Abstract
This study evaluated the impacts of different grazing treatments (continuous (C) and rotational (R) grazing) on tallgrass prairie landscape, using high-spatial-resolution aerial imagery (1-m at RGB and near-infrared bands) of experimental C and R pastures within two replicates (Rep A and Rep B) in the southern Great Plains (SGP) of the United States. The imagery was acquired by the National Agriculture Imagery Program (NAIP) during the agricultural growing season of selected years (2010, 2013, 2015, and 2017) in the continental United States. Land cover maps were generated by combining visual interpolation, a support vector machine, and a decision tree classifier. Landscape metrics (class area, patch number, percentage of landscape, and fragmentation indices) were calculated from the FRAGSTATS (a computer software program designed to compute a wide variety of landscape metrics for categorical map patterns) based on land cover results. Both the metrics and land cover results were used to analyze landscape dynamics in the experiment pastures. Results showed that both grass and shrubs of different pastures differed largely in the same year and had significant annual dynamics controlled by climate. High stocking intensity delayed grass growth. A large proportion of bare soil occurred in sub-paddocks of rotational grazing that were just grazed or under grazing. Rep A experienced rapid shrub encroachment, with a large proportion of shrub at the beginning of the experiment. Shrub may occupy 41% of C and 15% of R in Rep A by 2030, as revealed by the linear regression analysis of shrub encroachment. In contrast, shrub encroachment was not significant in Rep B, which only had a small number of shrub patches at the beginning of the experiment. This result indicates that the shrub encroachment is mainly controlled by the initial status of the pastures instead of grazing management. However, the low temporal resolution of the NAIP imagery (one snapshot in two or three years) limits our comparison of the continuous and rotational grazing at the annual scale. Future studies need to combine NAIP imagery with other higher temporal resolution imagery (e.g., WorldView), in order to better evaluate the interannual variabilities of grass productivity and shrub encroachment.
Highlights
Tallgrass prairie provides various ecological and economic benefits for people in the Great Plains of the United States
This study evaluated the impacts of different grazing treatments on tallgrass prairie landscape, using National Agriculture Imagery Program (NAIP) imagery of the experimental pastures
We initially evaluated the performances of the support vector machine (SVM), maximum likelihood (ML), and artificial neural network (ANN) in identifying shrub and the various levels of grass cover after the manually-identified tree, water body, and man-made feature classes were masked from the images
Summary
Tallgrass prairie provides various ecological (e.g., carbon sequestration and biodiversity) and economic (e.g., forage) benefits for people in the Great Plains of the United States. The distribution and condition of the tallgrass prairie is threatened by agricultural land use (e.g., conversion to crops) and land management (e.g., different grazing management systems). The tallgrass prairie provides nutritious forage for cattle grazing [1] in the Great Plains, which is a major revenue stream for farmers in the region [2]. Continuous grazing with a moderate stocking rate (around six acres per animal unit for a tallgrass prairie, with an average production of 2,885 kg per acre in the region [5]) and rotational grazing with a relatively high stocking rate are two main grazing management systems nowadays. Cattle may preferentially graze specific locations the majority of the time, which can cause landscape fragmentation and uneven grass coverage
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