In recent years, red spruce decline, sugar maple dieback, and other signs of trouble in the forest have all been attributed to acid deposition, which reaches the forest in rain, snow, fog, and dust. Studies of individual trees in small areas have tended to confirm this link, but measurements of the effects of pollution over a wide area have been difficult to come by. Just what are the consequences of the shower of acidifying sulfur and nitrogen from coal-burning power plants, cars, and other sources on our forests overall?
In order to get a handle on this, the governors of the New England states and the premiers of the eastern Canadian provinces have jointly established the Forest Mapping Group (FMG), a large group of scientists charged with determining how bad the damage really is and calculating what changes will be needed to keep eastern forests productive and healthy. In September, the FMG released a preliminary report, which examines Vermont and Newfoundland in detail. The findings are very disturbing: one-third of the forestland in Vermont is currently at risk of being seriously damaged – possibly irreparably – by acid deposition.
How did the FMG make this determination? Scientists combined data on sulfur and nitrogen deposition from the atmosphere, statewide maps of soils, bedrock and surficial geology, state and federal surveys of tree-harvesting rates, and models that analyze soil chemistry and forest tree composition to determine two crucial indices: the critical load index and the deposition index.
The critical load is simply the amount of sulfur and nitrogen that can rain down on a particular section of forest without stripping plant-essential nutrients from the soil. Since some soils are naturally able to resist acid deposition better than others, the critical index varies widely across Vermont, with much of the forest along the Green Mountain spine being the most susceptible and the forest of the Connecticut River being the least susceptible.
The second crucial factor, the deposition index, compares the critical load to the amount of acid deposition that a particular section of forest is actually receiving. In other words, the deposition index identifies which parts of the state are receiving more acidity than they can handle. Because most of the sulfur and nitrogen in the atmosphere over Vermont arrives on prevailing winds from the south and west, Bennington County receives, in general, more acid deposition than does the Northeast Kingdom.
According to the FMG, the deposition index reveals that 31 percent of the forested landscape in Vermont is now receiving more acidic deposition than the forest can tolerate without declines in nutrients that are essential to plants. Acidification causes nutrients (especially calcium) to be leached out of the soil, which means that more nutrients are being lost than can be replenished by the natural chemical breakdown of minerals in the soil and bedrock. Calcium is a key nutrient in many tree functions and has been shown to be an important part of the process that trees use to avoid freezing injury.
Different tree species have different nutritional requirements. The problems that acid deposition has caused to red spruce, which grows on poor soils at high elevation, are well known. You might think that more-fertile soils – those that host sugar maple, for instance – might be able to tolerate some acidification. They can, but the problem is that sugar maple grows on rich soils for a reason: it needs more calcium than do many of its forest associates, such as beech and birch. This study identified a considerable proportion of Vermont’s sugar maple as being at-risk. In fact, it may be that sugar maple will be the next tree to show declines at present pollution levels.
Field checks have confirmed that the model’s predictions are on the right track. In many cases, areas where deposition exceeds the critical load do indeed seem to be showing signs of decline and reduced growth. A recent Canadian study confirmed that tree growth is reduced on sites mapped as sensitive but is normal on sites mapped as having a tolerable critical load.
As well as mapping the magnitude of the problem, the FMG’s study looks for solutions. If there were a 50 percent reduction in the amount of sulfur and nitrogen that now falls on Vermont forests, 78 percent of the areas currently mapped as sensitive would be mapped as not sensitive. Instead of having 31 percent the forest destined to decline, only 7 percent would be in that category.
A large number of experts and forest managers have provided data and reviewed the methods used for this report. Results from much of the rest of the study area (remaining Canadian Provinces, Massachusetts, New Hampshire, Rhode Island, Connecticut, and Maine) are expected to be available in 2004 and 2005. See www.ecosystems-research.com/fmi for the whole story.