In his book, "The Dying of the Trees," Charles Little presents details of a pandemic of sick and dying trees all over North America. This book focuses on a topic often ignored in forestry: Death. Death is usually not addressed in literature or research, other than as a limit or a temporary medical shortcoming. In fact, a forest is as much dead as it is alive; there is a rhythm of death and replacement from the cellular to the ecosystem level.
Trees in a forest are always dying, either individually or in groups, waves, cohorts, or systems. Forests also may die, if enough of the trees die, as a result of catastrophic change or of too rapid or too much change. Mortality is a normal part of the life cycle. Mortality in forests usually occurs from a combination of factors. Lightning and wind cause tree death and injury. Injury and disease cause many tree deaths. The causes, rates, and patterns of death in tree species are poorly known, according to Jerry Franklin, despite a hundred years of forest research. That presents a problem: If we do not know the normal rates of mortality in a forest, how will we recognize abnormal ones?
Part of the life cycle of a tree is death. The dead trees keep contributing to the life of the forest, standing for a while (1 to 150 years), then falling and decaying (over 20-200 years). Ecological forestry accepts a typical percentage of death as the normal condition, necessary for the renewal of the forest. The rate of death per year in a mature forest is remarkably constant at about 1-2 percent, even with wind storms, fires, disease outbreaks, and animal damage. Rotting and burning are an integral part of the cycle of life and death in the forest. Tree mortality from pathogens occurs on various scales: gap phases (small scale), forest development (large scale), and landscape patterns (immense scale). Yet, even catastrophic disturbances like hurricanes rarely damage more than 5 percent of a forest. More than being agents of mortality, insects, diseases, and animals are native components of complex food webs in ecosystems that contribute to the selection of certain kinds (including healthy) and ages of trees (that determine the composition of the forest, which changes over time). Mammals and birds disseminate seeds. Insects pollinate some trees and overwhelm others (rarely more than 1 percent of a forest). Diseases remove stressed trees (also probably a low percentage on the order of 1 percent). Pathogens are one of the determinants of growth and development. Their effect on the long-term health of a forest can only be regarded as positive.
In compiling anecdotal evidence, Little shows that trees are dying throughout every continent, in greater than normal percentages and for a variety of reasons: Acid rain in New England, New York, North Carolina, Tennessee, Georgia, Ohio, Indiana and Kentucky; smog in California; excessive ultraviolet light (through a damaged ozone layer) in Arizona and New Mexico; rising temperatures and sea levels in Alaska and Florida; destructive forestry practices, such as clearcutting, in Colorado, Oregon and Washington; pesticides or toxic heavy metals (released by burning coal and oil) in many other places, or combinations of all these factors.
Little cites studies by Hubert Vogelmann, a botanist at the University of Vermont, who wanted to study an undisturbed forest; in 1965 he made a thorough survey of Camel¹s Hump in Vermont¹s Green Mountains. He thought he was describing a healthy ecosystem; he measured the types and sizes of the trees, as well as various other aspects of the ecosystem. Periodically, he resurveyed Camel¹s Hump, and a pattern began to emerge: The trees were dying. His survey in 1979, compared to the baseline study of 1965, showed a 48% loss of red spruce; a 73% loss of mountain maple; a 49% loss of striped maple, and a 35% loss of sugar maple. Vogelmann was able to show that the health of Camel¹s Hump had begun to decline in the period 1950-1960. Similar studies in the Black Forest of Germany, and in southern Canada, revealed that the most likely cause was acid rain. (Acid rain was not ³discovered² until 1972, by Eugene Likens and F. Herbert Bormann, although it had been falling on New York, New England, and southern Canada for about 20 years, as a result of the massive rise in use of fossil fuels, coal and oil.)
Vogelmann was able to show how acid rain affects the soil and thus the entire ecosystem, including trees. Soil contains large amounts of aluminum, in the form of aluminum silicates, which is not available to the roots of plants in that form. But acid rain dissolves the silicates, releasing the aluminum and making it available. When trees get aluminum into their roots and their vascular system, the roots clog, preventing them from taking up adequate nutrients and water. The trees are weakened, and may then be susceptible to extreme cold, insects or pathogens. Acid rain not only releases aluminum, it also releases other minerals ‹calcium, magnesium, phosphorus‹required by trees; the minerals are washed out of the soil, leaving it depleted of nutrients.
Furthermore, acid rain kills mycorrhizal fungi, thus further reducing the ability of trees to absorb water and nutrients. The tree roots provide sustenance to the mycorrhizal, and the mycorrhizal helps the tree roots gather water and nutrients from the soil. Acid rain also kills off portions of the detritus food chain. The detritus food chain is composed of microscopic creatures that compost leaves, twigs, and pine needles, turning them back into soil. Because the detritus food chain is damaged by acid rain, forest litter builds up on the floor of the forest. In deep litter, seedlings cannot take root in the soil. Furthermore, the litter promotes the growth of ferns, which give off substances that inhibit the growth of red spruce saplings.
Throughout the book, Little describes studies and statements by the US Forest Service that downplay the importance of tree disease and death. For example, in 1991 the Procter Maple Research Center at the University of Vermont pinpointed acid rain and other air pollution as an important cause of decline of sugar maples in Vermont. The following year the US Forest Service issued a report saying that 90% of the sugar maples surveyed were healthy and the overall numbers and volume of sugar maples were increasing (but they had counted only standing dead trees, not those lying on the ground).
Although Little cites many instances of damage to trees by pollution, other scientists, such as John Innes in his book "Forest Health," states that pollution is not proven to be a major cause of tree death (while admitting many of the declines from ozone pollution that are also in Little¹s book); Innes suggests that evidence points to climatic stress and poor site matching. David Perry, in his book "Forest Ecosystems," argues that pollution is involved in the decline of many forests, as a contributing stress, especially in Germany, where 52% of the forests are classified as diseased (in 1985). Beginning in the 1940s, it became evident that the plantation system, with single species even-aged trees, was susceptible to catastrophic change‹wind, pollution, insects‹in a way that natural forests were not. The forests began to die as forests‹the Germans called this disturbing phenomenon Waldsterben (forest death). Even into the 1980s, scientists were trying to find the causes of forest death to preserve the plantation system.
One central question of forest decline or death is whether or not human influences have accelerated natural mortality rates or caused new mortality. Several scientific studies have claimed that the evidence for decline due to human causes is insufficient. The occurrences of forest decline are well-documented from prehistoric to present times. Over 5000 years ago, elms declined in NW Europe, perhaps from disease or forest-clearing. From the 1930s to the 1950s, birches in eastern Canada and northeastern U.S. declined; no single cause has been identified, but stresses from severe weather are suspected. Starting in the late 1960s, there was a decline in Ohia trees in Hawaii, possibly from cohort senescence (especially in a short-lived species colonizing recently disturbed areas, where even-aged stands get stressed by conditions in a mature forest). In the 1970s and 80s in Germany, forest death in firs and spruce‹the primary factors in this ³forest death² seem to be air pollution, acidification, and toxic metals; it also occurs in Austria, Czechoslovakia, France, Italy, Switzerland, Scandinavia, Poland, and England. In the 1970s and 80s, declines of balsam fir in ³fir waves² in Newfoundland and New England in the US were probably caused by ice damage on the leading edge of the dieback. Symptoms vary between species, as they should. The causes seem to be combinations of biotic and abiotic factors. A major factor might be rates of forest clearance, destruction, and fragmentation, which have been accelerating in the last 100 years.
While scientists admit seeing declines in the vigor of mature forests, leading to stand-level mortality, they say they cannot make conclusive statements about ³causal factors.² They mention that forest declines occur in contaminated airsheds, but also where pollution is not important. There is overwhelming evidence of pollution-caused death in Europe and eastern United States. The fact that there are other causes that work independently or with pollution does not invalidate the other evidence. To argue so, as some scientists do, is based on a logical fallacy, the semantic fallacy of complexity.
The problem with scientific reasoning is that proof may take many decades and cost billions of dollars. Although more long-term studies are needed, it might be better to shift the burden of proof on the safety of industrial processes and products instead. The concluding paradox that Little identifies: Trees are abundant everywhere, but dying everywhere. People see trees everywhere and perceive that there is not a problem with trees dying. The grass is green everywhere too, but there are fewer species, fewer natives, younger plants, and lower quality material. Perceptions need to be changed. Little offers details, although some reviewers have criticized him for not offering enough scientific evidence. Even if Little has not supported his argument adequately, even if he may be wrong is some instances, the preponderance of evidence is not easily dismissed. Regardless of the exact causes or interactions of causes, trees and forests are dying.
His overall suggestions, such as controlling human population growth or stopping cutting forests, are good common sense suggestions. The greatest threats facing forests are not just disease organisms or pollution, but the synergistic effects of fragmentation, pollution, and climate change. And these are best addressed all at once. Little has described conditions that we need to know about, that we need to correct or address. Maybe forest death has not been conclusively demonstrated, but there is sufficient reason to take corrective action now.
ed.