Myron J. Mitchell

Who needs environmental monitoring

Environmental monitoring is often criticized as being unscientific, too expensive, and wasteful. While some monitoring studies do suffer from these problems, there are also many highly successful long-term monitoring programs that have provided important scientific advances and crucial information for environmental policy. Here, we discuss the characteristics of effective monitoring programs, and contend that monitoring should be considered a fundamental component of environmental science and policy. We urge scientists who develop monitoring programs to plan in advance to ensure high data quality, accessibility, and cost-effectiveness, and we urge government agencies and other funding institutions to make greater commitments to increasing the amount and long-term stability of funding for environmental monitoring programs.

Insect Defoliation and Nitrogen Cycling in Forests

O of defoliating insects can have dramatic effects on forest ecosystems. Studies have shown that defoliation can decrease transpiration and tree growth and increase tree mortality, light penetration to the forest floor, and water drainage (Stephens et al. 1972, Campbell and Sloan 1977, Houston 1981). The allocation of carbon to various parts of the tree may be altered, production of defensive compounds in foliage may increase (Schultz and Baldwin 1982), and seed production may decline for many years after defoliation (McConnell 1988, Gottschalk 1990). Shifts in tree species composition (Doane and McManus 1981, Glitzenstein et al. 1990) and changes in the population size of insectivorous birds and other wildlife may also occur (Holmes et al. 1986, USDA Forest Service 1994). Several studies of insect outbreaks have also indicated an increased loss of nitrogen (N) from forest ecosystems in drainage water following defoliation, suggesting an increase in soil-available nitrogen that is subject to leaching (Swank et al. 1981, McDonald et al. 1992, Webb et al. 1995, Eshleman et al. 1998, Reynolds et al. 2000). Large losses of nitrogen via leaching would reduce long-term forest production in Nlimited ecosystems. In addition, the export of nitrate (NO3 –) to stream water can acidify downstream waters (Webb et al. 1995) and contribute to eutrophication of coastal waters and estuaries (Fisher and Oppenheimer 1991). At first glance, the view held by many investigators that forest ecosystems leak N in large quantities after defoliation fits the general notion of nitrogen behavior in disturbed ecosystems. Significant nitrogen losses have been observed in response to disturbances such as intensive harvesting (Likens et al. 1970), fire (Bayley and Schindler 1991), and severe windstorms (Schaefer et al. 1996). However, defoliation differs qualitatively from these other disturbances in three ways. First, most of the trees usually remain alive with their woody structure intact after defoliation by insects. (Exceptions are the high mortality rates caused by repeated severe defoliations of hardwood trees or by severe defoliation of conifers.) Second, physical disturbance of the soil is minimal and significant erosion is therefore unlikely to occur. And third, if the trees are not killed, the time for substantial canopy recovery is often measured in weeks rather than years. In this article we examine the mechanisms and magnitudes of N-cycle perturbations by defoliation, drawing heavily on the considerable body of research on the gypsy moth (Lymantria dispar L.), an introduced lepidopteran that has been the major defoliator of hardwood forests in the northeastern United States during the last 5 or 6 decades (Doane and McManus 1981). We attempt to establish a more coherent view of the likely consequences of defoliation for N cycling, and we make the case that, contrary to the commonly held view, the response of forest ecosystems to defoliation is primarily one of redistribution, rather than loss, of nitrogen.

Winter in northeastern North America: a critical period for ecological processes

Ecological research during winter has historically been a low priority in northeastern North America, an oversight that stems from the commonly accepted notion that there is little biological activity when temperatures drop below freezing. However, recent research has shown that winter can be an especially important period for ecological processes, providing evidence that “dormant season” is a misnomer. Uncertainties about the effects of climate change on ecosystems are highlighting the need for a more thorough understanding of winter ecology. The failure to collect winter data in northeastern North America has meant that researchers are ill-equipped to make predictions about how ecosystems will respond to future climate change. A more focused, integrative ecological winter monitoring and research effort will enable us to better prepare for, and respond to, future climate change.

Effects of acidic deposition on forest and aquatic ecosystems in New York State.

Acidic deposition is comprised of sulfuric and nitric acids and ammonium derived from atmospheric emissions of sulfur dioxide, nitrogen oxides, and ammonia, respectively. Acidic deposition has altered soil through depletion of labile pools of nutrient cations (i.e. calcium, magnesium), accumulation of sulfur and nitrogen, and the mobilization of elevated concentrations of inorganic monomeric aluminum to soil solutions in acid-sensitive areas. Acidic deposition leaches essential calcium from needles of red spruce, making this species more susceptible to freezing injury. Mortality among sugar maples appears to result from deficiencies of nutrient cations, coupled with other stresses such as insect defoliation or drought. Acidic deposition has impaired surface water quality in the Adirondack and Catskill regions of New York by lowering pH levels, decreasing acid-neutralizing capacity, and increasing aluminum concentrations. Acidification has reduced the diversity and abundance of aquatic species in lakes and streams. There are also linkages between acidic deposition and fish mercury contamination and eutrophication of estuaries.

The biogeochemistry of sulfur at Hubbard Brook

A synthesis of the biogeochemistry of S was done during 34 yr(1964–1965 to 1997–1998) in reference and human-manipulated forestecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have beensignificant declines in concentration (−0.44µmol/liter-yr) and input (−5.44mol/ha-yr)of SO42− in atmospheric bulk wet deposition, and inconcentration(−0.64 µmol/liter-yr) an d output (−3.74mol/ha-yr) of SO42− in stream water ofthe HBEF since 1964. These changes arestrongly correlated with concurrent decreases in emissions of SO2from the source area for the HBEF. The concentration and input ofSO42− in bulk deposition ranged from a low of 13.1µmol/liter (1983–1984) and 211 mol/ha-yr(1997–1998) to a high of 34.7 µmol/liter(1965–1966) and 479 mol/ha-yr (1967–1968), with along-term mean of 23.9 µmol/liter and 336mol/ha-yr during 1964–1965 to 1997–1998. Despiterecentdeclines in concentrations, SO42− is the dominantanion in both bulk deposition and streamwater at HBEF. Dry deposition is difficult to measure, especially inmountainousterrain, but was estimated at 21% of bulk deposition. Thus, average totalatmospheric deposition was 491 and 323 mol/ha-yr during1964–1969 and 1993–1998, respectively. Based on the long-termδ34S pattern associated with anthropogenic emissions,SO42− deposition at HBEF is influenced by numerousSO2sources, but biogenic sources appear to be small. Annual throughfall plusstemflow in 1993–1994 was estimated at 346 molSO42−/ha. Aboveground litterfall, for thewatershed-ecosystemaveraged about 180 mol S/ha-yr, with highest inputs (190 molS/ha-yr) in the lower elevation, more deciduous forest zone. Weatheringrelease was calculated at a maximum of 50 mol S/ha-yr. Theconcentration and output of SO42− in stream waterranged from a low of 42.3µmol/liter (1996–1997) and 309 mol/ha-yr(1964–1965), to a high of 66.1 µmol/liter(1970–1971) and 849 mol/ha-yr (1973–1974), with along-term mean of 55.5 µmol/liter and 496mol/ha-yr during the 34 yrs of study. Gross outputs ofSO42− in stream water consistently exceeded inputsin bulkdeposition and were positively and significantly related to annualprecipitationand streamflow. The relation between gross SO42−output and annual streamflow changed with time asatmospheric inputs declined. In contrast to the pattern for bulk depositionconcentration, there was no seasonal pattern for streamSO42− concentration. Nevertheless, stream outputs ofSO42− were highly seasonal, peaking during springsnowmelt, andproducing a monthly cross-over pattern where net hydrologic flux (NHF) ispositive during summer and negative during the remainder of the year. Nosignificant elevational pattern in streamwaterSO42− concentration was observed. Mean annual,volume-weightedsoil water SO42− concentrations were relativelyuniform by soil horizon andacross landscape position. Based upon isotopic evidence, much of theSO42− entering HBEF in atmospheric depositioncycles throughvegetation and microbial biomass before being released to the soil solution andstream water. Gaseous emissions of S from watershed-ecosystems at HBEF areunquantified, but estimated to be very small. Organic S (carbon bonded andestersulfates) represents some 89% of the total S in soil at HBEF. Some 6% exists asphosphate extractable SO42− (PSO4).About 73% of the total S in the soilprofile at HBEF occurs in the Bs2 horizon, and some 9% occurs in the forestfloor. The residence time for S in the soil was calculated to be ∼9 yr, butonly a small portion of the total organic soil pool turns over relativelyquickly. The S content of above- and belowground biomass is about 2885mol/ha, of which some 3–5% is in standing dead trees. Yellowbirch, American beech and sugar maple accounted for 89% of the S in trees, with31% in branches, 27% in roots and 25% in the lightwood of boles. The pool of Sin living biomass increased from 1965 to 1982 due to biomass accretion, andremained relatively constant thereafter. Of current inputs to the availablenutrient compartment of the forest ecosystem, 50% is from atmospheric bulkdeposition, 24% from net soil release, 11% from dry deposition, 11% from rootexudates and 4% is from canopy leaching. Comparing ecosystem processes for Sfrom 1964–1969 to 1993–1998, atmospheric bulk deposition decreasedby 34%, stream output decreased by 10%, net annual biomass storage decreased by92%, and net soil release increased by 184% compared to the 1964–1969values. These changes are correlated with decreased emissions of SO2from the source area for the HBEF. Average, annual bulk deposition inputsexceeded streamwater outputs by 160.0 ± 75.3 SD molS/ha-yr,but average annual net ecosystem fluxes (NEF) were much smaller, mostlynegativeand highly variable during the 34 yr period (−54.3 ± 72.9 SDmol S/ha-yr; NEF range, +86.8 to −229.5). While severalmechanisms may explain this small discrepancy, the most likely are netdesorption of S and net mineralization of organic S largely associated with theforest floor. Our best estimates indicate that additional S from dry depositionand weathering release is probably small and that desorption accounts for about37% of the NEF imbalance and net mineralization probably accounts for theremainder (∼60%). Additional inputs from dry deposition would result fromunmeasured inputs of gaseous and particulate deposition directly to the forestfloor. The source of any unmeasured S input has important implications for therecovery of soils and streams in response to decreases in inputs of acidicdeposition. Sulfate is a dominant contributor to acid deposition at HBEF,seriously degrading aquatic and terrestrial ecosystems. Because of the strongrelation between SO2 emissions and concentrations ofSO42− in both atmospheric deposition and streamwater at HBEF,further reductions in SO2 emissions will be required to allowsignificant ecosystem recovery from the effects of acidic deposition. Thedestruction or removal of vegetation on experimental watershed-ecosystems atHBEF resulted in increased rates of organic matter decomposition andnitrification, a lowering of soil and streamwater pH, enhancedSO42− adsorption on mineral soil and smallerconcentrations andlosses of SO42− in stream water. With vegetationregrowth, this adsorbedSO42− is released from the soil, increasingconcentrations andfluxes of SO42− in drainage water. Streamwaterconcentration ofSO42− and gross annual output ofSO42−/ha are essentially the same throughout theHubbard BrookValley in watersheds varying in size by about 4 orders of magnitude, from 3 to3000 ha.

NITROGEN SATURATION IN JAPANESE FORESTED WATERSHEDS

Biogeochemistry of nitrogen was evaluated in a series of small watersheds in Gunma Prefecture, 100 km northwest of Tokyo in Japan. The forest vegetation ranged in age from 7 to 86 yr and included conifer plantations and naturally regenerated hardwoods. In a watershed with 24-yr-old stands of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa), stream water NO3− concentrations were lower (21 μmol/L) than watersheds composed of 7-, 84-, and 86-yr-old stands (62, 64, and 100 μmol/L, respectively). These differences among watersheds were mostly attributable to differences in the increments of N by forest vegetation. The absence of seasonal variation of stream NO3− concentration in watersheds with either coniferous or hardwood forests suggests that N availability was in excess of biotic demands during all seasons. Over 6 yr, inorganic N output by stream water (13.5 kg N·ha−1·yr−1) exceeded N input as bulk precipitation (10.5 kg N·ha−1·yr−1) in an 86-yr-old coniferous stand. The high N output was rela...

Consequences of climate change for biogeochemical cycling in forests of northeastern North America.

A critical component of assessing the impacts of climate change on forest ecosystems involves understanding associated changes in the biogeochemical cycling of elements. Evidence from research on northeastern North American forests shows that direct effects of climate change will evoke changes in biogeochemical cycling by altering plant physiology, forest productivity, and soil physical, chemical, and biological processes. Indirect effects, largely mediated by changes in species composition, length of growing season, and hydrology, will also be important. The case study presented here uses the quantitative biogeochemical model PnET-BGC to test assumptions about the direct and indirect effects of climate change on a northern hardwood forest ecosystem. Modeling results indicate an overall increase in net primary production due to a longer growing season, an increase in NO3– leaching due to large increases in net mineralization and nitrification, and slight declines in mineral weathering due to a reduction i...

Spatial patterns of precipitation quantity and chemistry and air temperature in the Adirondack region of New York

Regional assessments are critical to evaluate resources at riskfrom disturbances, such as acidic deposition, which occur in large spatial extent. It is imperative to accurately quantify atmospheric deposition to the regions like the AdirondackMountains of New Yorkwhere soils and surface waters are highly sensitive to inputs of strong acids. Spatial patterns in precipitation quantity and concentrations of major ions in precipitation in the Adirondackregion were estimated based on the locations of monitoring sites using the data from 1988 to 1999. Mean monthly minimum and maximum daily air temperatures were also predicted to characterize site conditions. The trends in precipitation quantity, temperature, and most ion concentrations during the period examined were not significant or minimally significant, suggesting that the mean values over the period could be used for the regression models to describe spatial patterns. The spatial variations in the mean annual and monthly precipitation amounts, monthly mean minimum and maximum daily temperatures, and annual and quarterly sulfate and nitrate concentrations were generally explained by the regression models. Comparisons with the regression models of Ollinger et al. (US Ecological Applications 3(3) (1993) 459; US Department of Agriculture, Forest Service, Radnor, PA., 1995, 30p.) for the northeastern US suggest local variations in climate variables for the smaller Adirondackregion. Further, precipitation quantity, minimum and maximum temperatures, and concentrations and deposition of ions in precipitation were predicted for the entire Adirondackregion, using digital elevation models among others. Precipitation quantity and the sulfate and nitrate concentrations generally increased from the northeast to the southwest. Precipitation quantity and sulfate concentrations also generally increased with elevation. Minimum and maximum temperatures decreased from the southeast to the northwest and with elevation. r 2002 Elsevier Science Ltd. All rights reserved.

Paleoecological investigation of recent lake acidification in the Adirondack Mountains, N.Y.

Paleoecological analysis of the sediment record of 12 Adirondack lakes reveals that the 8 clearwater lakes with current pH < 5.5 and alkalinity < 10 μeq l-1 have acidified recently. The onset of this acidification occurred between 1920 and 1970. Loss of alkalinity, based on quanitative analysis of diatom assemblages, ranged from 2 to 35 μeq l-1. The acidification trends are substantiated by several lines of evidence including stratigraphies of diatom, chrysophyte, chironomid, and cladoceran remains, Ca:Ti and Mn:Ti ratios, sequentially extracted forms of Al, and historical fish data. Acidification trends appear to be continuing in some lakes, despite reductions in atmospheric sulfur loading that began in the early 1970s. The primary cause of the acidification trend is clearly increased atmospheric deposition of strong acids derived from the combustion of fossil fuels. Natural processes and watershed disturbances cannot account for the changes in water chemistry that have occurred, but they may play a role. Sediment core profiles of Pb, Cu, V, Zn, S, polycyclic aromatic hydrocarbons, magnetic particles, and coal and oil soot provide a clear record of increased atmospheric input of materials associated with the combustion of fossil fuels beginning in the late 1800s and early 1900s. The primary evidence for acidification occurs after that period, and the pattern of water chemistry response to increased acid inputs is consistent with current understanding of lake-watershed acidification processes.

Sugar maple and nitrogen cycling in the forests of eastern North America

Sugar maple (Acer saccharum) is the most dominant and widely distributed tree species of the northern hardwood forests of the northeastern US and southeastern Canada. Recent studies have shown that sugar maple is also a unique and critical species with regard to nitrogen cycling in forest ecosystems, because forest stands dominated by sugar maple tend to have high rates of nitrification and nitrate leaching to surface waters. In some areas, sugar maple populations may be increasing due to reduction of one of their main competitors, American beech (Fagus grandifolia). However, several factors threaten populations of sugar maple in the near future, including acid deposition, climate change, and the introduction of a new insect pest. Changes in the abundance of sugar maple could lead to major alterations in nitrogen retention by forested watersheds in eastern North America.