Timothy J. Fahey

BELOWGROUND ECTOMYCORRHIZAL FUNGAL COMMUNITY CHANGE OVER A NITROGEN DEPOSITION GRADIENT IN ALASKA

Nitrogen availability may be a major factor structuring ectomycorrhizal fungal communities. Atmospheric nitrogen (N) deposition has been implicated in the decline of ectomycorrhizal fungal (EMF) sporocarp diversity. We previously characterized the pattern of decreased sporocarp species richness over an anthropogenic N deposition gradient in Alaska (USA). To determine whether this change in sporocarp community structure was paralleled below ground, we used molecular and morphological techniques to characterize the ectomycorrhizal community of white spruce (Picea glauca) over this gradient. We then related patterns of richness and relative abundance of taxa to various N-affected environmental parameters. Species richness of EMF declined dramatically with increasing N inputs. Over 30 taxa were identified at the low-N sites, compared with nine at the high-N sites. Low-N site dominants (Piloderma spp., Amphinema byssoides, Cortinarius spp., and various dark-mantled Tomentella spp.) disappeared completely at th...

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

Fine Root Dynamics in a Northern Hardwood Forest Ecosystem, Hubbard Brook Experimental Forest, NH

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Soil freezing alters fine root dynamics in a northern hardwood forest

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Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere processes.

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Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest

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