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Wednesday, Dec. 20, 2000
Glaciers prove ecological succession
By MARK BRAZIL
That powerful forces have shaped the world we live in is somehow easier to grasp when one lives in a country wracked by earthquakes, dotted with calderas and pocked with active volcanoes.
A look at an atlas on a geological time-scale reveals what our fragile Earth appeared like as we wind back the clock not through millennia, but through hundreds of millions of years.
In such maps the continents appear in unfamiliar clusters in parts of the globe far removed from where they are now. Even a modern atlas shows features that reveal forces past and present; I am thinking in particular of the ever-rising mountain range that epitomizes Asia -- the Himalayas.
Tectonic plates shift, mountain ranges rise and fall, but the pace of change has been so slow that species, in fact whole communities of species, have been able to steadily adapt to changing conditions, evolving in situ in some of the new habitats formed by these powerful forces.
Other forces at work in the world also appear slow, merely because our own life-spans, measured only in decades, are so infinitesimally short in comparison. In the human time frame any event that takes more than a 100 years slips "beyond living memory" and so the consequences of any process that continues over thousands of years is generally beyond the immediate grasp of anyone other than a specialist.
The arrival of winter's ice and snow has set me thinking of the slow-motion behavior of glaciers.
While the seasons tick-tock past, swinging from one extreme to another like a rapidly flicking metronome, the rate of advance and retreat of glaciers occurs at a far slower pace (though not in comparison with the glissading of continents).
If only we had time-lapse photography over several hundreds of thousands of years so that we could watch the glaciers at work!
Imagine watching the flickering accumulation of mountains of ice, of continental and polar ice-caps expanding, spreading, then flowing downhill. Imagine seeing them scouring, gouging and bull-dozing their way through entire mountain ranges.
It would be like watching a river in flood, only to see its movement slow, reduced first to a trickle then to nothing, then reverse so that the river retreated before one's eyes, leaving behind an altered landscape.
The accumulated debris carried by glaciers is dropped in heaps and piles; scattered melt-water lakes form in the depressions left by the great ice rivers. Then, after a short pause as the glaciers reach their smallest size, the whole process is repeated. Time and time again it has happened, like the tidal motion of waves rising and falling across a beach.
Alas, with no such time-lapse film available, we are limited to sifting historical records and relying on the writings of glacial scientists to understand how fascinating glaciers are and to get a feel for their powerful influence on life.
The power of a glacier is hard to imagine, but having been fortunate enough to camp within earshot of glaciers in both Iceland and Argentina, to walk to them in New Zealand and on them in Canada and Austria, to sail to them in Alaska and to fly over them in Antarctica, I can assure you that they are astounding in their understated immensity and strength.
At close range, especially in the still of the night, the sounds they produce are incredible: groaning, creaking, crashing and shrieking as the ice itself bulges, deforms, cracks and shifts and as the rubble of past destruction is scraped over the underlying rocks like sandpaper.
I have also viewed the glacier in Los Glaciares National Park, Argentina. The great wall of ice at the nose was both pure white and deeply blue, and the blueness within the glacier was of a startlingly clear, pale intensity that I have only witnessed elsewhere in the occasional special blue sea ice that drifts ashore along the Okhotsk Sea coast.
The face was carved into towers and turrets of ice, and at the foot of the ice wall was a lake of glacially cold blue water filled with suspended silt -- the glacial flour that colors entire rivers and in places flows out to sea in a visible plume.
While I watched, an enormous Andean condor soared past on outstretched wings. A glider in its element, it passed within a few meters of me, but that moment was lost in the excitement of seeing a great tower of ice stretching the full height of the glacial face fall away and crash into the lake below, sending up a huge gout of spray: a glacier in action!
One can imagine their action by watching for their signs -- smoothly scoured planes of bedrock, deeply grooved by boulders carried beneath a glacier, and heaps of boulders and smaller rock forming a loose rampart at the point of a glacier's furthest advance.
Smaller lines and ridges of rubble indicate where a melting glacier dumped loads of rocks that it had been carrying from the mountains, as it shrank back toward the peaks.
A glacier's life comes from the steady accumulation of snow at higher altitudes. The weight of that snow forms ice which pushes downward and outward, finding the easiest path and ultimately flowing lower as far as the pressure behind can force it.
A typical mountain glacier resembles a hand; the palm is the main part of the glacier, where accumulation is taking place, and the fingers the rivers of ice flowing down various valleys and away from the mountains.
In simple terms, when the rate of melting at the tips of the "fingers" is lower than the rate of accumulation, then the glacier grows and extends. Conversely, when the rate of melting is faster than the rate of accumulation, then the tips retreat.
Apart from their immediate appeal because of their power, glaciers provide a fantastic opportunity to grasp the processes of ecological succession. Over the last 200 years or so, most of the glaciers in the Northern Hemisphere have been retreating, leaving behind bare areas of rock that they have scraped clean, along with heaps of rubble and soil, which are known as moraines.
The succession of vegetation on the moraines reveals fascinating patterns. As the ice retreats, the newly exposed areas are first colonized by mosses. Once they have become established, the mosses serve to trap moisture and contribute their own organic matter to the developing soil.
This new soil can then be colonized by pioneering flowering plants such as fireweed. In turn the fireweed contributes its own organic material to the soil, improving it so that other flowering plants and low, mat-forming trees such as willows may get a toe-hold.
During the early stages, from its first exposure through the first phases of colonization, the soil is chemically basic, with a pH of around 8.0-8.4, and that is enough to prevent most plant species from being able to live there. However, once the creeping willows have become established, taller Alder trees are then able to invade quickly, and they spread rapidly, forming dense thickets.
The successional change from bare soil to alder thicket takes around 50 years, but once alders are established, then serious change is afoot.
Alders are very special pioneer trees that lead to a significant shift in the soil environment. Alders are able to fix atmospheric nitrogen, so their presence leads to a rapid increase in soil nitrogen.
Furthermore, their leaves are acid, so as they fall and decompose, they contribute acidity to the soil, shifting the balance from a pH of about 8.0 to 5.0 over three to five decades. The alders' mere presence thus brings about changes in soil nitrogen and acidity that make the old moraine habitable for many more plant species.
Of course as plant species accumulate, so too do invertebrates and vertebrates. Biodiversity becomes steadily richer, with mammals and birds soon able to colonize too.
Once the alder thickets are well established, then spruce trees can invade, taking as much as 120 years to grow into a dense forest.
The towering stature of those trees provides protection for other species to grow, and larger deciduous trees are the final phase. They invade, compete with the spruce and after perhaps another 80 years or so a relatively mixed forest of conifers and deciduous broad-leaved trees now covers the old moraine.
The whole process from bare soil to climax forest takes perhaps 200-250 years. That is too long for any one person to study, but the process can be pieced together by studying moraines of different ages from historically recorded glaciers.
It is astonishing to see these towering forests on soil that has formed entirely since the last passing that way of a wall of ice. It is even more amazing to think that at some distant point in the future the river of ice may return, sweeping away forest and soil and forcing the process to begin all over again.