FEATURE ARTICLE
Cliffs as Natural Refuges
Rocky precipices around the world provide a surprisingly sheltered environment for plants and animals
Douglas W. Larson, Uta Matthes, Peter Kelly
Any Port in a Storm
Realizing that the trees on the cliffs are exceedingly old, we turned our attention in 1990 to the smaller plants and animals living with them, in an effort to determine whether these parts of the biota reflect the ancient habitat. We soon ascertained that the arctic grasses in the northern parts of the escarpment and certain ferns—such as the rock polypody (Polypodium virginianum), wall-rue (Asplenium ruta-muraria), green spleenwort (Asplenium viride) and the smooth cliff-brake (Pellaea glabella)—are all more or less restricted to the cliffs, as are other rare plants such as the bird's eye primula (Primula mistassinica), butterwort (Pinguicula vulgaris) and the lakeside daisy (Hymenoxys herbacea). These small, rare plants in some cases were much more abundant decades ago, before people eliminated much of their habitat.
Finding so many types of vegetation living on the escarpment surprised us. We, like most other botanists, had thought of cliffs as dry and impoverished in nutrients. Indeed, we conducted an elaborate experiment to test our hunch that the lack of either water or nutrients had caused the trees on the cliffs to grow so very slowly. If this hypothesis were true, we reasoned, it should be possible to boost growth by force-feeding the trees with water or fertilizer. But supplying a solution of plant nutrients to the vertical face of a cliff was not going to be easy. A watering can would not do the trick.
After much discussion, we decided to employ the same equipment physicians use to rehydrate sick people. And so with a series of slightly awkward phone calls, we procured several hundred sets of used IV equipment and installed each near the roots of a tree that was growing on a cliff in an abandoned limestone quarry situated nearby. We used this novel arrangement to infuse the cracks in the rock with either plain water or a fertilizing solution, administering treatments to our green patients every second day for more than two years. By then it became abundantly clear that our painstaking efforts to water the trees had no effect at all and that supplying nutrients had increased the rate of growth by only a tiny amount. We concluded that the cliff-dwelling trees were not stunted for lack of water or nutrients.
These trees grow slowly, we now surmise, simply because their roots are hemmed in. Like those of carefully cultivated bonsai trees, the roots have much less mass than do those of a typical tree. And like a houseplant stuck in too small a pot, a tree rooted in a crevice responds by growing slowly, even with water and nutrients in ample supply. We have yet to find a proper way to test this explanation rigorously, but we are reasonably confident that this mechanism for limiting growth applies.
In retrospect, we probably should have realized that water and nutrients were being amply furnished to the trees on the Niagara Escarpment. In fact, these life-giving substances are usually found in more steady supply on cliffs than elsewhere, because the vast amount of groundwater stored under the neighboring plateau tends to leach nutrients from the rock and leak out of the open face. Although our experiment did not confirm our initial hypothesis, the effort was valuable for a bit of serendipity it provided when we harvested the trees at the end of our study.
During this work, we would frequently pull a chunk of limestone from the face of the rock, exposing a thin ribbon of bright green that lay only about one millimeter below the surface. Seeing this layer, we were instantly reminded of photographs of cryptoendolithic life (microbes that reside inside rock) within the sandstones of the dry valleys in Antarctica. The moment we saw the telltale green band, we began to wonder whether similar organisms could be hiding in the limestone cliffs of comparatively balmy Ontario. With the help of colleagues Joe and John Gerrath at the University of Guelph, we were able to answer that question: A community of single-celled plants, fungi and photosynthetic bacteria invisible from the surface was living just under the skin of these rocks. Although we have not yet identified the types of fungi, we have already added more than 30 taxa of photosynthetic algae and cyanobacteria to the growing list of species using the cliffs as a refuge.
» Post Comment