James W. Hornbeck

Long-term depletion of calcium and other nutrients in eastern US forests

Both harvest removal and leaching losses can deplete nutrient capital in forests, but their combined long-term effects have not been assessed previously. We estimated changes in total soil and biomass N, Ca, K, Mg, and P over 120 years from published data for a spruce-fir site in Maine, two northern hardwood sites in New Hampshire, central hardwood sites in Connecticut and Tennessee, and a loblolly pine site in Tennessee. For N, atmospheric inputs counterbalance the outputs, and there is little long-term change on most sites. For K, Mg, and P, the total pool may decrease by 2%–10% in 120 years depending on site and harvest intensity. For Ca, net leaching loss is 4–16 kg/ha/yr in mature forests, and whole-tree harvest removes 200–1100 kg/ha. Such leaching loss and harvest removal could reduce total soil and biomass Ca by 20%–60% in only 120 years. We estimated unmeasured Ca inputs from rock breakdown, root-zone deepening, and dry deposition; these should not be expected to make up the Ca deficit. Acid precipitation may be the cause of current high leaching of Ca. Although Ca deficiency does not generally occur now in acid forest soils, it seems likely if anthropogenic leaching and intensive harvest removal continue.

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.

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH4-N + NO3-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN− DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kghaminus 1 yrminus 1) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO3-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO3-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.

Streamwater chemistry and nutrient budgets for forested watersheds in New England: variability and management implications

Chemistry of precipitation and streamwater and resulting input-output budgets for nutrient ions were determined concurrently for three years on three upland, forested watersheds located within an 80 km radius in central New England. Chemistry of precipitation and inputs of nutrients via wet deposition were similar among the three watersheds and were generally typical of central New England. In contrast, chemistry and nutrient outputs in streamwater varied dramatically between watersheds, with chemistries ranging from acidic to alkaline. Comparisons with data reported for 159 other upland, forested watersheds in central New England show that our study watersheds span the regional range likely to be encountered in stream chemistry. The regional variability stems in part from past natural disturbances such as wildfire, and variations in source of soil parent material. An approach is presented for predicting the important influence of glacial till on stream chemistry, including acid-base relationships, aluminum content, and nutrient outputs. Knowledge of streamwater chemistry and controlling factors can serve as an index of how terrestrial and aquatic ecosystems will respond to forest management activities and atmospheric deposition.

Effects of intensive harvesting on nutrient capitals of three forest types in New England.

Abstract Effects of whole-tree clearcutting are being studied in three major forest types in the northeastern United States: a spruce-fir forest in central Maine, a northern hardwood forest in New Hampshire, and a central hardwood forest in Connecticut. At each site we sampled total and extractable nutrient capitals, inputs and outputs of nutrient ions in precipitation and streamflow, nutrient removals in harvested products, and nutrient accumulation in regrowth. Depending upon location, combined losses of nutrients in harvested products and increased leaching to streams were in the ranges of 374–558 kg ha−1 for Ca, 135–253 kg ha−1 for K, 50–65 kg ha−1 for Mg, 248–379 kg ha−1 for N, and 19–54 kg ha−1 for P. Opportunities for replacing these losses over the next rotation are best for N. Data on inputs in precipitation versus outputs in streamflow indicate that, once effects of harvest subside, most N in precipitation will stay within the forest. By contrast, Ca shows a net output of 8–15 kg ha−1 year−1 from uncut watersheds, and the added leaching losses due to harvest may have a serious impact on Ca capital. This is especially the case for the Connecticut site, where total site capital for Ca is only about 4000 kg ha−1.

The buffer capacity of forest soils in new England

We measured buffer capacity for major horizons of forest soils from four locations in New England by titration of field-moist samples with either HCl or NaOH. Titration curves for O horizons were nearly linear over a wide pH range, that is, buffer capacity was independent of pH. Titration curves for mineral horizons were S-shaped with ambient pH roughly in the middle of the least buffered part of the curve. We also measured exchangeable acid cations and NH4+ in unbuffered KCl extractions and exchangeable bases in NH4OAc extraction at pH 7. Ca+2 and Mg+2 in KCl extractions at ambient pH were only slightly less than in NH4OAc extractions at pH 7, implying that exchangeable bases did not depend much on the extraction pH. The O horizons were generally highly base saturated at ambient pH even though their pH was low; mineral soils had lower base saturation. Buffer capacity measured over the first 0.5 pH unit to the acid side depended strongly on organic matter fraction in the sample. All soil materials studied had buffer capacities per unit organic mass of about 100 meq kginf0sup−1 pH−1. Acid rain at pH 4.0 in New England would take at least several decades to lower pH of the soil profile by a whole pH unit.

Hydrometeorological database for Hubbard Brook Experimental Forest: 1955-2000

The 3,160-ha Hubbard Brook Experimental Forest (HBEF) in New Hampshire has been a prime area of research on forest and stream ecosystems since its establishment by the USDA Forest Service in 1955. Streamflow and precipitation have been measured continuously on the HBEF, and long-term datasets exist for air and soil temperature, snow cover, soil frost, solar radiation, windspeed and direction, and humidity. This information has provided the basis for hundreds of publications by Forest Service and cooperating scientists on numerous aspects of forest hydrology research as part of the ongoing Hubbard Brook Ecosystem Study. This report updates the tables, methods, watershed descriptions, and other pertinent data in ?Thirty Years of Hydrometeorological Data at the Hubbard Brook Experimental Forest, New Hampshire? (General Technical Report NE-141).

Use of stable isotope ratios for evaluating sulfur sources and losses at the Hubbard Brook Experimental Forest

Anthropogenic S emissions have been declining in eastern North America since the early 1970s. Declines in atmospheric S deposition have resulted in decreases in concentrations and fluxes of SO42− in precipitation and drainage waters. Recent S mass balance studies have shown that the outflow of SO42− in drainage waters greatly exceeds current S inputs from atmospheric deposition. Identifying the S source(s) which contribute(s) to the discrepancy in watershed S budgets is a major concern to scientists and policy makers because of the need to better understand the rate and spatial extent of recovery from acidic deposition. Results from S mass balances combined with model calculations and isotopic analyses of SO42− in precipitation and drainage waters at the Hubbard Brook Experimental Forest (HBEF) suggest that this discrepancy cannot be explained by either underestimates of dry deposited S or desorption of previously stored SO42−. Isotopic results suggest that the excess S may be at least partially derived from net mineralization of organic S as well as the weathering of S-bearing minerals.

Biomass and nutrient removals from commercial thinning and whole-tree clearcutting of central hardwoods

The objective of this research was to evaluate the impacts of increasing product removal on biomass and nutrient content of a central hardwood forest ecosystem. Commercial thinning, currently the most common harvesting practice in southern New England, was compared with whole-tree clearcutting or maximum aboveground utilization. Using a paired-watershed approach, we studied three adjacent, first-order streams in Connecticut. During the winter of 1981–82, one was whole-tree clearcut, one was commercially thinned, and one was designated as the untreated reference. Before treatment, living and dead biomass and soil on the whole-tree clearcut site contained 578 Mg ha−1 organic matter, 5 Mg ha−1 nitrogen, 1 Mg ha−1 phosphorus, 5 Mg ha−1 potassium, 4 Mg ha−1 calcium, and 13 Mg ha−1 magnesium. An estimated 158 Mg ha−1 (27% of total organic matter) were removed during the whole-tree harvest. Calcium appeared to be the nutrient most susceptible to depletion with 13% of total site Ca removed in whole-tree clearcut products. In contrast, only 4% (16 Mg ha−1) of the total organic matter and ⩽2% of the total nutrients were removed from the thinned site. Partial cuts appear to be a reliable management option, in general, for minimizing nutrient depletion and maximizing long-term productivity of central hardwood sites. Additional data are needed to evaluate the long-term impacts of more intensive harvests.

Chemical weathering and cation loss in a base-poor watershed

Weathering of minerals in soils provides important plant nutrients and consumes acidity, yet mineral weathering is difficult to observe and quantify. We derived present-day weathering flux estimates for soil minerals in a forested watershed in central New Hampshire using a geochemical mass balance. The Cone Pond watershed is characterized by low-alkalinity surface waters and acidic Spodosols developed on a thin mantle of locally derived till, making it susceptible to continued inputs of acid precipitation. Weathering reactions were developed on the basis of observed patterns of mineral abundance in the soil profile and measured mineral chemistry. The dissolution of approximately 171 mol ha −1 yr −1 of plagioclase feldspar is the predominant weathering reaction at Cone Pond. Weathering fluxes of hornblende, biotite, chlorite, and potassium feldspar ranged from 3 to 20 mol ha −1 yr −1 , an order of magnitude lower. When normalized to their abundances in the soil, however, hornblende and chlorite had the fastest weathering rates, as measured by cation release. Chemical weathering, mainly of plagioclase, resulted in the neutralization of only 52% of incoming acidity. Furthermore, silicate weathering could only account for the release of about 53 mol ha −1 yr −1 of Ca, compared to the observed net output (stream loss minus bulk precipitation input) of 75 mol ha −1 yr −1 . On the basis of these observations and a companion study, we conclude that current acid inputs exceed the ability of Cone Pond soils to neutralize hydrogen ion, and that depletion of approximately 22–53 mol ha −1 yr −1 of Ca from labile soil pools is occurring in this area. The rate of Ca depletion is one-third to one-eighth of the rate estimated for a more base-rich watershed nearby.