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State and trends


a permanent shift from tree cover to non-tree cover has occurred and is documented over a longer time series of earth observations (Reams et al. 2010). Similarly, the term ‘afforestation’ should be reserved for situations where there is a permanent shift to tree cover from herbaceous or shrub cover.


The figures also show that transitions between cultivated crops and pasture are much smaller than the tree-cover changes. There are some changes back and forth between pasture, that is more intensively managed grass cover, and herbaceous, unmanaged grass cover, while some herbaceous and pasturelands move into and out of cultivation. Interestingly, there is a significant segment of red on the loss side of the cultivated crops category, indicating loss of cropland to development. That red segment appears larger than the loss of tree-covered land to development, and the transition matrix data confirm that it is – a loss of 3.84 million hectares, − 0.47 per cent, of cultivated cropland and pasture to development between 2001 and 2011 compared to the loss of 2.33 million hectares, − 0.25 per cent, of tree-covered land to development. What is also clear is that in the developed- land categories, there was also a significant movement of what had been open space or low density development to more intensely developed areas in 2011. This set of transitions in land cover to developed land reinforces the messages discussed in the land-use section. An increasing percentage of the US’s population resides in developed areas which are growing in size at the expense of other land-cover types, while also increasing in development intensity.


US grazing land cover and cover change


The National Land Cover Dataset (NLCD) databases for 2001 and 2011 were used to estimate changes in the area of shrubland and grassland by classes of ownership (Reeves and Baggett 2014; Table 2.2.2; Figure 2.2.8). The research suggests that virtually all the changes occurred on the land of non-federal government owners; ranchers, farmers, and tribal governments.


Drivers of these land-cover changes vary by region. In the Great Plains and Eastern Prairies, market shifts in crop prices, such as the increasing value of maize for producing ethanol, and beef prices, played a significant role. Farmers responded to these market signals by bringing grasslands and shrublands under cultivation. Another factor important in the Great Plains and Interior West was the increase in energy development for oil and gas production. Conversely, the shrublands of the Interior West showed the least decline. This is the area where a federal-private partnership is devoted to improving habitat for species such as the greater sage grouse (Centrocercus urophasianus).


Reeves and Baggett (2014) analyzed the degradation of grasslands and shrublands in the northern and southern Great Plains, using a process that evaluated


satellite


imagery, to seek places with significant reductions in productive capacity, estimated by the Normalized Difference Vegetation Index (NDVI). Imagery from 2000 was used as a baseline for comparison with imagery from 2012. They found very little change in land cover since 2000, leading them to conclude that most of the degradation still observable across the landscape occurred prior to 2000. Further, they concluded that land management policies and practices in these regions were probably not contributing to further degradation, although they could not determine whether the policies and practices were supporting recovery.


When the analysis was extended to other western rangelands to assess the impacts of recent droughts, they concluded that northern Texas, eastern New Mexico, and central California have seen the strongest declines in vegetation abundance and productivity. About 16.5 million hectares—15 per cent of all rangelands—have exhibited declining trends in vegetation abundance since 2000. In contrast, increased vegetation productivity was found in the northern Great Plains. The best explanations for these changes are that climate change is extending the growing season in the northern Great Plains while reducing net primary productivity in the southwestern US due to lower precipitation and increased evapotranspiration. If the future brings increased frequency of droughts and fewer years of above-normal precipitation,


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