Thomas G. Siccama

The Hubbard Brook Ecosystem Study: Forest Biomass and Production

A small watershed in the White Mountains of New Hampshire bearing meso- phytic, cool-temperate, broadleaf-deciduous forests was studied. Acer saccharum, Betula lutea, and Fagus grandifolia are dominant, but toward higher elevations Picea rubens and A bies balsamea also occur and indicate the transition toward subalpine climate. The stands are young (following cutting in 1909-17) but contain older trees; stand composition is thought reasonably representative of the climax. For application of the Brookhaven system of forest dimension analysis, 93 sample trees of major species were cut and roots excavated. Mean dimensions of sample trees, and the constants for the system of logarithmic regressions relating volume, surface, mass, and growth to diameter at breast height and other independent vari- ables, show decrease in tree sizes and height/diameter ratios toward higher elevations. Stand characteristics, based on application of the regressions to forest samples, show trends of decrease for the elevation belts from low to high: stem basal area 26.3, 23.7, and 22.0 m2/ha, weighted mean tree height 16.9, 16.7, and 10.8 m, weighted mean age 124, 95, and 83 yr, stem wood volume 176, 155, and 103 m3/ha, aboveground biomass (dry matter) 162, 152, and 102 t/ha, estimated volume increment 379, 365, and 223 cm3/m2/yr, aboveground net primary productivity (1956-60) 1127, 1041, and 790 g/m2/yr, and leaf area ratio 6.2, 5.7, and 5.5 m2/m2. Biomass (and, presumably, production) of root systems is 18%-21% of that aboveground. Different estimations suggest that a mean climax biomass for the watershed may be around 350 t/ha, aboveground. Net ecosystem production (i.e., addition to the pool of woody biomass in the community) is estimated as 350 g/m2/yr aboveground and 85 below- ground for 1956-60, 238 and 52 g/m2/yr for 1961-65. Analysis of stem wood volume incre- ments reveals an abrupt and striking (18%) decrease in volume growth and productivity from 1956-60 to 1961-65. The net primary productivity of the former period, with a weighted mean for the watershed of 1110 g/m2/yr above and below the ground, is thought more nearly normal for the forest. Both drought and effects of increasing air pollution (notably increasing acidity of rainfall) may be responsible for the recent decrease in productivity.

The biogeochemistry of calcium at Hubbard Brook

AbstractA synthesis of the biogeochemistry of Ca was done during 1963–1992in reference and human-manipulated forest ecosystems of the Hubbard BrookExperimental Forest (HBEF), NH. Results showed that there has been a markeddecline in concentration and input of Ca in bulk precipitation, an overalldecline in concentration and output of Ca in stream water, and markeddepletion of Ca in soils of the HBEF since 1963. The decline in streamwaterCa was related strongly to a decline in SO

The Export of Nutrients and Recovery of Stable Conditions Following Deforestation at Hubbard Brook

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Element fluxes and landscape position in a northern hardwood Forest watershed ecosystem

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The Hubbard Brook Ecosystem Study: Composition and Dynamics of the Tree Stratum

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The biogeochemistry of potassium at Hubbard Brook

in stream water during the period. The soildepletion of Ca was the result of leaching due to inputs of acid rain duringthe past 50 yr or so, to decreasing atmospheric inputs of Ca, and tochanging amounts of net storage of Ca in biomass. As a result of thedepletion of Ca, forest ecosystems at HBEF are much more sensitive tocontinuing inputs of strong acids in atmospheric deposition than expectedbased on long-term patterns of sulfur biogeochemistry. The Ca concentrationand input in bulk precipitation ranged from a low of 1.0 µmol/ℓand 15 mol/ha-yr in 1986–87 to a high of 8.0 µmol/ℓ and 77mol/ha-yr in 1964–65, with a long-term mean of 2.74 µmol/ℓduring 1963–92. Average total atmospheric deposition was 61 and 29mol/ha-yr in 1964–69 and 1987–92, respectively. Dry depositionis difficult to measure, but was estimated to be about 20% of totalinput in atmospheric deposition. Streamwater concentration reached a low of21 µmol/ℓ in 1991–92 and a high of 41 µmol/ℓ in1969–70, but outputs of Ca were lowest in 1964–65 (121mol/ha-yr) and peaked in 1973–74 (475 mol/ha-yr). Gross outputs of Cain stream water were positively and significantly related to streamflow, butthe slope of this relation changed with time as Ca was depleted from thesoil, and as the inputs of sulfate declined in both atmospheric depositionand stream water. Gross outputs of Ca in stream water consistently exceededinputs in bulk precipitation. No seasonal pattern was observed for eitherbulk precipitation or streamwater concentrations of Ca. Net soil releasevaried from 390 to 230 mol/ha-yr during 1964–69 and 1987–92,respectively. Of this amount, weathering release of Ca, based on plagioclasecomposition of the soil, was estimated at about 50 mol/ha-yr. Net biomassstorage of Ca decreased from 202 to 54 mol/ha-yr, and throughfall plusstemflow decreased from 220 to 110 mol/ha-yr in 1964–69 and1987–92, respectively. These ecosystem response patterns were relatedto acidification and to decreases in net biomass accretion during the study.Calcium return to soil by fine root turnover was about 270 mol/ha-yr, with190 mol/ha-yr returning to the forest floor and 80 mol/ha-yr to the mineralsoil. A lower content of Ca was observed with increasing elevation for mostof the components of the watershed-ecosystems at HBEF. Possibly as a result,mortality of sugar maple increased significantly during 1982 to 1992 at highelevations of the HBEF. Interactions between biotic and abiotic controlmechanisms were evident through elevational differences in soil cationexchange capacity (the exchangeable Ca concentration in soils wassignificantly and directly related to the organic matter content of thesoils), in soil/till depth, and in soil water and in streamwaterconcentrations at the HBEF, all of which tended to decrease with elevation.The exchangeable pool of Ca in the soil is about 6500 mol/ha, and itsturnover time is quite rapid, about 3 yr. Nevertheless, the exchangeablepools of Ca at HBEF have been depleted markedly during the past 50 years orso, >21,125 mol/ha during 1940–1995. The annual gross uptake oftrees is about 26–30% of the exchangeable pool in the soil.Some 7 to 8 times more Ca is cycled through trees than is lost in streamwater each year, and resorption of Ca by trees is negligible at HBEF. Of thecurrent inputs to the available nutrient compartment of the forestecosystem, some 50% was provided by net soil release, 24% byleaching from the canopy, 20% by root exudates and 6% byatmospheric deposition. Clear cutting released large amounts of Ca tostream water, primarily because increased nitrification in the soilgenerated increased acidity and NO