Status and change in rare plants of the Apostle Islands National Lakeshore: 1990s–2019

Johnson, Sarah

doi:10.36967/nrr-2287739

Of the approximately 1,800 native plant species in Wisconsin, about 19% are listed as rare by the Wisconsin Natural Heritage program. Of these rare species, 41 have been documented within the Apostle Islands National Lakeshore in Lake Superior. This report summarizes the current status and changes in abundances and number of locations where 33 of these rare plants occur in the national lakeshore, from the early 1990s to 2019. The Apostle Islands are an archipelago of 22 islands, and the national lakeshore encompasses 21 islands plus part of the adjacent mainland shoreline. Diverse geologic landforms on islands ranging from 3 to 10,000 acres, and the moderating influence of Lake Superior along 160 miles of shoreline together support rare species in unique maritime microclimates. The lakeshore is in a sub-boreal region at the intersection of where multiple floristic elements migrated into the area after glaciers receded about 10,000 years ago. Several of the park’s rare species are at the southern edge of their boreal ranges, and some represent small disjunct populations of species from the arctic, western North America, or the Atlantic Coast. A few species are at the northern extent or central within their broader ranges. Climate change is a primary concern for the survival of rare species in the Apostle Islands, especially for species at the southern edge of their range and for species with very small population sizes. Lake Superior is warming, ice cover has declined, wind speeds are increasing, prevailing winds are shifting to the north and northeast, and extreme storm events are increasing in frequency. Average lake levels are not expected to change, but rapid or prolonged lows or highs in annual lake levels are a concern for coastal ecosystems. Emmet Judziewicz established detailed records of the location and abundance of rare species he found in the park in 1991–1992. He resurveyed the islands in 1996 and in 2001 and added rare species accounts. Other NPS-funded botanists conducted partial assessments on the status of a subset of State Threatened and Endangered species since 2001. I targeted 33 species for field resurveys in 2019, making this report the most comprehensive resurvey of rare plants in the park since the 1990s. Some species have been monitored more frequently than others. For most species, though, there is information from the early 1990s and 2019, providing a snapshot of up to 28 years of change. Judziewicz and others applied complete censuses to many species and rough estimates for others (e.g., coastal wetland sedges). In addition to compiling tables of plant counts among all known sites, I assigned a change rating to each target species if adequate data were available. I categorized each species as increasing, decreasing, or as remaining stable in total abundance and in site occupancy within the lakeshore. The scorecard system I developed used a conservative magnitude of change to account for sampling error that can accumulate in long-term monitoring programs due to variation in detection probability (e.g., monitoring cliff species from a moving boat), differences in effort, and phenology. Due to low sample sizes for some rare species, some data gaps, and because some of these surveys were complete population censuses instead of subsamples, these monitoring data generally do not lend themselves to statistical analyses.

Status and change in rare plants of the Apostle Islands National Lakeshore: 1990s–2019

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