Site Description

The Hubbard Brook Experimental Forest is in the southern part of the White Mountain National Forest in central New Hampshire. It lies in the towns of Ellsworth, Thornton, Warren and Woodstock, all in Grafton County, and is near the village of West Thornton.

The eastern boundary of most of the HBEF is about 800 m west of U.S. I-93 and about 210 km north of Boston. However, the highway intersects the Mirror Lake watershed.

The Atlantic Ocean is about 116 km to the southeast. The only vehicle access to the HBEF is from Rt. 3, which runs parallel to I-93. With the exception of the easternmost boundary of the Hubbard Brook Valley, all surrounding land is in the National Forest. Thus, there is complete administrative control over the entire Experimental Forest and a forest buffer surrounds most of the area.

West Thornton, the nearest village, has a population of about 200 people, and is 2 km from the forest. Lincoln and North Woodstock are about 15 km north of the forest and offer complete services for travelers. Plymouth is the largest nearby town and is about 25 km to the south.

The HBEF is characteristic of much of the White Mountain National Forest. The National Forest has hilly, occasionally steep topography; coarse, acidic, glacially-derived soils; bedrock dominated by metamorphic rock of igneous and sedimentary origin; northern hardwood forests on lower slopes and spruce-fir on upper reaches; and continental climate of long, cold winters and mild to cool summers. The HBEF is one of several sites in the White Mountain National Forest which offer opportunities for specialized research on the northern hardwood forest ecosystem. The Bowl Natural Area, about 26 km from Hubbard Brook, is an old-growth forest tract. Others sites include the Bartlett Experimental Forest, about 38 km away, and Cone Pond watershed, about 20 km from the HBEF. Other nearby areas with interesting geomorphological features for ecosystem research are Mt. Washington, Mt. Moosilauke, Pemigewasset Wilderness, the Great Gulf and Presidential - Dry River Wilderness Areas, and numerous lakes and rivers.

Physical Features:
Climate

Although the climate of Hubbard Brook varies with altitude, some major features include: (1) large and rapid changes in weather, (2) broad ranges in daily and annual air temperature, and (3) uniform monthly precipitation (about 100 mm/mo; Figure 3). Major air flow over the forest is either (1) continental polar air from subarctic North America (the predominant direction), (2) maritime tropical air from the Caribbean and Gulf of Mexico from the south or southwest, or (3) maritime air from the North Atlantic out of the east or northeast. In spite of the proximity of Hubbard Brook to the ocean (116 km), the climate is predominantly continental. The diverse character of the air masses influincing central New Hampshire produce a climate which is highly variable.

Annual precipitation averages about 1,400 mm, of which about one third to one quarter is snow. Approximately 111 separate storms occur each year, or about 2 storms per week. A snowpack usually persists from mid-December until mid-April, with a peak depth in March of about 1,020 to 1,270 mm, having about 250 to 300 mm of water content.

Winters are long and cold. January averages about -9oC, and long periods of low temperatures from -12˚C to -18˚C are common. Even though temperatures are low most of the time, occasional midwinter thaws result in elevated streamflo. Short, cool summers are the rule. The average July temperature is 18oC.

The average number of days without killing frost is 145; however, the growing season for trees is considered to be from 15 May, the approximate time of full leaf development, to 15 September, when the leaves begin to fall.

The estimated annual evapotranspiration (ET) is about 500 mm, determined by difference between precipitation and streamflow. This calculation is a reasonable approach for Hubbard Brook because of the apparent minimal deep seepage and annual removal of summer soil-water deficits by autumnal rains and spring snowmelt.

Geology
The eastern portion of the Experimental Forest (watersheds 1-6, and 9 included) is underlain by a complex assemblage of metasedimentary and igneous rocks. The major map unit is the Silurian Rangeley Formation, consisting of quartz mica schist and quartzite interbedded with sulfidic schist and calc-silicate granulite. Originally deposited as mudstones, sandstones and conglomerates, these rocks have been metamorphosed to sillimanite grade and have undergone four stages of deformation. Deformation style evident in outcrops is primarily tight isoclinal folds. However joints, slickensides and mylonites indicate brittle deformation as well. The metamorphic rocks were later intruded by a variety of igneous rocks including the Devonian Concord Granite, pegmatites, and Mesozoic diabase and lamprophyre dikes. The western portion of the forest (portions of watersheds 7 and 8 included) is underlain by the Devonian Kinsman Granodiorite, a foliated granitic rock with megacrysts of potassium feldspar.

 

streamflow precipitation 
Figure 3: Average monthly precipitation and streamflow for the Hubbard Brook Experimental Forest (1956-1988). The vertical bars are + one standard deviation of the mean. Precipitation inputs are evenly distributed over the year. Stream outflow is low during the summer due to high evapotranspiration and high in the spring during snowmelt (after Federer et al. 1990).


Continental glaciers, which blanketed the region during the Pleistocene and retreated some 13,000 years ago, removed most preexisting soils. Glacial movement was primarily in a southeasterly direction as indicated by striations on bedrock surfaces, and fragments of rocks in the till which are typical of bedrock to the northwest of the Hubbard Brook Valley. Materials deposited by the glacier are highly variable in degree of sorting and grain size, ranging from clays to 10m diameter boulders. The depth of glacial deposits ranges from zero on ridgetops and in stream valleys (resulting in bedrock outcrops) to 50m in the vicinity of Mirror Lake. Poorly sorted glacial till, commonly 2m thick, covers the bedrock in most of the valley. Ice contact terraces in the lower valley, consisting of well sorted sands and gravels, are typically 10's of meters thick. Detailed studies of the sediment in Mirror Lake (some 12-13 m thick) have revealed much about the glacial history of the area.

Topography
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.

Soils
Soils at Hubbard Brook are predominantly well-drained Spodosols, more specifically, Typic Haplorthods, derived from glacial till, with sandy loam textures. There are no residual soils, (i.e., derived from weathered bedrock). Principal soil series are the sandy loams of the Berkshire series, along with the Skerry, Becket, and Lyman series. These soils are acidic (pH about 4.5 or less) and relatively infertile (Table 1). A 20- to 200-mm thick forest floor layer is present, except where the soil surface has been disturbed by fallen trees. Long-term measurements suggest that the forest floor is at steady-state (Figure 4). This layer permits rapid infiltration of water and protects the soil from freezing before snow accumulates in winter (except in rare winters when snowfall is light). There is virtually no overland flow at the HBEF.

Table 1. Summary soils data for watershed 5 at the HBEF (after Johnson et al. 1991a,b).
Soil Types Typic Haplorthod, Typic Dystrochrept
Soil Series Berkshire, Skerry, Becket, Lyman, Tunbridge
Avg. Depth (cm):
      Forest Floor
      Mineral Soil

6.9
50.3
Mass (kg/m2):
      Forest Floor
      Mineral Soil

8.8
317.0
Soil Organic Matter (%):
      Forest Floor
      Mineral Soil

60.0
10.0
pH in Water:
      Oa Horizon
      Mineral Soil

3.9
4.3
Cation Exchange Capacity (cmol/kg):
      Oa Horizon
      Mineral Soil

18.0
5.0
Base Saturation (%):
      Oa Horizon
      Mineral Soil

50.0
12.0


forest floor OM mass  
Figure 4: Long-term trends in organic matter content of the forest floor in forested watersheds at the HBEF (W5, W6). Mean values and standard deviations are indicated. The mass of organic matter in the forest floor has remained constant since at least 1975. The "Big Dig" is forest floor sampled from 0.5 m2 plots on W5. Gosz et al. (1976) sampled to a lesser depth than the later surveys. When this difference is taken into account, there seems to be no difference in organic matter content over the entire record. Note that the forest floor mass measured in 1992 was somewhat higher than in previous years. This difference was likely due to two factors: 1) wet conditions which made separation of the forest floor with the mineral soil difficult, and 2) three samples with very thick forest floor depth. (T. Siccama, unpublished data).


Soil depths, including unweathered till, average about 2.0 m surface to bedrock, although this is highly variable. Soil on the ridgetops may consist of a thin accumulation of organic matter, resting directly on bedrock.

The separation between the pedogenic zone and the virtually unweathered till and bedrock below is distinct. Depth to the C horizon averages about 0.6 m. At various places in the Forest, the C horizon exists as an impermeable pan. These layers restrict root development and water movement. Rocks of all sizes are scattered throughout the soil profile. In many locations boulder fields are prominent features.

A prominent feature of the surface topography throughout the HBEF is the rough pit-and-mound appearance caused by the uprooting of trees. Such uprooting mixes mineral soil from below with nutrient-rich organic surface layers and/or deposits the lower mineral layers directly on top of the forest floor humus layers without mixing, creating buried horizons. This natural disturbance changes seedbed conditions for regenerating species, and affects weathering and biogeochemical cycles.

Streams
The most conspicuous streamflow characteristic is the large volume of flow in spring and very low flow in late summer-early autumn (Figure 3). These yearly highs and lows reflect seasonal spring snowmelt (that often occurs over a few days or weeks) and the slow progressive decrease in flow from the transpirational draft in summer, respectively. The numerous streams in the HBEF range from small ephemeral channels that often dry up during summer to a large perennial 5th-order stream (Hubbard Brook). Because of the shallow soil depths and high soil porosity, the stream channels quickly swell during large storms (e.g., 80 mm or more). As much as 60 to 80% of storm precipitation can pass through the stream as storm flow. In a record storm, the discharge from an 42.4 ha watershed yielded maximum flows of 43 L/s-ha, and the main Hubbard Brook (3,076 ha) yielded 45 L/s-ha.

Although stream channels occupy only 1% of the land area, the streams play an important role in many processes throughout the HBEF.

Because of the coarse-grained texture of the soil, streamwater is usually characterized by low concentrations of suspended materials. Mineral and organic particulate material is suspended and carried in storm periods, but once the flow recedes these materials quickly settle, leaving the stream with low concentrations of suspended solids (Figure 5). About 28 kg/ha of particulate matter (both inorganic and organic) are transported out of the watershed by streams each year. Streamwater is generally not highly colored due to the limited extent of wetlands in the forest.

  concentrated discharge  
Figure 5: The change in the concentration of dissolved and particulate matter with increasing stream flow in a mature northern hardwood forest ecosystem (after Bormann et al. 1969).


Most of the stream channels have exposed bedrock at some locations. However, the 1st to 3rd order streams have channels primarily made up of mineral and organic particulate matter lodged behind organic debris dams. These dams form a stair-step pattern in stream channels, and they play important roles in regulating many physical, chemical, and biological processes in the streams.

Stream temperatures in intact forests range from about 0o to 17oC, and are about the same as soil temperatures at the 31-cm depth. Streamwater chemistry is dilute and acidic (Figure 6). Specific conductivity ranges from 20 to 30 umhos/cm at 25oC and pH varies between 4.5 and 6.4, depending on season, storm and location. Alkalinity or acid neutralizing capacity is very low, about -5 ueq/L.

The first stream-gauging station was installed in the summer of 1955, with discharge records beginning in 1956 (Table 2). For a few succeeding summers, additional stream-gauging stations were established. The first gauging stations consisted of concrete structures built on bedrock with sharp-crested V-notch blades for discharge control sections (Table 3). Stations constructed later coupled the V-notch weir with a modified San Dimas flume. Size and configuration of the combination gauging stations were designed to fit the watershed size and topography at the construction site. By June 2001, 327 watershed-years of streamflow data had been amassed. In spite of extremely cold temperatures in winter (as low as -30oC), streamflow is monitored year-round, using propane heaters to prevent freezing in gauging station wells and enclosures.

Mirror Lake

Mirror Lake is a 15.0-ha lakelocated at the eastern end of the Hubbard Brook Valley, and is contiguous with the HBEF. Part of the drainage to the lake originates in the Experimental Forest. The State of New Hampshire has access to the lake at its outlet and the town of Woodstock owns a public beach on the southern shore. Otherwise the lake boundary is owned privately or by the USDA Forest Service.

The lake water is soft, slightly acidic and quite clear. Mirror Lake is classified as oligotrophic, with low productivity due primarily to the paucity of nutrients in the water. However, cultural development around the lake, including the construction of an interstate highway (I-93) through a portion of the lake watershed, has progressed at a rapid rate. This development has resulted in corresponding changes in the characteristics of the lake Figure 8.

w6 stream  
Figure 6: Average annual volume-weighted values of sum of basic cations (CB, Ca2+, Mg2+, Na+, K+) and SO42- (a) and pH (b) in W6 streamwater. There has been a marked decline in stream concentrations of SO42- ( and CB since the mid-1960's. The decline in SO42- is consistent with decreases in SO2 emissions. The near stoichiometric declines in SO42- and CB (the major anion and cations in streamwater) has resulted in little change in pH (updated from Driscoll et al. 1989).


mirror lake bathymetric  
Figure 7: Bathymetric map of Mirror Lake (from Likens et al. 1985).

Cl concentrations  
Figure 8: Time series of Cl- concentrations from the NE inlet of Mirror Lake. Note that marked increases in the concentration of Cl- have occurred since the construction of I-93 in 1969. This increase in Cl- is likely due to application of salt for deicing on I-93 (after Likens 1992).

Table 2. Characteristics of monitored watersheds at the HBEF.
Watershed No. Size (ha) Year Started Treatment
Experimental Watersheds
1 11.8 1956 Calcium manipulation in 1999; 1.2 metric tonnes/ha of CaSiO3 (wollastonite) was applied.
2 15.6 1957 Clear felled in winter 1965-66; no products removed; treated with herbicides summers of 1966, 1967, 1968. Left to regrow from 1969.
3 42.4 1958 None; hydrologic reference watershed
4 36.1 1961 Clear-cut to a 2 cm minimum diameter, by strips in three phases, 1970, 1972, 1974. Timber products removed.
5 21.9 1962 Whole-tree clear-cut to 5 cm diameter, 1983-1984. Timber products removed.
6 13.2 1963 None; biogeochemical reference watershed
7 76.4 1965 None
8 59.4 1969 None
9 ~68 1994 None
101 12.1 1970 Clear-cut to a 5 cm minimum diameter, as a block in 1970. Timber products removed. Note: streamflow quantity is not monitored, only water quality.
Mirror Lake Watersheds
ML-NE 20 1965 Highway construction 1969-71
ML-NW 34.6 1965 None
ML-W 24 1965 None
ML-Outlet 103 1965 None


Table 3. Characteristics of stream-gauging stations.
Watershed Type of Control Section size Capacity (approx.)
ft   cm ft3/sec m3/sec
1
90˚ sharp-crested V-notch weir
2 depth 61 14.0 0.4
2
120˚ sharp-crested V-notch weir
2 depth 61 24.2 0.7
3
120˚ sharp-crested V-notch weir
2 depth 61 24.2 0.7
4
120˚ sharp-crested V-notch weir
2 depth 61 24.2 0.7
5
Combination: 90˚ sharp-crested V-notch, and modified San Dimas, flume
1
3
3.3
depth
width
depth
30.5
91
101
2.5
105
0.1
3.0
6
Combination: 90˚ sharp-crested V-notch, and modified San Dimas, flume
1
2
3.0
depth
width
depth
30.5
61
91
2.5
31
0.1
0.9
7
Combination: 120˚ sharp-crested V-notch, and modified San Dimas, flume
1.25
4
4.5
depth
width
depth
38
122
137
7.7
212
0.2
6.0
8
Combination: 120˚ sharp-crested V-notch, and modified San Dimas, flume
1.75
4
4.5
depth
width
depth
52
122
137
17.4
212
0.5
5.2
9
120˚ sharp-crested V-notch weir
3.0
depth 91 65 1.8


Basic limnological studies on Mirror Lake were initiated in the mid 1960's (several years before the major road developments occurred), and a wide range of relevant physical, chemical, and biological data are available. For example, data on the hydrology, nutrient concentrations (Table 4), phytoplankton community structure, fish and salamander populations, zooplankton community structure, macrophyte productivity and phytosociology, and benthic invertebrates are available. In addition, intensive examinations of numerous sediment core samples, including analyses such as pollen, chemistry, pigment degradation products, texture, animal fossils, etc., have been conducted.

Table 4:Chemical characteristics of Mirror Lake (after Likens et al. 1985).
  mg/L uEq/L
Ca2+
2.4 119
NH4+
0.035 1.8
NO3-
0.09 1.5
SO42-
5.7 119
HCO3-
4.0 66
PO42-
0.0056 0.18
DOC
2.75 --
pH
6.4

 

Biological Features:
Major Tree Species

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 white 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. Logging operations ending around 1915-1917 removed large portions of the conifers and better quality, accessible hardwoods. The present second-growth forest is even-aged and composed of about 80 to 90% hardwoods and 10 to 20% conifers.

The total forest biomass has stopped accumulating since the early 1980's and is currently about 235 tons/ha (Figure 9). Present basal area is about 26 m2/ha but varies according to elevation, habitat and stand history.


Fauna

Fauna common to northern hardwood forests occur within the Hubbard Brook Valley. More than 90 species of birds have been recorded (Figure 10), and mammals such as snowshoe hare, moose, fox, black bear, beaver and white-tailed deer are present. A checklist of all organisms found at the HBEF and Mirror Lake, including the vascular flora, and a herbarium specimen collection of the major floral components on the HBEF are available.

Experimental Manipulations:
Watershed Manipulations

W2 The first of the HBEF manipulations was undertaken to evaluate the role of forest vegetation in regulating the hydrologic and element output of a northern hardwood forest watershed (Figure 11).

In December 1965, all trees, shrubs and woody vegetation were cut with chain saw on watershed 2 (15.6 ha) and left on the ground. The forest floor was virtually undisturbed, since the trees were felled on a snow surface of about 50 cm, no products were removed and no vehicles were allowed in the area. Bromacil, a nonspecific woody herbicide, was broadcast sprayed over the watershed by helicopter in June 1966 to kill regrowth. For the next two summers, 2,4,5-T was sprayed from the ground on the persistent regrowing vegetation with backpack mist blowers. Beginning in 1969, vegetation was allowed to regrow (Likens et al. 1970; Nodvin et al. 1988; Reiners 1992).

W4 The next watershed manipulation was conducted to assess the effects of strip cutting on water yield and nutrient input-output budgets.

Watershed 4 was divided into 49 east-west strips (almost following topographic contours) 25 m wide. In the autumn of 1970, every third strip was cut. The second series of strips was cut in 1972, leaving one series of strips uncut. Finally in 1974, the last set of strips was cut. Except for a variable width buffer of trees that was left along the main stream channel, the entire watershed was clear-cut in these three phases. All trees to a minimum of 5 cm were felled, and all products of value to the logging contractor were removed by rubber-tired skidder. Dense natural regrowth is occurring. Generally, regrowth of vegetation in northern hardwood stands in the area is profuse (Hornbeck et al. 1987).

 
Figure 9: Trends in total tree biomass in W6 at the HBEF. Observations suggest that above-ground biomass has slowed in forest watersheds, in recent years (updated from Likens et al. 1994).
bird population trends  
Figure 10: Trends in bird populations within forest watersheds at the HBEF. Data show a decline in bird populations in recent years (Holmes, unpublished data).

stream water chemistry  
Figure 11: Annual volume-weighted concentrations of solutes in streams draining W2 (dashed line) and W6 (solid line). Shaded area indicates treatment years. A large increase in NO3- was evident in W2 during and immediately following devegetation and herbicide treatment due to mineralization/nitrification. This coincided with a period of cation loss and SO42 retention. During regrowth NO3- was strongly retained, and cation loss diminished. Sulfate retention/release observed in W2 is consistent with pH-dependent adsorption/desorption from soil (updated from Nodvin et al. 1988).


W101 Watershed 101 (12.1 ha and ungaged) was clear-cut as a block in the autumn of 1970 along with W4 to compare the block cutting with strip cutting. All trees to a minimum of 5 cm were cut and all merchantable products removed by the same operator who cut W4. Rubber-tire skidders were used to remove the stems.

W5 This watershed manipulation was designed to assess the effects of a commercial whole-tree harvest on nutrient cycles. Watershed 5 (21.9 ha) was whole-tree clear-cut during the autumn of 1983 through the spring of 1984. All trees larger than 10 cm in diameter at breast height were harvested by removal of whole trees (bole and tops) using a feller-buncher machine on accessible slopes and chain saws on steep inaccessible slopes. Trees were removed, unlimbed, using skidders (Fahey et al. 1988; Johnson et al. 1991a,b; Dahlgren and Driscoll 1994).

W1 In November 1999, wollastonite (CaSiO3), pelletized with a 4% lignosulfonate binder, was applied by helicopter in a effort to replace the calcium that has been depleted from soil over the last 50 years due to inputs of acid rain. Forty-five tons of wollastonite (1.2 tonnes/ha) were added to W1 to increase the base saturation of soil from approximately 10% to 19%. The added wollastonite has a distinctive calcium to strontium ratio and strontium stable isotope ratio which will enable us to track the fate of the added material.


Stream Manipulations

Reaches of several tributary streams at the HBEF have been chemically or physically manipulated in an intensive series of experiments. These treatments include additions of HCl, H2SO4, AlCl3, KH2PO4, NH4Cl, KNO3, sucrose and leaf leachate, debris dam manipulations and artificial lighting (e.g. Meyer 1989; Bilby and Likens 1980; Hall et al. 1980, 1985; Meyer et al. 1981; Richey et al. 1985; McDowell 1985; Hedin et al. 1990).


Lake Manipulations

To assess diffusion across the thermocline as well as the hydraulic residence time, Mirror Lake was manipulated by a pulse input of tracers LiBr and sulfur hexafluoride broadcast uniformly throughout the lake in autumn of 1989. These tracers are non-toxic to humans or other biologic organisms.