoming Big Sagebrush Ecological Site. The green boxes represent very resilient vegetative phases that present little or no ecological risk to the integrity of the state. That is, any community phase they change toward will not result in transition to a different vegetative state. If a disturbance or management action removes the desired sagebrush, the perennial bunchgrasses are dense enough to reoccupy the site and exclude the annual invasive grasses, the sprouting shrubs, or few juniper trees.
The framework needed to develop a state and transition model for vegetation management and change.
Figure 8. The framework needed to develop a state and transition model for vegetation management and change. Courtesy of Brad Shultz
Management actions and/or normal successional pathways across decades could result in phase changes from communities described in the green boxes to communities in the yellow boxes. These may or may not be desired community changes, depending on management goals. Regardless, the changes represent increased risk to the integrity or resilience of the ecological site. These added biological risks are from increased sprouting shrubs, sagebrush cover above 15% to 20%, associated declines in the density of bunchgrasses, and the establishment of juniper trees. An increase in the woody species above some site-specific level is an ecological risk because large woody plants out-compete the smaller bunchgrasses for soil moisture, and the bunchgrasses eventually decline. At some point in the successional process, there will be insufficient bunchgrasses to completely occupy the site immediately after a catastrophic disturbance, and the site will likely transition to a cheatgrass community.
The yellow boxes represent community phases that may meet management goals but are communities with an increased risk of transition across a threshold to an undesired alternative state. The yellow lines that approach the threshold represent the reversible portion of the transition process. A change in management at this stage of plant succession may return the community to a phase with more resilience and less ecological risk. Once the transition crosses the threshold, none of the alternative states can move back into a desired phase of the ecological site without extensive and expensive inputs.
The black arrows between the community phases are pathways between each phase. The specific management actions and/or ecological processes that interact to change the vegetation from one phase to another can be documented and used to direct management toward the desired community phase.
Figure 9. State and transition model for a Loamy 8-10 ecological site in Central Nevada. Courtesy of Brad Shultz