The Vertical Forest

Canadian researchers have found a bizarre, ancient forest that may be the slowest-growing on Earth. They didn't have to travel to Yakutsk or Tasmania to find it--they had only to look at a cliff not far from Toronto.

By Mark Wheeler|Thursday, February 01, 1996
To get to work in the summertime, Douglas Larson, an affable 46- year-old botanist from the University of Guelph in Ontario, hikes to the top of a steep cliff, straps on a harness, and leaps into the abyss.

The cliff he jumps from is part of the Niagara Escarpment, a meandering wall of limestone and dolomite that begins near Niagara Falls, then wends its way north, passing within miles of the Guelph campus on its way to the Bruce Peninsula, which juts into Lake Huron; from there the escarpment hooks around Lake Michigan and turns south for 496 miles before petering out 125 miles north of Chicago. Formed 450 million years ago, the escarpment was once the edge of an ancient sea that sat roughly where the Great Lakes sit today, making it of obvious interest to a geologist or perhaps a paleontologist. The reason it is of interest to a botanist, however, is that on its ancient, harsh, inhospitable face is the most extensive and undisturbed old-growth forest east of the Rockies.

Larson and his colleagues have found that the forest is an ecosystem unlike anything growing on the horizontal parts of Earth. It is dominated by the eastern white cedar, a tree that lives only 90 years or so on the ground but, as Larson has discovered, has reached 1,600 years on his cliffs. Stunted and twisted, often growing upside down, the trees of the escarpment are some of the slowest-growing plants in the world. The researchers have also found that among the other strange residents of the cliffs are organisms that live inside the rock--organisms normally found in places like frigid Antarctic plains or infernal Middle Eastern deserts. To top it off, it seems that this forest, sitting smack-dab in the middle of an industrialized area of 7 million people, may provide a record of climate patterns dating back more than 2,700 years, potentially offering desperately needed answers to questions of global warming.

Small wonder, then, that Larson, along with dendrochronologist Pete Kelly (My tree-ring guy, as Larson fondly describes him) and the varying gaggle of graduate students that make up Larson’s Cliff Ecology Research Group, is willing to spend most of his summers garbed in helmet and harness. Larson, in fact, considers himself one fortunate fellow. Before his cliff work he had spent a decade studying lichens, but in the mid-eighties money for that line of research began drying up. Larson’s rescue came in the fall of 1985, when an obstreperous graduate student named Steven Spring came into his life, looking for a research project in ecology to use for a master’s thesis.

Spring was a recreational rock climber, and he frequently clawed his way up the Niagara Escarpment, the only good-size cliff in the otherwise flat landscape of southern Ontario. He had seen the trees scattered on the cliff face, and he was curious about them. He also politely, but flatly, refused to study lichens. I believe he said something along the lines of ‘You’d have to be nuts to study lichens,’ says Larson. All the same, Larson was reluctant to let Spring have his way. If you’re an expert in lichens, says Larson, there’s not much motivation for you to let someone go off by himself to study something out of your expertise. It’s just easier to keep everyone on the same page.

Around the time that Spring was making his preferences known, though, a new postdoc named Ute Matthes-Sears joined Larson’s lab. She was interested in how organisms survive in marginal places, such as Antarctica, that offer few nutrients. It doesn’t take a botanist to know that a cliff is a marginal environment, and when another graduate student said she too was interested in marginal life, their collective mass of opinion persuaded Larson to form a cliff research group.

The team looked at a number of sites around northern Ontario, but in the end they settled on the escarpment, despite its intimidating 200- foot-high cliffs. We chose the escarpment because it was close to home, says Larson. You want to keep your students nearby; that way, if they screw up the research, it’s not expensive to send them back out to the field.

Over the next three years they began to study the community of plants that made the cliffs their home, measuring their mortality and productivity. But it wasn’t until 1988 that they decided to check the age of the white cedars that dominated the cliffs. With the assistance of Ceddy Nash, an undergraduate, Larson started taking measurements near the town of Milton, just southeast of Guelph and 60 miles west of Toronto. Attached to ropes, the researchers leaned over the side of the cliff to reach cedars growing just below the precipice. If a tree was dead, they simply sawed off a slice--Larson calls such a sample a cookie. If a tree was alive, they bored out of it a pencil-size rod.

With these samples they could measure and count the trees’ growth rings. Each year, a tree adds a new ring to its trunk; the width shows how much wood the tree added to its trunk that year, while the total number of rings shows the tree’s age. On the third day out, Larson cut a cookie from a twisted little tree measuring only six feet tall and seven inches thick. To his perplexity, there weren’t any tree rings to be seen.

I took it back to the lab and sanded it down with some fine-grit sandpaper, and I could just make out what looked like rings, says Larson. So then I sanded it with some finer stuff we use for polishing. He put the sample under a microscope, and lo and behold, there were all these tightly packed rings. Larson began counting. Because he wasn’t a dendrochronologist, he counted very carefully. Then he counted again, and again. Each time, the count came up the same: this little tree had lived to the ripe old age of 350 years.

It was an age Larson had been completely unprepared for. Not only were white cedars thought to live less than a century, but almost all the old-growth forests of the Northeast were thought to have been leveled long ago by farmers and loggers. In Ontario the forests that exist today are at least second- or third-generation growth, and no more than 60, maybe 70, years old. So a botanist discovering a white cedar in the middle of urbanized Ontario that sprouted in the 1640s was, in Larson’s words, like a journalist getting on a bus and finding Elvis sitting in the next seat.

You have to understand: the conventional wisdom, including my own conventional wisdom, was that no old-growth forest remained in this area. So it was simply unimaginable that you could have that many rings in a tree, and from a site where you could see the skyline of downtown Toronto. When it dawned on me what we had found, I just about--well, let’s just say I was literally jumping up and down.

Several weeks and more than 400 samples later, Larson and Nash had cataloged many ancient trees, ranging up to 700 years old. After the university issued a press release on Larson’s discovery, a fair amount of hoopla ensued. The media came calling and, according to Larson, other researchers began carping. People were asking, ‘What the hell does Larson think he’s doing? He’s a lichenologist--he wouldn’t know a tree if he walked into one.’

Larson realized he needed to do two things: he had to shore up his credibility, and he had to see if he had discovered a lone stand of presettlement trees or an entire forest. He did both by bringing in Kelly, the dendrochronologist. At nine sites along the cliffs, Larson and Kelly hung old fire hoses painted with three-foot-long white and black stripes from the tops of the cliffs to the rocky debris at the base, known as the talus. They counted the cedars along the lengths of the markers and took samples for dating. That in itself was no easy task. While they could sit on the larger trees, the smaller ones required them to brace their feet against the cliff face or, if they were lucky, on a ledge where they could wedge their boots. But even here rope and caution were called for--the narrow ledges can serve as platforms for cranky bobcats and dozing snakes.

Combining their results with earlier work by Spring, they realized that across the entire length of the escarpment there existed a previously unknown, complete ancient forest ecosystem. The pattern of plants (such as elderberry shrubs, lichens, and ferns) growing on the tops, middles, and bottoms of the cliffs remained similar all along the escarpment’s length. Trees 300 to 800 years old were common, with the ages creeping up the farther north the team went. The oldest living tree they’ve found so far is 850 years old, located on the north shore of the Bruce Peninsula. But the living cedars, the researchers discovered, were actually pikers compared with the ancestors they found. The oldest dead tree was an astonishing 1,653 years old when it died. (Just as astonishing was the find, through radiocarbon dating, that the tree met its end some 900 years ago, around 1082. How dead cedars could last through the millennium in the wet Ontario climate remains a mystery.)

Kelly suspects there are living trees on the cliffs more than 850 years old, but he’s having a hard time proving it. Part of the problem in dating living trees, besides the obvious trouble with dangling from a rope while you’re doing it, is the difficulty in drilling all the way through to the pith, the tissue at the tree’s center. Ninety percent of the escarpment cedars grow asymmetrically--if you look at a tree trunk’s cross section, you see not a normal circular bull’s-eye pattern but instead an amoeba-like shape, with the pith on one side, far from the center, and the trunk spreading out in irregular lobes.

This odd growth pattern, Larson and Kelly discovered, is a consequence of the tree’s unusual plumbing. Unlike the root systems in most trees, those in eastern white cedars have what are known as sectored hydraulic pathways. Other trees are like upside-down funnels, with all the water and nutrients pouring in from the wide-ranging roots mixing and flowing together throughout the trunk. In eastern white cedars, however, different groups of roots are dedicated to specific sections of the trunk. The effect was clearly demonstrated when Larson’s team brought cedar seeds from the cliffs to their lab and grew them in pots; later they injected two colored dyes into two roots. The dyes both traveled the length of the tree but never commingled, making the tree look like a deviant barber pole with vertical stripes. The chairman of my department saw this tree right after the dye was injected, says Larson, and swore we had drawn the two colors on with felt markers. It showed up that distinctly.

For life on a cliffside, this arrangement offers some advantages. It’s a clever system, says Larson. If a rock gives way and detaches a root, only the portion of the trunk that was connected to that specific root will die. A typical tree on a cliff will lose roots ten times in its lifetime, as rocks fracture and fall away or loads of winter ice tear the root out of the rock. If a normal root system were damaged in this way, the whole tree would suffer the effects; but with its sectored pathways, an eastern white cedar can isolate the damage and survive.

Even with such evolutionary cleverness, though, it is still hard to fathom how such a forest, so different from a normal one, can exist. Growing above and below the escarpment in Larson’s neck of the woods is a typical deciduous forest, filled with a thick growth of ash, beech, cherry, and maple. The trees dig into a luscious thick organic soil covered by an accumulated detritus of leaves, twigs, branches, scat, and seeds. There’s a thick, leafy canopy above and a wide variety of animal life sprinkled among the rich flora on the forest floor below--birds, squirrels, chipmunks, frogs, snakes, mice, ants, spiders, centipedes, millipedes, earthworms, beetles--hundreds of animals, big and small, chewing and being chewed.

There’s no comparable vertical bed of detritus to feed the cedars. The trees, scattered in small stands across the cliff face, are subject to temperature fluctuations far more extreme than those confronting other forests. In summer, as the dolomite and limestone bake in the sun, they reach temperatures as high as 110 degrees. In winter the cliffs, lacking any of the protection provided by soil or even the insulation of a snow pack, plunge to -20 degrees--closer to the environment of the Arctic tundra than to the temperate climes in the surrounding forests. In fact, of the 25 or so herbaceous plants and ferns that grow on the cliffs, 5 also thrive in Arctic regions.

White cedars are not limited to life on a cliff, but they fare pretty poorly on horizontal terrain. They simply can’t compete against their more aggressive peers. When a forest recolonizes a cleared patch of land--say, a farmer’s abandoned field--the first trees to take over are those like crab apples and pin cherries, whose seeds are carried quickly to the new habitat by birds. Later other trees become dominant by virtue of their long-term strengths; sugar maples, for example, have a deep root system that can efficiently suck up a soil’s nutrients. Beeches cover the ground with their litter, keeping down seedlings of other trees while they send up new shoots from their roots.

Without these advantages, cedars are relegated to marginal environments like rocky soil or wet swamps--or even cliffs. Here the trees’ sectored pathways may give them an edge, and the other trees have certain weaknesses that may make the habitat inhospitable for them. All these species have little personality flaws, says Larson. Maples get too big and push themselves out of the rock, like a spring uncoiling out of your pocket. In contrast, the cedar can stay small, so it makes very few demands on its environment. The cedars haven’t specifically adapted to the cliff. Plant a cliffside cedar in a more normal habitat and it grows like a normal cedar.

In those normal habitats, researchers have found the average life span of an eastern white cedar to be 90 years, during which time the tree grows to an average height of 50 feet. Larson suspects, but hasn’t yet proved, that this age limit is architectural. These trees suffer from their own success; cedars are one of the weakest and lightest woods in North America. So it may be that they get big fast, then blow over. (There’s one consolation for a cedar in toppling like this; if the fall doesn’t kill this tree, its branches can penetrate the ground and grow new roots, which then send up new shoots.)

On the cliff, though, eastern white cedars must grow far slower. As some sectored pathways die off and others grow larger, the trees twist into convoluted shapes, and falling rocks, crushing ice, and the overwhelming effects of gravity stunt their further growth. Even the oldest trees usually grow no more than 10 feet tall and 1 foot in diameter. At 20 years old, they look like seedlings. One little tree born 155 years ago stands less than 4 inches high, having added an average of .11 gram of wood each year. That makes it the slowest-growing tree ever documented. Think bonsai without the pots.

The trees survive by snaking their roots into solution hollows- -small indentations that cup bits of soil and moisture. Recently Ute Matthes-Sears tried to determine whether the fluctuating and unreliable availability of water and nutrients in those hollows was at least partially responsible for the cedars’ slow growth. She set up shop on a 50-foot-tall cliff wall in a former prison rock quarry next door to the Guelph campus. Growing from the quarry’s cliff was a stand of cedars that had sprouted up 40 years earlier when the last felon swung his sledgehammer and the quarry closed down. Matthes-Sears hung up hospital-issue IV drip bags on the cliff face and ran the tubes into the crevices where roots had wedged themselves. She loaded up the cracks with water and nutrients and let the trees take their fill. Surprisingly, the abundance of food had no significant effect. The trees simply didn’t need it.

We’re starting to think of the cliffs as vertical swamps, Larson explains. They always seem to have enough water. We even tested the trees during a drought using an instrument that measures humidity inside a tree’s trunk, and they never showed water stress.

In part, the cedars’ tenacity may be explained by the help they receive from other members of the cliff ecosystem. The researchers discovered that the roots of the trees are infiltrated by a symbiotic web of fungi that can collect phosphorus and nitrogen from the rocks and pump them into the tree. They also found that there are organisms living inside the rock, which appear as a dark green band just below the surface. This collection of algae and fungi has been found before only in such unforgiving environments as Antarctica and the deserts of the Middle East, where the organisms may well have evolved to live inside the rock as a defense mechanism against the harsh conditions outside. Some of the rock- bound algae in the escarpment can convert nitrogen into compounds that plants can assimilate. While the prevalence of the algae makes the researchers almost certain that the trees exploit their nitrogen, they have no idea how the trees get at it.

Exactly how the cliff forests defy all ecological odds, it seems, may take years to discover. But Larson and his co-workers are already beginning to reap a scientific harvest from the cliffs: a 2,700-year-long record of climate. Many factors can make a tree’s growth fluctuate: changes of rainfall, soil nutrients, temperatures. In the case of the cliff forests, as Matthes-Sears has shown, rises and falls in water and nutrients don’t affect growth very much, which leaves climate as the primary driving force. In the tree rings, we’re seeing that the cooler the temperature, the bigger the growth; warmer temperatures show slower growth, says Larson. In other parts of the world, researchers are studying tree rings to see how the climate varied before humans pumped greenhouse gases into the atmosphere, but most researchers assumed this was impossible in the Northeast, since all the old trees had been axed. Now, however, the cliff cedars are making just such an analysis possible.

Larson’s team has only preliminary results at this point, but they suggest that while global warming is now occurring, it’s not happening at a rate that exceeds previous warming periods that the Northeast has experienced in the past. Such a finding doesn’t contradict the possible impact of global warming, but it can help to ground it in the actual history of Earth’s climate. If nothing else, that alone may put Larson, Kelly, and the Lilliputian forest of the North on the map.
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