Although they are often remote and scenic, it has long been known that mountain ecosystems receive higher doses of pollutants and nutrients than do nearby valleys. Known as atmospheric deposition, these doses of pollution are delivered through rain, snow, cloud, fog, rime ice or as gases and dry particles. Because gathering data from mountains can be expensive and difficult, measurements of atmospheric deposition are often made from single points, such as towers at the tops of mountains.
But a new study published this month reveals that these current methods of measurement may be inadequate. The researchers, writing in the journal Ecological Applications (Volume 10, No. 2), say that estimates of atmospheric deposition need to take into account the complex landscapes of mountains, and the effects that mountain terrain may have on the way atmospheric deposition makes its way to the surface of the earth. One area of a mountain, they discovered, can receive dramatically different amounts of pollutants and excess nutrients than another, and those differences can create depositional hotspots where ecosystems may be particularly susceptible to stress.
The study's authors, Kathleen Weathers, Gary Lovett, Gene Likens and Richard Lathrop, focused their work on the Catskill Mountains of New York. The Catskills provided an ideal research site because they are within 150 km of the massive New York-New Jersey metropolitan area where emissions of both pollutants and nutrients are high. Although the Catskill Mountains are close to these metropolitan areas, some areas of these mountains have been largely undisturbed for the past 100 years. In addition, the Catskill region is of particular interest because the area's watersheds provide 90% of New York City's water supply.
Soil samples were taken from Hunter Mountain, which is the second highest peak in the Catskills. Sites of different elevations on the northern, southern, eastern and western aspects of the mountain were sampled, as were some from the edge to the interiors of forested stands. The team also collected soil from below both coniferous and deciduous tree stands at both low and high elevations. They then measured the amount of lead found in each sample.
Lead was used because the element is a simple indicator of total atmospheric deposition for a number of chemical species of interest. For example, patterns of lead in the forest floor have been found to correlate closely with sulfur and nitrogen deposition, the two main ingredients in acid rain (also referred to as acid deposition). Acid deposition can damage ecosystems, and is therefore of considerable ecological interest. The researchers used this lead index to determine patterns of total deposition, and related those patterns to total sulfur and nitrogen deposition.
When examining the effect that forest edges had on total deposition, the team found that edges received approximately two times as much deposition as the interior of the forest.
Another factor in the increase of deposition is the large number of coniferous trees found at the top of Hunter Mountain. Few previous studies have examined the relative effect of tree type on atmospheric deposition, but coniferous trees were long thought to be more efficient scavengers of pollution and nutrients because they retain needles year round. Those needles capture particles more efficiently than do deciduous leaves. The pollutants that are trapped by these canopies are then deposited to the forest floor, when the captured precipitation falls to the ground and when the tree loses it needles.
The research team in this study found that deposition to high-elevation coniferous stands was 2.5 times greater than deposition to high-elevation deciduous stands. And the deposition to high-elevation coniferous stands was greater than in both types of low-elevation stands.
After they collected data on patterns of deposition across such landscape features as edges, elevation, aspect and vegetation type, the team used a geographic information system (GIS) database to scale up their estimates to the whole of the Hunter Mountain landscape.
The team also found that deposition at the high elevation region was 13-43% greater than the adjacent low elevation region. Deposition became especially significant at sites that were over 1000 m above sea level. This increase is probably due, says Weathers, to higher rainfall and cloud immersion at these elevations.
One of the most interesting aspects of the study was its revelation that some parts of the mountain receive extraordinarily high amounts of deposition. Referred to as "hotspots," these areas may receive as much as four times the deposition of nearby low elevation areas. Although they are small in area, these hotspots may be of great ecological importance, because nutrient and pollutant loads of such magnitude have been shown to affect ecosystem function.
"Our study shows that it is critically important to take into account landscape features when estimating atmospheric inputs to mountainous areas," says Weathers. "By not taking these factors into account, we may be drastically misinterpreting the amount of deposition taking place in sensitive, high elevation areas."
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