Steve Archer
Grasslands, shrublands and savannas, collectively represented in the term ‘rangelands', constitute ca. 50% of the Earth's land surface. Although typically characterized by low and highly variable annual rainfall, these geographically extensive arid and semi-arid landscapes represent a substantial fraction of terrestrial net primary productivity (30-35%), contain a significant fraction of the world's human population (>30%) and support the majority of the world's livestock production . As such, they play an important role in global carbon, water, and nitrogen cycles and human health. In addition, these landscapes provide habitat for game and non-game wildlife and provide a myriad of ecosystem goods and services. Rangelands thus have considerable, multi-dimensional conservation value. Stewardship of vegetation composition, cover, and production is at the foundation of sustainable rangeland management. A key component of rangeland vegetation management lies with maintaining vegetation ground cover and productivity and the balance between herbaceous and woody plants.
One of the most striking land cover changes in rangelands worldwide over the past 150years has been the proliferation of trees and shrubs at the expense of perennial grasses. In some cases, native woody plants are increasing in stature and density within their historic geographic ranges; and in other cases non-native woody plants purposely or accidentally introduced into areas are becoming dominant. In arid and semi-arid regions, increases in the abundance of unpalatable, xerophytic shrubs at the expense of mesophytic grasses may represent a type of desertification; and in semi-arid and subhumid areas, the encroachment of shrubs and trees into grasslands and savannas can profoundly alter ecosystem structure and function. At the global scale, woody plant encroachment stands in stark contrast to deforestation, which has received substantially more attention. Drivers of woody plant encroachment are the topic of active research and include interactions between changes in climate, fire and grazing regimes, atmospheric CO2 enrichment and nitrogen deposition.
This shift in vegetation structure has implications for the sustainability of pastoral and commercial livestock production systems and may influence climate and atmospheric chemistry via impacts on the carbon, nitrogen and water cycles. Documentation of shifts in woody plant abundance is poor and causes are not well understood. As a result, our ability to anticipate the rate, direction and magnitude of future changes is limited. My research has concentrated on interactions between grasses and woody plants in relation to soils, climate and disturbance.
Population, transition probability and dynamic ecosystem simulation models are used in conjunction with remote sensing, GIS, dendrochronology and stable isotope chemistry to reconstruct vegetation history and to predict future changes and the consequences of such changes on sustainability of grazing systems, ecosystem biogeochemistry and land surface-atmosphere interactions. Field and laboratory experiments on the population biology of grasses and shrub growth forms are emphasized in the context of landscape ecology, succession and historical land-use practices.
Increasing woody plant abundance in the worlds drylands stands in stark contrast to the widespread decreases in woody plant abundance associated with deforestation.
Changes in the spatial pattern and amount of shrub cover in northern Texas between 1937 and 1999 are related to changes in land use.
When woody plants increase in abundance in drylands, emissions of non-methane hydrocarbons such as isoprene may increase and affect tropospheric chemistry.
