This column originally appeared in the May 13, 2007 issue of the Poughkeepsie Journal.

We all know rain falls from the sky, but we may not notice that even on a clear day, pollutants fall from the atmosphere onto the land. The fate of these pollutants depends on where they fall. Do they land in a lake or stream? Or do they wind up falling on a forest or field?

Land acts like a sponge, absorbing and modifying what it receives from the sky. Forests can serve as important filters, removing pollutants before they reach nearby lakes and streams. Some of the pollutants that fall on forests are trapped in the trees and soil; in a forest sponge. When this sponge is full, pollutants can leak into ground and surface water. A forest's ability to trap pollutants is influenced by the chemistry of inputs, resident trees and microbes, soil conditions, and where it sits in the landscape-on the top of a hill or in a valley, for example.

For two decades, scientists at the Institute of Ecosystem Studies have been investigating how atmospheric pollutants and forest composition influence water quality in the Catskill Mountains. We are interested in how the forest responds to pollutants, such as nitrogen. Does the forest retain nitrogen emitted from sources near and far? What influences the quality of the water leaving the Catskills? Do landscape features, such as vegetation and slope, affect the way that watersheds process nitrogen?

Why do we care so much about nitrogen? Essential to life, this nutrient was once in limited supply for plant growth. Humans have dramatically increased available nitrogen through agricultural activities, driving automobiles, and burning fossil fuels. Now a pollutant in many parts of the world, nitrogen can saturate a forest, causing excess nitrogen to leak into nearby water bodies.

The Catskills receive some of the highest doses of atmospheric pollutants in the Northeast. Our research shows forests leak between 10 percent to 50 percent of the nitrogen pollution that falls on them. This excess nitrogen can have a number of negative ecological effects, such as stripping important nutrients from the soil, acidifying streams, and polluting coastal waters downstream.

Why do some forests absorb more nitrogen than others? The leakiness of the landscape is influenced, in part, by the types of trees present, their age, and the composition of the soil beneath them. Our research suggests that when excess nitrogen falls on a watershed with oak or beech forests, it stays longer than when it falls on sugar maple forests.

Human landscape effects - forest management, invasive species and climate change - threaten to shift tree composition in Catskill forests. Beech bark disease and the hemlock wooly adelgid are already killing trees in New England forests. What happens when a major tree species disappears? If trees that are good at removing pollutants are replaced by less effective species, forests may no longer buffer pollutants.

A number of the processes performed by ecosystems benefit humans, clean drinking water is among them. When a forest is made less effective at cleansing water, whether through the effects of pollution, the influence of introduced pests, or some other disturbance, its ability to provide this important ecosystem service is diminished.

Catskill forests influence the quality of freshwater resources vital to New Yorkers. Streams that drain from Catskill watersheds provide drinking water to tens of millions of people. The long-term viability of this resource depends on minimizing pollution deposited in the Catskills and understanding and preserving the forests that buffer pollutant inputs.

Dr. Kathleen Weathers is an ecologist at the Institute of Ecosystem Studies in Millbrook, NY.