Assessment of decadal vegetation changes in select wetlands at Apostle Islands National Lakeshore

Meeker, James

This report describes the vegetation changes observed at select wetlands within Apostle Island National Lakeshore from the period 1998–2002 to 2010. Sampling in 2010 focused on vegetation changes in both coastal wetlands and island interior marshes and bogs in light of lake level changes (a potential climate related impact), as well as invasion by aggressive plant species. Sites included an interior open peatland on Stockton Island; additional interior peatlands on Devils and Sand Islands; the lagoons of Stockton, Outer, Michigan Islands; the Sand River estuary on the mainland; and seven wetlands on Long Island. Permanently marked transects using 1 m × 1 m quadrats were established in 1998–2002 that summarized the different cover-types, but through re-sampling in 2010 were used to monitor changes. Of particular interest in this study was to demonstrate whether vegetation in these wetlands tracked changes in water levels by behaving in the manner suggested by the “pulse stable” concept. Pulse stability would be supported if vegetation changes were cyclic, that is constantly changing in species abundances in the short-term, but over longer time scales remaining stable as specific wetland types. Alternatively, wetlands could be undergoing the beginnings of directional change to generally different wetland types. The results reported here demonstrate that there have been increases in woody and semi-woody plant taxa along with concomitant decreases in more obligate wetland taxa, as Lake Superior levels have dropped over the last 15 years. Over the 8- to 11-year sampling period 21 of the 24 woody taxa increased in abundance, while all of the 13 “aquatic” taxa lost ground. These were predictable results, as the two extreme ends of the water level gradient (the aquatics on the wet end) and the woody plants on the dry end should respond quickly to decadal changes in water levels. In addition, the non-Sphagnum wetlands (sedge meadows, shore fens) have changed more over the sampling period in comparison to the Sphagnum dominated (bog) sites. However, the results of this monitoring––following only one half of a full high-water to low-water back to high-water cycle––falls short in demonstrating that the natural cycle of wetland vegetation change (pulse stability) is still operating. Lake Superior has not experienced a return of high water to re-sample, during which we might expect an increase in abundance of the aquatic taxa (that is, a return to the original abundance). It will take another high-water period to determine if we are experiencing a period of directional change or cyclic change, and the water levels on the big lake just have not cooperated. If climate is indeed changing, resulting in a long-term period of lower water levels, we may be observing the beginnings of directional change.

Assessment of decadal vegetation changes in select wetlands at Apostle Islands National Lakeshore

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