At the 8,acre Hubbard Brook Experimental Forest in New Hampshire, long-term studies of air, water, soils, plants, and animals have produced major discoveries about human and natural disturbances to the forested landscape of the northeastern United States. In a collaborative research project spanning nearly six decades, scientists have discovered the existence and origins of acid rain; unlocked the mysteries of lead, salt, and nitrogen pollution in streams and lakes; and charted the rise and fall of bird populations because of climate change and other threats.
Research findings at Hubbard Brook provide the raw material for education and policy-outreach programs that deliver authentic data to students, policymakers, and members of the public who care deeply about our natural world. Validation of the small watershed-ecosystem approach as a powerful scientific tool in tackling problems at the landscape-scale of complexity ecological, hydrological, biogeochemical.
Confirmation that clearcutting and other major forest disturbances cause severe disruptions in the vital nitrogen cycle of terrestrial ecosystems. Major depletion of base cations, primarily calcium, from soil pools by acid rain. Dramatic decreases in chemistry of precipitation and stream water to extremely low concentrations.
That food limitation, climate, and forest structure account for most dramatic changes in the abundances of neotropical migrant birds.
More than 1, scientific publications, 12 books, and 8 monographs have been published. HBES has developed into a relatively complex matrix of projects involving a large number of scientists from diverse disciplines. Individuals or groups of researchers are supported by competitive grants to pursue a variety of specific research studies in the HBES in cooperation with the LTER program. From , over publications have been produced through the HBES, providing a wealth of information on the structure, function and development of forest, stream and lake ecosystems.
The framework for the HBES is the small watershed approach which provides precise quantitative water and element input-output budgets for the forested ecosystems. In a catchment where the underlying bedrock is impermeable, water falling on the catchment leaves only by evapotranspiration ET or as stream discharge.
By quantifying the latter, annual ET can be calculated by difference:. For chemical elements without a gas phase at Earth temperature, the small watershed also allows estimation of soil weathering Johnson et al. Here, the stream output and atmospheric input of an element are determined as the product of water flux and the element concentration in the water.
Also, some elements enter the ecosystems as dry rather than wet deposition e. For elements with gaseous phases e. Experimental manipulations e. The internal processes determining the whole ecosystem responses are measured at the plot scale at selected locations within or nearby the small watershed. Together these measurements form the basis for much of the research in the HBES including long-term monitoring as detailed below.
Figure 2. Hubbard Brook rain gauge. Atmospheric deposition of elements to the ecosystem is the sum of wet and dry deposition. Measurement of wet deposition has been conducted on a weekly basis since at a network of sampling sites across the HBEF Figure 2.
All major dissolved solutes are measured and together with precipitation water volume from standard rain gauges, element fluxes in wet deposition are determined. Dry deposition of particles and gases is more difficult to quantify than wet deposition and can constitute a substantial proportion of the input of several important elements, especially nitrogen and sulfur , that are derived partly from pollutant emissions. Some of the dry deposition can be captured on artificial samplers or as canopy throughfall, but the calculation of dry deposition flux is complex and uncertain, and we refer the reader to detailed explanations in the literature Likens et al.
Figure 3. Hubbard Brook weir at watershed 3. Stream discharge of water from nine small watersheds in the HBEF is measured continuously with 90 o or o V-notch weirs located at the base of the surveyed catchments Figure 3. The weirs continuously measure stage height in a stilling pond above the weir outlet. Each weir is calibrated by directly measuring the relationship between stage height and water flux. To calculate element fluxes, water samples for chemical analysis are collected weekly just above each weir.
More frequent measurements to more precisely quantify storm-event fluxes have been conducted periodically using automated samplers. Flux of particulate matter is measured by removing accumulated material in the weir stilling pond periodically. The flux of fine suspended solids during large storm events also has been measured using permeable bags positioned below the weir.
The composition, biomass and element content of the vegetation is measured periodically within the gauged watersheds and throughout the entire HBEF. The basic approach is to measure the diameter of trees across the whole watershed e.
Forest biomass is estimated using allometric equations relating DBH to other tree dimensions e. These equations were developed at the site by directly measuring trees harvested during experimental manipulations Whittaker et al. The pool or stock of an element is estimated from the product of mass and element concentration in various tissues. Thus, changes in the stock of an element in the forest biomass are calculated typically at a 5-year intervals.
The mass and element stocks of soils have been estimated using the quantitative pit approach. The soil is carefully excavated manually from a 0. Soil mass in each depth increment is weighed and sieved in the field and subsamples are taken for chemical analysis in the laboratory. Because of the large stocks and high spatial variability of elements in soil, detection of changes through time or in response to treatments is challenging.
Nevertheless, we have been able to estimate the net soil release for major elements resulting from acid deposition and forest harvest on the basis of quantitative pit sampling Johnson et al. Figure 4. Hubbard Brook flux tower. The small watershed approach does not, by itself, account for outputs in gas phase, an important flux pathways for the nutrient element nitrogen.
The principal gaseous flux of N results from denitrification, a process that is notoriously variable in space and time. The flux chamber approach, in which the accumulation of gaseous N diffusing from soil is measured over short time intervals e. Unfortunately, because of the high background in the atmosphere this approach is not effective for measuring N 2 flux.
Laboratory, isotopic and modeling approaches have provided indirect estimates of N 2 flux from soils e. Fluxes of carbon dioxide and water vapor from the forest ecosystem can be estimated using aerodynamic approaches with eddy flux towers. Recently, an eddy flux tower has been installed in the lower valley at the HBEF and began collecting data in Figure 4. The facility will soon provide the first direct estimates of gross primary production, autotrophic and total ecosystem respiration, and net ecosystem productivity for the forest in the footprint of the tower.
The processes regulating the biogeochemistry of the forest ecosystem include a variety of major internal element fluxes which are monitored in selected intensive plot locations in and around the experimental watersheds at the HBEF. In particular, element leaching through soils is measured using zero-tension lysimeters positioned beneath the forest floor, B h and B s horizons. Water fluxes at each depth, estimated with a hydrologic model, are used with measured element concentrations in soil solutions to estimate soil leaching fluxes Cho et al.
Litterfall is measured using permanent litter traps co-located in plots with soil lysimeters. Litter is sorted by species, providing annual estimates of leaf biomass and leaf area index in these plots.
Periodically, the chemistry of leaf litter is measured; combined with chemistry of live foliage from the plots, these measurements provide estimates of nutrient resorption from senescing foliage, a large internal ecosystem flux especially for N and P Ryan and Bormann Deposition of woody litter as coarse woody debris is measured annually on cleared 2. The chemistry of canopy throughfall and element fluxes in canopy leaching also have been measured in these intensive plots Lovett et al.
Bormann, F. Likens, T. Siccama, R. Pierce and J. The export of nutrients and recovery of stable conditions following deforestation at Hubbard Brook.
Ecological Monographs 44 3 Cho, Y. Driscoll, C. Johnson, and T. Biogeochemistry Fahey, T. Templer, B. Anderson, J. Battles, J. Campbell, C. Driscoll, A. Fusco, M. Green, K. Kassam, N. Rodenhouse and L. The promise and peril of intensive site based ecological research: insights from the Hubbard Brook ecosystem study.
Ecology, 96 4 Johnson, C. Johnson, T. Huntington and T. Whole-tree clear-cutting effects on soil horizons and organic-matter pools. Soil Science Society of America Journal Johnson, N. Driscoll, J. Eaton, G. Likens and W. Geochimica et Cosmochimica Acta 45 9 Likens, G. Driscoll, D. Buso, M. Mitchell, G. Lovett, S. Bailey, T. Siccama, W. Reiners and C. The biogeochemistry of sulfur at Hubbard Brook. Biogeochemistry 60 3 Lovett, G. Nolan, C. Driscoll and T.
Factors regulating throughfall flux in a New Hampshire forested landscape. Canadian Journal of Forest Research Ryan, D. Nutrient resorption in northern hardwood forests.
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