Plants on the Edge: Monitoring rare, disjunct, and range edge species on Lake Superior Islands 03.15.2022

The plants Dr. Johnson spoke about are on the edge in two senses of the word. Many grow in the dynamic zone between water and land, and they are among the rarest plants of the region. She has studied these rare plants at the Apostle Islands National Lakeshore, which includes 22 Islands and a mainland unit, and at Isle Royal National Park, which includes approximately 400 satellite islands in addition to the main island.

Dr. Johnson described the various habitats found along these island shorelines. At Isle Royal it is primarily basalt bedrock with crevices and pools that are repeatedly scoured by waves and ice. There are also bedrock shorelines in the Apostles, but of sandstone, often in cliffs. The Apostles also have eroding clay bluffs and some sand dunes on the quiet side of the islands, sometimes with wetland lagoons protected by sandspits.

The rare plants of these shorelines are significant in different ways. Some are at the edge of their range, like Lake Cress (Rorippa aquatica), which mostly grows to the southeast. Others are part of separate Great Lakes populations of species that primarily grow in the arctic, western mountains, or the Atlantic coast. These are called disjuncts, and butterwort (Pinguicula vulgaris) is an example. It is mainly a species of the arctic and northern coastlines, but also grows in bedrock crevices around the Great Lakes, including Minnesota. It has rosettes of sticky, bright, lime-green leaves that catch small insects and gorgeous purple flowers in June.

Given the precarious environments these plants inhabit, climate change presents a variety of potential threats. There is less ice and warmer water than in the past, which may make conditions less hospitable to the arctic species. Winds are getting faster, with more big storms and heavy rains. While these plants are adapted to disturbances, the disturbances could increase beyond their capacity to survive. In addition, the water level of Lake Superior has recently experienced more extreme fluctuations, which can reduce and increase the habitat available. Recent high levels have also resulted in significant shoreline erosion in some places.

Wisconsin botanist Emmet Judziewicz surveyed the National Park lands for rare plants in the 1990s and again in the 2000s when the lake was very low. Dr. Johnson set out revisit all the populations he had documented and assess their status. She described her crew searching the rocks and bluffs for weeks at a time. They became quite adept at spotting particular plants perched on cliff faces with binoculars from a rocking boat.

Dr. Johnson spoke about many individual species and how they are faring, illustrated with fabulous photos. Many populations fluctuated along with the lake level. Butterwort, for instance, increased during the low water of Judziewicz’s second survey, but Johnson found they have since decreased back to the 1990s level. On the other hand, Elegant Groundsel (Packera indecora) plummeted during low water, but has since rebounded part way. Overall, Johnson concludes that so far most populations have been resilient to changes in water level and other conditions. This is good news for the distinct genetics these isolated populations likely possess, which may prove important to the adaptive capacity of these species in the face of impending climate upheaval.

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