Introduction:
Prairie Plant Adaptations

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Most prairie grasses and forbs are adapted to drought, fire, and grazing. They are perennials that grow back every year and have their growing points underground. During a prairie fire, the above-ground portion of the plant is destroyed, but the below-ground growing structures are unharmed because the soil protects the underground structures. Some trees and shrubs are also resistant to fire, but through different mechanisms. Many of the oaks, especially bur oak (Quercus macrocarpa), have very thick bark, which offers high resistance to fire. Other trees such as aspen (Populus tremuloides) and northern pin oak (Quercus ellipsoidalis) have thin-barked above-ground parts that are killed by fire, but they readily resprout following fire. Many shrubs, notably hazel (Corylus americana), also resprout readily after fire. In fact, the shrub prairie owes its existence to the resprouting ability of shrubs.
Prairie Grasses Fire increases growth and productivity in the dominant tallgrass prairie grasses such as big bluestem (Andropogon gerardii) and switch grass (Panicum virgatum) (Knapp, 1985; Gibson, 1988). Becker (1989) reported a post-burn increase in native forb cover, which he attributed to increased vigor in existing plants and establishment of new plants resulting from the fires. Rohn and Bragg (1989) showed decreased time to germination in rosin weed (Silphium integrifolium) on burned sites. They also showed decreased germination times for major prairie grasses such as Indian grass, big bluestem, side-oats grama, and switch grass on burned sites. A fall burn can have a positive effect on spring germination, because the dark, exposed surface warms more quickly in the spring. On the other hand, prairie coneflower (Ratibida columnifera) exhibited decreased reproduction and productivity in burned sites (Hartnett, 1991). Other studies have also found that fires depress forbs in favor of grasses (Zimmerman and Kucera, 1977).


Frequently burned tallgrass prairie suffers from nitrogen limitation, but many nitrogen-fixing legumes common in prairies show positive growth following fire, thus restoring nitrogen to the ecosystem.

The dominant grasses of the tallgrass prairie are adapted to withstand dry conditions and high daytime temperatures. The roots of big bluestem and Indian grass reach depths of 1.5-2.5 m, and those of switch grass, extend downward to a depth of 2.5-3.5 meters (Weaver, 1954). The roots of these grasses, which form thick networks beneath the prairie, effectively absorb moisture during dry periods. Prairie grasses have narrow leaves, which lose less water to evapotranspiration than flat, broad leaves.


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