Houghton's goldenrod (Solidago houghtonii) COSEWIC assessment and status report: chapter 5
Habitat
Habitat requirements
Houghton’s goldenrod is closely associated with the Great Lakes shorelines, particularly Lake Huron and Lake Michigan. The continental climate of the area is moderated by the effect of Lake Huron. Both Manitoulin Island and the Bruce Peninsula have milder winters and cooler summers than areas of similar latitude in Ontario. Average January temperature is -10.0°C, average July temperature is 19.1°C, and the area receives 808.9 mm of precipitation each year (Environment Canada climate data for Gore Bay).
Houghton’s goldenrod is found on calcareous beach sands, interdunal wetlands, or seasonally wet limestone pavements (Morton, 1979). The latter kind of habitat, also known as alvar, is by far the most common habitat for Houghton’s goldenrod growing in Ontario. Over 86% of Ontario’s known populations of Houghton’s goldenrod occur on alvars (Catling, 1995). This is in contrast to Michigan, where the species is mostly associated with dunes (Morton, pers. comm.) but perhaps more specifically in interdunal wet meadows (Penskar, 1997).
Alvars are distinct ecosystems characterized by thin soil over level limestone bedrock, a sparse but unique vegetation community made up of shrubs and herbs, and the absence of trees (Catling and Brownell, 1995). The poor drainage on the level limestone results in spring flooding due to snowmelt, or periodic flooding in the case of shoreline alvars, but standing water quickly evaporates from the thin soils, and drought conditions are common during the summertime. These growing conditions give rise to an unusual community of plants and animals adapted to drought and alkaline soils that would be excluded by competition in other habitats (Figure 4).
Little else is known about particular microclimate requirements, but the persistence of Houghton’s goldenrod in alvars indicates it is likely to be drought-resistant (Penskar, 1997). It is also limited to calcareous areas, suggesting that the species may have a high calcium requirement.
Figure 4. Houghton’s goldenrod (Solidago houghtonii) habitat at Cabot’s Head.
Trends
The original alvar habitat is estimated to have once occupied 50 km² on the Bruce Peninsula and 400 km² on Manitoulin Island (Catling and Brownell, 1995). Much of this habitat still exists, though some may have been degraded due to livestock grazing. There are currently estimated to be 25 alvar sites on the Bruce Peninsula, and 75-100 on Manitoulin Island, although it appears that only a limited proportion of these contain Houghton’s goldenrod.
Protection/ownership
Most of the Houghton’s goldenrod occurrences on record with the Ontario Natural Heritage Information Centre (NHIC) are privately owned by individuals or are part of First Nations’ territories. Much of the alvar habitat on the La Cloche Islands is owned by one family. The Strawberry Island site is owned by the Canadian Coast Guard, and the Cabot Head site is part of the Cabot Head Provincial Nature Reserve. As such, the Cabot Head site is the only site on protected land.
In the United States, about nineteen populations in Michigan grow on state, federal or private protected land, including 4 populations that are believed to be the largest in existence (Michigan Natural Features Inventory, 1996).