Fire-grazing interactions in a mixed grass prairie

Grasslands are characterized by recurring disturbances such as fire and grazing occurring against a background of topoedaphic heterogeneity and climatic variability. The result is a complex, multi-scaled disturbance regime, in which fire and grazing often have interactive roles, yet they have usually been studied independently. Relationships between climate, fire and simulated grazing (=mowing) were explored to determine the roles these disturbances play in shaping patterns and processes in southern mixed-grass prairie. A field experiment investigated the potential effects of these disturbances on above and belowground plant productivity, patch dynamics, and soil respiration over a 2-year period characterized by drought (1998) and normal (1999) rainfall.
Spring burning and mowing had interactive effects on aboveground net primary production (ANPP). Consistent with published single factor studies, burning without mowing doubled ANPP, whereas mowing in the absence of burning had neutral effects. However, subsequent mowing on burned plots reduced ANPP gains to levels comparable with all unburned plots. Drought reduced ANPP by 22% relative to a normal rainfall year.
In contrast to the traditional model of root response to defoliation, burning and mowing each stimulated root length recruitment measured with minirhizotrons. However, subsequent mowing on burned plots did not produce additional root recruitment. Fire and mowing appear to interact by affecting different components of root recruitment (production and mortality, respectively). Root biomass recovered from ingrowth cores were not correlated with minirhizotron results, and responded only to drought, suggesting that methodological differences have contributed to the varied root responses reported in the literature. Drought suppressed soil respiration, diminished soil moisture, and enhanced soil temperature, whereas fire and/or mowing had little effect. Results suggest that any fire or mowing effects on soil respiration in southern mixed-grass prairie may be highly constrained by moisture limitations during dry periods. In summary, patch level response to fire is a pulse of root recruitment followed by increased ANPP, unless subsequent grazing offsets these gains. Grazing alone produces a pulse of root recruitment, perhaps to replace consumed foliage. This study demonstrates the interactive nature of fire and grazing in grasslands, and the perils of single-factor studies.