Woody plant encroachment into the grassland ecosystems of North America has been a long-documented phenomenon of ecological and economical significance. Due to anthropogenic manipulation, simply monitoring percent woody cover over time does not fully show the dynamics of woody cover growth and decline. Woody plant removal on one property may be offset by encroachment on another. In this study, we spatially analyzed aerial imagery over a 77-year period in five watersheds located in the Limestone Cut Plains ecoregion of central Texas. Common woody species include deciduous oak, live oak, juniper, mesquite, and elm species. Imagery was collected for 1937/40, 1958, 1974/75, 1980/82, 1995, 2004, 2008, 2010, 2012, and 2014. Using the ENVI GIS system, all imagery was classified into shrub and non-shrub categories using an object-based classification model. Classified imagery was then used to calculate the total woody cover, woody cover growth and loss from one temporal period to the next, and total attrition of the 1937 woody plant cover. Four watersheds experienced a drastic decrease in woody cover from 1937 to 1958. Over the next 56 years, each watershed had varying amounts of shrub cover increase and decline, with the exception of one watershed having relatively equal amounts of addition and loss for each time period. However, only two of the watersheds had a total net increase of woody cover over from 1937 to 2014. Attrition of the 1937 woody cover was quite substantive for the entire dataset. Less than 10% of the 1937 woody cover remained. Even in periods of increasing woody plant cover, attrition of the 1937 cover was occurring. Our analysis underscores the importance of accounting for human dimensions in evaluating woody plant encroachment and changes in woody plant cover over time.
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