Thomas G. Huntington

Controls on soil respiration: Implications for climate change

The disruption of the global C cycle by human activity in both developed and developing countries is one of the key environmental issues facing human populations as we move into the 21st century.

Changes in the timing of high river flows in New England over the 20th Century

The annual timing of river flows is a good indicator of climate-related changes, or lack of changes, for rivers with long-term data that drain unregulated basins with stable land use. Changes in the timing of annual winter/spring (January 1 to May 31) and fall (October 1 to December 31) center of volume dates were analyzed for 27 rural, unregulated river gaging stations in New England, USA with an average of 68 years of record. The center of volume date is the date by which half of the total volume of water for a given period of time flows past a river gaging station, and is a measure of the timing of the bulk of flow within the time period. Winter/spring center of volume (WSCV) dates have become significantly earlier ðp , 0:1Þ at all 11 river gaging stations in areas of New England where snowmelt runoff has the most effect on spring river flows. Most of this change has occurred in the last 30 years with dates advancing by 1 ‐ 2 weeks. WSCV dates were correlated with March through April air temperatures ðr ¼ 20:72Þ and with January precipitation ðr ¼ 20:37Þ: Three of 16 river gaging stations in the remainder of New England had significantly earlier WSCV dates. Four out of 27 river gaging stations had significantly earlier fall center of volume dates in New England. Changes in the timing of winter/spring and fall peak flow dates were consistent with the changes in the respective center of volume dates, given the greater variability in the peak flow dates. Changes in the WSCV dates over the last 30 years are consistent with previous studies of New England last-frost dates, lilac bloom dates, lake ice-out dates, and spring air temperatures. This suggests that these New England spring geophysical and biological changes all were caused by a common mechanism, temperature increases. Published by Elsevier Science B.V.

Changes in the Proportion of Precipitation Occurring as Snow in New England (1949–2000)

Abstract The ratio of snow to total precipitation (S/P) is a hydrologic indicator that is sensitive to climate variability and can be used to detect and monitor hydrologic responses to climatic change. Changes in S/P ratio over time could influence the magnitude and timing of spring runoff and recession to summer baseflow. The S/P ratio for 21 U.S. Historical Climatology Network sites in New England was examined. Eleven out of twenty-one sites in New England had significant decreasing annual S/P ratios from 1949 to 2000. Annual trends in S/P are predominantly a result of decreasing snowfall, and to a lesser extent, increasing rainfall. The most consistent trends were in northernmost New England where all four sites had decreasing ratios, and in the coastal and near-coastal areas where five out of eight sites had significantly decreasing ratios. The four sites in northernmost New England, which had the strongest and most coherent trends, showed an average decrease in annual S/P ratio from about 0.30 in 194...

Analysis of the Arctic System for Freshwater Cycle Intensification: Observations and Expectations

Abstract Hydrologic cycle intensification is an expected manifestation of a warming climate. Although positive trends in several global average quantities have been reported, no previous studies have documented broad intensification across elements of the Arctic freshwater cycle (FWC). In this study, the authors examine the character and quantitative significance of changes in annual precipitation, evapotranspiration, and river discharge across the terrestrial pan-Arctic over the past several decades from observations and a suite of coupled general circulation models (GCMs). Trends in freshwater flux and storage derived from observations across the Arctic Ocean and surrounding seas are also described. With few exceptions, precipitation, evapotranspiration, and river discharge fluxes from observations and the GCMs exhibit positive trends. Significant positive trends above the 90% confidence level, however, are not present for all of the observations. Greater confidence in the GCM trends arises through lowe...

Climate and hydrological changes in the northeastern United States: recent trends and implications for forested and aquatic ecosystems

We review twentieth century and projected twenty-first century changes in climatic and hydrologic conditions in the northeastern United States and the implications of these changes for forest ecosystems. Climate warming and in- creases in precipitation and associated changes in snow and hydrologic regimes have been observed over the last century, with the most pronounced changes occurring since 1970. Trends in specific climatic and hydrologic variables differ in their responses spatially (e.g., coastal vs. inland) and temporally (e.g., spring vs. summer). Trends can differ depending on the period of record analyzed, hinting at the role of decadal-scale climatic variation that is superimposed over the longer- term trend. Model predictions indicate that continued increases in temperature and precipitation across the northeastern United States can be expected over the next century. Ongoing increases in growing season length (earlier spring and later autumn) will most likely increase evapotranspiration and frequency of drought. In turn, an increase in the frequency of drought will likely increase the risk of fire and negatively impact forest productivity, maple syrup production, and the in- tensity of autumn foliage coloration. Climate and hydrologic changes could have profound effects on forest structure, com- position, and ecological functioning in response to the changes discussed here and as described in related articles in this issue of the Journal.

The potential for calcium depletion in forest ecosystems of southeastern United States: Review and analysis

Biogeochemical mass balance assessments of calcium status in southeastern forests indicate that losses through harvesting and soil leaching often exceed inputs from atmospheric deposition and weathering. Many forest soils of the southeastern United States are particularly sensitive because these soils and the underlying saprolite from which these soils are derived are largely depleted of weatherable calcium. At most of the intensively studied sites in the southeastern United States, it is estimated that calcium depletion has already reduced or will likely reduce exchangeable soil calcium reserves to less than the estimated requirement for a merchantable forest stand in 150 years or less. At most sites, calcium uptake into merchantable wood equals or exceeds soil leaching losses. Chronic atmospheric deposition of sulfate and nitrate and declining atmospheric deposition of calcium are likely to accelerate calcium depletion. The southeastern U.S. regional distribution of soil calcium pools and calcium fluxes (deposition and uptake in merchantable wood) indicates that the depletion status of the intensively studied sites is representative of a substantially larger area. Where weathering inputs are insufficient to replace leaching and uptake losses, there is a potential for a regional problem in forest nutrition over the long term.

Redistribution of soil nitrogen, carbon and organic matter by mechanical disturbance during whole-tree harvesting in northern hardwoods

Abstract To investigate whether mechanical mixing during harvesting could account for losses observed from forest floor, we measured surface disturbance on a 22 ha watershed that was whole-tree harvested. Surface soil on each 10 cm interval along 81, randomly placed transects was classified immediately after harvesting as mineral or organic, and as undisturbed, depressed, rutted, mounded, scarified, or scalped (forest floor scraped away). We quantitatively sampled these surface categories to collect soil in which preharvest forest floor might reside after harvest. Mechanically mixed mineral and organic soil horizons were readily identified. Buried forest floor under mixed mineral soil occurred in 57% of mounds with mineral surface soil. Harvesting disturbed 65% of the watershed surface and removed forest floor from 25% of the area. Mechanically mixed soil under ruts with organic or mineral surface soil, and mounds with mineral surface soil contained organic carbon and nitrogen pools significantly greater than undisturbed forest floor. Mechanical mixing into underlying mineral soil could account for the loss of forest floor observed between the preharvest condition and the second growing season after whole-tree harvesting.

Historical trend in river ice thickness and coherence in hydroclimatological trends in Maine

We analyzed long-term records of ice thickness on the Piscataquis River in central Maine and air temperature in Maine to determine whether there were temporal trends that were associated with climate warming. The trend in ice thickness was compared and correlated with regional time series of winter air temperature, heating degree days (HDD), date of river ice-out, seasonal center-of-volume date (SCVD) (date on which half of the stream runoff volume during the period 1 Jan. to 31 May has occurred), water temperature, and lake ice-out date. All of these variables except lake ice-out date showed significant temporal trends during the 20th century. Average ice thickness around 28 February decreased by about 23 cm from 1912 to 2001. Over the period 1900 to 1999, winter air temperature increased by 1.7 °C and HDD decreased by about 7.5%.Final ice-out date on the Piscataquis River occurred earlier (advanced), by 0.21 days yr−1 over the period 1931 to 2002, and the SCVD advancedby 0.11 days yr−1 over the period 1903 to 2001. Ice thickness was significantly correlated (P-value <0.01) with winter air temperature, HDD, river ice-out, and SCVD. These systematic temporal trends in multiple hydrologic indicator variables indicate a coherent response to climate forcing.

Identifying fluorescent pulp mill effluent in the Gulf of Maine and its watershed

Using fluorescence spectroscopy and parallel factor analysis (PARAFAC) we characterized and modeled the fluorescence properties of dissolved organic matter (DOM) in samples from the Penobscot River, Androscoggin River, Penobscot Bay, and the Gulf of Maine (GoM). We analyzed excitation-emission matrices (EEMs) using an existing PARAFAC model (Cory and McKnight, 2005) and created a system-specific model with seven components (GoM PARAFAC). The GoM PARAFAC model contained six components similar to those in other PARAFAC models and one unique component with a spectrum similar to a residual found using the Cory and McKnight (2005) model. The unique component was abundant in samples from the Androscoggin River immediately downstream of a pulp mill effluent release site. The detection of a PARAFAC component associated with an anthropogenic source of DOM, such as pulp mill effluent, demonstrates the importance for rigorously analyzing PARAFAC residuals and developing system-specific models.

Climate Warming-Induced Intensification of the Hydrologic Cycle: An Assessment of the Published Record and Potential Impacts on Agriculture

Abstract Climate warming is expected to intensify and accelerate the global hydrologic cycle resulting in increases in evaporation, evapotranspiration (ET), atmospheric water-vapor content, and precipitation. The strength of the hydrologic response, or sensitivity of the response for a given degree of warming, is a critical outstanding question in climatology and hydrology. In this review chapter, I examine the published record of trends in various components of the hydrologic cycle and associated variables to assess observed hydrologic responses to warming during the period of observational records. Global and regional trends in evaporation, ET, and atmospheric water-vapor content and several large river basin water-balance studies support an ongoing intensification of the hydrologic cycle. Global trends in precipitation, runoff, and soil moisture are more uncertain than the trends in the variables noted above, in part because of high spatial and temporal variability. Trends in associated variables, such as systematic changes in ocean salinity, the length of the growing season, and the rate of precipitation recycling are generally consistent with intensification of the hydrologic cycle. The evidence for an increase in the frequency, intensity, or duration of extreme-weather events like hurricanes is mixed and remains uncertain. The largest potential impacts to agricultural systems depend greatly on the responses of hydrologic variables that are the most uncertain; for example, intensity and duration of heavy rainfall events; frequency, intensity, and duration of major storms and droughts; and rates of erosion. Impacts on agriculture will depend greatly on how insects, diseases, weeds, nutrient cycling, effectiveness of agrichemicals, and heat stress are affected by an intensification of the hydrologic cycle.