Abstract

The Hubbard Brook Experimental Forest was established by the U.S. Forest Service in 1955 as a major center 

for hydrologic research in the Northeast. The Hubbard Brook Ecosystem Study originated 8 years later with 

the idea of using the small watershed approach to study element flux and cycling and the response of 

forest ecosystems to disturbance. Since that time, the research program at Hubbard Brook has expanded to 

include various physical, chemical and biological measurements collected by researchers from a number 

of cooperating institutions. Collaborative, long-term data are the keystone of the Hubbard Brook Ecosystem 

Study and have provided invaluable insight into how ecosystems respond to disturbances such as air 

pollution, climate change, forest disturbance, and forest management practices. This report highlights longterm ecological trends at Hubbard Brook, provides explanations for some of the trends, and lists references 

from the scientific literature for further reading.

Acknowledgments

We would like to thank Kimberley M. Driscoll of Syracuse University for compiling data and developing initial 

graphs used in this publication. We also thank David Sleeper of the Hubbard Brook Research Foundation for 

advice and encouragement. The Hubbard Brook Research Foundation provided support for the photography, 

design, and layout of this report, through a grant from the Northeastern Ecosystem Research Cooperative. This 

manuscript is a contribution of the Hubbard Brook Ecosystem Study. Hubbard Brook is part of the Long-Term 

Ecological Research (LTER) network, which is supported by the National Science Foundation. The Hubbard 

Brook Experimental Forest is operated and maintained by the U.S. Forest Service, Northern Research Station, 

Newtown Square, Pennsylvania.

The design of this report was done by RavenMark, Inc. of Montpelier, Vermont. 

Cover photos, top two: U.S. Forest Service Archives. Bottom two: Buck Sleeper. 

Photographs throughout this report are by Buck Sleeper, unless otherwise noted.

The Authors

JOHN L.CAMPBELL and CHRISTOPHER EAGAR are research ecologists and AMEY S. BAILEY is a forest 

technician with the Northern Research Station, Durham, NH. CHARLES T. DRISCOLL is a university professor 

of environmental systems engineering and CHRIS E. JOHNSON is an associate professor of civil and 

environmental engineering, Syracuse University, Syracuse, NY. GENE E. LIKENS is an ecologist and 

distinguished senior scientist of the Institute of Ecosystem Studies, Millbrook, NY. THOMAS G. SICCAMA is 

a professor emeritus of forest ecology, Yale School of Forestry and Environmental Studies, New Haven, CT. 

TIMOTHY J. FAHEY is a professor of natural resources, Cornell University, Ithaca, NY. STEVEN P. HAMBURG 

is Director of the Global Environment Program at the Watson Institute for International Studies, Brown 

University, Providence, RI. RICHARD T. HOLMES is a professor emeritus of biology, Dartmouth College, 

Hanover, NH. DONALD C. BUSO is a biologist and manager of field research at the Hubbard Brook 

Experimental Forest, for the Institute of Ecosystem Studies, Millbrook, NY.

Published by:

USDA FOREST SERVICE

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Newton Square, PA 19073-3294

October 2007

www.nrs.fs.fed.us

For additional copies:

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Fax: (704) 368-0152

Covers.indd 2 
Contents

Hubbard Brook Experimental Forest ................................................................................................... 1

Site Description and Characteristics .................................................................................................... 1

Map of the Hubbard Brook Experimental Forest in the 

White Mountain National Forest, New Hampshire ........................................................................ 2

Research Focus .................................................................................................................................... 3

Report Overview ................................................................................................................................. 5

Biological Measurements 

	 Bird abundance ...............................................................................................................................	 5	

	 Tree species biomass ......................................................................................................................	 7	

	 Tree stem counts within and between health classes ...................................................................	 9	

Physical Measurements 

	 Precipitation, stream runoff, and evapotranspiration ....................................................................	 11	

	 Mean annual air temperature ..........................................................................................................	 13	

	 Maximum snow depth and water content 

	 	 and snow cover duration ...........................................................................................................	 15	

	 Days of ice cover for Mirror Lake ...................................................................................................	 17	

Chemical Measurements 

	 Chloride in Mirror Lake ...................................................................................................................	 19	

	 Sulfur dioxide emissions versus sulfate concentrations .................................................................	21	

	 Stream water concentrations of base cations,

	 	 sulfate and nitrate, and pH ........................................................................................................	 23	

	 Concentrations of forms of aluminum in soil solutions ................................................................	 25	

	 Lead in precipitation, stream water, and the forest floor ..............................................................	 27	

	 Forest floor carbon and nitrogen ....................................................................................................	 29	

Experimental Manipulations 

	 Nitrate concentrations in stream water ..........................................................................................	 31	

	 Whole tree harvest effects on exchangeable calcium ...................................................................	 33	

	 Calcium concentration in wood fern fronds .................................................................................	 35	

	 Aboveground biomass.....................................................................................................................	 37	

	 Changes in water yield after forest cutting ....................................................................................	 39
Long-term Trends from 

Ecosystem Research at the 

Hubbard Brook 

Experimental Forest


Hubbard Brook Experimental Forest

The Hubbard Brook Experimental Forest (HBEF) was established in 1955 as a site of

hydrologic research in New England. The site is located within the White Mountain

National Forest in central New Hampshire. The emphasis of early studies at the HBEF

was the impact of forest management on water yield and quality, and flood control.

The Northern Research Station of the U.S. Forest Service manages the site for longterm ecosystem research and operates an on-site field station, the Robert S. Pierce

Ecosystem Laboratory.

The northern hardwood forest of the HBEF is representative of much of the Northern

Forest region, which covers more than 10.5 million hectares stretching from the

northern woods of Maine to the Adirondack Mountains and Tug Hill regions of New

York. The Northern Forest is the largest contiguous block of forest land in the eastern

United States and includes a mixture of mountain ranges, rivers, lakes, and wetlands

inhabited by many wildlife species (e.g., moose, pine marten, Canada lynx, song birds,

peregrine falcons, common loons, bald eagles). The key issues facing the sustainable

management of the Northern Forest include the impacts of forest management, land

development, air pollution, climate change, introduced species, timber and fiber

production, water supply and quality, and management of carbon stocks.

The Hubbard Brook Ecosystem Study (HBES) originated with the idea of using

the small watershed approach to study element flux and cycling. A joint research

program between the U.S. Forest Service and Dartmouth College was established

by a cooperative agreement in 1963. In 1988 the HBEF was designated a Long

Term Ecological Research (LTER) site by the National Science Foundation. Ongoing

cooperative efforts among diverse educational and research institutions and

government agencies have resulted in one of the longest and most extensive

continuous databases on the hydrology, biology, geology, and chemistry of natural

ecosystems. Today, more than 40 scientists from about 20 institutions participate

in the HBES.

The primary goals of the HBES are: 1) to advance scientific understanding of

forest and aquatic ecosystems, and their response to natural and human-induced

disturbances; 2) provide scientific information required for making sound management

and policy decisions; 3) to offer educational and research opportunities to students;

and 4) to promote greater public awareness of ecosystem science, with a focus on

the northern hardwood forest.

Site Description and Characteristics

The HBEF is a 3,160-ha, bowl-shaped valley with hilly terrain, ranging from 222 to

1,015 m altitude; the Forest is located in the towns of Woodstock and Ellsworth,

NH. The HBEF has a network of precipitation and stream-gaging stations, weather

instrumentation, as well as soil and vegetation monitoring sites on small first order

watersheds. There are nine first-order gaged watersheds at the HBEF, including several

used for long-term experiments. Watersheds are numbered (Fig. 1) and throughout

this report are referred to in the shorthand: W1, W2, etc.

The northern 

hardwood forest 

of the HBEF is 

representative of 

much of the Northern 

Forest region, 

which covers more 

than 10.5 million 

hectares stretching 

from the northern 

woods of Maine 

to the Adirondack 

Mountains and 

Tug Hill regions 

of New York.


Average annual precipitation is about 1,400 mm, with one-third to one-quarter as

snow. Approximately 111 separate precipitation events occur each year. A snowpack

usually persists from late-December until mid-April, with a peak depth in March.

January averages about –9 °C, and long periods of low temperatures are common.

The average July temperature is 18 °C. The average number of days without killing

frost is 145; however, the growing season for trees is considered to be from 15 May

to 15 September. The estimated annual evapotranspiration is about 500 mm.

Soils at the HBEF are predominantly well drained Spodosols, Typic Haplorthods,

derived from glacial basal till, with sandy loam textures. These soils are acidic (pH

about 4.5 or less) and relatively infertile (base saturation of mineral soil ~ 10 percent).

A 20- to 200-mm thick forest floor layer is present, except where the soil surface has

been disturbed by fallen trees. Soil depths, including unweathered till, are highly

variable but average about 2.0 m. Soil on the ridgetops may consist of a thin

accumulation of organic matter resting directly on bedrock. The separation between

the pedogenic zone and unweathered till and bedrock below is indistinct. Average

depth to the C horizon is about 0.6 m. At various places in the HBEF, the C horizon

exists as an impermeable pan. The unsorted till includes coarse fragments (rocks) of

all sizes scattered throughout the soil profile, though with increasing density with

depth. In many locations, surficial boulders are prominent.

Figure 1. Site map of the Hubbard Brook Experimental Forest in the White Mountain National 

Forest, New Hampshire. Shown are the network of rain gages, experimental watersheds, 

and Mirror Lake.


The HBEF is entirely forested, mainly with deciduous northern hardwoods: sugar

maple (Acer saccharum), beech (Fagus grandifolia), and yellow birch (Betula 

allegheniensis), and some white ash (Fraxinus americana) on the lower and middle

slopes. Other less abundant species include mountain maple (Acer spicatum), striped

maple (Acer pensylvanicum), and trembling aspen (Populus tremuloides). Red spruce

(Picea rubens), balsam fir (Abies balsamea), and mountain paper birch (Betula 

papyrifera var. cordifolia) are abundant at higher elevations and on rock outcrops.

Hemlock (Tsuga canadensis) is found along the main Hubbard Brook. Pin cherry

(Prunus pensylvanica), a shade intolerant species, dominates all

sites for the first decade following a major forest disturbance.

The presettlement forest was dominated by red spruce, beech

and birch. Logging operations began in the 1880s with major

removals in the 1910s when the remaining spruce and large

portions of the better quality hardwoods were removed. The

1938 hurricane and subsequent salvage logging resulted in

additional tree removal and understory release. The present

second-growth forest is uneven aged and comprised of about

80 to 90 percent hardwoods and 10 to 20 percent conifers.

Research Focus

The small watershed ecosystem approach to research on nutrient cycling was

pioneered at the HBEF. This approach uses the forest as a single integrated landscape

unit with which scientists can conduct experiments on a watershed level, monitoring

long-term changes in streamflow, nutrient cycling, forest growth, and composition.

Experimental manipulations have been used extensively at the HBEF. Many whole

watershed, stream, and lake manipulations have been conducted to test hypotheses,

obtain quantitative information on the impacts of management options, and to validate

process-based ecosystem models. Whole ecosystem manipulations conducted at the

HBEF include experiments to examine forest management practices, (clearcutting,

strip cutting, whole tree harvesting), the role of biological uptake on nutrient cycles

(herbicide application for 3 years) and mitigation of air pollution (addition of calcium

silicate).

Scientists track climatic conditions and chemical inputs through the air, rain, and

snow. Researchers use long-term measurements, long-term experiments, models, and

cross-site studies to investigate the effects of these conditions and how the ecosystem

responds to changes in these conditions. These data are used to document and assess

the ecological effects of regional and global environmental disturbances. At Hubbard

Brook there are major research themes that encompass much of the ongoing research

at the site, including perturbations from air pollution (such as acid rain and mercury),

and the response and recovery from catastrophic (such as clearcutting) and

noncatastrophic forest perturbations (such as ice storms, forest disease).

A strength of the HBES is the long-term monitoring program, which has demonstrated

that short-term observations are often misleading and that decades may be required

to detect real changes in complex ecosystems. The long-term record at the HBEF

provides: 1) insight into ecosystem function; 2) empirical data for testing models and

generating hypotheses; 3) a record of extreme or unusual events; and 4) information

that is relevant to regional, national, and global issues.

Bluebead lily (Clintonia borealis) fruit


Table 1. Current long-term monitoring data sets developed through the Hubbard Brook 

Ecosystem Study. The institution responsible for the data and initial year of data collection 

is indicated.

Measurement Institution Year

Hydrometeorological Monitoring 

Instantaneous streamflow (9 stations) U.S. Forest Service 1956

Daily streamflow (9 stations) U.S. Forest Service 1956

Daily precipitation (25 stations) U.S. Forest Service 1956

Air temperature: mean, min, max (8 stations) U.S. Forest Service 1955

Solar radiation U.S. Forest Service 1958

Wind speed and direction U.S. Forest Service 1965

Vapor pressure U.S. Forest Service 1966

Weekly snow depth and snow water equivalent U.S. Forest Service 1959

Hourly canopy surface wetness Inst. of Ecosystem Studies 1989

Mirror Lake precipitation (2 stations) Inst. of Ecosystem Studies 1974

Mirror Lake groundwater Inst. of Ecosystem Studies 1978

Mirror Lake streamflow (outlet and 3 inlets) Inst. of Ecosystem Studies 1978

Mirror Lake thermal profiles Inst. of Ecosystem Studies 1967

Mirror Lake ice in/out dates Inst. of Ecosystem Studies 1967

Solution and Air Chemistry 

Weekly bulk precipitation (6-10 stations) Inst. of Ecosystem Studies 1963

Monthly soil solution chemistry within W1 and W6 Syracuse University 1984

Weekly stream chemistry at weirs of W1-9 Inst. of Ecosystem Studies 1963

Monthly stream chemistry within W1 and W6 Syracuse University 1982

Mirror Lake water column chemistry Inst. of Ecosystem Studies 1967

Mirror Lake precipitation chemistry (2 stations) Inst. of Ecosystem Studies 1974

Mirror Lake groundwater chemistry Inst. of Ecosystem Studies 1980

Mirror Lake inlets/outlet chemistry (outlet and 3 inlets) Inst. of Ecosystem Studies 1967

Air chemistry (sulfur dioxide, nitric acid, particulates, ozone) Inst. of Ecosystem Studies 1988

Organisms 

Bird populations Dartmouth College 1969

Phytophagous insect populations Dartmouth College 1969

Salamander populations Inst. of Ecosystem Studies 1970

Snail populations Brown University 1996

Small mammals Dartmouth and Wellesley 1986

W2 Vegetation, biomass, chemistry University of Wyoming 1970

W4 Vegetation, biomass U.S. Forest Service 1970

W5 Vegetation, biomass, chemistry Cornell/Yale University 1985

W6 Vegetation, biomass, chemistry Yale University 1965

Valley-wide vegetation Cornell University 1997

Vegetation structure and composition (Bird Transect Area) Dartmouth and Wellesley 1970

Fine-root, biomass, chemistry Cornell University 1992

Microbial biomass and activity Inst. of Ecosystem Studies 1992

Litter-Fall Cornell University 1988

Phenology U.S. Forest Service 1989

Soils 

Forest floor mass, chemistry (W1, W5, and W6) Yale/Brown University 1968

Chemical and physical properties from soil pits Syracuse/Cornell/ 1984

(W5, valley-wide) Brown University

Sediment yield in weir basin U.S. Forest Service 1956

Soil frost U.S. Forest Service 1956

Soil temperature and moisture U.S. Forest Service 1959

 
Report Overview

In this report, we highlight some results of long-term measurements and experiments

conducted at the HBEF. The figures included show some of the more interesting

long-term trends; however not all of the long-term Hubbard Brook data are presented.

A complete list of long-term measurements is provided in Table 1. This report includes

graphs, a description of the data and trends, the principal investigator(s) and related

references. The figures are grouped into four major categories: 1) biological measurements; 2) physical measurements; 3) chemical measurements; and 4) experimental

manipulations. The report targets a broad audience, including land managers,

environmental protection agencies and organizations, educators and students,

research scientists, and the general public.

Biological Measurements

Bird abundance 

Bird censuses are conducted annually between late

May and early July on a 10-ha plot west of W6. The

census methods, which have been used consistently

since the inception of the study, consist of timed

censuses along transects, mist net capture, and

systematic observations on individual birds. Results from this study show that the

abundance of birds has declined from more than 200 individuals per 10 ha in the

early 1970s to 70 to 100 per 10 ha from the early 1990s to the present. Shifts in the

Figure 2. Number of adult birds (males and females, all species combined) breeding at 

the HBEF, 1969-2005.

Bird Abundance 

Black-throated blue warbler (Dendroica 

caerulescens). Photograph by

Nicholas Rodenhouse
composition of the bird community that have occurred since sampling began in

1969 can be explained in part by changes in vegetation, resulting from natural forest

succession and local disturbances. Other important factors that influence bird

abundance are food availability and events that occur during migration and winter

periods. However, long-term declines in the overall abundance of birds remain largely

unexplained and may depend upon shifts at multiple levels as the ecosystem responds

to compounded stresses and perturbations. Figure updated from Holmes and Sherry

(2001).

Principal Investigator:

Richard T. Holmes, Dartmouth College

Online Access: 

Bird abundances – http://www.hubbardbrook.org/data/dataset.php?id=81

Associated Databases:

Lepidoptera, small mammals

Further Reading:

Holmes, R.T.; Sherry, T.W. 1988. Assessing population trends of New Hampshire forest 

birds: Local versus regional patterns. The Auk. 105: 756-768.

Holmes, R.T.; Sherry, T.W. 2001. Thirty-year bird population abundance in an 

unfragmented temperate deciduous forest: Importance of habitat change.

The Auk. 118: 589-609.

Holmes, R.T.; Sherry, T.W.; Sturges, F.W. 1986. Bird community dynamics in a 

temperate deciduous forest: long-term trends at Hubbard Brook. Ecological

Monographs. 56: 201-220.

Holmes, R.T.; Sturges, F.W. 1975. Bird community dynamics and energetics in 

a northern hardwoods ecosystem. Journal of Animal Ecology. 44: 175-200.

Undergraduate students updating the bird census computer database
Figure 3. Tree species biomass for live and dead trees on the reference watershed W6.

Tree Species Biomass

Tree species biomass

Tree density and diameter at breast height (d.b.h.=1.37 m above ground level) have been

measured at 5-year intervals on the reference watershed (W6) since 1965. Biomass is

calculated using allometric equations that relate tree diameter to the mass of each species.

Biomass calculations for dead trees include estimates of decay (see Siccama et al. 2007).

Recent unexpected declines in total live biomass (> 10 cm d.b.h.) on W6 reflect a

combination of decreased growth rates and increased mortality rates of the dominant

species (sugar maple, American beech, and yellow birch). One possible explanation for

the decline in sugar maple biomass is depletion of soil calcium associated with acidic

deposition. While American beech biomass increased steadily from 1965 to 1997, the

recent declines in the large size class (> 10 cm d.b.h.) are due primarily to beech bark
disease. Beech trees affected by the disease are gradually girdled by cankers and often

respond with vigorous root sprouts. Beech bark disease typically does not affect the

growth of smaller trees (2 to 9 cm d.b.h.). Of the three dominant tree species, yellow

birch has shown the least amount of change in biomass over the long-term record.

Total live tree biomass in W6 reached a maximum earlier and at a lower level than

previously predicted (see Wittaker et al. 1974), which clearly shows the importance

of testing such estimates with carefully collected long-term data.

Principal Investigators:

Thomas G. Siccama, Yale University

Timothy J. Fahey, Cornell University

John J. Battles, University of California - Berkeley

Chris E. Johnson, Syracuse University

Online Access:

1965 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=29

1977 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=30

1982 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=31

1987 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=32

1992 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=33

1997 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=34

2002 W6 forest inventory – http://www.hubbardbrook.org/data/dataset.php?id=35

Associated Databases: 

Forest inventory data (W1, W5, and Bird Transect Area)

Further Reading:

Whittaker, R.H.; Bormann, F.H.; Likens, G.E.; Siccama, T.G. 1974. The Hubbard Brook 

Ecosystem Study: Forest Biomass and Production. Ecological Monographs. 44: 233-254.

Siccama, T.G.; Denny, E. 2006. Long-term changes in the calcium concentration of 

wood fern fronds [Online]. Available at http://www.hubbardbrook.org/yale/

watersheds/w6/biomass-stop/how-to-quantify.htm. (accessed 12 Sept. 2006).

Siccama, T.G.; Fahey, T.J.; Johnson, C.E.; Sherry, T.; Denny, E.G.; Girdler, E.B.;

Likens, G.E.; Schwarz, P. 2007. Population and biomass dynamics of trees in a northern 

hardwood forest at Hubbard Brook. Canadian Journal of Forest Research. 37: 737-749.

The northern hardwood forest at the HBEF
Tree Stem Counts Within and Between Health Classes

Tree stem counts within and between health classes

Figure 4. Tree stem counts within and between health classes at the 10-ha Bird 

Transect Area.

The fate of more than 5,000 trees has been tracked at the 10-ha Bird Transect Area

(see Figure 2) at the HBEF as part of a continuing forest inventory that started in 1991.

At the beginning of the study, all stems greater than 10-cm in the entire 10-ha area

were marked with aluminum tags and the status of each tree was recorded. Trees are

reassessed at 2-year intervals and these data are used

to calculate the transfers of trees among five categories:

1) live and healthy; 2) unhealthy (thin crowns and few

or yellowed leaves); 3) standing dead (most branches

still present); 4) snag (dead trees without major

branches, or broken off above breast height [1.37 m]);

or 5) downed (uprooted or broken off below 1.37 m).

In each 2-year resurvey, ingrowth trees, defined as

those reaching the 10-cm d.b.h. threshold, are also

tagged. In the diagram above, which includes 10 years

of data (1991-2001), the numbers in boxes represent

the number of trees in each category for each sampling

year and the values associated with arrows represent

the number of trees changing categories between

the 2-year measurement interval. To keep the diagram

legible, transfers to the downed wood category are not shown by arrows. Instead,

they are given in brackets at the bottom of the figure, ordered according to the source

Tree tagged with identification number
10

category [live (including ingrowth), unhealthy, standing dead, snag]. Of the 4720 live

trees (4662 healthy and 58 unhealthy) tagged in 1991, 11 percent died over the 10-year

period. Most of the dying trees entered the standing dead pool. Thereafter, a majority of

the standing dead trees passed through the snag

pool before finally falling to the ground. An

average standing dead tree remained standing in

this forest for ~7.5 years while an average snag

remained standing for ~15 years. The relatively

short existence of standing dead and snag trees in

mature northern hardwood forests results in a low

density of deteriorating trees for wildlife that use

this habitat. Further calculations and summaries,

including basal area, stem density, and species

composition can be made using an online

interactive program (see http://www.

hubbardbrook.org/w6_tour/biomass-stop/

phytobird.htm). Figure adapted from Siccama et

al. 2007.

Principal Investigators:

Thomas G. Siccama, Yale University

Timothy J. Fahey, Cornell University

Chris E. Johnson, Syracuse University

Ellen G. Denny, Yale University and U.S. Forest

Service

Online Access:

Bird Transect Area forest inventory –

http://www.hubbardbrook.org/data/dataset.

php?id=43

Associated Databases: 

Forest inventory data (W1, W5, and W6)

Further Reading:

Siccama, T.G.; Fahey, T.J.; Johnson, C.E.; Sherry,

T.; Denny, E.G.; Girdler, E.B.; Likens, G.E.;

Schwarz, P. 2007. Population and biomass 

dynamics of trees in a northern hardwood forest at 

Hubbard Brook. Canadian Journal of Forest

Research. 37:737-749.

Deteriorating trees provide habitat for wildlife
11

Figure 5. Precipitation, streamflow, and evapotranspiration at the hydrologic reference 

watershed (W3) from 1957 to 2005.

Precipitation, Stream Runoff, and Evapotranspiration

Physical Measurements

Precipitation, stream runoff, and evapotranspiration

Precipitation and streamflow have been measured at the hydrologic reference

watershed (W3) since 1957 and at eight other watersheds at the HBEF with starting

dates ranging from 1956 to 1995. Precipitation is measured with rain gages located

in and around each watershed, and is weighted to determine the value for the entire

watershed. Streams are gaged at weirs located at the outlet of each watershed,

allowing for precise measurements of streamflow. Evapotranspiration, which is a term

that describes water evaporated directly from the leaves, soil or snow, is calculated by

subtracting streamflow from precipitation. Variation in precipitation and streamflow is

much greater than variation in evapotranspiration. Over the long-term, 61 percent of

the precipitation that enters W3 leaves as streamflow, while 39 percent is returned to

the atmosphere via evapotranspiration. There are no significant long-term trends in

precipitation, streamflow, or evaporation although some indices suggest that the past

4 to 5 decades have been relatively wet.
12

Principal Investigators:

John L. Campbell, U.S. Forest Service

Amey S. Bailey, U.S. Forest Service

Christopher Eagar, U.S. Forest Service

Online Access:

Daily precipitation by watershed – http://www.hubbardbrook.org/data/dataset.

php?id=14

Daily streamflow by watershed – http://www.hubbardbrook.org/data/dataset.php?id=2

Associated Databases: 

Instantaneous streamflow, daily precipitation by rain gage

Further Reading:

Bailey, A.S.; Hornbeck, J.W.; Campbell, J.L.; Eagar, C. 2003. Hydrometeorological 

database for Hubbard Brook Experimental Forest: 1955-2000. Gen. Tech. Rep. NE-305.

Newton Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern

Research Station. 36 p.

Measurements from stream-gaging stations are

recorded and analyzed.

Photographs: (top) U.S. Forest Service Archives;

(bottom) Hubbard Brook Research Foundation

Archives.
13

Figure 6. Mean annual air temperature at the Robert S. Pierce Ecosystem Laboratory 

and Station 14.

Mean Annual Air Temperature

Mean annual air temperature

Air temperature is measured at seven rain gage clearings (Robert S. Pierce Ecosystem

Laboratory and Stations 1, 6, 14, 17, 23, and 24) located throughout the area of the

experimental watersheds (see Figure 1). The oldest air temperature record dates back

to 1955 at Station 1. Since that time, temperature measurements have been made

consistently using hygrothermographs housed in standard shelters. Although mean

annual temperature is quite variable, all locations show an upward trend, consistent

with temperature records from elsewhere in the region over the same time period,

as well as over a longer (>100 year) time period. Winter air temperatures at the HBEF

are warming more rapidly than summer temperatures, and have greater interannual

variability. In the graph above, air temperature data show significant increases

(seasonal Kendall tau test, p<0.01) and give an indication of the range in air

temperatures arising primarily from differences in elevation and aspect. A more

thorough understanding of long-term temperature trends at the HBEF is crucial to

predicting how ecosystem processes will respond to future climate change.
14

Principal Investigators:

John L. Campbell, U.S. Forest Service

Amey S. Bailey, U.S. Forest Service

Christopher Eagar, U.S. Forest Service

Online Access:

Air temperature – http://www.hubbardbrook.org/data/dataset.php?id=58

Associated Databases: 

Solar radiation, soil temperature

Further Reading:

Bailey, A.S.; Hornbeck, J.W.; Campbell, J.L.; Eagar, C. 2003. Hydrometeorological 

database for Hubbard Brook Experimental Forest: 1955-2000. Gen. Tech. Rep. NE-305.

Newton Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern

Research Station. 36 p.

View of the Hubbard Brook Valley during fall. Photograph by Jerry Franklin
15

Figure 7. Maximum snow depth and water equivalence and snow cover duration at Station 2.

Maximum Snow Depth and Water Content and Snow Cover Duration

Maximum snow depth and water content and snow cover duration

Snow measurements have been collected weekly at 21 locations at the HBEF for various

time periods. Snow depth and water content are measured at “snow courses” that are

located under the forest canopy adjacent to select rain gages. A “snow course” consists

of a transect of 10 points spaced at 2-m intervals. Each week, snow depth is recorded at

each point and a core of the snowpack is collected and weighed to determine snow water

content. The following week, an undisturbed parallel transect, 2-m from the previous

transect, is used. Mean annual maximum snow depth for the 50-year record at Station

2 is 726 mm and mean annual maximum snow water content is 189 mm. On average

continuous snow cover lasts from December 25 to April 16, a total of 112 days. Despite

high interannual variability, there have been slight, but statistically significant long-term
16

Snow cores are collected and weighed to measure the water

content of snow. Photographs: U.S. Forest Service Archives

declines in snow depth (Mann-Kendall test, p=0.04), water content (p=0.03) and snow

cover duration (p=0.04). Over the 50-year record, on average, mean annual maximum

snow depth has decreased by 243 mm, snow water equivalence by 65 mm and snow

cover duration by 23 days. These trends for snow are consistent with other indicators

of climate change at the HBEF.

Principal Investigators:

John L. Campbell, U.S. Forest Service

Amey S. Bailey, U.S. Forest Service

Christopher Eagar, U.S. Forest Service

Online Access:

Snow depth – http://www.hubbardbrook.org/data/dataset.php?id=27

Snow water equivalence – http://www.hubbardbrook.org/data/dataset.php?id=28

Associated Databases: 

Air temperature, soil frost

Further Reading:

Bailey, A.S.; Hornbeck, J.W.; Campbell, J.L.; Eagar, C. 2003. Hydrometeorological 

database for Hubbard Brook Experimental Forest: 1955-2000. Gen. Tech. Rep. NE-305.

Newton Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern

Research Station. 36 p.
17

Days of ice cover for Mirror Lake

Mirror Lake (area, 15 ha; max. depth, 11 m) is located within the Hubbard Brook

Valley and has been the subject of numerous continuous limnological investigations

since the early 1960s. Routine measurements of ice cover on Mirror Lake began in

1967; however, the exact date of onset (ice in) in 1967 is uncertain so the record

shown here begins in 1968. Dates for ice in and dissipation (ice out) of ice cover have

been recorded using criteria that have been followed faithfully since the beginning of

the study. The date of ice in is taken when more than 50 percent of the lake surface is

covered with ice and remains covered. The date of ice out is taken when more than

50 percent of the lake surface is open water and the lake’s surface does not refreeze.

The long-term trend in ice covered days (number of days between ice in and ice out)

shows an overall decline of about 0.5 days per year during the long-term record.

The ice out date is in April and has been occurring significantly earlier on average,

whereas there has been no significant change in the ice in date. The earlier ice out

date in April with time for Mirror Lake is correlated with increased average air

temperatures in April. This pattern is consistent with the pattern of global warming,

but the interannual variability is large and most of the significant change occurred

before 1985. Figure adapted from Likens (2000).

Figure 8. Days of ice cover for Mirror Lake from 1968 to 2004.

Days of Ice Cover for Mirror Lake
18

Principal Investigator: 

Gene E. Likens, Institute of Ecosystem Studies

Online Access:

Mirror Lake ice cover - http://www.hubbardbrook.org/data/dataset.php?id=118

Associated Databases: 

Air temperature, Mirror Lake thermal profiles

Further Reading:

Likens, G.E. 2000. A long-term record of ice-cover for Mirror Lake, New Hampshire: 

Effects of global warming? Verhandlungen Internationale Vereinigung für theoretische

und angewandte Limnologie. 27: 2765-2769.

Ice out at Mirror Lake. Photograph by Donald Buso
19

Chloride in Mirror Lake

Figure 9. Chloride concentrations at the northeast inlet to Mirror Lake and at the 

Mirror Lake outlet.

Chemical Measurements

Chloride in Mirror Lake

The increase in chloride concentrations at the northeast inlet and Mirror Lake outlet is

caused primarily by runoff of road salt used to de-ice Interstate 93 (I-93). Much of the

road salt is transported to Mirror Lake via the northeast inlet stream, which provides

approximately 30-50 percent of all the chloride to the lake but only a small fraction

of total streamflow (2 percent). Chloride concentrations at the northeast inlet began to

increase in 1970, when I-93 opened, despite the installation of an earthen diversion

dam. The decrease in concentrations in 1995-96 was likely due to dilution from higher

than usual precipitation. After 2000, further declines resulted from installing a plastic

liner adjacent to the highway to divert contaminated runoff away from the lake.

Unlike the northeast inlet, chloride concentrations at the lake outlet have continued

to increase because of small increases in salt use on local roads within the west and

northwest inlets to the lake, which carry 47 percent of the water inflow.
20

Mirror Lake and its gaged outlet.

Photographs: (left) U.S. Forest Service Archives,

and (right) Judy Brown

Principal Investigator: 

Gene E. Likens, Institute of Ecosystem Studies

Online Access:

Mirror Lake northeast inlet chemistry – http://www.hubbardbrook.org/data/dataset.

php?id=87

Mirror Lake outlet chemistry – http://www.hubbardbrook.org/data/dataset.php?id=86

Associated Databases: 

Chemistry of Mirror Lake water column, west inlet, northwest inlet

Further Reading:

Bormann, F.H.; Likens, G.E. 1985. Air and watershed management and the aquatic 

ecosystem. In: Likens, G.E., ed. An ecosystem approach to aquatic ecology: Mirror

Lake and its environment. New York: Springer-Verlag: 436-444.

Rosenberry, D.O.; Bukaveckas, P.A.; Buso, D.C.; Likens, G.E.; Shapiro, A.M.;

Winter, T.C. 1999. Movement of road salt to a small New Hampshire lake. Water, Air,

and Soil Pollution. 109: 179-206.
21

Figure 10. Sulfur dioxide emissions versus sulfate concentrations in precipitation and 

stream water.

Sulfur Dioxide Emissions Versus 

Sulfate Concentrations

Sulfur dioxide emissions versus sulfate concentrations

In the national debate about acid rain during the 1980s, there was insufficient data to

show how sulfur emitted to the atmosphere from pollution sources is related to sulfur

concentrations in precipitation and stream water. Since that time, there have been

improvements in the methods for determining the quantity of sulfur emitted to the

atmosphere and there are now longer records of weekly precipitation and stream

water chemistry data at the HBEF. These factors have made it possible to show that

there is a strong significant relationship between sulfur dioxide emissions from source

areas in the United States and Canada and sulfate concentrations in bulk precipitation

at the HBEF. Long-term data (1965-2003) from the HBEF show that reducing emissions

of sulfur dioxide decreases the concentration of sulfate in bulk precipitation, ultimately

reducing sulfate concentrations in stream water. Figure adapted from Likens et al.

(2002).
22

Principal Investigator: 

Gene E. Likens, Institute of Ecosystem Studies

Online Access:

Stream water chemistry (W6) – http://www.hubbardbrook.org/data/dataset.php?id=8

Bulk precipitation chemistry (W6) – http://www.hubbardbrook.org/data/dataset.

php?id=20

Associated Databases: 

Stream water chemistry (W1-5, and W7-9), Bulk precipitation chemistry (W1-5, and

W7-9)

Further Reading:

Likens, G.E.; Buso, D.C.; Butler, T.J. 2005. Long-term relationships between SO2 and 

NOX emissions and SO4

2

- and NO3- concentration in bulk deposition at the Hubbard 

Brook Experimental Forest, New Hampshire. Journal of Environmental Monitoring.

7: 964-968.

Likens, G.E.; Driscoll, C.T.; Buso, D.C.; Mitchell, M.J.; Lovett, G.M.; Bailey, S.W.;

Siccama, T.G.; Reiners, W.A.; Alewell, C. 2002. The biogeochemistry of sulfur at 

Hubbard Brook. Biogeochemistry. 60: 235-316.

Sampling precipitation chemistry.

Photograph: Hubbard Brook Research

Foundation Archives
23

Figure 11. Stream water concentrations of base cations, sulfate and nitrate, and pH at W6.

Stream Water Concentrations of Base Cations, 

Sulfate and Nitrate, and pH

Stream water concentrations of base cations, sulfate and nitrate, 

and pH

Stream water chemistry has been monitored at the outlet of the biogeochemical

reference watershed (W6) since 1963. Declines in stream water sulfate during this time

have coincided with decreases in atmospheric deposition of sulfur. During the last 35

years, nitrate concentrations have also declined in stream water. However, the trend in

stream water nitrate has not been clearly linked to atmospheric nitrogen deposition,

and the cause of the trend is not yet well established. The recent trend of decreasing

nitrate in W6 was interrupted by two noncatastrophic disturbance events: a soil

freezing event in 1989 and an ice storm in 1998. Declines in nitrate and sulfate have

been balanced to some degree by declines in concentrations of base cations. The pH

of stream water has increased slightly as streams recover from acidic deposition.

However, recovery has been slower than anticipated because base cations have been

depleted from the soil due to acidic deposition and to a lesser extent, a reduction in

atmospheric inputs of base cations. Additionally, years of high sulfur deposition have

caused sulfur to accumulate in the soil, which is now being released to surface

waters as sulfate. The slow recovery response demonstrates the need for long-term

measurements. Figure adapted from Likens et al. (1996).

0

20

40

60

80

100

120

140

160

4.7

4.8

4.9

5.0

5.1

Water year

pH Concentration (μeq/L)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
24

Sampling streams for chemical analyses

Photograph: Hubbard Brook Research Foundation Archives

Principal Investigator: 

Gene E. Likens, Institute of Ecosystem Studies

Online Access:

Stream water chemistry (W6) – http://www.hubbardbrook.org/data/dataset.php?id=8

Associated Databases: 

Stream water chemistry (W1-5, and W7-9)

Further Reading:

Likens, G.E.; Driscoll, C.T.; Buso, D.C. 1996. Long-term effects of acid rain: response 

and recovery of a forest ecosystem. Science. 272: 244-246.

Likens, G.E.; Driscoll, C.T.; Buso, D.C.; Mitchell, M.J.; Lovett, G.M.; Bailey, S.W.;

Siccama, T.G.; Reiners, W.A.; Alewell, C. 2002. The biogeochemistry of sulfur at 

Hubbard Brook. Biogeochemistry. 60: 235-316.

Likens, G.E.; Driscoll, C.T.; Buso, D.C.; Siccama, T.G.; Johnson, C.E.; Lovett, G.M.;

Fahey, T.J.; Reiners, W.A.; Ryan, D.F.; Martin, C.W.; Bailey, S.W. 1998. The 

biogeochemistry of calcium at Hubbard Brook. Biogeochemistry. 41: 89-173.

Driscoll, C.T.; Lawrence, G.B.; Bulger, A.J.; Butler, T.J.; Cronan, C.S.; Eagar, C.;

Lambert, K.F.; Likens, G.E.; Stoddard, J.L.; Weathers, K.C. 2001. Acidic deposition in the 

northeastern United States: Sources and inputs, ecosystem effects, and management 

strategies. BioScience. 51: 180-198.
25

Concentrations of forms of aluminum in soil solutions

Shown are two forms of dissolved aluminum in soil waters: total dissolved aluminum

and organic aluminum. The difference between total dissolved and organic aluminum

represents the inorganic form of dissolved aluminum. Dissolved inorganic aluminum is

considered to be toxic at high concentrations. Organic aluminum is considered to be

less toxic. Figure 12 shows trends in aluminum in forest floor (Oa horizon) and lower

mineral soil (Bs horizon) solutions. The mobilization of aluminum is a consequence of

inputs of acidic deposition to forest soils. In soil solutions draining the upper organic

horizon, aluminum is largely in the nontoxic organic form. In the mineral soil, toxic

inorganic aluminum is mobilized and is ultimately transported to streams. Shown are

Concentrations of Forms of Aluminum in Soil Solutions

Figure 12. Concentrations of total dissolved and organic aluminum in soil solutions at 

the high elevation hardwood zone in W6.
26

soil solutions collected in the high elevation hardwood zone. This is an area of the

HBEF where effects of acidic deposition on soils are greatest. Over the longterm,

concentrations of both forms of aluminum have decreased, consistent with declines

in inputs of acidic deposition to the HBEF.

Principal Investigator: 

Charles T. Driscoll, Syracuse University

Online Access:

Soil solution chemistry (W6) – http://www.hubbardbrook.org/data/dataset.php?id=62

Associated Databases: 

Precipitation and stream water chemistry (W6)

Further Reading:

Driscoll, C.T.; van Breemen, N.; Mulder, J. 1985. Aluminum chemistry in a forested 

Spodosol. Soil Science Society America Journal. 49: 437-444.

Lawrence, G.B.; Fuller, R. D.; Driscoll, C.T. 1986. Spatial relationships of aluminum 

chemistry in the streams of the Hubbard Brook Experimental Forest, New Hampshire.

Biogeochemistry. 2: 115-135.

Driscoll, C.T.; Johnson, N.M.; Likens, G. E.; Feller, M.C. 1988. The effects of acidic 

deposition on stream water chemistry: a comparison between Hubbard Brook, New 

Hampshire and Jamieson Creek, British Columbia. Water Resources Research. 24:

195-200.

Lawrence, G.B.; Driscoll, C.T.; Fuller, R.D. 1988. Hydrologic control of aluminum 

chemistry in an acidic headwater stream. Water Resources Research. 24: 659-669.

Driscoll, C.T.; Postek, K.M. 1995. The chemistry of aluminum in surface waters.

In: Sposito, G., ed. The Environmental Chemistry of Aluminum. Chelsea, MI: Lewis

Publishers: 363-418.

Palmer, S.M.; Driscoll, C.T. 2002. Acidic deposition. Decline in mobilization of toxic 

aluminum. Nature. 417: 242-243.

Palmer, S.M.; Driscoll, C.T.; Johnson, C.E. 2004. Long-term trends in soil solution and 

stream water chemistry at the Hubbard Brook Experimental Forest: relationship with 

landscape position. Biogeochemistry. 68: 51-70.
27

Lead in Precipitation, Stream Water, and the Forest Floor

Figure 13. Lead in precipitation, stream water, and the forest floor (±SE).

Lead in precipitation, stream water, and the forest floor

In the 1970s, the sale of gasoline containing lead additives was restricted in the

United States under the Clean Air Act. Since then, the amount of lead emitted to the

atmosphere has declined resulting in lower concentrations of lead in precipitation

and stream water. However, despite lower inputs, lead continues to accumulate in

the forest ecosystem at the HBEF due to extremely low losses in drainage water. Since

lead was assumed to have a long residence time in the forest floor, the declining rate

of deposition was expected to simply slow the rate of accumulation. However, studies

at the HBEF and elsewhere in the northeastern United States have shown significant

net decreases in the amount of lead in the forest floor. Lead now appears to be

accumulating in the mineral soil. It is unclear what caused the increase in forest

floor lead in 1992 at the HBEF; however, it may be related to the high forest floor

organic matter mass measured in samples collected that year. Based on these field

observations, it is clear that lead is more mobile than previously thought, suggesting

a continued threat of lead pollution in drainage water.
28

Principal Investigator: 

Chris E. Johnson, Syracuse University

Thomas G. Siccama, Yale University

Online Access:

1976 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=69

1977 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=70

1978 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=71

1982 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=72

1987 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=73

1992 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=74

1997 forest floor mass and chemistry (W6) –

http://www.hubbardbrook.org/data/dataset.php?id=75

Associated Databases: 

Forest floor mass and chemistry (W1 and W5)

Further Reading:

Johnson, C.E.; Petras, R.J. 1998. Lead and zinc fractionation in a forest Spodosol.

Soil Science Society of America Journal. 62: 782-789.

Johnson, C.E.; Petras, R.J.; April, R.H.; Siccama, T.G. 2004. Post-glacial lead dynamics 

in a forest soil. Water, Air and Soil Pollution: Focus. 4: 579-590.

Johnson, C.E.; Siccama, T.G.; Driscoll, C.T.; Likens, G.E.; Moeller, R.E. 1995. Changes 

in lead biogeochemistry in response to decreasing atmospheric inputs. Ecological

Applications. 5 :813-822.

Siccama, T.G.; Smith, W.H. 1978. Lead accumulation in a northern hardwood forest.

Environmental Science and Technology. 12: 593-594.

Smith, W.H.; Siccama, T.G. 1981. The Hubbard Brook Ecosystem Study: biogeochemistry 

of lead in the northern hardwood forest. Journal of Environmental Quality. 10: 323-333.

Wang, E.X.; Bormann, F.H.; Benoit, G. 1995. Evidence of complete retention of 

atmospheric lead in the soils of northern hardwood forested ecosystems. Environmental

Science and Technology. 29: 735-739.

Yanai, R.D.; Ray, D.G.; and Siccama, T.G. 2004. Lead reduction and redistribution in 

the forest floor in New Hampshire northern hardwoods. Journal of Environmental

Quality. 33: 141-148.
29

Forest Floor Carbon and Nitrogen

Figure 14. Forest floor carbon and nitrogen (±95 percent confidence interval) at W6.

Forest floor carbon and nitrogen

The forest floor (Oi, Oe and Oa horizons) of W6 has been sampled and analyzed for

carbon and nitrogen content seven times since 1976. All samples were collected from

randomly selected grid cells in W6, and the sampling intensity steadily increased

from 58 samples per year in 1976 to 100 in 2002. Samples from 1976 and 1977 were

combined to make the sample size more comparable to those in later years. Over the

25 years of monitoring, there has been no significant change in either the carbon or

nitrogen content of the forest floor. Linear regression analysis shows an insignificant

long-term increase of 95 kg carbon per ha/yr and an increase of 1.7 kg nitrogen per

ha/yr. There is a significant increase in the carbon:nitrogen ratio for the whole forest

floor over 25 years, reflecting the greater increase in carbon relative to nitrogen.

Though the changes in forest floor carbon and nitrogen contents are subtle, the

shifting carbon:nitrogen ratio suggests that the forest floor has been dynamic over

the 25 years of record. This change in the carbon:nitrogen ratio may slow organic

matter decomposition if sufficient nitrogen to support microbial growth cannot be

obtained from the decomposing material or soil solution. Forest floor samples are

permanently stored in the sample archive at Hubbard Brook, along with many other

types of samples, not only to construct longer records for monitoring, but also to

apply new analytical techniques to preserved samples.
30

The forest floor is the dark horizon near

the surface. It is sampled with a pin block

(below) which is used to extract the

intact organic matter for analyses.

Photographs: U.S. Forest Service Archives

Principal Investigator: 

Steven P. Hamburg, Brown University

Thomas G. Siccama, Yale University

Associated Databases: 

Forest floor mass and chemistry (W1 and W5)

Further Reading:

Huntington, T.G.; Ryan, D.F.; Hamburg, S.P. 1988. Estimating soil nitrogen and carbon 

pools in a northern hardwood forest ecosystem. Soil Science Society of America

Journal. 52: 1162-1167.

Yanai, R.D.; Stehman, S.V.; Arthur, M.A.; Prescott, C.E.; Friedland, A.J.; Siccama T.G.;

Binkley, D. 2003. Detecting change in forest floor carbon. Soil Science Society of

America Journal. 67: 1583-1593.
31

Nitrate Concentrations in Stream Water

Figure 15. Nitrate concentrations in stream water at W2, W4, W5, and W6.

Experimental Manipulations

Nitrate concentrations in stream water

Biologically important nutrients, such as nitrogen, are usually strongly retained in

northern hardwood forest ecosystems. When northern forest ecosystems are disturbed,

however, the nitrogen cycle is disrupted, resulting in high leaching losses of nitrate.

This stream water nitrate response is demonstrated by comparing long-term trends of

annual volume-weighted concentrations of nitrate in the reference (W6), and cut (W2

devegetation and herbicide treatment, 1965-1968; W4 strip-cut, 1970-1974; W5 clearcut,

1983-1984) watersheds. The increase in stream water nitrate after cutting generally

only lasts a few years because nitrate is readily retained by regrowing vegetation. In

addition to these results, there have been some unexpected patterns in long-term

nitrogen retention data from these experimentally cut watersheds. During the 10 to

15 year aggrading phase of W2 and W4, inputs of nitrogen were strongly retained and

nitrate leaching losses were low. In recent years, this pattern has shifted such that

nitrate loss in cut watersheds exceeds values observed in the reference watershed.

If this pattern remains consistent across cut watersheds, nitrate concentrations in W5

would soon exceed concentrations in the reference watershed. Without long-term

studies such unexpected and interesting findings would not be discovered.
32

Stream samples are stored in the archive

building at the HBEF for future analyses.

Photographs: (top) U.S. Forest Service

Archives, and (bottom) Buck Sleeper

Principal Investigator: 

Gene E. Likens, Institute of Ecosystem Studies

Online Access:

Stream water chemistry (W2) - http://www.hubbardbrook.org/data/dataset.php?id=4

Stream water chemistry (W4) - http://www.hubbardbrook.org/data/dataset.php?id=6

Stream water chemistry (W5) - http://www.hubbardbrook.org/data/dataset.php?id=7

Stream water chemistry (W6) - http://www.hubbardbrook.org/data/dataset.php?id=8

Associated Databases: 

Stream water chemistry (W1, W3 and W7-9)

Further Reading:

Bormann, F.H.; Likens, G.E.; Fisher, D.W.; Pierce, R.S. 1968. Nutrient loss accelerated 

by clear-cutting of a forest ecosystem. Science. 159: 882-884.

Likens, G.E.; Bormann, F.H.; Johnson, N.M.; Fisher, D.W.; Pierce, R.S. 1970. Effects of 

forest cutting and herbicide treatment on nutrient budgets in the Hubbard Brook 

watershed-ecosystem. Ecological Monographs. 40: 23-47.

Pardo, L.H.; Driscoll, C.T.; Likens, G.E. 1995. Patterns of nitrate loss from a 

chronosequence of clear-cut watersheds. Water, Air, and Soil Pollution. 85: 1659-1664.
33

Whole-tree Harvest Effects on Soil Exchangeable Calcium

Figure 16. Whole-tree harvest effects on soil exchangeable calcium at W5.

Whole-tree harvest effects on soil exchangeable calcium

Clearcutting northern hardwood forests typically causes enhanced export of mineral

nutrients, such as calcium, in drainage waters. When these drainage losses are added

to the amount of nutrients removed in timber, the total amount of nutrients removed

from the ecosystem can be substantial. Thus, changes in soil chemical properties after

logging are important in determining the long-term implications of logging on nutrient

availability and site fertility. During the winter of 1983-84, W5 was logged by removing

whole trees greater than 5 cm d.b.h. Soil exchangeable calcium was measured in

mineral and organic soil before (1983) and after (1986, 1991, 1998) the whole-tree

harvest. Long-term data from this study indicate that the whole-tree harvest had little

effect on the total pool of exchangeable calcium. In the 15 years since the forest was

harvested, the cut did not result in the depletion of exchangeable calcium pools, even

though a considerable amount of calcium was removed from the ecosystem via

increased stream water export and biomass. Soil is the principal source of calcium in

these forests, and whole-tree harvesting does not appear to have much of an influence

on this nutrient reserve. While there is some evidence of soil calcium depletion in

adjacent uncut watersheds, possibly caused by acidic atmospheric deposition, the

effects are not evident in the soil exchangeable pool of the aggrading forest at W5.
34

Whole-tree harvest at W5. Photographs: U.S. Forest Service Archives

Principal Investigator:

Chris E. Johnson, Syracuse University

Thomas G. Siccama, Yale University

Associated Databases: 

Quantitative pit soil carbon and nitrogen (W5), Continuous revegetation survey

data (W5)

Further Reading:

Johnson, C.E.; A.H. Johnson, A.H.; Siccama, T.G. 1991. Whole-tree clear-cutting effects 

on exchangeable cations and soil acidity. Soil Science Society of America Journal. 55:

502-508.

Johnson, C.E.; Romanowicz, R.B.; Siccama, T.G. 1997. Conservation of exchangeable 

cations after clear-cutting of a northern hardwood forest. Canadian Journal of Forest

Research. 27: 859-868.

Likens, G.E.; Driscoll, C.T.; Buso, D.C.; Siccama, T.G.; Johnson, C.E.; Lovett, G.M.;

Fahey, T.J.; Reiners, W.A.; Ryan, D.F.; Martin, C.W.; Bailey, S.W. 1998.

The biogeochemistry of calcium at Hubbard Brook. Biogeochemistry. 41: 89-173.
35

Figure 17. Calcium concentrations (±SE) in wood fern fronds at W1 and west of W6.

Calcium Concentration in Wood Fern Fronds

Calcium concentration in wood fern fronds

Wood fern (Dryopteris spinulosa) is the most abundant herbaceous species at the

HBEF comprising nearly half of the biomass of that stratum of vegetation. Annual

wood fern sampling began in 1985 using consistent methods in an area west of

the weir at W6. Each July, fronds are collected and analyzed for several elements

including calcium. In the early 1990s, there was a decline in the calcium concentration

of wood fern fronds. This pattern is consistent with the idea that calcium is being

depleted from soils at the HBEF due to base cation leaching associated with acidic

deposition. An experiment was conducted in W1 designed to replace this lost

calcium. Pelletized wollastonite (CaSiO3) was added to the entire watershed by

aerial application during the fall of 1999. Wood fern was among the first species

of vegetation to show a calcium response, with frond concentrations increasing

immediately following the wollastonite addition. Despite the general patterns

described here, there are several unexplained values, such as the extremely high

concentrations at W6 in 2000 and 2004. Further monitoring will help determine

what causes these anomalies in the long-term record.
36

Wood ferns (Dryopteris spinulosa)

Principal Investigator: 

Thomas G. Siccama,Yale University

Associated Databases: 

Temporal canopy leaf chemistry (W1 and W6)

Further Reading:

Peters, S.C.; Blum, J.D.; Driscoll, C.T.; Likens, G.E.

2004. Dissolution of wollastonite during the 

experimental manipulation of Hubbard Brook 

Watershed 1. Biogeochemistry. 67: 309-329.

Siccama, T.G.; Denny, E. 2006. Long-term changes in 

the calcium concentration of wood fern fronds 

[Online]. Available at http://www.hubbardbrook.org/

yale/misc/ferncycl.htm (accessed 29 Sept. 2006).
37

Figure 18. Aboveground biomass at W2 and W5.

Aboveground Biomass

Aboveground biomass

Aboveground biomass has been measured at plots on two of the experimentally

harvested watersheds (W2 and W5) to examine regrowth following disturbance.

Watershed 2 was deforested in 1965 followed by 3 successive summers of herbicide

treatment. Vegetation was allowed to begin regrowing in 1969 and biomass was

measured in years 1, 2, 3, 5, 11, 20 and 31 of regrowth. Watershed 5 underwent a

whole-tree harvest in 1984–1985 and aboveground biomass was measured in years

2, 3, 5, 7, 11, 16 and 21 of regrowth. The fast-growing pioneer species, pin cherry,

dominates the vegetation on cutover sites at the HBEF and results in particularly high

rates of biomass accumulation. For example, in the first 5 years of regrowth, the rate

of biomass accumulation on W5 significantly exceeded that on nearby W2, where

the abundance of pin cherry was reduced by repeated treatment with herbicides.

Reduction in vegetative sprouting and decline in site fertility due to high nutrient

leaching probably also contributed to the difference in biomass accumulation between

W2 and W5. These differences in vegetation and soils associated with the different

treatments appear to have long-lasting effects. During the second decade after

disturbance, rates of biomass accumulation accelerated on W5 compared to W2.

After 21 years of regrowth, more biomass accumulated on W5 than the amount

accumulated after 31 years on W2. Figure adapted from Fahey et al. (2005).
38

Aggrading forest stand at the HBEF. Photograph: Nicholas Rodenhouse

Principal Investigators:

Timothy J. Fahey, Cornell University

William A. Reiners, University of Wyoming

Thomas G. Siccama, Yale University

Online Access:

Vegetation recovery (W2) - http://www.hubbardbrook.org/data/dataset.php?id=52

1982 forest inventory (W5) - http://www.hubbardbrook.org/data/dataset.php?id=36

1990 forest inventory (W5) - http://www.hubbardbrook.org/data/dataset.php?id=37

1994 forest inventory (W5) - http://www.hubbardbrook.org/data/dataset.php?id=38

1999 forest inventory (W5) - http://www.hubbardbrook.org/data/dataset.php?id=39

Associated Databases: 

Forest inventory (W1, W6, and Bird Transect Area)

Further Reading:

Fahey, T.J.; Siccama, T.G.; Driscoll, C.T.; Likens, G.E.; Campbell, J.; Johnson, C.E.;

Battles, J.J.; Aber, J.D.; Cole, J.J.; Fisk, M.C.; Groffman, P.M.; Hamburg, S.P.; Holmes,

R.T.; Schwarz, P.A. Yanai, R.D. 2005. The biogeochemistry of carbon at Hubbard Brook.

Biogeochemistry. 75: 109-176.

Johnson C.E.; Driscoll C.T.; Fahey T.J.; Siccama T.G.; Hughes J.W. 1995. Carbon 

dynamics following clearcutting of a northern hardwood forest. In: McFee W.W.; Kelly

J.M. (eds), Carbon forms and functions in forest soils. Madison, WI: American Society

of Agronomy: 463-488.

Likens, G.E.; Bormann, F.H.; Johnson, N.M.; Fisher, D.W.; Pierce, R.S. 1970. Effects of 

forest cutting and herbicide treatment on nutrient budgets in the Hubbard Brook 

watershed-ecosystem. Ecological Monographs. 40: 23-47.

Reiners W.A. 1992. Twenty years of ecosystem reorganization following experimental 

deforestation and regrowth suppression. Ecological Monographs. 62: 503-523.
39

Changes in Water Yield After Forest Cutting

Figure 19. Changes in water yield after forest cutting at W2, W4, and W5.

Changes in water yield after forest cutting

Experiments were carried out on three of the gaged watersheds to examine the effect

of forest cutting on water yield. Experiments included clear-felling and 3 successive

years of herbicide applications (W2), strip-cutting (W4), and whole-tree harvesting

(W5). Responses in annual water yield varied among treatments, but increased initially

and then decreased as the forest regenerated. The increase in water yield following

the treatments was short-lived, with the greatest losses occurring at W2 where

vegetation regrowth was suppressed by the application of herbicides. Unexpected

decreases in annual water yields were evident after the clearfelling and herbicide

treatment and after the strip-cutting treatment. These decreases in water yield occurred

because newly developing stands dominated by pin cherry and birches have the

capacity to transpire more than mature forests, leaving less water available for

streamflow. Interestingly, no sustained decreases in water yield occurred on W5

following the whole-tree harvest. Several possible explanations for the different
40

response at W5 include a lack of regeneration on skid roads, heavy moose browse

near the top of the watershed, and a greater proportion of American beech in the

regenerating forest. All these factors could reduce transpiration rates without

decreasing water yield.

Principal Investigators:

John L. Campbell, U.S. Forest Service

Amey S. Bailey, U.S. Forest Service

Christopher Eagar, U.S. Forest Service

Online Access:

Daily streamflow by watershed - http://www.hubbardbrook.org/data/dataset.php?id=2

Associated Database: 

Instantaneous streamflow

Further Reading:

Bailey, A.S.; Hornbeck, J.W.; Campbell, J.L.; Eagar, C. 2003. Hydrometeorological 

database for Hubbard Brook Experimental Forest: 1955-2000. Gen. Tech. Rep. NE-305.

Newton Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern

Research Station. 36 p.

Hornbeck, J.W.; Martin, C.W.; Eagar, C. 1997. Summary of water yield experiments at 

Hubbard Brook Experimental Forest, New Hampshire. Canadian Journal of Forest

Research. 27: 2043-2052.
41

The use of trade, firm, or corporation names in this publication is for the information and 

convenience of the reader. Such use does not constitute an official endorsement or approval 

by the U.S. Department of Agriculture or the Forest Service of any product or service to the 

exclusion of others that may be suitable. 

This publication/database reports research involving pesticides. It does not contain 

recommendations for their use, nor does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by appropriate State and/or Federal, 

agencies before they can be recommended. 

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and 

fish or other wildlife—if they are not handled or applied properly. Use all pesticides selectively 

and carefully. Follow recommended practices for the disposal of surplus pesticides and 

pesticide containers.
Abstract

The Hubbard Brook Experimental Forest was established by the U.S. Forest Service in 1955 as a major center 

for hydrologic research in the Northeast. The Hubbard Brook Ecosystem Study originated 8 years later with 

the idea of using the small watershed approach to study element flux and cycling and the response of 

forest ecosystems to disturbance. Since that time, the research program at Hubbard Brook has expanded to 

include various physical, chemical and biological measurements collected by researchers from a number 

of cooperating institutions. Collaborative, long-term data are the keystone of the Hubbard Brook Ecosystem 

Study and have provided invaluable insight into how ecosystems respond to disturbances such as air 

pollution, climate change, forest disturbance, and forest management practices. This report highlights longterm ecological trends at Hubbard Brook, provides explanations for some of the trends, and lists references 

from the scientific literature for further reading.

Acknowledgments

We would like to thank Kimberley M. Driscoll of Syracuse University for compiling data and developing initial 

graphs used in this publication. We also thank David Sleeper of the Hubbard Brook Research Foundation for 

advice and encouragement. The Hubbard Brook Research Foundation provided support for the photography, 

design, and layout of this report, through a grant from the Northeastern Ecosystem Research Cooperative. This 

manuscript is a contribution of the Hubbard Brook Ecosystem Study. Hubbard Brook is part of the Long-Term 

Ecological Research (LTER) network, which is supported by the National Science Foundation. The Hubbard 

Brook Experimental Forest is operated and maintained by the U.S. Forest Service, Northern Research Station, 

Newtown Square, Pennsylvania.

The design of this report was done by RavenMark, Inc. of Montpelier, Vermont. 

Cover photos, top two: U.S. Forest Service Archives. Bottom two: Buck Sleeper. 

Photographs throughout this report are by Buck Sleeper, unless otherwise noted.

The Authors

JOHN L.CAMPBELL and CHRISTOPHER EAGAR are research ecologists and AMEY S. BAILEY is a forest 

technician with the Northern Research Station, Durham, NH. CHARLES T. DRISCOLL is a university professor 

of environmental systems engineering and CHRIS E. JOHNSON is an associate professor of civil and 

environmental engineering, Syracuse University, Syracuse, NY. GENE E. LIKENS is an ecologist and 

distinguished senior scientist of the Institute of Ecosystem Studies, Millbrook, NY. THOMAS G. SICCAMA is 

a professor emeritus of forest ecology, Yale School of Forestry and Environmental Studies, New Haven, CT. 

TIMOTHY J. FAHEY is a professor of natural resources, Cornell University, Ithaca, NY. STEVEN P. HAMBURG 

is Director of the Global Environment Program at the Watson Institute for International Studies, Brown 

University, Providence, RI. RICHARD T. HOLMES is a professor emeritus of biology, Dartmouth College, 

Hanover, NH. DONALD C. BUSO is a biologist and manager of field research at the Hubbard Brook 

Experimental Forest, for the Institute of Ecosystem Studies, Millbrook, NY.

Published by:

USDA FOREST SERVICE

11 Campus Blvd., Suite 200

Newton Square, PA 19073-3294

October 2007

www.nrs.fs.fed.us

For additional copies:

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Publications Distribution

359 Main Road

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Fax: (704) 368-0152

Covers.indd 2 
Capitalizing on the strengths of existing 

science capacity in the Northeast and 

Midwest to attain a more integrated, cohesive, 

landscape-scale research program