J M Briggs, Arizona State University, Tempe, AZ, USA A K Knapp, Colorado State University, Fort Collins, CO, USA S L Collins, University of New Mexico, Albuquerque, NM, USA © 2008 Elsevier B.V. All rights reserved.
Grasslands Grassland Types The Grassland Environment Fire in Grasslands
Grazing in Grasslands
Threats to Grasslands and Restoration of Grasslands Further Reading
Steppes and prairies (grasslands) are ecosystems that are dominated by grasses and to help understand grasslands, it is important to know something about grass morphology and growth forms. The remarkable ability of grasses to thrive in so many ecological settings and their resilience to disturbance is largely attributable to their growth form. Grasses are characterized by streamlined reduction and simplicity with tillers being the key adaptive structural element of the plant (Figure 1). Tillers originate from growing parts (meristems) typically just near, at, or below the surface of the soil. The meristems that produce tillers are generally well protected by their location near or beneath the soil surface. It is the location of the meristem that
explains much of the resilience of grasses and thus grasslands to disturbance.
Grass leaves are narrow and generally well-supplied with fibrous supporting tissue that has thick-walled cells. These features, along with a capacity to fold or roll the leaves along the vertical plane, permit the plant to endure periods of water stress without collapse. Another feature of grass leaves is the presence of siliceous deposits and sili-cified cells (phytoliths). Although silica is present in many plant families, phytoliths are characteristic of grasses. Phytoliths often have distinctive forms within taxonomic groups and since they persist in soil profiles for a very long time, they can be used by paleobotanists to determine shifts in dominance from one grass form to another. Silica also makes grass forage very abrasive and it is now generally accepted that the evolution of abrasion-resistant teeth present in many modern grazing animals was an evolutionary response to tooth-wearing effects of a diet high in grass. This also suggests that the grasses and their megaherbivore grazers are highly coevolved. But recent discovery of grass phytoliths in Late Cretaceous dinosaur coprolites in India suggest that grasses were already substantially differentiated and that abrasive phytoliths were present in many grasses before the explosion of grazers in the Oligocene and Miocene time periods.
Grasses show a very large variation in the way tillers are aggregated as they expand from their origin, but two general forms of grasses are recognized: bunch-forming (caespitose) and sod-forming (rhizomatous). This description captures the major features of the dominant grass species but there are some species and groups that deviate from this general pattern. The most obvious include the woody bamboos (some of which can reach tree size and for the most part are restricted to forest habitats in the tropics and subtropics).
In addition to growth form, grasses can also be roughly divided into two categories based upon their photosynthetic pathways: cool season (C3) and warm season (C4). C4 photosynthesis is a variation on the typical C3 pathway and is thought to have an advantage in high-light and -temperature environments typical of many grassland regions worldwide. Throughout the world today, tropical, subtropical, arid, semiarid, and mesic grasslands are typically dominated by C4 grasses while in cooler high-elevation or northern climates, C3 grasses are more common.
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