Exotic Grass and Woody Plant Encroachment

Exotic Grass and Woody Plant Encroachment in Southwestern Rangelands

Although characterized by low and highly variable annual rainfall, arid and semi-arid landscapes represent a substantial percentage of terrestrial net primary productivity, contain >30% of the world’s human population, and support the majority of global livestock production. As such, rangelands play an important role in global biogeochemical cycles and human health. Rangelands thus have considerable, multi-dimensional conservation value. A key component of rangeland ecosystem management is maintaining vegetation within a desirable mix of herbaceous and woody plants. In recent decades, this balance has been disrupted and shifted in favor of unpalatable shrubs and “woody weeds.” Historic brush management treatments were relatively short-lived, ineffective in the long run, and unsustainable. These realizations led to the development of integrated brush management systems (IBMS), which are long-term planning processes that move away from a purely livestock production perspective and towards management of rangelands for multiple uses and values.

Traditionally, brush management has been a reactive approach that seeks to reduce the cover of woody plants once they dominate a landscape. However, brush management practices are often expensive, have a short effective lifespan, and are economically unjustifiable. Similarly, it is clear that non-native perennial African grasses are highly resistant to brush management through a series of positive feedbacks that reinforce their persistence and spread. Although most would agree that an ounce of prevention is worth a pound of cure where the proliferation of woody plants and invasive species are concerned, there has been little investment in the development of prevention strategies. For shrub encroachment, the vast amount of research has been on post-encroachment brush management treatment efficacy.

This project focuses on the critical seedling establishment phase and asks “How do herbivory and precipitation interact to influence the probability of shrub and non-native grass seedling establishment in native grassland?” Armed with this knowledge, we are developing a proactive management paradigm focused on curtailing shrub encroachment and non-native grass establishment before it becomes a problem.

Our research objectives and hypotheses are tested with a large-scale field experiment that manipulates in a factorial design: (a) incoming precipitation, (b) seed and seedling accessibility to ants, rodents, and small mammals, (c) and levels of native grass defoliation.

These experiments take place at two Southwestern sites that differ in mean annual precipitation. Ultimately, this research explicitly addresses abiotic and biotic interactions and determine how livestock grazing interacts with precipitation, and constraints imposed by native herbivores to influence the probability of shrub and non-native grass establishment. Results from our experiments will enable us to anticipate and forecast state-transitions so that proactive management can be adjusted to prevent them. As a result, this study will provide a basis for developing cost-effective, proactive management strategies aimed at preventing shrub encroachment and non-native grass invasion and the state-transitions that result.

Osvaldo Sala
Steve Archer

US Department of Agriculture National Institute of Food and Agriculture – Agricultural Food Research Initiative

May 2016 – ongoing