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Counting Pitcher's thistles


Pitcher’s thistle or Dune Thistle is a native thistle that grows on open sand dunes and beach ridges along the shores of the Great Lakes, including Lakes Michigan, Superior, and Huron. Its only habitat is the great lakes beaches and it grows nowhere else in the world. The only remaining natural populations occur in Indiana, Michigan, and Wisconsin and Ontario Canada. Recently, a population has been re-established in Lake County, Illinois. This species was listed as a federally threatened plant by the US Fish and Wildlife Service in 1988. A threatened species is one that could become at risk of extinction (category endangered) in the near future.

On Saturday, 20 September 2003, a group made up of UW--Green Bay students and faculty, and residents from the Green Bay community joined people from other parts of Wisconsin, including Door County, Stevens Point, Madison, and even as far away as East Lansing, Michigan to count all the Pitcher's thistles on the beach at Whitefish Dunes State Park. The park is host to the largest population of this threatened species in Wisconsin. It is the only population where every individual plant is actually counted, and where every plant is counted each year, which gives managers much more detailed information on its status and how to better preserve it. Yearly counts have been conducted since 2001. Last year over 9000 plants were counted on the beach. This year's count was lower than expected with a beach total of 6,710. P Carolyn Rock, Natural Resource Educator at the park will present this data along with a summary of the health of the population at the Natural Areas Conference in Madison, Wisconsin.

Pitcher’s thistle grows for five to eight years before it flowers and dies. It spends most of its life as a non-flowering rosette or cluster of silvery leaves. When it flowers, the plant typically has one stem up to three feet tall with many flower tipped branches. The leaves are deeply lobed and appear silvery because they are covered with tiny white hairs that help the plant survive the extreme climate on the beach. The dense covering of hairs provide insulation that lowers water evaporation, while their white coloration reflects dangerous UV radiation. The plants have a taproot that can be as long as six feet. This another adaptation that allows the plant to store nutrients and water until it flowers. Like other thistles it has spines along the edges of leaves. Both non-flowering and flowering plants have a long taproot, up to 6 feet long. The flowers are cream or pink and are found on the plant from June to September. They are pollinated by a variety of insects including bees, butterflies, and flies. The seeds set in the fall and are windblown throughout the beach area. The seeds that land in relatively open areas will germinate the following spring.

Pitcher's thistle are valuable as a dune species that is unique and is part of or disappearing Great Lakes Dune community. Plants like the thistles hold down soil and are part of an successional community that changes from the open beach to dunes to dune ridge to forest. As you drive into Whitefish Dunes State Park. The maple beech forest you are driving through is actually ancient sand dunes. Beech trees cannot grow on sand, so they depend on the transitional stages provided by earlier successional species to transform that sand to soil. A plant related to Pitcher's thistle is being reviewed as a commercial source of natural rubber and may help limit U.S. dependence on imported natural rubber. Saving species preserves their unique characteristics and genetic information that may be important in the future.

Pitcher's thistles are threatened because their habitat is being lost as people develop and change beach habitat. Dune habitats are by their nature very sensitive due to the instability of the shifting sands. They are extremely intolerant of disturbance. The plants are important in maintaining the beach ridge habitat and help hold the sand and forming soils in place. Even one person climbing a dune ridge can unintentially destroy many plants by trampling and burial as they scramble for a foothold on the steep incline of the dunes. These eroded areas increase in size and form "blowouts" that are difficult to restore because they continue to erode as the wind blows away the sand. And once a dune is damaged they take a long time and require extensive measures to recover. Shoreline development that destroys or modifies beaches and dunes, like conversion into residential housing and marinas, eliminates Pitcher's thistle and other dune plants as well as destroys its habitat. Road construction over dunes destroys and fragments dune habitats and increases access for off-road vehicles and hiking that trample plants and create erosion. The dunes at Whitefish Dunes state park are beautiful and host to an interesting array of rare plants specifically because people are not allowed to climb them. If hiking and climbing was allowed in the dune ridge areas our dunes would be uninteresting and mostly barren piles of sand.

You can see living Pitcher's thistles from the boardwalk near the nature center at Whitefish Dunes. Inside the nature center is a display with more information on this unique plant. Or volunteer for next year's thistle count by calling the park. The count is held in September.

Other rare species the group saw while counting the thistles

  • prairie sand reed (Calamovilfa longifolia var. magna--WI threatened)
  • thick-spiked wheat grass (Elytrigia dasystachya subsp. psammophila--WI threatened)
  • dune goldenrod (Solidago simplex-- WI threatened)
  • seaside spurge (Chamaesyce polygonifolia--WI special concern)


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Last updated on March 20, 2015