Although forest decline was occasionally reported in several European countries earlier this century, the current concern implicating air pollution was first reported in Germany during the early 1970s. Initially characterized by needle loss and chlorosis in silver fir at high elevations, reports of decline affecting a number of other species, including Norway spruce, scots pine, and European beech, have been identified.
There is much confusion about the actual killing agent. The suggested evidence is that many factors, including pollution, are causing tree decline and mortality. Both biotic and abiotic agents are at work but cannot be singled out as a direct cause. The European conclusion is that there is more to learn about tree mortality and "forest decline".
United States
There is no conclusive evidence of a link between hardwood tree mortality and air pollution according to several government reports. The reports speculate that an apparent increase in decline and mortality of some hardwood species in the last few decades may be due to the maturation of the forest itself, or may simply be a function of increased reporting.
A report of the National Forest Health Monitoring Program concluded that forests in the northeast US are generally healthy and that there is no indication of regional forest decline, although forest health concerns were identified for particular species some areas. The high-elevation spruce-fir forests of the northeast US appears to be one of these ecosystems where decline is chronic and quite extensive. Serious declines were reported for red spruce in the 1980s across the northeastern Appalachian mountains and in some regions of the southern Appalachians and in the cloud forests.
The U.S. Forest Service researches and publishes information on a multitude of forest health problems. They are somewhat silent on the issue of forest decline. They do mention it in their latest Forest Health Highlights but suggest that primary causes for the decline cannot yet be scientifically supported. The United States conclusion is that there is more to learn about tree mortality and "forest decline".
Canada
To date, there is no conclusive evidence to link forest decline in Canada directly to air pollution, at least not because of "direct cause and effect relationships". This is partly because the burden of scientific proof is so "onerous". And Canada is showing growth throughout her forests by most recent surveys.
What's needed, according to Canadian officials, "are a few more years of conscientious monitoring, a better understanding of the climate/tree growth relationships, and further controlled environment experimentation to explore the relationship between [pollution] exposure and the physiological mechanisms controlling winter hardiness and drought response." The Canadian conclusion is that there is more to learn about tree mortality and "forest decline".
For more information on forest decline and health issues:
