Opuntia Fragilis
The Little Roaming Cacti

by Ernie Boyd

Imagine a time near the end of the last great ice age, some eight thousand years ago. Most of Canada, is just emerging from under a sheet of ice as much as a kilometer thick that has covered nearly all of the northern hemisphere for 10,000 years. Only the tops of the highest mountain ranges have been visible above the ice. The landscape is completely barren and devoid of life. No plants, not even mosses or lichens, have yet taken hold. Only sand, alluvial gravels, streams and rivers, vast shallow lakes and rock polished smooth by the immense pressure and movement of ice. A world so bare and unobstructed that winds howl across the landscape.

As the ice sheet melts, small hardy plants begin to inhabit suitable places along the foot of mountains in what is now Nevada and Colorado. Gradually, wind and flows of meltwater move spores, seeds, uprooted plants and other plant material along the eastern flanks of the mountains as the ice recedes. Vegetation creeps slowly northward along the foothills of the Rockies and eventually eastward into the Great Plains. Only the most hardy plants gain ground. By about four thousand years ago, Opuntia fragilis has worked its way northward well into the dry foothills of the central Rockies. Colonies soon occupy every suitable microclimate across central North America and the Great Plains. Most cacti inhabit the dry desert areas. But a few of the hardiest specimens, those able to endure long periods under snow and temperatures as low as minus fifty degrees Celsius, finally reach into Canada.

Amazingly, Opuntia fragilis has today spread completely across the Canadian Great Plains well into the area along the Manitoba - Ontario border. Some of the colonies of this cactus are found not only in the sandy alluvial soils and dry environments normally associated with these species, but grow instead on granite barrens within the Canadian Shield in both Manitoba and Ontario. One small population has even been found not far from Ottawa.

In 2000 while visiting a longtime friend, Hart Schmidt, at his home near Rennie, Manitoba he mentioned that he had heard of a 'cactus island' in the Whiteshell Provincial Park. Knowing that my curiosity would be aroused he had offered to take me there to find this site. This was not possible then but in August 2003 we arranged to travel by boat to a cluster of small, low-lying granite islands on Big Whiteshell Lake. After carefully searching several promising areas we were finally rewarded with a find of a small patch of Opuntia fragilis.

These plants are on an unoccupied island only about one hectare in area. There is an overburden of alluvial sands, gravel and granite boulders covering most of the top of the island. This lean soil supports a thick cushion of mosses and lichens, scrub brush, shrubby pine trees, aspens, alders and willows, a few choke cherry and saskatoon bushes, and a variety of sedges and grasses. The south face of the island is wind swept, quite steep and is smooth, bare, red granite. The cacti are on a slightly sloping ledge about three meters above the lake surface. The patch of plants is small, about two meters by perhaps six meters. Remarkably, they are not growing in sand or a crevice, as one would expect, but in very shallow depressions containing moss, lichens and a very thin layer of decomposed vegetation. This part of Manitoba had received little rain through the summer and the lichens on the island were completely dry to the point of brittleness. The cacti however, were green, plump and not at all desiccated.

Most of these cacti were very small with only a few joined pads on each rooted plant. Many were single pads. Some of these were already rooted but many others lay loose on the surface. The entire patch had apparently produced only two flowers, both dry and still attached. No seed pods were present. The situation of this little show of Opuntia fragilis seems very precarious given its small size and exposed location. Careless visitors or plant collecting (illegal in the park) could easily cause the disappearance of this unique plant population.

Later at the Schmidt residence a neighbour told us of another island, this one at White Lake in the park where his mother many years earlier had found a few cacti. We searched out this location as well and found a much larger show of Opuntia fragilis. This one also was on an area of red granite in conditions almost identical to those described above. The main difference was that this island was larger, with more gravel and sand overburden on its top. This was sufficient to support many pine trees. Fallen pine needles in considerable quantities were present where the cacti grew. Once again, nearly all of the cacti were growing on a south-facing slope, in patches of dry moss and lichens. We visited this second site after a brief thunderstorm and the moss and lichens were moist and quite spongy. In this situation, the cacti are able to absorb enough water periodically and to tolerate long dry periods. The seeds of other plants which might land in these sites will germinate when the lichens are damp, but no doubt promptly die from the effects of direct sunlight and drying winds. There is therefore no competition from other plants that might overshadow the cacti. While O. fragilis can be found in southern Alberta, Saskatchewan and south-western Manitoba, the soils in these areas are often alkaline. The fact that the Whiteshell cacti are found growing on granite, with some exposed to decomposing pine needles, suggests that these plants can adapt readily to both alkaline and acid soil conditions.

How O. fragilis has managed to find its way onto islands in the Whiteshell Provincial Park and other Ontario locations is a matter of interest and considerable speculation. The cacti pads break off from the parent plant very easily. Their spines are sharp and once they penetrate skin or other materials are quite difficult to remove. They will attach themselves readily to passing animals or the footwear of human visitors. One patch of cacti on the White Lake island contained the partially decomposed, dry and mummified remains of a field mouse or some other small rodent. This unfortunate creature had ventured onto the patch of cacti and a spine had penetrated deeply into its rear foot. Apparently struggling and unable to escape, it had become impaled on other spines and had perished.

Botanists who have studied the distribution of O. fragilis have noted that the distribution of these plants is often along the rivers in western Canada. It has been noted that the pads of these cacti float readily and can withstand immersion in water for extended periods. Flood conditions in springtime could account for some of the distribution of these plants. Animals such as beaver, bear, or moose moving through a patch of cacti might easily pick up a few pads and transport them considerable distances. Considering that these and several other mammals tend to feed along river banks and inhabit the islands of the Canadian Shield, one may speculate that these animals possibly account for distribution of the cacti well above the high water line on remote islands in the park. Finding these cacti growing and thriving on islands, far from human habitation or other O.fragilis, and in a region identified as USDA Zone 2 was an interesting and pleasant surprise.

Previously published in Ottawa Valley Rock Garden & Horticultural Society Newsletter, December 2003. Reproduced here with permission. www.ovrghs.ca

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