Researchers turn back time for acid-rain ravaged forest

ANN ARBOR—Heavy trucks and helicopters aren’t the kind of equipment one usually associates with scientific experiments, but they played key roles in an ambitious project that’s helping researchers assess the effects of acid rain on forests. The first results of the experiment, a joint project of the University of Michigan, Syracuse University and the Institute of Ecosystem Studies in Millbrook, N.Y., appear in the February/March issue of the journal Biogeochemistry.

“We know that one of the effects of acid rain has been to reduce the amount of calcium that’s available to plants in the forest, and a fundamental question is, how does that calcium loss affect the structure and function of forest and aquatic ecosystems?” said U-M geochemist Joel Blum. One way of finding out is to add back to the ecosystem the amount of calcium lost due to acid rain and watch what happens, and that’s exactly what Blum and collaborators are doing with a 29-acre (11.8-hectare), wooded watershed at the Hubbard Brook Experimental Forest in northern New Hampshire, which scientists have been studying for more than 30 years. “If there’s a single best place to do this experiment, it’s at Hubbard Brook, where we think we understand the biogeochemical system better than anywhere else,” said Blum.

Simple as the idea sounds, the actual experiment has presented a host of challenges, both scientific and logistical. First, the scientists had to figure out what form of calcium to use. “Most geological materials dissolve so slowly that in our lifetimes we wouldn’t get enough calcium becoming available to plants to study it,” said Blum. Lime (calcium carbonate) dissolves quickly, but researchers ruled it out because it changes soil pH—a measure of acidity—too much, making it difficult to separate the effects of added calcium from the effects of changing pH. Another mineral, plagioclase (a calcium-aluminum silicate), was rejected because its aluminum content would introduce another variable that might muddy up interpretation of experimental results. Finally, the research team settled on wollastonite, a brilliant white calcium silicate that’s used in the ceramics industry to make dishes and decorative pottery whiter.

A mine in upstate New York supplied a customized batch of extremely pure, finely powdered wollastonite, but the researchers couldn’t just go strewing it around the forest. “If we sprinkled it out of a helicopter, it would be taken by the wind and spread over a wide area, instead of being focused right on the watershed,” said Blum. Not only could that dilute the effects of the experiment, it could cause problems because the experimental watershed is close to a pristine watershed that researchers have been studying for decades and use as a reference for assessing changes in other watersheds. “It’s the sacred watershed of all watersheds, and we had to be very careful not to compromise it, because the chemical record for it is the longest continuous record in the world,” Blum said.

The solution was to have the wollastonite powder made into pellets using a water-soluble binder, but finding someone to do that was another hurdle. The researchers approached numerous companies that make pelleted products such as fertilizer and rabbit chow, but no one wanted to chance ruining their equipment by running the mineral through their mills. “Finally, we found a guy in a small town in the middle of Illinois who had a small, custom fertilizer factory,” said Blum. “We had to pay to modify his little plant to be able to run this material through it.” But first, the wollastonite—three tractor-trailer truckloads of it—had to be transported from New York to Illinois. Then the pellets had to be shipped back from Illinois to New Hampshire. “The logistics were unbelievable,” said Blum. “I learned a lot about trains and trucks and helicopters.”

Finally, with the help of a helicopter pilot who specializes in precisely applying pesticides to cranberry bogs and blueberry fields using a global positioning system to insure accuracy, the scientists applied about 55 tons of wollastonite to the watershed. Now, they’re carefully monitoring the effects by following the movement of calcium through the ecosystem, and that’s where several other unique features of wollastonite come in handy. The mineral contains strontium, an element that behaves much like calcium in living systems, but has built-in properties that allow the wollastonite calcium to be distinguished from other calcium in the watershed. Both the calcium-to-strontium ratio in wollastonite and the ratio of the various isotopes of strontium (atoms of strontium with slightly different masses) in the wollastonite are very different from those of the minerals that release calcium into the watershed under natural conditions and thus different from what was already in the watershed before the experiment began. As a result, researchers can use the calcium-to-strontium ratio and the strontium isotope ratio to trace the fate of the artificially applied calcium.

“We can follow tiny little amounts with high accuracy as they move through the system and cycle through different pathways, and we can test all sorts of hypotheses related to calcium—the path it takes through the system, the rates at which it moves through different soil layers and through groundwater, the rates at which it’s taken up by different types of trees and how different trees vary in their ability to take up calcium,” said Blum. “It opens up a million projects, and I have at least a half dozen students and collaborators from different areas of ecology and geology who are working with us on various aspects of the experiment.”

The report published this month in the journal Biogeochemistry documents how the tracer returned the stream water pH to pre-acid rain values, follows the processes by which the wollastonite dissolved and traces the storage of the applied calcium in the stream channel. Though most of the ecological results are still preliminary, they’re very promising, said Blum. “We’re already seeing biological responses—certain species of trees that were previously declining are coming back in large numbers, and we’re seeing that different species of trees have differing abilities to access the calcium and take it up into their foliage.”

The point of the research is not necessarily to come up with fixes that will reverse the effects of acid rain—though foresters have inquired about the feasibility of dropping loads of wollastonite on other woodlands. “The idea is to improve our understanding of how the system works so that we can make better predictions about what the road to recovery will be like—how long it will take under various scenarios for pollution emissions, for example,” said Blum. “Each piece of the puzzle enhances that understanding.”

For more information:

Hubbard Brook Ecosystem Study >

Photo essay on the wollastonite project >

Joel Blum >

Hubbard Brook Ecosystem Study >Photo essay on the wollastonite project >Joel Blum >