Early warning signals of transitions in spatial data.

Leading spatial indicator, the primary underlying phenomenon, the expected trend along a degradation gradient, and the original references in which these were proposed. The trend of spatial skewness depends on the nature of test data: It can show a nonmonotonic behavior (thus, a peaking) for transition from a low density state to higher density state. For discrete data, it typically shows a monotonic behavior (increasing or decreasing depending on whether it is transitioning from fully covered to bare state or the other way transition).