Elon S. Verry

Long-term impacts of forest treatments on water yield: a summary for northeastern USA

Abstract Long-term changes in annual water yield are summarized and compared for 11 catchment studies in the northeastern USA. Substantial increases in water yield of up to 350 mm year−1 were obtained in the first year by clearing forest vegetation and controlling regrowth with herbicides. Commercial clearcutting with natural regrowth resulted in initial increases in water yield of 110–250 mm year−1. This range in response was due to differences in precipitation and configuration of cuttings. Unless regrowth was controlled with herbicides, yield increases declined quickly after cutting, seldom persisting for more than 10 years. However, yield increases were readily extended over 20 years or more with intermediate cuttings and/or repeated control of regrowth with herbicides. Nearly all increases in water yield occur during the growing season as augmentation of baseflow. Changes in species composition after forest cutting on several study catchments eventually resulted in decreased water yields compared with those from uncut, control catchments. Results are discussed in terms of implications for surface water supplies, global climate change, nutrient cycling, hydrological modeling, and long-term research.

Environmental Factors Controlling Methane Emissions from Peatlands in Northern Minnesota

Controls on methane emission from peatlands in northern Minnesota were investigated by correlation to environmental variables and by field manipulation. From September 1988 through September 1990, methane flux measurements were made at weekly to monthly intervals at six sites in the Marcell Experimental Forest, northern Minnesota (two open bog sites, two forested bog sites, a poor fen, and a fen lagg). Flux was related to water table position and peat temperature with simple correlations at individual sites and multiple regression on all sites together. The effect of water table was also investigated experimentally in “bog corrals” (open-ended metal enclosures set in the peat) in which water table was artificially raised to the surface in the driest peatland. Temperature largely controlled variation in flux within individual ecosystems at Marcell, but hydrology distinguished between-site variation. Water table position, peat temperature, and degree of peat humification explained 91% of the variance in log CH4 flux, predicted annual methane emission from individual wetlands successfully, and predicted the change in flux due to the water table manipulation. Raising the water table in the bog corrals by an average of 6 cm in autumn 1989 and 10 cm in summer 1990 increased emission by 2.5x and 2.2x, respectively. Just as expanding the scale of investigation from a single habitat in a wetland to several wetlands necessitates incorporation of additional variables to explain flux (water table, peat characteristics), modeling flux from several wetland regions, if possible, will require the addition of climate parameters.

WATERBORNE NUTRIENT FLOW THROUGH AN UPLAND-PEATLAND WATERSHED IN MINNESOTA'

Water and nutrient flow were measured on a complex upland-peatland watershed in north central Minnesota. Annual water budgets for upland and peatland components and for the total watershed were developed. Nutrient input and output budgets were developed for each component on a seasonal basis, using net precipitation inputs, and an annual nutrient budget was developed for the entire watershed, using gross precipitation and total outputs. Both components evapotranspire water near potential rates. The upland converts 34% of the water input to water yield, while the peatland (a bog in a lower topographic position) converts 55% of its water input to water yield. The upland annually retains some N, P, K, and Ca from net precipitation, but passes through Mg and supplies Na in excess of inputs. The peatland is a nutrient trap retaining 36-60% of all nutrient inputs annually. There are striking differences in the seasonal retention of nutrient forms between the upland and bog. The total watershed accumulates P and (apparently) N but loses more K, Ca, Mg, and Na than it receives in gross precipitation. Nutrient flow is interpreted for the design of nutrient-added technologies (fertilization and sewage treatment) and as a bench mark for nutrient-depleted technology (whole-tree harvesting).

Eddy correlation measurements of methane flux in a northern peatland ecosystem

A pilot study to measure methane flux using eddy correlation sensors was conducted in a peatland ecosystem in north central Minnesota. A prototype tunable diode laser spectrometer system was employed to measure the fluctuations in methane concentration.The logarithmic cospectrum of methane concentration and vertical wind velocity fluctuations under moderately unstable conditions had a peak nearf = 0.10 (wheref is the nondimensional frequency) and was quite similar to the cospectra of water vapor and sensible heat. Daytime methane flux during the first two weeks of August ranged from 120 to 270 mg m-2 day-1. The temporal variation in methane fluxes was consistent with changes in peat temperature and water table elevation. Our results compared well with the range of values obtained in previous studies in Minnesota peatlands.These field observations demonstrate the utility of the micrometeorological eddy correlation technique for measuring surface fluxes of methane. The current state-of-the-art in tunable diode laser spectroscopy makes this approach practical for use in key ecosystems.

Streamflow Chemistry and Nutrient Yields from Upland‐Peatland Watersheds in Minnesota

Twenty-two water quality parameters were determined for the streamflow from complex but typical upland-peatland watersheds over a period of 5 yr. Five watersheds with oligotrophic peatlands and one with a minerotrophic peatland were studied. Concentrations of organically derived nutrients are highest in the streamflow from watersheds containing oligotrophic peatlands; while concentrations of nutrients derived from solution of aquifer minerals are higher in streamflow from the watershed containing a minerotrophic peatland. However, flow-weighted concentrations of organically derived nutrients are similar for both watersheds. Annual nutrient yields (kilogram/hectare per year) calculated for two of the tight uplandoligotrophic peatland watersheds are generally low and are quite similar to values for other forested areas in North America which do not contain peatlands.

Atmospheric Inputs of Mercury and Organic Carbon into a Forested Upland/Bog Watershed

Inputs of mercury (Hg) and dissolved organic carbon (DOC) in throughfall and stemflow waters were measured for an upland/bog watershed in northern Minnesota, and were compared to the deposition in a nearby opening to determine the influence of tree canopies on Hg and DOC deposition. Twice as much Hg and seven times as much DOC was deposited in the forested watershed compared to the opening. Mass balance studies that are based on wet-only deposition in openings severely underestimate atmospheric deposition of Hg in forests. Conifer canopies are more efficient filters of airborne particulates than are deciduous canopies as indicated by much higher Hg concentrations and total deposition in throughfall and stemflow waters under conifers. Significant positive relationships existed between Hg and DOC in both throughfall (36–57% of the variation) and stemflow waters (55–88% of the variation). Hg complexation by DOC appears to be related to the contact time between precipitation and carbon sources.

Stream flow and ground water recharge from small forested watersheds in north central Minnesota

In hydrologic studies of forested watersheds, the component of the water balance most likely to be poorly defined or neglected is deep seepage. In the complex glaciated terrain of the northern Lake States, subsurface water movement can be substantial. On the Marcell experimental forest (MEF) in north-central Minnesota, ground water table elevations measured in observation wells in recharge areas were used to calculate rates of ground water recharge. In northern Minnesota winters, precipitation is stored on the surface as snow and ground water recharge ceases. Water table elevations in recharge areas decline over winter at calculable rates. Deviations from these rates during other times of the year are due to ground water recharge. On 10-50 ha watersheds on the MEF, ground water recharge varies among watersheds but constitutes about 40% of the total water yield. Annual ground water recharge amounts were found to vary linearly with precipitation. Even in high precipitation years, the infiltration capacity of the watersheds was not exceeded. Regression equations were developed relating yearly ground water recharge, stream flow, and total water yield, to seasonal precipitation amounts, summer and autumn precipitation during the previous year, and non-winter air temperature.

Relating chamber measurements to eddy correlation measurements of methane flux

Methane fluxes were measured using eddy correlation and chamber techniques during 1991 and 1992 at a peatland in north central Minnesota. Comparisons of the two techniques were made using averages of methane flux data available during 1-week periods. The seasonal patterns of fluxes measured by the two techniques compared well. Chamber flux, in 1991, was about 1.8 mg m−2 hr−1 greater than the flux obtained by the eddy correlation technique. In 1992, the chamber flux was about 1.5 mg m−2 hr−1 higher than the eddy correlation flux prior to midseason and 1.0 mg m−2 hr−1 lower than the eddy correlation flux after midseason. Chamber data from individual hummock and hollow pairs were used to calculate the averaged dF/dZ (rate of change of methane flux with surface height). During midseason in 1991, the magnitude of dF/dZ ranged between 10 and 100 (mg m−2 hr−1)m−1. We speculate that high water table conditions caused a decrease in the magnitude of dF/dZ after midseason of 1992. As compared to 1991, greater variability of dF/dZ in 1992 probably resulted from less frequent sampling. To obtain a more valid comparison of the results from the two measurement techniques, chamber data were adjusted to account for the spatial variation in methane flux. Accordingly, the chamber flux values were “scaled up” using the dF/dZ values and distributions of surface heights representative of the footprint of the eddy correlation sensors. The scaling procedure reduced the chamber fluxes by an average of 1.8 mg m−2 hr−1 in 1991 and 1.0 mg m−2 hr−1 in 1992. The comparison of eddy correlation and chamber fluxes was improved both before and after midseason in 1991. The slope of the linear regression between eddy correlation and chamber fluxes decreased from 1.49 to 1.14 (r2 increased from 0.53 to 0.75). During 1992, the scaling of chamber fluxes slightly improved their comparison with eddy correlation fluxes only prior to midseason. The lack of improvement after midseason in 1992 is likely the result of scaling assumptions when the water table was above the hollow surface. Results suggest that the adjustment of chamber flux data for spatial variations on microtopographical scales does provide fluxes more representative of a larger area. However, more information is needed on factors controlling spatial variation of methane flux to help refine the assumptions involved in the scaling procedure.

Retention and mobility of cations in a small peatland: trends and mechanisms

Inputs and net storage or release of cations can have a large impact on the acid-base balance of any ecosystem. Because of the absence of mineral soils, ion exchange and accretion in biomass are the major internal processes governing cation accumulation in peatlands. It is widely believed that peatlands have a large capacity to store cations, and thus to affect the acidity of surface waters. A long-term record of hydrologic and chemical inputs and outputs to the S-2 peatland in the Marcell Experimental Forest was used to construct annual and monthly mass balances for the four major cations. Bulk deposition and throughfall measurements suggest that deposition of Na, Ca and Mg has increased over the period 1971–1984; wet-only and dry bucket measurements do not show such a change. Seasonal and annual variations in cation inputs are not visible in the cation export from the peatland because the large cation reservoir on exchange sites in peat buffers the surface waters and prevents them from responding rapidly to changing inputs. Nevertheless, ion exchange represents a relatively minor source of acidity to this peatland, and accumulation of cations in woody biomass accounts for the majority of the cation retention. The major mechanism of retention in the peatland is different for each cation; accumulation in woody biomass is most important for Ca, storage on ion exchange sites in peat is most important for Mg, green plant tissues are the dominant site of K storage, and Na is bound by unknown mechanisms in the peat. Retention efficiencies show large annual variations but average 50% for Ca, 22% for Mg, 29–44% for Na, and 21% for K.