Tree Islands

Tree islands are an integral component of the Everglades, but they have undergone extensive damage from extreme flooding, drought, fire, tropical storms, and invasive species. These islands are also sensitive to ongoing small to large-scale restoration activities of the Comprehensive Everglades Restoration Plan. Changes in hydrologic regimes due to restoration projects, including construction of Tamiami Bridges and other Central Everglades Project Planning project components, are likely to alter the impact of drivers and stressors on tree islands. While such alterations in the impact of these stressors at the broader scale influence the spatial distribution pattern of tree islands within the landscape, the hydrologic alterations also affect the plant community structure and function on individual tree islands.

See the results for tree islands in the Greater Everglades.

Photo credit: South Florida Water Management District.

How is it measured?

This indicator is measured in the Greater Everglades. The four tree islands annually monitored from 2012–2017 were the subset of a network of 16 tree islands that were studied for varying periods within both the ridge and slough and marl prairies landscapes in the Everglades National Park. Three islands (Black Hammock, Gumbo Limbo Hammock, and Satinleaf), are in Shark River Slough, and have been monitored since 2001. The fourth island, SS-81, monitored since 2007, is within the Northeast Shark River Slough, downstream of the 1-mile bridge that has been built along the Tamiami Trail. The monitoring plots ranged from 300 m² to 625 m². Mean elevation of tree island heads in SRS varies between 1.190 ± 0.094 m and 2.663 ± 0.191 m. Plot elevations within individual islands were highly variable. Among four islands, SS-81 had higher within-plot variability than other islands, with low spots frequently occupied by swamp forest trees. From 2012–2017, the mean annual relative water level across all islands was approximately 66 cm below the ground surface. However, though rarely, the water-table rose enough to saturate the surface soils of several tree islands. 

The tree islands indicator includes three metrics; Regeneration, Tree Basal Year Change, and Invasive Exotic Species. 

Regeneration: Ingrowths-Mortality (percent/year), Tree (<= 5 cm dbh) ingrowth minus mortality percentage in hardwood hammock plots.

Tree Basal Year Change (m²/ha/year): Tree (<= 5 cm dbh) growth represented by annual change in basal area per ha in hardwood hammock plots.

Invasive-Exotic Species (% Freq. of Occurrence): Frequency (Percent occurrence) of invasive exotic woody species/climbers observed on each transect on 12 sampled tree islands.