When talking about land cover changes over time the terms “loss” and “change” are often used interchangeably, but they may mean quite different things. Depending on what is being mapped and the story being told, these terms need to be clarified.
Change versus loss
This is best highlighted by comparing maps of land cover versus land use. Land cover captures the physical state of land resources, while land use denotes how the land is being used, or planned for use (see previous post for more info). For example, a parcel of land may be owned, and managed, specifically for the purpose of growing and harvesting timber, and be classified as Forested in terms of land use. As this parcel is cut and the trees removed, it still may be called Forested (land use), because it is planned to be replanted and eventually harvested again. In this case, there may be no loss or change of Forested land.
Another change example we commonly see occurs in the wetland categories. Much of the Forested land C-CAP maps are Palustrine (wetland) forests. When we tally the amount of wetlands mapped by C-CAP, these Palustrine Forests are sometimes binned as a more general category of wetlands. As these lands are cut, the land typically becomes Palustrine Emergent (another wetland category).
Depending on how you look at it, there could be a loss of wetlands (Palustrine Forest cut) or wetland gained (Palustrine Emergent), and in the end there may be no net change in the amount of wetlands. What is very important to note though, while it may look like there is no net change in wetland area, there may have been significant changes within the wetland features. A Forested wetland may provide very different ecological services (e.g., wildlife habitat and carbon sequestration) than an Emergent wetland.
Another example can be seen in the image to the left in Collier County, FL. This map is depicting areas (red) that have changed from wetlands in 1996 to developed in 2010. C-CAP considers these areas as wetland losses due to the fact once an area is developed, it rarely becomes undeveloped (i.e., it is a permanent change).
Not only are these wetland lost, the ecosystems services (storm water control, wildlife habitat, water quality) provided by these wetland are lost as well.
To wrap-up with some actual numbers, I will refer to land cover data from the Coastal Change Analysis Program and highlight several uses of these terms. The table below represents C-CAP data for Charleston County, SC from 2006-2010 as presented in the C-CAP Land Cover Atlas. Based on this table, I could truthfully state that 10.01 sq miles of Forested land was lost over this time period. I could also state that 5.55 sq miles of Forested land was gained. A little explanation is required here. The loss of Forested land is indicative of a specific feature that was forest in 2006, but not forest in 2010. These areas may have been harvested or converted to agriculture. The gain of Forested land is the exact opposite. A parcel that was scrub in 2006, but forest in 2010, would be counted as a gain. By combining these two numbers, you end up with a net change, thus I could state that there was a net loss of Forested land of 4.46 sq miles. The losses and gains offset each other.
One thing that is constant in the world is change. Forest are cut and regrown, crops are harvested, and cities are expanding. As one land cover type is changed or lost, it must be replaced by another type of land cover. If a forest is cut down for a shopping mall, it is pretty safe to assume that forest is permanently lost. The ecological services provided by that forest, carbon sequestration, wildlife habitat, water and air quality benefits, are gone. If that forest is harvested and replanted, there is a change in the forest and ecological services, but ideally they will return. Hopefully when viewing data and reports detailing changes and/or loss you are now better prepared to understand and digest this information. It is important to understand what exactly is being reported.