Fine scale mechanisms and effects of fire grazing interactions

Background/Question/Methods   Fire and grazing are critical disturbances in the ecology and maintenance of native grasslands. The spatial and ecological interactions of these disturbances create vegetation patterns that are described as shifting mosaics. These interactions depend upon broad and fine scale mechanisms; such interactions also have broad and fine scale effects. While broad scale mechanisms are more obvious (fire and grazing), finer scale mechanisms are more varied. Our objective was to examine fine scale mechanisms that contribute to fire grazing interactions. We specifically quantified vegetation quantity and quality, light environments, and plant morphology and physiology. To measure vegetation quantity and quality, we harvested aboveground biomass in areas that differed in time since fire and grazing. We recorded dry biomass and analyzed samples for crude protein. We quantified light environments of the same areas using a ceptometer at various heights above ground. We also recorded various plant morphology (total leaf area, specific leaf area, leaf length, leaf angles, etc.) and physiology (light response curves, max photosynthesis) of Andropogon gerardii in areas that differed in time since fire.

Results/Conclusions   Vegetation quality (crude protein) was highest in areas that were recently burned and grazed (ca. 18%) throughout the growing season and decreased in areas with greater time since fire (ca. 4%). Vegetation quantity was lowest in recently burned areas and increased with time since fire. Attraction to burned areas is likely due to increased nutritional content and reduced biomass. High light environments were present in recently burned areas, and were maintained beyond the growing season. Total leaf area of A. gerardii was lowest in recently burned areas (ca. 8 cm2; maintained throughout the growing season) and increased with time since fire (ca. 80 cm2). In contrast, maximum photosynthesis of A. gerardii was highest in recently burned and grazed areas (ca. 50 µmol CO2 m-2 s-1) and decreased with time since fire (ca. 20 µmol CO2 m-2 s-1). A tradeoff in plant productivity appears, as leaf area is maximized with greater time since fire, but at the cost of reduced photosynthesis. These fine scale mechanisms are the result, but also contribute to, the broader scale interactions of fire and grazing. Fire grazing interactions are complex ecological processes, influencing many aspects (animal behavior, plant physiology, thermal environments, etc.) of the ecosystem.