Fire, Wind, And The
Changing Forest
Walking the middle section of the Greenstone Trail,
one passes through dense young forests of birch and aspen. Here and
there a dead pine stub rises above the living trees, and charred stumps
twist upward among the white trunks. All these are the present signs of
the most destructive event on Isle Royale in this century, and of
nature's enormous powers of recuperation.
In the summer of 1936, Minnesota, Wisconsin, and
Michigan lay in the grip of a deep drought. On Isle Royale, slash from
logging operations in the Big Siskiwit River area covered the ground. On
July 25, man or lightning started a fire near the logging camp at the
head of Siskiwit Bay. The fire eventually burned a large area west of
the camp, jumped northward and consumed forests all the way from Lake
Desor to Moskey Basin. Not until mid-August, when a shift in wind and
heavy rains helped the 1800 firefighters, was the fire put out. It left
27,000 acres a jumble of charred logs.
But the dense green forest now covering these slopes
shows that nature can cope with fire. Fire, in fact, is a part of
nature, an element to which plant and animal species have become
adapted. Other forces, too, attack the forests, and form a part of the
total pattern of life. Wind blows trees down; insects devour leaves and
tunnel through trunks; disease enters; animals browse saplings or cut
trees. All these agents keep the forests in slow turmoil, in continual
cycles and sub-cycles of destruction and re-creation.
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The 1936 burn, seen from Greenstone Ridge, is undergoing a slow process
of recovery.
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Indeed, more than half of the forest on Isle Royale
is in some stage of recovery from fire and other destructive
forcesnot yet, that is, in a mature stage in which certain tree
species have attained long-term dominance. The process of succession
toward that state is continuous; but disturbances, particularly the
strong winds, work against the attainment of a mature, stable forest
over the whole island. Instead, there is a patchwork of forest types,
generally following the linear pattern of ridges and valleys but made
more random by the chance effects of destructive forces.
On most of Isle Royalethat part most affected
by the layer of cool, moist air over Lake Superiorthe trend is
toward a forest of white spruce and balsam fir. Sizable patches
approaching this type, though containing much birch and aspen, occur
near the shorelines. Forests of old aspen and paper birch in which
spruce and fir have not yet become dominant occupy a larger area. On the
central ridges in the southwestern part of the island, where soils are
fairly deep and the climate is somewhat warmer, mature forests of sugar
maple and yellow birch have stood for a long time, apparently little
affected by fire. This type and spruce-fir represent the two "climax"
forest types on Isle Royale. Most of the 1936 burn area should progress
toward these two kinds of forest. Other types on the island are swamp
forests of black spruce, white cedar, and fir; and small areas of jack
pine on some rocky, south-facing slopes and ridgetops. These forests may
eventually join the dominant white spruce-fir forest, as swamp soils dry
and spruce grows up in the shade of the pines.
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These spruces and firs will eventually replace the birches under which
they started. (Photo by R. Janke)
Thimbleberry is a shade-tolerant plant of the forest floor. (Photo by Robt. G. Johnsson)
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When fire strikes hard, burning trees as well as
ground cover, it sets back succession to an early stage. Where fire is
particularly intense, it may consume all organic matter in the soil and,
together with the erosion that often follows, strip the surface down to
bare rock. But in most places some ash-covered soil is left. From this,
grasses, fireweed, pearly everlasting, and other herbaceous plants soon
sprout, speckling the blackened earth with green. These are followed by
woody plants: thickets of hazelnut, seedlings of juneberry, pin cherry,
and choke cherry. Paper birches, aspens, and mountain ashes, killed
aboveground, resprout from stem and roots below ground, sending up
several shoots in place of each single stem burned. In a few years an
open grassy area, dotted with shrubs and small trees, has developed.
Grasshoppers, flies, bees, chipping sparrows and song sparrows find
such places to their liking.
As shrub clumps and tree sprouts grow over the
fire-felled logs, making a dense tangle of greenery,
snowshoe hares take advantage of the combination of food and cover, and
red foxes take advantage of the snowshoe hares. Moose find a lot of browse in
such places, and wolves find moose.
If conditions are right, the trees grow rapidly.
Gradually the shrubs are shaded out, the logs rot, and lower tree branches
die, reducing the amount of food that hares can reach and eliminating many
of their hiding places. After 30 years or so, the trees may be too tall
to supply food even for moose.
At this stage, the forest is a monotonous, even-aged stand of close-packed
young birches and aspens, with a few small spruces and an occasional fir
underneath, the result of seeds blown or carried in from older forests.
Here and there a white pine shoots up, perhaps one day to tower above
all the other trees. The forest floor, now fairly well shaded, is likely
to be covered with thimbleberry, large-leaved aster, bracken fern, or
wild sarsaparilla, with perhaps a few bunchberries and bluebead-lilies
underneath them. As might be expected from the lack of variety in
habitats, animal life is rather scarce. One may see only an occasional
red squirrel or moose, and a few birds such s red-eyed vireos,
ovenbirds, and chickadees. But variety will grow with age.
This stage may be slow in developing, however,
because of heavy browsing by moose. Aspens and birches in some of the
1936 burn area are still quite suppressed. Through all stages of growth,
moose strongly affect the structure and composition of the forest.
On some burned areas, particularly dry slopes and
ridgetops, spruce and jack pine, rather than birch and aspen, are the
pioneer trees. In such places there may be little change in the tree
species present as the stand grows older.
In a typical stand of young
birches and aspens, competition among the trees for water, light, and
minerals gradually thins the stand as the survivors grow taller. Spruces
and firs, now germinating more abundantly, begin to create a dark layer
beneath the birches and aspens. The firs, however, will be under
continual pressure from moose browsing, and few will "escape" to become
part of the overstory.
A hundred years or more after the fire, the spruces,
along with some firs, may gain dominance over the deciduous trees.
Gradually the aspens reach the end of their life span. Where the
conifers stand close together the aspens leave few offspring, for here
the forest floor is too shady for survival of the seedlings. Paper
birches, usually more numerous, survive longer as a forest component,
but their seedlings and root sprouts have the same problem. Given enough
time and lack of disturbance, most of the birch would disappear, too,
and a dark forest of spruce and fir would develop. For a number of
reasons (which were stated earlier and which we will examine later) this
seldom happens on Isle Royale.
These maturing forests, with their mixture of
conifers and broad-leaved trees, provide homes for much more wildlife
than do the dense young stands. Red squirrels, attracted by the seeds
and thick cover of conifers, become common. Warblers, thrushes,
red-breasted nuthatches, winter wrens, white-throated sparrows, and many
other birds occupy the variety of niches now available.
If the area is above the influence of the layer of
cool moist air that lies over Lake Superior, it may change from a young
paper birch-aspen stand into a sugar maple-yellow birch forest rather
than spruce-fir. In this case, a dense crop of sugar maple seedlings
develops under the birches and aspens, and the maples, with a sprinkling
of yellow birches, eventually take over. In some areas, red maples, red
oaks, and white pines may be important in the successional stages. In
the mature maple-birch forest, which shelters a few conifers in moist
places, squirrels and birds are also much more abundant than in the
earlier stage of the forest, though this type seems to have less variety
of wildlife than does spruce-fir forest.
The relative variety and abundance of life in the
later stages of forest succession is due partly to the several layers
within them. There is the canopy, high above the ground and struck with
sunlight. There is an understory, composed mostly of young of the canopy
trees. There are shade-loving shrubs and small plants near ground level.
But the variety is due also to destructive forces, which create openings
in the forest and thus provide additional habitats.
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Snowshoe hares abound on old burns where shrubs and fallen trees make
thick cover. (Photo by R. Janke)
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Besides firewhich burns small areas much more
often than large oneswind, insects, larger animals, and disease contribute to
the constant change in forest structure. Wind, aided by thinness of
soils and trunk weakening diseases, probably takes the lives of more
trees than does any other agent. Over most of Isle Royale, the last
glacier was stingy with its deposits, and there has not been enough time
since then for deep soils to develop. The shallow root systems of spruce
and fir, anchored in thin, often wet soil over rocks, provide poor
resistance to strong winds. Particularly during fall storms or high
winds in winter, when trees may be laden with ice or snow, trees fall by
the hundreds. Balsam fir is frequently afflicted with heart rot and
this adds to its susceptibility, often causing it to snap off in high
wind. Blowdowns are thus a common sight on Isle Royale, creating
barriers for the hiker, cover and food for animals, and many openings in
the forest.
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Feeding by the aspen leaf roller (caterpillar of the
large aspen tortrix moth) caused heavy damage to aspen forests in the
early 1970's. (Photo by R. Janke)
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Insect outbreaks periodically add their effects to
those of other agents of destruction. In the early part of the 20th
century many tamaracks were killed by larch sawfly larvae, which eat the
needles, and tamaracks are still scarce on the island. In the 1930's the
spruce budworm destroyed many firs (which it prefers over spruce), in
the same manner, but it is apparently doing little damage today. The
chief insect scourge of the early 1970's was the large aspen tortrix (a
close relative of the spruce budworm), which eats the leaves of aspen
before making its cocoon inside a rolled-up leaf. Many acres of aspens,
particularly on the southwest end of the island, were defoliated; but
most of the trees leafed out again. On a small scale every year, and on
a large scale some years, insects contribute to the constant forest
turnover.
Less dramatic but perhaps more important in the total
effect on trees are diseases. Heart rot, mentioned earlier, is caused by
fungi that enter the heartwood of trees. White pine blister rust,
another fungus, kills white pines by destroying needles and girdling
stems. Hundreds of other fungi, bacteria, and viruses attack trees,
continually challenging their fitness to live and to dominate the area
they shade.
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In this view from Minong Ridge near Linklater Lake, spruces and firs
form dark patches amid the lighter aspens and birchesa typical
pattern on Isle Royale. (Photo by Robt. G. Johnsson)
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The role played by vertebrates in forest change is
especially evident on Isle Royale. Moose and beaver drastically affect
the structure and composition of the forest, while other mammals, though
by comparison unimportant agents, nevertheless have measurable
effects.
In winter, moose feed extensively on the foliage and
twigs of fir. The island's high population of moose has left browse
lines on most stands of fir, denuding many of the branches from the
snowline to about eight feet above the ground. They often break the tops
of small firs to reach the foliage there. This heavy browsing of fir has
the net result of decreasing the fir component of the forest and
increasing the proportion of spruce, which moose do not eat. Moose are
also especially fond of aspen, mountain-ash, paper birch, pin cherry,
mountain maple, and red maple; in some areas they keep saplings of
these species perennially browsed back to shrub height. Earlier in this
century, moose virtually removed the American yew, an evergreen shrub
that once formed dense thickets on the island. Only on certain offshore
islands, such as Raspberry and Passage, where moose seldom or never
go, does yew continue to flourish. Elsewhere, since moose eat the more
conspicuous individuals, only small specimens of yew can be found. In a
later chapter we will further examine the moose-vegetation relationship
on Isle Royale.
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Two moose forage in the shallows of Washington
Creek. (Photo by Robt. G. Johnsson)
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Beavers, of course, dramatically change the forest in
the vicinity of their ponds, which on some parts of Isle Royale form a
large part of the landscape. Trees are killed in the flooded area, and
many aspens and birches within 150 feet or more of the pond are felled.
The beavers thus maintain open areas in the forest, and to some extent
encourage the growth of spruce and fir through the removal of aspen and
birch. Intense suckeringsprouting of shoots from the base of a
trunkoccurs when beavers cut aspens.
Snowshoe hares feed on many of the same plants that
moose eat; but their comparative effect on vegetation is small. As with
moose, they have the greatest impact on the forest in winter, when only
woody stems, twigs, and bark are available to the hares. Since they can
reach about two feet above the surface of the snow, they browse up to a
height of four or five feet.
Insects, birds, and mammals also play a productive
role in the forest. Insects aid in seed production by pollinating
flowers. Birds and mammals disperse seeds, sometimes inadvertently, and
aid in germination by burying some and softening the coats of others in
their digestive tracts.
The forest, then, is an ever-changing mixture of
plants, acted upon and influenced by all the forces of the environment,
including the whole spectrum of animal life that dwells within it. While
hiking on Isle Royale, particularly on trails that cut across the
topographic grain, one sees a mosaic of forest types and stages: damp
shoreline forests of fir, spruce, and birch dripping with beard moss;
open, shrub-dotted stands of tall aspens and birches; dense, dark swamps
of black spruce and white-cedar; sun-baked grassy ridgetops, slowly
recovering from some long-ago fire. Each part of the forest is coming
from somewhere and going somewhere. None of it is standing still.
One purpose of national parks is to preserve places
where nature is allowed to operate as much as possible without the
interference of man. On Isle Royale, this laissez-faire policy
extends even to fire. Though human-caused fires are put out, lightning
fires are allowed to burn unless they threaten some campground or other
developed area. Most of these are very small and go out soon. (This does
not mean that fires are not carefully watched, however. Fire flights and
fire tower operators quickly spot and follow the course of all fires.)
Similarly, insect outbreaks are not checked. Nature eventually does this
herself, through the effects of weather, food shortage, and predation
by other insects and birds. For instance, the population explosion of
the large aspen tortrix in the early 1970's was inhibited, if not
checked, by an army of birds, including red crossbills, blackbirds, many
kinds of warblers, and even woodpeckers that awkwardly crept out on the
twigs and picked caterpillars off the leaves.
On Isle Royale, as in all wild places, the forces of
creation and destruction work in some ultimate balance. This equal
struggle of life against death creates many different patterns of
existence in the woods and waters of this lonely, harsh, beautiful
island.
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