NIWOT RIDGE - The dominoes are tumbling one by one, and the end point will be dead fish and dead trees in Colorado's high country, according to University of Colorado biogeochemist Mark Williams.

"The wilderness is not going to fall apart in the next year, but we're kicking over the first dominoes now," said Williams, a fellow at CU's Institute of Arctic and Alpine Research, known as INSTAAR.

He has studied the chemical changes in snowmelt runoff from the upper basins of the Green Lakes Valley near Niwot Ridge, about 30 miles west of Boulder.

The watershed, tucked up against the eastern boundary of the Continental Divide, is a significant contributor to Boulder's water supply.

Over the last 20 years, nitrogen compounds in the rain and snow have more than doubled there. As the snow melts, runoff acidity spikes, occasionally reaching concentrations strong enough to kill young fish, said Williams.

This "episodic acidification" was first detected in 1994 and usually lasts a couple of weeks. If the dominoes continue to fall, however, the Green Lakes Valley is heading toward chronic acidification, which can threaten the health of streams, forests and alpine tundra.

Along the way, wildflower diversity in the tundra is likely to decline as grasses and sedges replace flowers.

On the pH scale of acidity, distilled water has a neutral pH of 7.0. Rainfall is naturally somewhat acidic, with normal values between 5.0 and 5.6.

In the northeastern U.S., where emissions of sulfur compounds were blamed for acid rain that ravaged forests and waterways in the latter half of the last century, typical pH values range from 4.0 to 4.5, according to Meteorology Today by C. Donald Ahrens.

In the Green Lakes Valley, pH levels as low as 4.8 have been detected, Williams said. That's low enough to kill young trout, although there are no fish in the highest catchments where the most acidic water has been collected.

Other parts of the country, such as the Northeast, receive more nitrogen pollution than Colorado. But this state's alpine areas, with their granitic bedrock, sparse vegetation, thin soils and short growing season, are much more sensitive to even small nitrogen additions.

Unlike deciduous forests in the East, Colorado's alpine outposts just can't digest all that extra nitrogen.

Once chronic acidification spreads through the Green Lakes Valley, tainted waters will start moving downslope, into streams and lakes teeming with trout, said William Bowman, director of CU's Mountain Research Station.

More dominoes will fall, but the timetable is unclear. It could take decades.

"The acidification will become more sustained, rather than episodic," Bowman said. "The stream insects will begin to disappear, then you'll see a loss of the stream vertebrates - the fish.

"I doubt it would get all the way down to Boulder, but it could get into some heavily used recreational streams with trout," Bowman said during a recent hike to the top of 11,600-foot Niwot Ridge.

Scientists affiliated with the Mountain Research Station have been conducting climate research on the ridges since 1952. Some of the first stream chemistry work was done by INSTAAR researcher Nel Caine.

In 1997, Bowman and his colleagues began spraying ammonium nitrate fertilizer onto small test plots in the alpine tundra to see how nitrogen deposition is likely to affect that ecosystem in the future.

On Niwot Ridge, the alpine tundra is a mix of grasses, sedge, wildflowers and other forbs, moss, lichen and low shrubs such as dwarf willow and birch.

Three hundred forty species of plants have been described at Niwot Ridge, which shares 40 percent of its plant species with the Arctic.

"This is one of the most diverse herbaceous ecosystems in the United States," Bowman said. "In an hourlong drive, you can get into a biotic zone similar to northern Canada."

The wildflowers - violet-blue harebells with oblong, toothed leaves; buttercups; funnel-shaped Arctic gentian with creamy, green-speckled petals - bloom during a three-month growth spurt that starts in late May.

Though most of them are no more than 6- to 8-inches tall, these perennials can live for 70 years or more, eking out an existence in a punishing environment.

Bowman's fertilization experiments have shown that alpine blue grasses and a type of curly-leafed sedge are better able to take advantage of excess nitrogen than other plants. In the test plots, weedy grasses and sedges are driving out the wildflowers and taking over.

The same effect is beginning to be seen outside the test plots, on the open alpine tundra. Grasses and sedges are sucking up the Front Range nitrogen compounds falling from the sky.

The end result will likely be fewer wildflower species and a less dazzling visual display.

But the effects go beyond aesthetics.

As grasses and sedges proliferate, soil properties change and more nitrogen is released into streams, accelerating the progression toward acidification.

The time it takes for all the dominoes to fall depends, in part, on whether urban dwellers rein in the nitrogen emissions.

"The usual strategy in Colorado is that we'll deal with problems when we have dead fish and dead trees," Williams said.

"But the scientists are saying, 'We know we're perturbing the system now. Do we really want to wait until we have dead fish and dead trees until we do something about it?' "

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