BACKGROUND

The Hubbard Brook clear-cut experiment is probably the most quoted ecosystem-scale study, and students are often immediately impressed by the scale (both temporal and spatial) of its design. Herb Bormann and Gene Likens chose this region of the White Mountains in New Hampshire because the Hubbard Brook Experimental Forest could be partitioned into reasonably sized, discrete watersheds drained by obvious streams. Impermeable bedrock beneath the site allowed the researchers to conduct input/output measurements by sampling precipitation (plus dry deposition and nitrogen fixation) and stream water at the base of the watersheds. For the biogeochemical studies they monitored nutrient loss from the 6 watersheds by placing a V-notched wier across the stream exiting a watershed; the wier created a small dam and stream gauges continuously measured flow and water height. Water samples collected at the notch gave an integrated measure of nutrient loss from the watershed.

In the Watershed 2 study all vegetation was cut during fall and winter of 1965; the forest floor was little disturbed since the trees were felled onto snow and no wood was removed. Bromacil, a nonspecific woody herbicide, was broadcast sprayed over the watershed by helicopter in June 1966 to kill regrowth. For the next two summers, 2,4,5-T was sprayed from the ground on the persistent regrowing vegetation with backpack mist blowers. Beginning in 1969, vegetation was allowed to regrow.

Export exceeded input of most nutrients for 2 reasons. First, the absence of evapotranspiration lead to a 40% increase in stream discharge and accelerated leaching of soil nutrients. The second phenomenon was decoupling of decomposition and plant nutrient uptake in the watersheds. Soil ammonium concentrations increased mainly due to decomposition of the pulse of organic matter (e.g. dead leaves, roots) and inhibition of plant uptake. Soil nitrifiers then oxidized the ammonium into nitrate, and the mobile nitrate leached into streams leaving the watershed. The dramatic result demonstrates the tight coupling of nitrogen cycle processes in this forest.

The researches link increased nitrate output with increased availability of hydrogen ions (as a result of nitrification) and subsequent leaching of cations from the system. In soil cations are mobilized as hydrogen ions replace them in soil exchange complexes.

According to Likens et al. (1970) "Our study shows that retention of nutrients within the ecosystem is dependent on constant and efficient cycling between the various components of the intrasystem cycle (i.e. organic, available nutrients, and soil and rock mineral compartments)….Blocking of the pathway of nutrient uptake by destruction of one subcomponent …. leads to greatly accelerated export of the nutrient capital of the ecosystem. " This Ecological Monographs paper concludes with a very useful 11 point summary of the study.

For additional information (including photographs) see the Hubbard Brook Data Set in this TIEE volume and http://www.hubbardbrook.org/research/overview/hbguidebook.htm.

Literature Cited

• Bormann, F. H., and G. E. Likens. 1970. The nutrient cycles of an ecosystem. Scientific American 223: 92-101.


• Bormann, F. H., and G. E. Likens. 1995. Biogeochemistry of a forested ecosystem. Spiringer-Verlag, New York.


• Likens, G. E., F. H. Bormann , R. S. Pierce, and W. A. Reiners. 1978. Recovery of a deforested ecosystem. Science 199: 492-496.


• Likens, G. E., F. H. Bormann, N. M. Johnson, D. W. Fisher, and R. S. Pierce. 1970. Effects of forest cutting and herbicide treatment on nutrient budgets in the Hubbard Brook watershed-ecosystem. Ecological Monographs 40:23-47.