Ecological interactions between grasses and shrubs in a tallgrass prairie

Woody vegetation has expanded into grasslands worldwide with a number of abiotic and biotic factors, many anthropogenic, purported to be causal. At the tallgrass prairie-deciduous forest ecotone in North America, alterations in fire, as well as climate and large herbivore activities may play key roles in this increase in woody vegetation, particularly shrub encroachment. The displacement of C4 grasses by C3 shrubs represents a fundamental shift in growth-form dominance, which is likely accompanied by changes in resource acquisition and use. The objectives of this research were to examine the mechanisms and consequences of shrub encroachment in mesic grasslands. Research focused on four aspects of shrub-grass interactions: (1) the functional equivalency of shrubs and grasses and their patterns of resource acquisition; (2) the effects of fire and grazing on shrub expansion; (3) mechanism and patterns of post-fire shrub recovery; and (4) changes in soil processes and properties as a consequence of shrub encroachment. Results from these studies suggest that different shrub species cannot be considered functional equivalents as a group, nor are they necessarily distinct from grasses in resource acquisition and use. Although fire has a clear negative effect on shrubs, burned shrub communities (islands) rapidly recovered from burning with increases in stem density and island size (area). This increased growth was attributed to post-fire increases in resource availability for developing stems. Indeed, grazing, which removed fuel and ameliorated fire damage to shrubs, slowed the growth and expansion of established shrubs. The presence of shrub islands in unburned tallgrass prairie did not alter N availability, but did decrease C cycling by reducing soil CO2 flux. Overall, the present-day expansion of woody plants into grasslands may have transformed this system to a point where even a return to historic fire or grazing intensities would not substantially decreased shrub abundance.