Very early it was recognized that watershed-ecosystems of Hubbard Brook had a great potential for multi-faceted ecosystem research. Some of the important attributes of the experimental watersheds include:
1. Watertight Bedrock.
The bedrock and glacial till limit deep seepage from experimental watersheds. As a result, precipitation entering the watershed can leave primarily by only two paths: streamflow and evapotranspiration. Careful and precise measurement of the inflow and outflow of water, and associated elements in a watertight watershed ecosystem allows the development of rather complete water and element budgets. Thus, intensive studies can be made of ecosystem processes with quantitative element budgets.
2. Uniformity of Watersheds. The quality and quantity of streamwater which drains an area reflects the climate, atmospheric deposition, vegetation, soils and geology of that area. Monitoring of streamflow and water chemistry has shown that the watersheds have similar characteristics. Within each watershed there are a variety of soils, vegetation, microtopographical features, and micro-climate. Nevertheless, the composition of these variables seems to be remarkably similar from watershed-to-watershed. Thus the effects of experimental manipulations of watersheds can be adequately evaluated by comparison with neighboring undisturbed watersheds.
3. Representative of Area. The soils, vegetation and climate at the HBEF are characteristic of the northern hardwood forest complex, which spans much of the north-central and northeastern U.S. and southeastern Canada. Streamflow and chemistry reflect the landscape characteristics of the drainage area. Consequently, results from the relatively small watersheds at the HBEF are to a first approximation representative of a much larger regional area.
4. Ownership and Administrative Control. The entire Experimental Forest is owned by the federal government and administered by the USDA Forest Service Northeast Research Station. The White Mountain National Forest is responsible for road construction and maintenance, timber sales, and fire suppression within the HBEF. All other decisions regarding use of the Forest rest with the Director of the Northeastern Research Station.
5. Isolation. The Experimental Forest is located in the midst of a large, almost unbroken tract of forest land (New Hampshire is 87% forested), and is almost surrounded by the National Forest. Large centers of major industrial pollution are generally more than 160 km from the HBEF.
6. Data Base. Beginning in 1956 when the first experimental watershed was completed and for several years thereafter as the remaining 13 gauged watersheds became established (9 watersheds in the experimental forest and 4 in the Mirror Lake watershed), a continuous data base of precipitation, streamflow and meteorology has developed (Figure 1). Continuous measurement of the chemistry of precipitation and streamwater began in 1963. Other data available include atmospheric chemistry, throughfall, soil water, soil chemistry, vegetation, microbes, animal populations and behavior of the forest ecosystem, and the limnology of the various streams and Mirror Lake in the Hubbard Brook Valley.
7. Physical Facilities. An established road and trail system provides ready access to most parts of the Forest. The existing Forest Service buildings (office, archive, meeting room, and residence complex) provide a nucleus of facilities.
8. Space for Research. Although about 98 ha have been manipulated in whole watershed experiments, more than 3,000 ha of intact forest still remains for future use. Since 1968, five non-manipulated watersheds in the Experimental Forest have been monitored continuously for input and output of water. From these data, reliable equations have been developed for predicting nutrient concentrations in streamwater. Thus, valuable watersheds have been developed for future research involving the manipulation of whole ecosystems. Some potential types of experiments might involve the following: (1) timber cutting and harvesting procedures; (2) applications of sewage effluent and sludge, fertilizers, trace metals and pesticides; (3) controlled burning; (4) artificial planting; (5) chemical manipulations (e.g., nutrient, acid addition); (6) simulation of climatic disturbance.
9. Mirror Lake. The location of Mirror Lake in the Hubbard Brook Valley has provided a unique opportunity to investigate and quantify air-land-water interactions. The lake is 15 ha in area and up to 11 m deep. There are three inlet streams and one outlet to Mirror Lake.