Reading the landscape: temporal and spatial changes in a patterned peatland

Abstract

The Everglades of south Florida is a patterned peatland that has undergone major hydrologic modification over the last century, including both drainage and impoundment. The Everglades ridge and slough patterns were originally characterized by regularly spaced elevated ridges and tree islands oriented parallel to water flow through interconnected sloughs. Many areas of the remaining Everglades have lost this patterning over time. Historical aerial photography for the years 1940, 1953, 1972, 1984, and 2004 provides source data to measure these changes over six decades. Maps were created by digitizing the ridges, tree islands, and sloughs in fifteen 24 km² study plots located in the remaining Everglades, and metrics were developed to quantify the extent and types of changes in the patterns. Pattern metrics of length/width ratios, number of ridges, and perimeter/area ratios were used to define the details and trajectories of pattern changes in the study plots from 1940 through 2004. These metrics characterized elongation, smoothness, and abundance of ridges and tree islands. Hierarchical agglomerative cluster analysis was used to categorize these 75 maps (15 plots by 5 years) into five categories based on a suite of metrics of pattern quality. Nonmetric multidimensional scaling, an ordination technique, confirmed that these categories were distinct with the primary axis distinguished primarily by the abundance of elongated ridges in each study plot. Strong patterns like those described historically were characterized by numerous, long ridges while degraded patterns contained few large, irregularly shaped patches. Pattern degradation usually occurred with ridges fusing into fewer, less linear patches of emergent vegetation. Patterning improved in some plots, probably through wetter conditions facilitating expression of the underlying microtopography. Trajectories showing responses of individual study plots over the six decades indicated that ridge and slough patterns can degrade or improve over time scales of a decade or less. Changes in ridge and slough patterns indicate that (1) patterns can be lost quickly following severe peat dryout, yet (2) patterns appear resilient at least over multi-decadal time periods; (3) patterns can be maintained and possibly strengthened with deeper water depths, and (4) the sub-decadal response time of pattern changes visible in aerial imagery is highly useful for change detection within the landscape. This analysis suggests that restoration of some aspects of these unique peatland patterns may be possible within relatively short planning time frames. Use of aerial photography in future Everglades restoration efforts can facilitate restoration and adaptive management by documenting sub-decadal pattern changes in response to altered hydrology and water management.