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Alder Thicket (Global Rank G4; State Rank S4)

Locations of Alder Thicket in Wisconsin. The deeper hues shading the ecological landscape polygons indicate geographic areas of greatest abundance. An absence of color indicates that the community has not (yet) been documented in that ecological landscape. The dots indicate locations where a significant occurrence of this community is present, has been documented, and the data incorporated into the Natural Heritage Inventory database.

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Overview: Distribution, Abundance, Environmental Setting, Ecological ProcessesAlder Thicket is a minerotrophic wetland community domi-nated by the tall shrub speckled (or “tag”) alder (Alnus incana). The community is widespread and common throughout the glaciated portions of northern Wisconsin, where it occurs on lake and stream margins, and in basins experiencing lateral movement of oxygenated and nutrient-enhanced ground-water. In central Wisconsin, Alder Thicket occurs in and on the margins of the poorly drained unglaciated areas formerly occupied by Glacial Lake Wisconsin (a huge proglacial lake). In southern Wisconsin, Alder Thicket is rare and local and sometimes associated with other plant communities more common in the north, such as conifer swamps. In the ungla-ciated “Driftless Area,” it may border small, free-flowing streams or occur at slope bases. Soils are usually mucks, and seepages or spring runs may be present within or along the edges of the community. Flooding is seasonal, usually occur-ring during the spring, following snowmelt.

Nutrient status is high, as speckled alder can fix nitrogen. Whatever the physical setting, the groundwater is neither stagnant nor oxygen depleted—some movement of ground-water is a characteristic hydrological attribute. Microtopogra-phy is subdued. Alder Thicket may grade into sedge meadow,

one of the northern fens, or lowland forest, depending on hydrology, soil moisture levels, and disturbance history.

Based on the accounts of explorers and traders who described conditions in the parts of Wisconsin they traveled through during the 1600s, Alder Thicket has apparently been stable on some sites (e.g., along the upper Brule River in what is now Douglas County) for centuries. Elsewhere, succession to forest may occur, most often to wet-mesic forest communi-ties dominated by northern white-cedar, tamarack, black ash, or mixtures of these species. Once lost due to logging or inun-dation, re-establishment of northern white-cedar dominance in the near future is very slow and unlikely or impossible at this time because of high white-tailed deer populations and heavy browse pressure on the palatable seedling and sapling northern white-cedars.

Community Description: Composition and Structure Speckled alder, which may reach heights approaching 5 meters, is the dominant tall shrub. Associates include red osier dog-wood (Cornus stolonifera), nannyberry (Viburnum lentago), common winterberry (Ilex verticillata), mountain holly (I. mucronata), nine-bark (Physocarpus opulifolius), currants (Ribes spp.), willows (Salix spp.), and sometimes the American mountain-ash (Sorbus americana) and showy mountain-ash (S. decora). Sapling or seedling trees, such as northern white-cedar (Thuja occidentalis), black ash (Fraxinus nigra), American elm (Ulmus americana), yellow birch (Betula alleghaniensis), bal-sam fir (Abies balsamea), tamarack (Larix laricina), and eastern white pine (Pinus strobus), may also occur in Alder Thicket.

Characteristic broad-leaved herbs and low shrubs are orange jewelweed (Impatiens capensis), sensitive fern (Onoclea sensibilis), Canadian wood-nettle (Laportea canadensis), spot-ted Joe-Pye-weed (Eupatorium maculatum), boneset (E. per-foliatum), rough bedstraw (Galium asprellum), arrow-leaved tear-thumb (Polygonum sagittatum), dwarf red raspberry (Rubus pubescens), marsh fern (Thelypteris palustris), jack-in-the-pulpit (Arisaema triphyllum), marsh skullcap (Scutel-laria galericulata), and several asters such as panicled aster (Symphyotrichum lanceolatum), swamp aster (S. puniceum), and flat-top aster (Doellingeria umbellatus). Dense beds of skunk cabbage (Symplocarpus foetidus) may form an almost continuous herbaceous “subcanopy” in stands with substantial groundwater movement. Near springs and seepages, marsh-marigold (Caltha palustris), swamp saxifrage (Saxifraga pensylvanica), American golden saxifrage (Chrysosplenium americanum), marsh pennywort (Hydrocotyle americana), and blue marsh violet (Viola cucullata) are often common. Rep-resentative graminoids include fowl manna grass (Glyceria striata), blue-joint grass (Calamagrostis canadensis), brome-like sedge (Carex bromoides), fringed sedge (C. crinita), nod-ding sedge (C. gynandra), and swollen sedge (C. intumescens). Mosses may be important members of this community.

Speckled alder lines the banks of the upper Brule River for several miles, grading into a tamarack-dominated conifer swamp, then an extensive stand of northern white-cedar. The wetlands border-ing the river are laced with springs and seepages. Animals of note inhabiting the alder include American Woodcock, Golden-winged Warbler, Alder Flycatcher, Veery, and wood turtle. The protruding tamarack trees are used as perches by the Olive-sided Flycatcher. Brule Spillway, Brule River State Forest, Douglas County, Northwest Sands Ecological Landscape. Photo by Eric Epstein, Wisconsin DNR.

Among the rare plants for which Alder Thicket provides important habitat are bog bluegrass (Poa paludigena), sweet colt’s-foot (Petasites sagittatus, listed as arrow-leaf sweet-colt’s-foot by the Wisconsin State Herbarium), and lesser wintergreen (Pyrola minor).

Animals of management interest that inhabit or use Alder Thicket include American black bear (Ursus americanus), bobcat (Lynx rufus), snowshoe hare (Lepus americanus), wood turtle (Glyptemys insculpta), American Woodcock (Scolopax minor), Ruffed Grouse (Bonasa umbellus), Veery (Catharus fuscescens), Golden-winged Warbler (Vermivora chrysoptera), and four-toed salamander (Hemidactylium scutatum). Frugivorous birds such as thrushes, catbirds, and waxwings may make heavy use of fruit-bearing shrubs such as blackberries, dogwoods, elderberries, and viburnums, any or all of which can be common associates of speckled alder.

Conservation and Management ConsiderationsAs a cover type, alder thicket may be more common now than it was historically. In some areas, it has replaced wet-mesic lowland forests such as northern white-cedar swamps and black ash-dominated hardwood swamps due to hydrologi-cal disruptions caused directly or indirectly by logging and population increases of American beaver (Castor canadensis). Partial drainage of herbaceous wetlands such as sedge mead-ows and fens has also allowed alder to spread into areas from which it was formerly excluded by high water tables.

Invasive plants that are especially problematic in Alder Thicket include reed canary grass (Phalaris arundinacea), glossy buckthorn (Rhamnus frangula), and stinging nettle (Urtica dioica). Thistles, especially Canada thistle (Cirsium arvense) and European swamp thistle (Cirsium palustre), and common burdock (Arctium minus), can alter stand structure and composition. To date such problems have been most acute in southern and central Wisconsin where grazing is and has been more prevalent than in the north.

In southern Wisconsin, many stands have been pastured, a practice that can alter or obliterate native herbs and low shrubs, lead to an increase in rank weedy generalists, facilitate the spread of exotics, compact the soil, and add to stream sedi-ment and nutrient loads. In recent years, the practice of main-taining alder thickets by shearing them to increase woody stem densities and aid in regeneration has become more common as a means of enhancing habitat for game species, especially for American Woodcock. In some cases alder shearing has also been done in the name of streambank stabilization. If such activities become more widespread, study and monitoring will be needed to clarify the impacts on other members of the com-munity and the extent of the vegetation that has been replaced.

The need for and effectiveness of shearing to regenerate and maintain stands of alder merits additional discussion and test-

ing. Several factors should be considered, including whether site occupancy by alder has been long term and apparently stable (e.g., along the upper reaches of the Bois Brule River in Douglas County) or, if alder represents a short-term succes-sional stage that followed the removal of a beaver dam or the heavy logging of a lowland forest. The proximity of invasive species, especially reed canary grass, glossy buckthorn, and European swamp thistle, and the likelihood of inadvertently aiding their spread, should be carefully assessed.

On the poorly drained, heavy clay soils of the Superior Coastal Plain Ecological Landscape, intensive logging may lead to “swamping.” Tree removal may change the water bal-ance, raise the water table, lead to tall shrub dominance, and render the site unsuitable for reoccupancy by forest vegeta-tion, at least in the foreseeable future. Management deci-sions on hydrologically compromised sites need to consider impacts broadly, both spatially and temporally, to ensure that site conditions do not deteriorate further or have unforeseen negative effects on site hydrology and water behavior.

Additional Information For additional information on natural communities simi-lar to Alder Thicket, see the descriptions in this chapter for Northern Hardwood Swamp, Northern Wet-mesic Forest, Shrub-carr, Forested Seep, and Northern Sedge Meadow. The U.S. National Vegetation Classification (US NVC) type cor-responding most closely to Alder Thicket is CEGL002381 Speckled Alder Swamp Shrubland (Faber-Langendoen 2001).

FROM: Epstein, E.E. Natural communities, aquatic features, and selected habitats of Wisconsin. Chapter 7 in The ecological land-scapes of Wisconsin: An assessment of ecological resources and a guide to planning sustainable management. Wisconsin Department of Natural Resources, PUB-SS-1131H 2017, Madison. For a list of terms used, please visit the Glossary. For a reference list, please see the Literature Cited.

http://dnr.wi.gov/topic/landscapes/documents/ELOWCh7/Glossary.pdfhttp://dnr.wi.gov/topic/landscapes/documents/ELOWCh7/LitCited.pdf

Shrub-carrAlder ThicketBog RelictMuskegOpen Bog