Print Window Close Window Death and Decay For some residents of the boreal forest, trees aren’t much use until they’re dead, broken, rotting, or full of holes. While humans may speak of “overaged” stands – trees past their prime as desirable timber – nothing could be more appealing to a squirrel, a marten, or a woodpecker. In fact, in recent years ecologists have become increasingly aware of the myriad of ways in which healthy forest life is dependent on death. Some areas within the boreal are prone to periodic burning; others are more fire-resistant. In either case, however, the natural forest pattern blends the dead with the living. Snags – dead standing trees – may remain after a fire has passed, since only in the hottest section of the blaze are trunks as well as crowns obliterated. But dead trees are common in unburned forest, too. Large snags occur in old-growth areas, where trees naturally senesce. In younger forest, they may be a relic of natural thinning, when a tree is overshadowed by a sturdier, taller member of its own species. In boreal stands, quick-growing light-loving birch and aspen are typically overtaken by shade-tolerant white spruce as the stand ages, and an understory of dead birch may be the eventual result. Trees may also be killed by insects or disease. Obviously, there is a happy medium between too little insect life and too much; the bark beetle epidemic in Southcentral Alaska showed what can happen in a severe infestation. On the other hand, it would be a mistake to eliminate all insect outbreaks and fungal infections in trees, even if we could. Fungal heart-rot is an enemy to loggers, because it makes a seemingly valuable tree economically worthless – but it is the perfect way to form hollow trees. Such trees are crucial habitat for flying squirrels, martens, and several species of bird. Perhaps not surprisingly, many cavity-nesting species prey on the insect pests that kill trees in the first place, thus achieving a tenuous balance. Fallen trees and rotting logs are crucial to the very lowest rungs of the food chain. Insects, fungus, and even small mammals such as mice help act as decomposers; not only do they obtain their own sustenance from the forest floor, they also enrich its composition. Without a constant influx of organic debris, the humus would soon degrade, becoming compacted and nutrient-poor. Small mammals also rely on logs as shelter and protection from predators. Thus, deadwood plays a fundamental role in forest ecology when it stays where it falls. But even in a wilderness setting, it doesn’t always stay put – it plays another role as well. Studies of the importance of logging buffers along rivers and streams show that large woody debris that falls into waterways is important in several ways: it slows, diverts, or changes channel flow; it provides hiding spots and habitat for fish; and it enriches the water as it breaks down. Forests managed for timber often lack the degree and variety of decay found in wild forests. At the extreme end of the spectrum, some European forests were traditionally harvested and “cleaned” so assiduously – often for local firewood use – that not a twig was allowed to rot on the forest floor. Hundreds of years later, forest managers have realized how species-poor and nutrient-starved the forests have become, and are belatedly trying to rectify the matter -- without complete success. In the Alaska Interior, we still have the chance to do things right the first time. Cutting local birch at 70 years and spruce at 100, as preferred by the Alaska Division of Forestry, has the long-term potential to reduce habitat availability and diversity. Although current harvest levels are low enough to make the threat minimal, it is nevertheless a problem that should not be overlooked. Soils Most northern Alaskans know that they’d rather build a house among birch trees than on land that grows only black spruce, but many would be hard-pressed to explain the complex interrelationship between tree species, climate, and forest soils. It is easy to observe that black spruce grows where it is boggy, or cold, or both. Birch grows in sunny, drier spots. But why is there so much variation over quite small areas? How do moisture and sunlight, permafrost and decay rates factor into the equation? And what makes some spots “productive forest” while others are labeled “unproductive”? Soil is nothing more than a layered mixture of rock fragments (rocks, gravel, sand and silt) and organic matter in various stages of decay. For ideal growth, most trees prefer soil conditions with ample room for root growth, plenty of available nutrients, aeration, and continual access to adequate moisture. However, soils in the Interior almost always lack at least one of these conditions, and sometimes lack all of them. In the Fairbanks area, soils are dominated by permafrost, with as much as 75% of the land underlain by perpetually frozen soil. From the point of view of a tree, permafrost might as well be solid rock. Roots cannot penetrate it. Thus, in some locations, roots cannot extend more than a few inches below the surface. Obviously, this poses a structural limitation on tree growth. But permafrost also has the effect of causing very poor soil drainage. Summer rains and winter snowmelt can’t trickle deep into the ground and reach the water table, so water tends to pool on the surface, creating bogs and fens. An excess of water drives air out of the soil. Non-aerated soil, in turn, does not favor decomposition, since many forms of decay require oxygen. The result is that boggy ground tends to be nutrient-poor. Water-logging leads to a slowing of decay – but so too do cold temperatures. Long, icy Fairbanks winters don’t just freeze the soil; they also affect the speed of decomposition. Nothing rots quickly here, meaning that dead leaves and needles, branches, whole trees, and the other varied detritus of the forest floor take years to break down. Almost 80% of the soils are classified as “inceptisols,” a term that implies that they are new, not terribly well developed, and unlikely to be nutrient-rich. The average Fairbanks temperature is about minus 3 degrees Celsius, but there is enough site-by-site variation to ensure that some locations have average temperatures above freezing -- south-facing slopes that get plenty of sun, or high spots that experience significant inversions in the winter, for example. River basins also experience a certain immunity from permafrost, at least along the flood plains of glacial waterways, because these rivers constantly erode existing banks and deposit the material elsewhere. The soils that form under such conditions are gravelly, sandy, and silty. There is little topsoil at first, but a succession of herbs, willow, and hardwoods such as balsam poplar gradually create conditions favorable to the largest and longest-lived local trees, white spruce. Once soils are tenable by white spruce, the spruce out-compete aspen, poplars, and birch because they need less light than these deciduous species, and can create a dense shade in which the other species cannot thrive. Oddly enough, this dense shade can, in some cases, cool the ground enough to change soil conditions. Old white spruce stands may start to develop permafrost soils in locations where there was no permafrost previously. In some cases, this can lead to the eventual succession of black spruce. From the point of view of timber harvest, black spruce is unproductive. The average white spruce stand has 3 to 4 times the biomass (total weight of living material) of a black spruce stand. The average aspen stand grows (gains biomass) five times faster than black spruce. But from an ecological perspective, black spruce is the ultimate survivor. It can grow with a shallow root system above permafrost; it can survive on thin soils near treeline, under conditions of extreme cold; it can live in nutrient-poor soil; it can be found in all but the wettest of bogs. In an environment where so much of the soil is thin, wet, frozen, or otherwise unfavorable for tree growth, it is the little black spruce that holds together the continuous expanse of our forests. This article incorporates some information from articles written by Glenn Juday, John Zasada, Leslie Viereck, CT Dyreness, Keith Van Cleve, and Joan Foote |