Browsing by Subject "fire"
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Item Fire-grazing interactions in a mixed grass prairie(Texas A&M University, 2004-09-30) Hubbard, John AndrewGrasslands 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.Item Tree-grass and tree-tree interactions in a temperate savanna(Texas A&M University, 2004-11-15) Simmons, Mark TrevorSavannas comprise over one eighth of the world's land surface with some 50 Mha in North America alone. They are productive systems supporting a high level of both faunal and floral diversity and are of increasing socioeconomic importance. The maintenance and formation of savannas have been attributed to climate, soils, herbivory and fire. However, the reasons for the coexistence of trees and the grass layer have still to be determined. These two contrasting life forms create a complex of intra- and interspecific positive, negative, and neutral interactions, few of which have been quantified. Under lower-than-average rainfall, tree effects on grasses in a Prosopis savanna in northern Texas were largely neutral with few measurable competitive or facultative effects from the tree canopy. However, grasses demonstrated increased productivity where belowground competition with neighboring trees was removed. Similarly, tree growth increased following the removal of grasses under and around individual trees, particularly on shallower soils, but only during a season of significant precipitation. Low intensity burning of grasses enhanced growth of adult trees, but patterns were inconsistent between two different sites. Moderate clipping around individual trees had no apparent effect on tree growth. Intraspecific competition between savanna trees was not evident, but may have been blurred by an extensive, lateral distribution of near-surface roots. Overall, tree intraspecific competition was neutral regardless of soil depth, suggesting lateral tree roots may be only used opportunistically. Although some competitive relationships were verified, the differences in the responses between the two years of study, and at different sites indicated that soil depth and climate may have overriding impacts on tree-grass interactions and savanna dynamics in this system.