Effects of fire and large herbivores on canopy nitrogen in a tallgrass prairie

Authors: LING, EDWARD - Kansas State University; Raynor, Edward; GOODIN, DOUGLAS - Kansas State University Agricultural Research Center-Hays; JOEM, ANTHONY - Kansas State University

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2019
Publication Date: 6/6/2019
Citation: Ling, E.J., Raynor, E.J., Goodin, D., Joem, A. 2019. Effects of fire and large herbivores on canopy nitrogen in a tallgrass prairie. Remote Sensing. 11:1364. https://doi.org/10.3390/rs11111364.
DOI: https://doi.org/10.3390/rs11111364

Interpretive Summary: Hyperspectral imagery is a tool for measuring vegetation quality and quantity in terrestrial ecosystems. In particular, easily obtainable and accurate measures of grassland structure are vital for understanding the ecological processes underpinning grassland function. In this study, the role of fire and grazing in shaping grassland canopy nitrogen distribution in an experimental landscape are assessed. A geospatial evaluation across burned and unburned watersheds with different grazing histories (grazed year-long by bison, season-long by cattle, and not grazed) provided insight into the interaction of prescribed fire, topography, and landscape grazing history on canopy nitrogen content and variability. Four hyperspectral images obtained during the 2011 growing season provided spatially-explicit measurements of canopy nitrogen in the Kansas Flint Hills. Results showed that fire greatly stimulated the plant productivity in the first year following the burn, and resulted in large uniform vegetation patches during the growing season. The interactive effects of fire and ungulate herbivores were observed in the watersheds burned in spring, where the fire-promoted herbivory reduced the canopy density and increased the interspersion of high canopy nitrogen patches. Moreover, ungulate grazing effects on the grassland heterogeneity varied with topographic positions and site grazer history. The differences in the spatial distribution of canopy nitrogen contents observed across topographic positions and grazing treatments suggested preference for the uplands by the ungulate grazers in the growing season. The differences observed between sites with long-term bison-grazing and recent cattle-grazing revealed the role of grazer history in establishing grassland resource heterogeneity. These findings are useful for further mapping grassland canopy features and modeling grassland dynamics involving interplays among fire, large grazers and vegetation communities.

Technical Abstract: Effects of fire and ungulate grazing on the structure and function of grassland vegetation communities are of special interest in grassland science. This study analyzed the spatial heterogeneity of grassland canopy nitrogen in a tallgrass prairie with different fire and grazing treatments. Variogram analysis was applied to the continuous remote sensing images to examine the spatial structure and variability in grassland canopy nitrogen distribution. Spatial heterogeneity metrics (e.g., contagion, Simpson’s diversity, and interspersion/juxtaposition index) were calculated from the categorical canopy nitrogen maps and compared among different fire and grazing treatments. Results indicated that fire enhanced plant productivity, resulting in a more uniform distribution of canopy nitrogen. The interactive effects of fire and ungulate grazing were most evident in the areas with shorter fire intervals. A high level of ungulate grazing reduced vegetation density, but promoted grassland heterogeneity, as indicated by greater interspersion/juxtaposition index values for high-nitrogen content patches. This study not only provided a qualitative evaluation of effects of fire and large herbivores on grassland heterogeneity, but also quantitatively documented the differences in canopy nitrogen distribution across varying fire and grazing treatments. Results of this study are useful for further mapping grassland canopy features and modeling grassland dynamics involving interplays among fire, large grazers and vegetation communities.