Grassland ecology

Abstract

Grasslands are one of Earth's major biomes and the native vegetation of up to 40 % of Earth's terrestrial surface. Grasslands occur on every continent except Antarctica, are ecologically and economically important, and provide critical ecosystem goods and services at local, regional, and global scales. Grasslands are surprisingly diverse and difficult to define. Although grasses and other grasslike plants are the dominant vegetation in all grasslands, grasslands also include a diverse assemblage of other plant life forms that contribute to their species richness and diversity. Many grasslands also support a diverse animal community, including some of the most species-rich grazing food webs on the planet. Grasslands allocate a large proportion of their biomass below ground, resulting in large root to shoot ratios. This pattern of biomass allocation coupled with slow decomposition and weathering rates leads to significant accumulations of soil organic matter and often highly fertile soils. Climate, fire, and grazing are three important drivers that affect the composition, structure, and functioning of grasslands. In addition to the independent effects of these factors, there are many interactions among grazing, fire, and climate that affect ecological patterns and processes in grasslands in ways that may differ from the independent effects of each driver alone. Grasslands occur under a broad range of climatic conditions, though water is generally limiting for some part of the year in most grasslands. Many grasslands experience periodic droughts and a dormant season based on seasonal dry or cold conditions. Grasslands are sensitive to climate variability and climate changes. There are well-documented shifts in the distribution of North American grasslands in response to past droughts, and both observational data and experiments suggest that grasslands will be affected by future changes in rainfall and temperature. Fire is a common occurrence, particularly in more mesic grasslands, due to the large accumulations of dry, highly combustible fine fuel in the form of dead plant material. Fire affects virtually all ecological processes in grasslands, from the physiology of individual plants to the landscape-level patterns, though the effects of fire vary with grassland productivity and the accumulation of detritus. All grasslands are grazed or have experienced grazing as a selective force at some point in their evolutionary history. The ecological effects of grazing vary with climate and plant productivity, and the associated evolutionary history of grazers in different grasslands. Grasslands have been heavily exploited by humans, and many temperate grasslands are now among the most threatened ecosystems globally. Widespread cultivation of grasslands was the major land-use change that impacted grasslands historically, while multiple global changes drivers (i.e., altered fire and grazing regimes, woody plant encroachment, elevated CO2, invasive species, fragmentation) contribute to the contemporary loss of grasslands. Grassland restoration aims to recover the diversity and ecosystem services that grasslands provide. While restored grasslands may attain productivity comparable to native grasslands and sequester carbon for extended periods, they typically support much less diversity than comparable native grasslands. Recovery of soil communities and properties is often very slow.