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Celebrating a Half-Century of Environmental Discovery in the Hubbard Brook Experimental Forest

In Review: Hubbard Brook: The Story of a Forest Ecosystem
Richard T. Holmes and Gene E. Likens
Yale University Press, 2016, 288 pages

In 1951, University of Wisconsin ecologist Arthur Hasler, while working at the Notre Dame Environmental Research Center that straddles Michigan and Wisconsin, built up an earthen dike and divided a single lake into two, transforming the one water body into Peter and Paul Lakes. Hasler was studying fresh water acidity and the food chain. He deposited lime, which is known to reduce acidity in Peter Lake and observed its effects on transparency, the food chain, and on the ecological conditions of the water body relative to Paul Lake. Hasler found the food chain changes he anticipated, but the investigation’s fame rests largely with its status as the first whole-lake experiment, setting the example for ecosystem-manipulation research to which scientists today are turning to understand climate change impacts. 

photo of a forest lakephoto by Mariel Carr, Chemical Heritage Foundation Videographer, Wikimedia CommonsThe Hubbard Brook Experimental Forest watersheds drain into mirror lake.

While Hasler’s Peter and Paul Lakes experiment and other large scale ecological explorations, such as Great Britain’s 170-year-old running Rothamsted Research center in southern England – famous for its soil and plant health agricultural investigations on small plots and in the news recently with biotechnology testing – have shed much light on nature and socio-ecological systems (e.g. agriculture and forestry), no American ecosystem study is as famed or as impactful as the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Hubbard Brook recently celebrated its 60th anniversary as a designated experimental forest and fifty years as a focused ecological study, making it a landmark ecology research hub: Scientists have monitored the 3160-hectare, nine-watershed site for six decades, conducting Hasler-type alterations and yielding findings, that have transformed the field of ecology and environmental policy, including the discovery of acid rain. 

Today, science at Hubbard Brook is active and robust, and on the occasion of its 60th anniversary, long-time Hubbard Brook scientists Richard Holmes and Gene Likens, who is a co-founder of the Hubbard Brook study, have put together a coffee-table book, Hubbard Brook: The Story of a Forest Ecosystem, published in 2016 by Yale University Press. The book has beautiful photos and many informative graphs, and Holmes and Likens have written lively chapters for a public readership on the science, policy impacts, and dynamic ecology of Hubbard Brook. 

Holmes and Likens begin the book with a sense-rich “Prologue” that describes the four seasons in the Hubbard Brook forest, which is representative of a northeast hardwood forest: the summer with copious tree canopies; the fall, when the denuding trees suck back nitrogen and other nutrients from their soon-to-fall leaves to retain through the winter; the dazzling winter, when the snow caught in the needles of evergreen trees “sublimates” (evaporates directly from solid to gaseous form); and finally the vibrant spring, when the winter snow pack melts, stream flows become torrents, and herbs sprout early in the soil, holding onto nutrients that would otherwise flow away before the trees’ full budding.

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This focus on nutrients and chemicals circulating through a forest watershed is the hallmark of Hubbard Brook research. More than a half century ago,  Hubbard Brooke ecologists started by looking at a single Hubbard Brook’s stream network and, as the authors write, “pose[d] questions based on a simple metaphor: that the chemistry of stream water could be used to diagnose a watershed-ecosystem in the same way that a physician uses the chemistry found in blood or urine to diagnose the health of a patient.” That approach and the discrete intactness of the Hubbard Brook ecosystem proved auspicious: By the mid-1960s, the Hubbard Brooke vision expanded to a larger focus on ecological dynamics and, famously, on the complex cycling of nutrients and other chemicals via precipitation, stream flow, rock weathering, soil, trees, microorganisms, birds, mammals, and other species. Altogether, the Hubbard Brook focus had a marked emphases on biogeochemistry, which looks at how the living organisms and nonliving matter (e.g. rocks) work together to create chemical cycles and trajectories ( e.g. chemical releases). Biogeochemistry is fundamental to understanding our environment. As Holmes and Likens explain, “The complex interactions among biology, geology, chemistry, and hydrology elucidated by the study of biogeochemisty provide the foundation for all life and for the functions in the watershed-ecosystems at Hubbard Brook.”

Nitrogen, calcium, sulfur, organic carbon, aluminum, and other chemicals and compounds have been measured and traced in the waters, soils, and organisms at Hubbard Brooke on a recurrent basis, after large scale manipulations (e.g. forest cuttings), and in connection to natural but non-typical occurrences such as winter soil freezing when there is unusually low snowfall, which would otherwise insulate the below-ground biology.

One recurrent Hubbard Brook measurement is stream and rainfall pH, reflecting degree of acidity. In the early sixties, the Hubbard Brook ecologists noticed strangely high acidity readings for rainfall, and they spent nine years measuring and studying acidity for their seminal 1972 paper, “Acid Rain,” which identified a national environmental problem. In the process, Hubbard Brook scientists established that acid rain resulted not only from sulfur dioxide emissions from Mid-Western power plants but also from nitrogen oxide emissions that come from both power generators and combustion-based transportation, especially cars.

The discovery led to other critical questions about acid rain, such as natural background acidity levels and locale of sources. And the Hubbard Brook scientists addressed them: they established monitoring sites as remote as Torres del Paine in southern Chile to derive the best approximation of preindustrial background acidity levels; they worked with scientists who flew in planes and drove on the ground to trace mid-western emissions; and in the 1980s, they even used Building 2 at New York City’s World Trade Center as a site for studying acidity in clouds. The ecologists also documented the impacts of acid rain at Hubbard Brook, including damaged trees and release of dangerous levels of aluminum from the soil. They participated in advocacy, with Likens briefing President Reagan and his cabinet in 1983. Ultimately, their persistent work paid off. After years of Hubbard Brook scientists informing the public and policy makers, in the early 1990s the United States enacted the Clear Air Act Amendments, which have significantly reduced acid rain.

Still, the acid rain legacy continues, and one enduring impact is that acid rain has stripped calcium from soil, hurting forest growth. To understand this process, Hubbard Brook scientists conducted one of their famous whole-watershed manipulations, adding ground Wollastonite, a mineral rich in calcium, to the experimental forest’s Watershed 1 in 1999. At Watershed 1, since the manipulation began, sugar maple and red spruce, which were badly damaged by acid rain, have displayed significant growth, this among other positive ecological signs. But biogeochemistry is usually complex, and since 2011, “an unexplained large increase in nitrate output in stream water from Watershed 1 since 2011” has been detected that remains a study focus.   

There are many other well-noted Hubbard Brook manipulations that have greatly informed science and policy, including famous deforestation experiments on a few Hubbard Brook watersheds, which provided seminal data on nutrient release and regrowth patterns following diverse forestry actions. Studies of resident and seasonal birds have also been influential, with a Hubbard Brook study documenting that most mortality for Hubbard Brook seasonal breeders occurs during the migration. A recently publicized Hubbard Brook experiment on abrupt freezing, with water sprayed on trees in cold weather, looks at prospective climate change impacts from extreme weather outbursts. This last Hubbard Brook example reflects where the field of biogeochemistry is headed, and is joined by similar undertakings like the projected 10-year US Department of Energy’s Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) project in Northern Minnesota, which focuses on climate impacts on the southern boreal biome. SPRUCE became fully commissioned in 2015 with ten large silos placed across a boreal ecosystem. Within the silos, carbon levels and temperatures are being manipulated to study changes in peatland, carbon release, and vegetative assemblage resulting from carbon and temperature variations.

In fact, Likens and Holmes end their book with a climate change focus and another tour of the seasons, this tour set in 2065. The last 50 years have seen unmitigated climate change, and while the scene at Hubbard Brook is not an environmental dystopia, much has been altered for the worse: vines that were never at Hubbard Brook entwine the trees; the sugar maples that offered the most vibrant fall colors are gone; and biting insects, viruses, and extreme weather disturbances stress the forest, its inhabitants, and recurrent visitors, including the seasonal breeding birds and the inveterate Hubbard Brook scientists. Yet, the authors envision that even if contemporary climate change warnings go unheeded, the Hubbard Brook science will continue. It has to – there are always better actions to take and new understandings of nature and its processes to develop. As the authors vividly lay out, with over 50 years of research and data, Hubbard Brook is a model of a scientific effort that can eventually lead to wiser stewardship of our local, regional, and global environs.   

Richard Blaustein
Richard Blaustein is a freelance science and environmental journalist. He can be followed on twitter @richblaustein.

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