Estimating catchment residence time using environmental tracers: modeling recharge and input uncertainty
The residence time of subsurface water in a catchment is an important control on stream water qua... more The residence time of subsurface water in a catchment is an important control on stream water quality. The residence time reflects the various flow pathways, storage volumes, and chemical interaction times of water transported through the catchment to the stream. Residence time estimates therefore provide a primary description of the hydrological system and catchment sensitivity to anthropogenic inputs and land-use
BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access t... more BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.
A major challenge in predicting Earth's future climate state is to understand feedbacks that ... more A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO 2 ). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.
Alexandra Fiord Mesic Tundra Ecosystem. Version 1.0
The data set presented here represents growing season (late-1999, 2000 and 2001) values of net ca... more The data set presented here represents growing season (late-1999, 2000 and 2001) values of net carbon dioxide exchange, photosynthesis, and respiration between the atmosphere and mesic tundra. The site is located at Alexandra Fiord on the east-central side of Ellesmere Island, Nunavut, Canada at 78.54N 75.55W, 50 m elevation asl. As part of the International Tundra Experiment (ITEX), Greg Henry of the University of British Columbia has been increasing air and soil temperatures at this site since 1993 using small open-topped chambers (OTCs). These OTCs are hexagonal in shape and are constructed of transparent fiberglass. They typically raise air and soil temperatures by 1 to 4C. The data set shows periodic carbon dioxide exchange data from the end of the 1999 growing season and throughout the 2000 and 2001 growing seasons in ambient plots and long-term (7-9 yrs) warmed (OTC) plots. The most complete data set (2001) begins in spring before winter snowmelt and ends at the end of summer...
This dataset contains Eurasian Plant Nutrient Survey Data, including the ecophysiological charact... more This dataset contains Eurasian Plant Nutrient Survey Data, including the ecophysiological characteristics of clonal Carex arctic plant species from the Eurasian Arctic. Data are leaf d13C, %N, and d15N of Carex ensifolia Krecz. ssp. arctisibirica Yurtsev, Carex lugens Holm, and Carex stans Drej from a series of tundra sites visited in summer 1994 as part of the Swedish-Russian Tundra Ecology -1994 expedition aboard the Russian ice breaker R/V Akademik Fedorov. Additional information about the expedition and location of sites are found in Ambio 28: No. 3 (May 1999).
Winter CO2 efflux - Basetraps (ASCII). Version 1.0
This dataset represents estimates of wintertime carbon efflux from various tundra and subarctic s... more This dataset represents estimates of wintertime carbon efflux from various tundra and subarctic soils over several winters. This dataset is also available in an Excel format. For more information, please see the read-me file. NOTE: This dataset contains the data in ASCII format.
This dataset includes mid-day measurements of CO2 exchange in hummocks and pools with and without... more This dataset includes mid-day measurements of CO2 exchange in hummocks and pools with and without open-top experimental warming chambers (OTCs). Measurements were made using a LI-6200 and a clear acrylic 20 x 20 x 20 cm chamber over a 30 second period beginning 15 seconds after chamber placement. The study was carried out in a fen near Thule (Pituffik), Greenland (76 deg 33' N, 68 deg 30' W). Coverage is from 20040706 - 20060818.
Thule Biocomplexity - Gross Ecosystem Photosynthesis, Version 1.0
Ecosystem carbon dioxide flux measurements were taken at a High Arctic prostrate dwarf-shrub, her... more Ecosystem carbon dioxide flux measurements were taken at a High Arctic prostrate dwarf-shrub, herb tundra near Pituffik (Thule), Greenland (76 deg 29'N, 68 deg 26'W; elevation 245 m asl) northwest Greenland. Between 1978 and 2004, mean annual air temperature and precipitation for the site were -11.4 deg C and 12.5 cm respectively. Over the same period, growing season (June, July and August) air temperatures averaged 3.8 deg C and approximately 50% of precipitation fell between October and April as snow. The soil surface of the study site was approximately 40% covered by vascular plants and 60% unvegetated. Dominant vascular plants include the deciduous dwarf-shrub Salix arctica, the graminoid Carex rupestris and the wintergreen dwarf-shrub Dryas integrifolia. The dataset covers the growing seasons of 2006 and 2007.
This dataset contains Thule Ecosystem Flux data and represents estimates of net carbon dioxide ex... more This dataset contains Thule Ecosystem Flux data and represents estimates of net carbon dioxide exchange, respiration and photosynthesis (umol m-2 s-1) from high arctic ecosystems on the northwest coast of Greenland, near Thule (76 31'N, 68 30'W). Measurements were taken in six different ecosystems: Polar desert, Polar semi-desert, Polar oasis, Polar desert deglaciated < 2 years, Polar desert deglaciated ~50 years, Polar desert deglaciated >100 years.
Toolik Inorganic Nitrogen Uptake of Snow Covered Plants. Version 1.0
This dataset examines the ability of snow-covered arctic plants in dry heath and moist tussock tu... more This dataset examines the ability of snow-covered arctic plants in dry heath and moist tussock tundra to take up inorganic Nitrogen during snowmelt. The data is tab delimited ASCII format. For more information, see the readme file.
Alexandra Fiord - Plant Nutrient Data. Version 1.0
This dataset contains growing season (2000) carbon and nitrogen dynamics of leaves of dominant hi... more This dataset contains growing season (2000) carbon and nitrogen dynamics of leaves of dominant high arctic plant species in dry, mesic, and wet tundra ecosystems at Alexandra Fiord, Ellesmere Island, Canada. Data is in .txt format.
δ15N and δ13C as proxy for diet of black oystercatchers (Haematopus bachmani) in the northern Gulf of Alaska
Black oystercatchers (Haematopus bachmani) are a species of concern throughout their range becaus... more Black oystercatchers (Haematopus bachmani) are a species of concern throughout their range because of their specialized habitat and small population size. The prey base of black oystercatchers, which consists of animals found in the intertidal zone, may shift in unknown ways as a result of climate change. To better understand the intra-season and long-term diet patterns of black oystercatchers, we analyzed whole blood and feathers of black oystercatchers as well as several species of potential prey items using carbon (13C) and nitrogen (15N) stable isotopes. Blood and feather samples were collected from several field locations in the northern Gulf of Alaska in 2012 and 2013. Additionally, archived blood collected in the early 2000s and archived feathers collected throughout the 1900s in the same locations were also analyzed. Potential prey items were collected during 2012 and 2013 field efforts. Mixing models suggest that the diet of individuals sampled in 2012 and 2013 is approxima...
Stable isotope analysis is a powerful tool for tracing biogeochemical processes in the carbon and... more Stable isotope analysis is a powerful tool for tracing biogeochemical processes in the carbon and water cycles. One particularly powerful approach is to employ multiple isotopes where the simultaneous assessment of the D/H, 18 O/ 16 O and/or 13 C/ 12 C in different compounds provide a unique means to investigate the coupling of water and carbon fluxes at various temporal and spatial scales. Here, we present a research update on recent advances in our process-based understanding of the utilization of carbon, oxygen and hydrogen isotopes to lend insight into carbon and water cycling. We highlight recent technological developments and approaches, their strengths and methodological precautions with examples covering scales from minutes to centuries and from the leaf to the globe.
Unexpected Effects of Energy and Water Additions on the Microclimate of a High Arctic Ecosystem
Changes in radiative forcing and precipitation may have important consequences for surface energy... more Changes in radiative forcing and precipitation may have important consequences for surface energy budgets and ecosystem processes in the Arctic. In this study, energy and water inputs were manipulated in prostrate dwarf-shrub, herb tundra near Pituffik, Greenland (76° 33&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; N, 68° 34&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; W). Infrared lamps were used to provide two levels of supplemental radiation. The higher radiation supplement was combined
Differential Photosynthetic Responses of two Deciduous Shrub Species to Short and Long Term Snow Accumulation in the Arctic Tundra of Northern Alaska
ABSTRACT Shrub abundance is increasing in many ecosystems in the arctic. This change is likely to... more ABSTRACT Shrub abundance is increasing in many ecosystems in the arctic. This change is likely to have large scale implications on carbon, water and energy balances both at local and global scales. A proposed mechanism underlying increased shrub abundance is a positive feedback where shrubs increase snow accumulation which insulates soils during the winter and increases nitrogen mineralization in these nutrient limited ecosystems. The deciduous shrub Betula nana has been implicated as the primary driver of changes in snow accumulation. We examined the leaf level photosynthetic rates of B. nana and another deciduous shrub species Salix pulchra after short (1 winter) and long-term (12 winters) snow accumulation treatments. Our results indicate differential photosynthetic responses of the two shrub species with B. nana showing a greater increase in photosynthesis in response to long term snow increases but not following 1 year of snow addition. Salix did not exhibit significant increases in leaf level photosynthesis in response to short or long term increases in snow depth. Higher leaf- level carbon fixation by Betula may partially explain it&#39;s gradual dominance under deeper snow conditions in winter and may be associated with greater N availability and higher leaf N.
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Papers by J. Welker