Land use and Land Use Change (LULUC)
The land use change data identifies grassland to cropland and cropland to grassland conversion. The methods use a unique land cover dataset that provides the per pixel percent cover of crop, water, urban area, and natural vegetation (specifically five plant functional types [PFT] which include annual grasses, perennial grasses, trees, shrubs and bare ground). The data are produced annually at 30m resolution for the contiguous United States.
Conversion is detected by iterating across years, from 2008-2022, and checking whether a pixel has converted from a grass class to crop class (or vice-versa). For grass to crop conversion the first conversion event detected in the series of years is recorded. If future conversion occurs (e.g. field is cultivated in 2011, then is left fallow for 4 years while grass cover increases, but field is then cultivated again in 2016) only the initial conversion is recorded, and that pixel will maintain a conversion year 2011.
For grass to crop conversion two checks were put in place to ensure the pixel was a grassland in 2008. First an independent land cover dataset (USGS NLCD) from 2008 is used to confirm the class of the pixel. Second, the pixel must have been classified as grass for the three consecutive years of 2006, 2007, 2008 in the PFT data.
If a crop to grass conversion is detected, that pixel must remain in a grass state for the remainder of the time series (and for at least 5 years) in order for the conversion to be recorded in the data. This is a generous assumption that a cultivated field, when abandoned or converted to grass, can be considered a grassland after 5 years. Many publications will argue at least 10-15 years are required before it has relatively comparable functionality of an undisturbed grassland and even a century before they recover their former species richness or belowground soil and structure development. Conservation guides on restoring crop fields to prairie/grassland often give a 5-year window as the establishment phase of restoration that requires consistent maintenance, after which longer term, less frequent management actions should continue.