Widespread abrupt shifts of grasslands to a wooded state: woody encroachment in a stable-state and socio-economic framework

Woody encroachment of global grasslands and savannas affects biodiversity and a variety of vital ecosystem services. To date, there have been few efforts to conceptualize woody encroachment in a common theoretical framework and to resolve debates over whether woody encroachment is the result of global and/or site-specific drivers. Using long-term trends in woody plant cover and global change drivers at 7 LTER sites, and 2 international sites, we found that transitions from a grass to a shrub-dominated state have been widespread and strikingly rapid (~2-20 years), especially compared to other ecological processes, such as succession or paleo-vegetation transitions. We argue that this transition to woody dominance often represents a transition to an alternative stable state, based on the observations that:

1. Increases in woody cover tend to be abrupt over time, even though the changes in exogenous forcing have often been gradual
2. Woody encroachment is usually associated with positive feedback mechanisms, which theoretical competition models predict should lead to community bi-stability and hysteresis.

Our results also reaffirm an emerging principle that ~5-10% woody cover is an unstable ecosystem state. Our theoretical/conceptual framework, based in dynamical systems theory, depicts how local and global-scale anthropogenic activities affect resilience to encroachment, providing a means to weigh woody encroachment as an externality of modern anthropogenic activities at multiple scales. This framework can be used to guide local management and policy decisions, when managing woody cover in grass-dominated systems is a priority. 

Related Materials and Graphics: 

Woody Encroachment Conceptual Framework