Rare Plants in the Highlands - Key to Glacial History

by: James Bridgland

The highlands of Cape Breton are well known for their magnificent scenery. With steep mountain valleys and river gorges, they never fail to inspire one's imagination. even more inspiring and intriguing is the plant life found in these valleys and gorges. In places the flora is quite unlike that found anywhere else in Nova Scotia. Over a third of the 210 species designated as rare in Nova Scotia are found in the Cape Breton Highlands, and half of these are found mainly in deep rocky gorges which dissect the plateau. The occurrence of these plants raises questions about the geological history of the province and about why such a large number of rare plants should be found in one type of habitat and in such a small area.

Outside Nova Scotia these rare plants have remarkably similar ranges. Over half are Arctic Alpine species normally found north of the continental tree line. In Nova Scotia and elsewhere south of the tree line they exist only in highlands or other mountainous areas, often as outlying populations widely separated from their main centres.

Another major group of plants, the Boreal Disjunct species, has ranges which centre in the cold coniferous forest zone stretching from Newfoundland to Alaska. The maritime populations of these plants are also widely separated from their main population centres.

A third, much smaller, group of Endemic species is found only in restricted localities around the edge of the gulf of St. Lawrence, including the Long range Mountains of western Newfoundland, the Shikshok Mountains on the Gaspe Peninsula, and the Cape Breton Highlands.

The large concentration of rare Arctic-Alpine, Boreal Disjunct and Endemic species found in the Cape Breton Highlands (and other regions around the Gulf) poses two questions. How did they get here and why do they persist?

While in human terms forest vegetation may appear very static and almost as fixed as the soil and rocks it grows on, in the context of geological time it is relatively ephemeral. During the Pleistocene Epoch (1,800,000 to 10,000 years B.P.) Canada was covered with ice 4 times. The last glacial episode, the Wisconsin glaciation, covered the Maritime provinces between 12,000 and 16,000 years. Well over 90%of our flora has expanded into Canada from the south since then. In the early 1900s the Harvard Botanist M.L. Fernald suggested that the occurrence of endemics, disjunct arctic and boreal species in the mountains surrounding the gulf of St. Lawrence was evidence that those mountains had stuck up above the continental ice sheet as ice-free islands or nunataks. The plants which now show such curious tops, he said, while elsewhere they were wiped out by the overriding ice.

This theory was controversial when he proposed it in 1925. Most geologists of the time believed that all of eastern Canada has been completely buried by glaciers during the Wisconsin period.

In the 1930s alternate theories were proposed to explain this peculiar distribution. The most influential of these was put forward by V. C. Wynne-Edwards in 1937. He observed that many of these plants were in fact calcium-loving species normally restricted to alkaline soils. He held that the distributions of the rare plants reflected their ecological isolation and the discontinuity of preferred habitat.

While it appeared to explain the current distribution of these plants, Wynne-Edwards' theory of "Ecological Isolation" failed to answer the question of how the plant got to the region in the first place. Frere Marie-Victorin (a Quebec botanist)proposed the "Rainbow Theory" in which the Arctic-Alpine and Boreal Disjunct floras were regarded as remnants of a single plant population or flora. This flora, he argued, had evolved in the Arctic, north of the ice sheet during or since the Pleistocene era, and had expanded southward on both sides of Hudson Bay over barren calcium-rich soils as the ice sheet melted. With time, the "arch" or "rainbow-shaped" distributions of these plants were disrupted by gradual leaching and acidification of glacial soils as well as by competition from the flora expanding northward from continental North America.

There is a problem with the alternative explanations of Wynne-Edwards and Frere Victorin. Not all of the sites where these plants are found have alkaline soils. Hounsell and Smith, two Nova Scotia botanists, examined many of the Cape Breton sites of Arctic-Alpine plants and found that most of these sites have acidic soils. What appears to be the common elements allowing these plants to persist in Cape Breton Highlands is a combination of deep shade in the upland gorges, cooling moisture from brooks, waterfalls and seepage, and very unstable substrates such as scree and talus slopes or river gravels. These conditions duplicate the Arctic environment - a short cold growing season, poorly developed soils disrupted by frost heaving and a lack of humus accumulation.

However, the question remains. How did these plants get here? Did they survive the Ice Age on nunataks? Or did they arrive with the first expansion of plants across a barren tundra after the retreat of the glaciers? On this question the jury is still out. Recent geological studies by Doug Grant (Geological Survey of Canada) suggest the possible existence of Wisconsin nunataks on North Mountain east of Pleasant Bay and at Cape Smokey. Rene Belland of the University of British Columbia has recently discovered several moss species in Cape Breton which show the same disjunctions as the rare flowering plants. Did they spend the Ice Age here as well? We may never know for sure, but speculation is bound to continue.