Methods to compare sites concerning a category’s change during various time intervals

This paper presents new methods to analyze a category’s change through a time series of maps, even when the time intervals have inconsistent durations. The methods include an option to facilitate comparison among sites by expressing results as an annual percentage of each site’s unified size. A site’s unified size is the union of where the category exists at any of the site’s time points. The methods also specify gross losses, gross gains, eight trajectories, and three components: Quantity, Exchange, and Alternation. The illustrative application compares maps of the marsh category for three Long-Term Ecological Research sites: Plum Island Ecosystems (PIE), Georgia Coastal Ecosystems (GCE), and the Virginia Coast Reserve (VCR). The application analyzes marsh’s changes during two time intervals that have unequal durations within each site’s distinct temporal extent. Results show that PIE has the fastest change during each site’s temporal extent. Gross change is more than double the quantity change for all sites. Exchange accounts for most of the change in GCE, while Alternation accounts for most of the change in PIE and VCR. The methods provide more information than popular methods that quantify annual net change. Our timeseriesTrajectories R package performs the analysis and is available for free at https://github.com/bilintoh/timeseriesTrajectories.