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Effects of Fire and Large Herbivores on Canopy Nitrogen in a Tallgrass Prairie

1
School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, China
2
Rangeland Resources and Systems Research Unit, Agricultural Research Service, Fort Collins, CO 80526, USA
3
Department of Geography, Kansas State University, Manhattan, KS 66502, USA
4
Division of Biology, Kansas State University, Manhattan, KS 66502, USA
*
Author to whom correspondence should be addressed.
Remote Sens. 2019, 11(11), 1364; https://doi.org/10.3390/rs11111364
Received: 4 April 2019 / Revised: 22 May 2019 / Accepted: 3 June 2019 / Published: 6 June 2019
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Abstract

This study analyzed the spatial heterogeneity of grassland canopy nitrogen in a tallgrass prairie with different treatments of fire and ungulate grazing (long-term bison grazing vs. recent cattle grazing). Variogram analysis was applied to continuous remotely sensed canopy nitrogen images to examine the spatial variability in grassland canopies. Heterogeneity metrics (e.g., the interspersion/juxtaposition index) were calculated from the categorical canopy nitrogen maps and compared among fire and grazing treatments. Results showed that watersheds burned within one year had higher canopy nitrogen content and lower interspersions of high-nitrogen content patches than watersheds with longer fire intervals, suggesting an immediate and transient fire effect on grassland vegetation. In watersheds burned within one year, high-intensity grazing reduced vegetation density, but promoted grassland heterogeneity, as indicated by lower canopy nitrogen concentrations and greater interspersions of high-nitrogen content patches at the grazed sites than at the ungrazed sites. Variogram analyses across watersheds with different grazing histories showed that long-term bison grazing created greater spatial variability of canopy nitrogen than recent grazing by cattle. This comparison between bison and cattle is novel, as few field experiments have evaluated the role of grazing history in driving grassland heterogeneity. Our analyses extend previous research of effects from pyric herbivory on grassland heterogeneity by highlighting the role of grazing history in modulating the spatial and temporal distribution of aboveground nitrogen content in tallgrass prairie vegetation using a remote sensing approach. The comparison of canopy nitrogen properties and the variogram analysis of canopy nitrogen distribution provided by our study are useful for further mapping grassland canopy features and modeling grassland dynamics involving interplays among fire, large grazers, and vegetation communities. View Full-Text
Keywords: spatial heterogeneity; fire; ungulate grazing; grassland canopy nitrogen; grassland dynamics; grazing history spatial heterogeneity; fire; ungulate grazing; grassland canopy nitrogen; grassland dynamics; grazing history
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Ling, B.; Raynor, E.J.; Goodin, D.G.; Joern, A. Effects of Fire and Large Herbivores on Canopy Nitrogen in a Tallgrass Prairie. Remote Sens. 2019, 11, 1364.

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