Cliffs as Natural Refuges

Douglas W Larson, Uta Matthes, Peter E Kelly. American Scientist. Volume 87, Issue 5. Sep/Oct 1999.

In 1904, Paul Vogler, a Swiss scientist who had noted the decline of the English yew (Taxus baccata) in Prussia, set out to write the botanical obituary for this species in his country. Intensive use of its valuable wood had fueled widespread clear-cutting in the 19th century, which had virtually eliminated this yew from large parts of Europe. Although the surviving trees were restricted to rather inaccessible spots-mostly rocky outcrops on the vertical faces of cliffs to his surprise and delight Vogler found that the English yew in Switzerland was nevertheless wide-ranging.

In the same year, Max Oettli, a colleague of Vogler, published a report about the plants growing on various crags and bluffs in the southern Alps. He concluded that for a variety of plant species, these precipices served as refuges. Both Vogler and Oettli, howe-ver, failed to recognize that their findings might be part of a general pattern. Decades passed with only occasional references in the botanical literature to the rare plants that liked to make a home of cliffs. Few scientists realized that these nearly vertical surfaces support rich communities that are quite distinct from their level-ground neighbors.

Only after World War II did a few people, such as Peter Davis at the University of Edinburgh and, more recently, Peter Wardle of the Department of Scientific and Industrial Research in New Zealand, fully grasp this point and draw attention to it. Yet most other naturalists remained oblivious. Even the well-known ecologist John Curtis of the University of Wisconsin at Madison, writing in his 1959 classic text, The Vegetation of Wisconsin, argued that cliffs “represent a geological feature, not a biotic community type.” Curtis, like most other scientists, did not see cliffs as proper places at all. We, too, held this prejudice until the mid-1980s, when a chain of intriguing events led us to alter our thinking-and, ultimately, to adopt the ecology of cliffs as the focus of our research.

At that time, we were trying to assemble a team of ecologists to address a central question: How does environmental stress organize a biological community? We anticipated that if we selected a particular habitat to study with the tools of our specialized subdisciplines, we could achieve a comprehensive answer, at least for one chosen setting.

Clearly, we needed to examine a place with a strong environmental gradient. The Niagara Escarpment of southern Ontario-which we each had seen many times, at least from a distance-appeared to offer an obvious contrast between the open cliffs and the adjacent woodlands. So we selected this locale for our program. Closer inspection of the trees on the 30-meter bluff revealed a striking pattern: The exposed wall of rock supports a sparse cover of eastern white cedar, but almost none of the tree species found in the surrounding forest (which is composed mostly of different kinds of hardwoods, including oaks, ashes and maples) reside on the cliffs.

This incongruity, we thought, signaled that the different types of trees had evolved to exploit specialized environmental conditions. Two years of work, however, showed that all these species preferred the rather lush properties of the adjacent forest; none had specific adaptations for life on the cliffs. What is more, a careful search of the bluffs for seedlings turned up almost no newly established plants. We could think of only one explanation: We imagined that all this land had been cleared sometime in the 19th century and that the more aggressive hardwoods took over the prime spots on level ground, leaving the cedars to fill the gap left on the cliff. Little did we know how wrong our guess would turn out to be.

Happy Trails

Like many scientific advances, the key to our understanding took form while we were applying ourselves to a different (and very practical) problem. We were trying to establish whether people hiking along the trails at the edge of the Niagara Escarpment were harming the adjacent vegetation. We began by carefully examining the trees situated near these well-trod paths. To gauge whether the recent visitors had done any damage, we needed to know the productivity of the trees in the years before the trails were established. Fortunately, it is not difficult to gain such historical records about a tree, which chronicles its life with a series of annual growth rings.

Measuring the width of tree rings without chopping through a trunk requires an instrument called an increment corer, essentially a slim metal tube with an auger at the tip. Although this device leaves a small hole, it does little harm to the tree being sampled. During the first day of field work, we extracted cores from some dozen trees, including a specimen the size of a small Christmas tree that was perched on a tiny ledge near the top of the vertical face. Normally, annual growth rings are plainly visible on a newly extracted core, but this slim plug of wood appeared to lack the usual light and dark banding. Only after polishing it with fine-grained sandpaper and placing it under a microscope could we see the rings. To our astonishment, we were able to count nearly 400 annual increments, some thinner than a human hair. Hurriedly we processed a few other samples from similar trees and realized that this 400-year-old Christmas tree was not a freak.

Such sluggish growth is unusual, but it is well within the range of the ancient Bristlecone Pines of the American West, which were made famous during the 1950s by University of Arizona botanist Edmund Schulman. Only at this point did we realize that the cliffs where we had been working for nearly three years constituted a thin strip of old-growth forest, a vestige of preColumbian North America that had survived in the heart of industrial Canada, not 60 kilometers from the nation’s largest city.

A New Old-Growth Forest

With some rapid grant writing, we secured funds to conduct a proper survey of these intriguing trees. For that study, we cored more than 800 trunks and counted their microscopic growth rings, producing impressive results: It turned out that the cliffs of the Niagara Escarpment shelter the most ancient forest to be found east of the Rocky Mountains.

Although this conclusion might not have surprised enthusiasts of bonsai, who know to look for diminutive old trees growing on rocky bluffs, it did come as news to the scientific community. Even dendrochronologists, specialists who study past environmental conditions using tree rings, had neglected cliffs entirely as they scoured the globe for ancient trees to sample. Edward Cook of the tree-ring laboratory of Lamont-Doherty Earth Observatory in New York was indeed quite skeptical when we first suggested a collaboration. But in the end, our work together produced a chronology from tree rings that shows variations in summer temperatures for southern Ontario since long before there were any European settlers present to record them.

So what was initially an attempt to measure the damage done by hikers had unearthed a goldmine for dendrochronologists studying climate change. We had also stumbled on the explanation for the lack of young seedlings on the cliffs: New trees rarely took root on the rocky escarpment, but even sporadic regeneration was adequate to support a forest that has been growing ever so gradually for centuries. And a forest it truly is. Although the cliffs appear largely bare, we count about 1,000 trunks on average protruding from each hectare of the rocky wall-roughly the same concentration as in most other mature forests, which appear denser only because the trees are normally so much taller and fuller. Also, as we discovered with our survey, the distribution of ages of the trees on these cliffs follows a negative exponential curve, mirroring what has been found for other undisturbed woodlands.

A sparse array of small, scraggly trees clinging to the vertical face of a rocky precipice certainly does not fit the popular image of an old-growth forest. Indeed, many of the trees look downright sickly, with considerable dead wood intertwined with live growth (an arrangement allowed by the peculiar sectored architecture of hydraulic pathways in the trunks). There is no competition here for the giant redwoods or majestic sequoia of the Pacific Northwest, for example. Mature cedars on the Niagara Escarpment are rarely more than a half a meter in diameter, even though some of these trees have been growing there since before the Norman conquest.

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.

Splendid Isolation

At this stage in our research, we began to interest other biologists in studying the animals that inhabit these cliffs. For example, Jeff Nekola of the University of Wisconsin at Green Bay examined the gastropods of the Niagara Escarpment in Canada and its extension into the U.S. He discovered that these cliffs support the richest snail faunas in the Great Lakes region, with some individual 10-by-10 centimeter plots containing 21 separate taxa. Jeff Matheson, a graduate student at our institution, examined the distribution of birds and small mammals on the Niagara Escarpment and compared these patterns with those he charted on the forested plateau. Matheson found that the diversity of these creatures was, in fact, higher around the cliffs than in the level woodlands nearby

By 1993, ecologists in distant parts of the world were beginning to publish studies of other cliffs, and their results had a familiar ring: Rocky precipices everywhere, it seemed, shelter an exceptionally rich collection of plants and animals. Many types of flies, spiders, salamanders, rodents and raptors, as well as large numbers of plant species appearing nowhere else were being sighted on cliffs in many countries. Interestingly, the genera-and in some cases even the species-matched those found on the Niagara Escarpment.

We saw this similarity ourselves in a study that we mounted in 1997 with the help of colleagues in the U.S., U.K., France and Germany. For this investigation, we sampled more than 200 mature Thuja, Juniperus and Taxus trees on 46 separate bluffs in North America and Europe. Virtually everywhere we looked, the cliffs presented us with slow-growing, ancient woodlands that were almost identical in appearance to the forest perched on the face of the Niagara Escarpment. Other cliff-dwelling plants, including ferns and cryptoendolithic algae also proved to be alike on the two sides of the Atlantic. This correspondence was telling us that the same ecological forces must be at play in these widely separated but similarly configured locales.

Modern Synthesis

Realizing that certain unifying principles were at work, we attempted to assemble the many pieces of the puzzle we had collected over the years. A few scientists, including Peter Davis and Peter Wardle, had already taken a few halting steps toward such a synthesis, as did Paul Vogler and Max Oettli decades before them. We were now able to bring what had been a fuzzy picture into clear focus. The key came from probing the trees, which record the passage of time and thereby attest to the antiquity of the whole system. The discovery of old-growth forests on so many different cliffs made us realize the degree to which these places remain constant in a changing world.

Whether a continent is cooling in advance of outright glaciation or warming and drying after its glaciers retreat, cliffs can buffer the climatic shifts. Daily excursions in temperature are minimal because the bare rock absorbs solar energy during the daytime and releases it back at night. Seasonal cycles are modest as well, because the summer sun, even at midday, strikes the vertical face of the rock at a low angle, keeping radiation loads moderate. In fact, south-facing cliffs in the northern hemisphere and north-facing cliffs in the southern hemisphere receive their most direct sunlight in midwinter, when the organisms they harbor can benefit from the added warmth.

Vertical forests are also far less prone to the scorching heat of fire than are typical woodlands, in part because they are well watered but mostly because the density of vegetation is too low to allow a conflagration to propagate. These forests have also been largely immune to people’s meddling, whether from logging, farming or building suburban subdivisions. Indeed, we suspect that the only widespread disruption of these unique refuges has occurred in Japan, where the popularity of bonsai has led collectors to destroy much of their country’s ancient heritage of small, cliff-dwelling trees.

In recent years, many of the vertical old-growth forests in North America and Europe have become threatened, perhaps for the first time in history, by a more modern hobby: rock climbing. In some places we have studied, hundreds of climbing trails crisscross the rocky bluffs, each one bringing scores of mountaineers each weekend to hammer and claw at the rocky crevices and the fragile roots they hold. Although most of the rock climbers we have met cherish the natural world, few of these people know that the roots they are tugging on may be centuries old. Indeed, we too were unaware of the venerability of these trees until we had counted growth rings spanning the rise and fall of empires.

Now when we look at cliffs, we no longer view them as inhospitable places populated with scraggly vegetation. Rather, we see the last remnants of a landscape untouched by humans, refuges for displaced organisms in a disrupted natural landscape, habitats of overwhelming importance to the maintenance of biodiversity that are worth our every effort to protect. We hope that more people will share our vision of cliffs before it is too late to preserve the treasure they still hold.