An experimental forest in New Hampshire where scientists clearcut entire watersheds in the 1960s to measure nutrient loss. The research proved that intact forests act as filters, holding soil and nutrients that wash away rapidly when trees are removed.
Mariel Carr, Videographer, CC BY-SA 3.0, via Wikimedia Commons
The Small Watershed Approach
The Hubbard Brook Experimental Forest, a 7,800-acre reserve in New Hampshire's White Mountains, was established by the U.S. Forest Service in 1955 to study forest hydrology. The site is a bowl-shaped valley containing a series of small, discrete watersheds. These watersheds are ideal for ecosystem experiments because their underlying bedrock is watertight, allowing scientists to precisely measure all precipitation entering the system and all water and nutrients leaving via a single stream.
In 1963, a team of scientists including Gene Likens, F. Herbert Bormann, Robert Pierce, and Noye Johnson launched the Hubbard Brook Ecosystem Study. Their goal was to use this "small watershed approach" to treat an entire ecosystem like a laboratory experiment. By monitoring a few watersheds as undisturbed controls and subjecting others to manipulations, they could quantify the effects of disturbance. Each watershed is outfitted with a V-notch weir, a concrete dam with a triangular opening that allows for precise measurement of streamflow. This setup was the basis for a significant experiment in forest ecology.
A Radical Experiment and Its Results
In the winter of 1965-1966, researchers conducted a dramatic experiment on Watershed 2 (W2), a 15.6-hectare forested area. They cut down every tree and shrub, leaving them on the ground to minimize soil disturbance. For the next three summers, they applied herbicides by helicopter and backpack sprayers to prevent any vegetation from regrowing. They wanted to see what would happen to the water and nutrient cycles when a forest's biological filter was completely removed.
The results were immediate and significant. With no trees to absorb water through their roots (transpiration), annual streamflow from W2 increased by as much as 40 percent. More startling were the chemical changes. The concentration of nitrate, an important nutrient, in the streamwater increased from less than 1 mg/L to over 50 mg/L within two years. The loss of other essential nutrients, such as calcium and potassium, also rose dramatically. The experiment showed that an intact northern hardwood forest is efficient at retaining nutrients, and that removing the forest causes these life-sustaining elements to wash out of the ecosystem.
The continuous data collection at Hubbard Brook also led to an accidental discovery. In the 1960s, scientists noticed that the rain and snow falling on the forest were unusually acidic. This monitoring led to the first documentation of acid rain in North America. Researchers traced the pollution to sulfur and nitrogen oxides from power plants hundreds of kilometers away in the Midwest. This work directly influenced the passage of the 1990 Clean Air Act Amendments.
💡Fun Facts
The Hubbard Brook Ecosystem Study, started in 1963, is one of the longest-running and most comprehensive ecosystem studies in the world.
Researchers here first documented acid rain in North America in the mid-1960s.
To study the long-term effects of acid rain depleting calcium, scientists in 1999 added calcium silicate mineral back into an entire watershed.
The forest is a living laboratory, with ongoing studies on climate change, bird populations, and the effects of ice storms.
The forest itself has no set hours, but guided tours for groups are available from May to October by appointment only.
Admission
Free.
Accessibility
Access into the experimental watersheds is primarily via unpaved single-lane roads and forest trails. Sturdy footwear is required for tours. The terrain can be uneven and steep in places.
