Forest Decline
Forest decline
In recent decades there have been observations of widespread declines in vigor and dieback of mature forests in many parts of the world. In many cases, pollution may be a factor contributing to forest decline, for example in regions where air quality is poor because of acidic deposition or contamination with ozone , sulfur dioxide , nitrogen compounds, or metals. However, forest decline also occurs in some places where the air is not polluted, and in these cases it has been suggested that the phenomenon is natural.
Forest decline is characterized by a progressive, often rapid deterioration in the vigor of trees of one or several species , sometimes resulting in mass mortality (or dieback) within stands over a large area. Decline often selectively affects mature individuals, and is thought to be triggered by a particular stress or a combination of stressors, such as severe weather, nutrient deficiency, toxic substances in soil , and air pollution . According to this scenario, excessively stressed trees suffer a large decline in vigor. In this weakened condition, trees are relatively vulnerable to lethal attack by insects and microbial pathogens . Such secondary agents may not be so harmful to vigorous individuals, but they can cause the death of severely stressed trees.
The preceding is only a hypothetical etiology of forest dieback. It is important to realize that although the occurrence and characteristics of forest decline can be well documented, the primary environmental variable(s) that triggers the decline disease are not usually known. As a result, the etiology of the decline syndrome is often attributed to a vague but unsubstantiated combination of biotic and abiotic factors.
The symptoms of decline differ among tree species. Frequently observed effects include: (1) decreased productivity; (2) chlorosis, abnormal size or shape, and premature abscission of foliage; (3) a progressive death of branches that begins at the extremities and often causes a "stag-headed" appearance; (4) root dieback; (5) an increased frequency of secondary attack by fungal pathogens and defoliating or wood-boring insects; and (6) ultimately mortality, often as a stand-level dieback.
One of the best-known cases of an apparently natural forest decline, unrelated to human activities, is the widespread dieback of birches that occurred throughout the northeastern United States and eastern Canada from the 1930s to the 1950s. The most susceptible species were yellow (Betula alleghaniensis ) and paper birch (B. papyrifera ), which were affected over a vast area, often with extensive mortality. For example, in 1951 at the time of peak dieback in Maine, an estimated 67% of the birch trees had been killed. In spite of considerable research effort, a single primary cause has not been determined for birch dieback. It is known that a heavy mortality of fine roots usually preceded deterioration of the above-ground tree, but the environmental cause(s) of this effect are unknown, although deeply frozen soils caused by a sparse winter snow cover are suspected as being important. No biological agent was identified as a primary predisposing factor, although fungal pathogens and insects were observed to secondarily attack weakened trees and cause their death.
Another apparently natural forest decline is that of ohia (Metrosideros polymorpha ), an endemic species of tree usually occurring in monospecific stands, that dominates the native forest of Hawaiian Islands . There are anecdotal accounts of events of widespread mortality of ohia extending back at least a century, but the phenomenon is probably more ancient than this. The most recent widespread decline began in the late 1960s and resulted in about 200 mi2 (518 km2) of forest with symptoms of ohia decline in a 1982 survey of 308 mi2 (798 km2). In most declining stands only the canopy individuals were affected. Understory saplings and seedlings were not in decline, and in fact were released from competitive stresses by dieback of the overstory.
An hypothesis to explain the cause of ohia decline has been advanced by D. Mueller-Dombois and co-workers, who believe that the stand-level dieback is caused by the phenomenon of "cohort senescence." This is a stage of the life history of ohia characterized by a simultaneously decreasing vigor in many individuals, occurring in old-growth stands. The development of senescence in individuals is governed by genetic factors, but the timing of its onset can be influenced by environmental stresses. The decline-susceptible, over mature, life history stage follows a more vigorous, younger, mature stage in an even-aged stand of individuals of the same generation (i.e., a cohort) that had initially established following a severe disturbance. In Hawaii, lava flows, events of deposition of volcanic ash, and hurricanes are natural disturbances that initiate succession . Sites disturbed in this way are colonized by a cohort of ohia individuals, which produce an even-aged stand. If there is no intervening catastrophic disturbance, the stand matures, then becomes senescent and enters a decline and dieback phase. The original stand is then replaced by another ohia forest comprised of an advance regeneration of individuals released from the understory. Therefore, according to the cohort senescence theory, the ohia dieback should be considered to be a characteristic of the natural population dynamics of the species.
Other forest declines are occurring in areas where the air is contaminated by various potentially toxic chemicals , and these cases might be triggered by air pollution. In North America, prominent declines have occurred in ponderosa pine (Pinus ponderosa ), red spruce (Picea rubens ) and sugar maple (Acer saccharum ). In western Europe, Norway spruce (Picea abies ) and beech (Fagus sylvatica ) have been severely affected.
The primary cause of the decline of ponderosa pine in stands along the western slopes of the mountains of southern California is believed to be the toxic effects of ozone. Ponderosa pine is susceptible to the effects of this gas at the concentrations that are commonly encountered in the declining stands, and the symptomalogy of damage is fairly clear.
In the other cases of decline noted above that are putatively related to air pollution, the evidence so far is less convincing. The recent forest damage in Europe has been described as a "new" decline syndrome that may in some way be triggered by stresses associated with air pollution. Although the symptoms appear to be similar, the "new" decline is believed to be different from diebacks that are known to have occurred historically and are believed to have been natural. The modern decline syndrome was first noted in fir (Abies alba ) in Germany in the early 1970s. In the early 1980s a larger-scale decline was apparent in Norway spruce, the most commercially-important species of tree in the region, and in the mid 1980s decline became apparent in beech and oak (Quercus spp.).
Decline of this type has been observed in countries throughout Europe, extending at least to western Russia. The decline has been most intensively studied in Germany, which has many severely damaged stands, although a widespread dieback has not yet occurred. Decline symptoms are variable in the German stands, but in general: (1) mature stands older than about 60 years tend to be more severely affected; (2) dominant individuals are relatively vulnerable; and (3) individuals located at or near the edge of the stand are more-severely affected, suggesting that a shielding effect may protect trees in the interior. Interestingly, epiphytic lichens often flourish in badly damaged stands, probably because of a greater availability of light and other resources caused by the diminished cover of tree foliage. In some respects this is a paradoxical observation, since lichens are usually hypersensitive to air pollution, especially toxic gases.
From the information that is available, it appears that the "new" forest decline in Europe is triggered by a variable combination of environmental stresses. The weakened trees then decline rapidly, and may die as a result of attack by secondary agents such as fungal disease or insect attack. Suggestions of the primary inducing factor include gaseous air pollutants, acidification , toxic metals in soil, nutrient imbalance, and a natural climatic effect, in particular drought . However, there is not yet a consensus as to which of these interacting factors is the primary trigger that induces forest decline in Europe, and it is possible that no single stress will prove to be the primary cause. In fact, there may be several "different" declines occurring simultaneously in different areas.
The declines of red spruce and sugar maple in eastern North America involve species that are long-lived and shade-tolerant, but shallow-rooted and susceptible to drought. The modern epidemic of decline in sugar maple began in the late 1970s and early 1980s, and has been most prominent in Quebec, Ontario, New York, and parts of New England. During the late 1980s and early 1990s, the decline appeared to reverse, and most stands became more healthy. The symptoms are similar to those described for an earlier dieback, and include abnormal coloration, size, shape, and premature abscission of foliage, death of branches from the top of the tree downward, reduced productivity, and death of trees. There is a frequent association with the pathogenic fungus Armillaria mellea, but this is believed to be a secondary agent that only attacks weakened trees. Many declining stands had recently been severely defoliated by the forest tent caterpillar (Malacosoma disstria ), and many stands were tapped each spring for sap to produce maple sugar. Because the declining maple stands are located in a region subject to a high rate of atmospheric deposition of acidifying substances, this has been suggested as a possible predisposing factor, along with soil acidification and mobilization of available aluminum . Natural causes associated with climate , especially drought, have also been suggested. However, little is known about the modern sugar maple decline, apart from the fact that it occurred extensively; no conclusive statements can yet be made about its causal factor(s).
The stand-level dieback of red spruce has been most frequent in high-elevation sites of the northeastern United States, especially in upstate New York, New England, and the mid- and southern-Appalachian states. These sites are variously subject to acidic precipitation (mean annual pH about 4.0–4.1), to very acidic fog water (pH as low as 3.2–3.5), to large depositions of sulfur and nitrogen from the atmosphere , and to stresses from metal toxicity in acidic soil.
Declines of red spruce are anecdotally known from the 1870s and 1880s in the same general area where the modern decline is occurring. Up to one-half of the mature red spruce in the Adirondacks of New York was lost during that early episode of dieback, and there was also extensive damage in New England. As with the European forest decline, the "old" and "new" episodes appear to have similar symptoms, and it is possible that both occurrences are examples of the same kind of disease.
The hypotheses suggested to explain the initiation of the modern decline of red spruce are similar to those proposed for European forest decline. They include acidic deposition, soil acidification, aluminum toxicity, drought, winter injury exacerbated by insufficient hardiness due to nitrogen fertilization, heavy metals in soil, nutrient imbalance, and gaseous air pollution. Climate change, in particular a long term warming that has occurred subsequent to the end of the Little Ice Age in the early 1800s, may also be important.
At present, not enough is known about the etiology of the forest declines in Europe and eastern North America to allow an understanding of possible role(s) of air pollution and of natural environmental factors. This does not necessarily mean that air pollution is not involved. Rather, it suggests that more information is required before any conclusive statements can be made regarding the causes and effects of the phenomenon of forest decline.
See also Forest management
[Bill Freedman Ph.D. ]
RESOURCES
BOOKS
Barnard, J. E. "Changes in Forest Health and Productivity in the United States and Canada." In Acidic Deposition: State of Science and Technology. Vol. 3, Terrestrial, Materials, Health, and Visibility Effects. Washington DC: U. S. Government Printing Office, 1990.
Freedman, B. Environmental Ecology. Second Edition. San Diego: Academic Press, 1995.
PERIODICALS
Mueller-Dombois, D. "Natural Dieback in Forests." Bioscience 37 (1987): 575–583.