Regime shifts and threshold dynamics in rangelands

Does grazing management influence threshold dynamics in California rangelands? Are transitions between different grassland states (annual forage grasses, native perennials, noxious weeds) triggered by grazing? Do plant-soil feedbacks accelerate these shifts?

Rangeland systems, particularly those in arid and semiarid regions, exhibit complex dynamics in response to disturbance and climate variation. Substantial shifts in vegetation for instance from acceptable forage species to noxious weeds – can occur with little forewarning, and once the shifts occur they are often difficult to reverse. Incorporating this complexity in the development and evaluation of management frameworks has been a long-standing challenge. Understanding how the presence of alternative states and thresholds affect community dynamics may be key to predicting how and when major transitions will occur.

We examine the role of thresholds to enhance grazing management in several lines of research:

State and Transition models: linking application and theory

Rangeland ecology has operationalized the inclusion of complex ecosystem dynamics through the development of state and transition models. State and transition models of rangeland vegetation dynamics split changes in rangeland ecosystems into discrete states and describe processes that cause transitions between states. They are well developed for many rangeland systems and are a valuable decision making tool for managers. For California rangelands, for example, high-intensity livestock grazing is believed to drive transitions between rangeland states from more favorable species into noxious weed dominated states. However, state and transition models are descriptive rather than mechanistic, and empirical tests of model predictions are lacking.

This project examines whether grazing management can change the resilience and threshold dynamics in a California rangeland system. We focus on three ecosystem states that differ in conservation and forage value: perennial native grasslands; annual exotic grasslands dominated by acceptable forage species; and annual exotic grassland dominated by noxious weeds such as Medusahead.

At the Sierra Foothills Research and Extension Center, we are experimentally manipulating grazing intensity to identify critical thresholds where rapid changes in rangeland vegetation and health may occur. We are also investigating how different types of pasture vegetation may promote positive feedbacks with the soil system, creating conditions where vegetation may seem stuck in a particular state regardless of management efforts. This work will validate and refine state and transition models, aid in the development of management frameworks, and test assumptions about the restoration of native perennial rangelands.

People involved: Claudia Stein, Dr. Stan Harpole, Iowa State University

Managing threshold dynamics in noxious weed invasions: cross-scale interactions and positive feedbacks

One critical component of threshold dynamics is interaction across spatial scales. Recent evidence suggests that cross-scale interactions may be critical to understanding catastrophic spread in patchy systems, including invasive species. For example, the pasture-wide conversion of productive rangelands to noxious weeds may be most likely when broad-scale processes feedback on fine-scale processes to reinforce spatial spread at both scales. In particular, cross-scale interactions may regulate pasture wide invasion if threshold dynamics inhibit changes in broad-scale processes when patches are small, and if broad-scale processes tend to reinforce fine-scale feedbacks. Thus, identifying feedbacks at multiple scales, as well as the links between them, could provide key information about how to avoid difficult to reverse shifts.

Because both fine and broad scale processes are determined by species-specific life history traits, the thresholds and feedbacks that can trigger pasture-wide invasion are likely to also be species-specific. This project focuses on four species that vary in dispersal mode and competitive ability. Medusahead (Taeniatherum caput-medusae) and Wild Barley (Hordeum murinum) are annual grasses with awns adapted for dispersal by animals while yellow starthistle (Centaurea solstitialis) and Italian thistle (Carduus pycnocephalus) are passively or wind dispersed. We hypothesize that density dependence mediated by intraspecific competition will play an important role at the fine-scale by regulating the establishment and spread of individuals within a patch, while dispersal, and particularly the presence of multiple dispersal modes will define how broad scale processes occur. We expect that the interaction across scales will be depend on the behavior of livestock that either choose to avoid or pass through patches depending on their size and the phenology of noxious weeds.

This research is being conducted at the Sierra Foothills Research and Extension Center, where we are experimentally manipulating grazing to identify critical thresholds where rapid changes in rangeland vegetation and health may occur. This research will provide ways to identify critical thresholds that can stymie sustainable rangeland management and offer tools to retain efficient functioning in rangeland systems.

People involved: Erica Spotswood, Claudia Stein, Dr. Scott Stephens, University of California Berkeley