Are Northern Forests a Byproduct of Acid Rain
Corvallis - Mar 18, 2002
Studies of pristine forests in South America found that the cycling of nitrogen, an essential nutrient, was quite different than expected, and it suggests that many forests of North America and Europe actually have an unnatural ecology driven largely by air pollution, acid rain and artificial nitrogen fertilization.
The research calls into question some basic concepts about the ecological function of forests near more populated regions, even those that are supposedly in wilderness areas. And it makes clear that the impact of humans on our natural forest ecosystems may already be much greater than previously realized.
The study was recently published in the journal Nature by Steven Perakis, now a courtesy professor of forest science at Oregon State University and researcher with the U.S. Geological Survey. A co-author on the research was Lars Hedin, now at Princeton University.
"This research challenges much of what we believed about nitrogen biogeochemistry in forests," Perakis said. "It indicates that unpolluted forests may be more uniform in nitrogen biogeochemistry than we ever considered, even across large geographic areas such as those we studied in temperate regions of South America. In contrast, air pollution and other human activities have overwhelmed natural controls over nitrogen cycling in many northern hemisphere forests."
Some of the changes are very difficult to predict, and since forests are responding to many factors at once, they seem to act in complex and idiosyncratic ways, Perakis said.
These changes may influence the plant species that can thrive in forests, affect tree susceptibility to disease and extreme climate events, and force other changes in forest or stream biogeochemistry that are well beyond most natural limits, he said.
The new findings also build on some studies at OSU more than 20 years ago, Perakis said, which suggested that the behavior of nitrogen in less-polluted western North American forests may not be what scientists had long assumed from studying more polluted forests in the East.
It may take some time to understand the full ecological implications of the new study, Perakis said, because they cut across many areas of forest growth and health, with implications for streams, lakes, fisheries and wildlife.
A study of nitrogen biogeochemistry in forests is fundamental to understanding the ecology of a region, scientists say, because it is such a critical plant nutrient and often regulates growth and biological diversity. Much of modern agriculture and even some forestry is built on the use of nitrogen fertilizers.
Prior to this research, most scientists believed that the primary forms of nitrogen that were cycled and lost from forests were simple inorganic nitrogen-containing compounds, such as nitrate or ammonium. The organic forms of nitrogen, or those associated with carbon, were not thought to be particularly important.
To test some of these assumptions, scientists spent five years doing studies of 100 temperate forest watersheds in Chile and Argentina that were truly pristine, almost totally unaffected by human activity, air pollution, urbanization or forest management.
What they found there was surprising. Up to 95 percent of the nitrogen lost in watershed streams occurred as organic nitrogen compounds. It was almost a reverse image of many forests in eastern North America and Europe, where most of the nitrogen found in forest ecosystems and streams is from inorganic chemical forms.
"Inorganic forms of nitrogen are more readily used by plants for growth, yet our eastern United States forests are releasing large amounts of inorganic nitrogen into streams," Perakis said. "We now suspect this is happening because we have been pouring high levels of inorganic nitrogen into these systems through air pollution. In unpolluted regions that have only small amounts of inorganic nitrogen in rainfall, such as our study areas of South America, the forests rapidly use what inorganic nitrogen is available and lose virtually none to streams."
At the same time, these clean forests can still lose large amounts of organic nitrogen from soils to streams, since complex organic compounds are difficult for plants to use unless first broken down by fungi and bacteria.
Over long time scales, Perakis said, such losses of organic nitrogen can drain nitrogen out of soils, keep the ecosystem nitrogen starved, and help explain why plant growth in so many areas is naturally limited by nitrogen. This nitrogen limitation on plant growth places an even higher premium on the use of simple inorganic nitrogen compounds, and may provide evolutionary pressure for plants to use simple organic nitrogen compounds as well.
There are several implications to this, Perakis said.
First, it suggests that some forests of North America and Europe are getting more nitrogen than they can use, affecting the plant life and possibly encouraging invasive species, including those that prefer large amounts of inorganic nitrogen which would not have been available prior to human activities.
It also indicates that many forests may start to become saturated with nitrogen, lose their ability to handle any more and fail to serve as a buffer against further pollution. As a result, much inorganic nitrogen is lost to streams and ends up as pollution in streams, lakes and estuaries, causing eutrophication and associated impacts on fish and wildlife.
Furthermore, Perakis said, the steady loss of inorganic nitrogen from these systems may be taking with it other nutrients essential to plant health, such as calcium or magnesium, which tend to bind to inorganic nitrogen compounds and literally "go with the flow." As these other nutrients are depleted, there may be associated declines in plant or tree health and attack by insects or disease.
The largely conifer forests of the Pacific Northwest, Perakis said, have so far been spared the flood of pollution nitrogen that is more prevalent in eastern forests. But that does not mean they are immune to the process or the concern about excess nitrogen.
"As the Willamette Valley population surges, we're likely to see increased industrial activity, automobile use and the other activities that put nitrogen into the atmosphere," he said. "In some important ways our Cascade Range conifer forests may start to behave in a similar fashion to more polluted forests in the Eastern U.S. We already see this in forests of coastal Oregon and Washington, where old soils and high rates of biological nitrogen input from red alder result in ecosystems that are excessively nitrogen rich."
The high nitrogen allows these coastal forests to initially grow quite fast, Perakis said, yet this rapid growth also allows short-rotation harvesting that may accelerate the development of nutrient imbalances in soils and vegetation. So there is a clear concern about our own forest biogeochemistry in this area, he said.
There's some indication, Perakis said, that the current outbreak of Swiss needle cast, a forest disease, could be linked to excess nitrogen. This is a native fungus that previously didn't cause major problems in Pacific Northwest forests, but is now an epidemic.
"While there is no doubt of excess nitrogen in many coastal forest plantations, a lingering question is whether widespread forest clearing, followed by substantial biological nitrogen input from red alder, has synchronously increased nitrogen availability over large areas of the Coast Range and provided a more favorable environment for the fungus," Perakis said.
"The real story here is that forest biogeochemistry is much more complicated and easily influenced than we may have appreciated," Perakis said. "The forests in South America provide us a window into the past, and what we see now in our own forests of North America and Europe is quite different."
Oregon State University
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