Communities and Ecosystems
The Hubbard Brook Ecosystem Studies
An In-Depth Analysis of the Small Watershed Technique
Summary of Biogeochemical Patterns
Much of the following summary of background information comes from the book by Likens and Bormann (1995).  See reference list at the end of this lecture.
Introduction
    Hubbard Brook was established as a hydrologic laboratory for the study of watershed management in 1955.  The area is named for the major stream that drains the watersheds, Hubbard Brook.  Each watershed though, has smaller streams that drain them and which eventually feed into Hubbard Brook.  The Hubbard Brook Experimental Forest (HBEF) contains eight watersheds, and 20 streams.  A small lake (Mirror Lake) is also a part of the system.  The watersheds drain eventually into the Atlantic Ocean.  The HBEF covers 3,160 ha, and ranges in altitude from 222 to 1015 m.  All the watersheds face south.  See Figure below for a view of HBEF.

     Early History
    In the early 1960s, Gene Likens (then at Cornell) and Herb Bormann (then at Dartmouth, and later Yale) thought that one approach to studying ecosystem processes, in particular nutrient cycling, or biogeochemical cycles, could be done by monitoring inputs and outputs to ecosystems.  If those ecosystems, in this case small watersheds, were underlain by impervious rock, then it would be relatively simple to quantify nutrient and hydrologic inputs and outputs.  Gaseous inputs and outputs would be difficult, but could be quantified with enough work.  In this scenario, the inputs would be either biological or meteorological (weathering, althoug important, would work at a slower time scale, but still, is measurable).  It was also assumed that animals would just as likely move into the watershed as out, so their net flux would cancel out to zero.  Thus, this black box approach allowed one to measure the inputs and outputs of an entire ecosystem.

    Bedrock and Soils
    The watersheds were covered by glaciers until about 12,000-13,000 ya.  The geologic substrate is bedrock and stony till, made up of primarily quartz-mica schist, and quartzite embedded with schist and calc-silicate rock.  Metamorphic rocks were transformed into granite in most places, and deep seepage through cracks is thought to be minimal.

    Soils are well-drained spodosols, with a sandy-loam texture.  There is a thick (3-15 cm) thick organic layer at the surface.  Most precipitation infiltrates the soils, and there is little run-off.  At the higher elevations, the soils are thinner, and bedrock can be seen in locations.  The pH is low (< 4.7) and the soils are relatively infertile.

    Vegetation and Fauna
    The HBEF is part of the northern hardwood zone.  Principal deciduous species include beech (Fagus grandifolia), sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), white ash (Fraxinus americana), basswood (Tilia americana), red maple (Acer rubrum), red oak (Quercus rubra) and elm (Ulmus americana); coniferous species include hemlock (Tsuga canadensis), red spruce (Picea rubens), and white pine (Pinus strobus).  In disturbed sites, pin cherry becomes very prominent (Prunus pennsylvanica).

    The forests were logged just after the turn of the century (1909-1919), and extensively damaged by the great New England hurricane of 1938.  There is no history of fire in these stands.  The NPP of the forest was estimated at 4.85 Mg ha-1 yr-1 between 1965-1977, but in recent years has declined significantly (0.89 Mg ha-1 yr-1 from 1987-1992) for reasons we will discuss later.  The basal area in 1965 was 24 m2/ha.  There are thought to be about 100 total plant species on the site.

    Many birds are native to these forests, as well as showshoe hare (Lepus americanus), red fox (Vulpes fulva), black bear (Ursus americanus), moose (Alces americana), and whitetail deer (Odocoileus virginianus).  Deer populations are generally low because of hunting pressure and very severe winters.

    Hydrology
    In order to analyze nutrient budgets, the researchers at HBEF needed to figure out what period during the year would constitute a water-year.  This is the period of time during the year that consistently gives the highest correlation between precipitation and streamflow.  At HBEF, streamflow is dependent on 1) precipitation, 2) amount of water stored in the soil, and 3) amount of water from the snowpack.
 
    After doing a large number of regressions, the researchers settled on the period from June 1 through May 31.  This time period also separates the year into growing and dormant seasons in large part.  Consistently gave high correlations throughout the study years corresponded to this time period and is explained by several phenomena:

                a) evapotranspiration causes variability in soil moisture from year to year, by fall, soils are recharged, since plants
                    are no longer transpiring,
                b) water storage at HBEF is about 10-15 cm, and
                c) adding 20-30 cm of water while the snowpack melts fully recharges the soil water content.

    HBEF gets 130 cm of precipitation on average each year.  Of this, about 62% becomes streamflow, and 38% is lost due to evapotranspiration.  It rain on average 111 days of the year, or about 1/3 of the days.  This gives an average rainfall intensity of about 1.2 cm/event/day.  However, average precipitation is of limited utility.  Between 1956-1974, only 9 yrs had rainfall within 10 cm of the mean.  The interannual variability is on the order of 12%.  The range between lowest and highest rainfall is 91 cm, and emphasizes the need to maintain long-term records of rainfall.  In essence, no year is "average"!

    Although yearly amounts vary quite a bit, over the long haul, the monthly variation is small.  Rainfall is essentially constant from month to month, so there is no extended dry period.  In the winter months, most of the precipitation falls as snow.  This is important, because snow is lighter than drops of rain, and have less erosive potential.  About 30% of the yearly precipitation falls as snow.

    Streamflow and Evapotranspiration
    As we saw at Coweeta, streamflow is measured with weirs, and evapotranspiration using standard meteorological techniques.  While rainfall is relatively constant during the year, streamflow tends to peak in the spring months, when there are abundant rains, and the snowpack is melting, and there is less transpiration by the plants.  Streamflow shows a strong correlation with annual precipitation, whereas evapotranspiration does not.  Once evapotranspirational needs are met, the rest of the water leaves the system through streamflow.  Transpiration from plants accounts for about 60% of total evapotranspiration, the rest leaving as evaporation from soil and plant surfaces, but not passing through plants.  Most of this transpiration must occur when the plants are active, between May and September.
 
    On an annual basis, variability in total streamflow is small (only by a factor of two or so), but individual events can alter streamflow by several orders of magnitude.  So, its important to monitor individual events, not just annual totals.  As streamflows go up, their erosive potential, and their ability to carry debris rise exponentially, so these events are important if you are monitoring fluxes of materials in the system.
 
    As streamflow is reduced, more of the water is retained in the soils, or passes through the vegetation.  Thus in low rainfall years, the nutrient cycles tend to be "tighter", that is, less is lost due to streamflow and leaching.

References

Federer, C.A., L.D. Flynn, C.W. Martin, J.W. Hornbeck, and R.S. Pierce. 1990. Thirty years of hydrometeorological
        data at the Hubbard Brook Experimental Forest, New Hampshire. USDA Forest Service, General Technical Report
        NE-141, 44 pp.

Likens, G.E. and F.H. Bormann. 1995. Biogeochemistry of a Forested Ecosystem, 2nd Edition. Springer-Verlag, NY.
        159 pp.


Return to Ecosystems Main Page                                        Return to Biology Homepage