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Hadley, Julian

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Hadley

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Julian

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Hadley, Julian
Author's Publications: search.filters.isAuthorOfPublication.ff85b780-6f54-4cf6-a565-ecf7a097aa02

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    Soil Respiration in a Northeastern US Temperate Forest: A 22-Year Synthesis
    (Ecological Society of America, 2013) Giasson, M.-A.; Ellison, Aaron; Bowden, R. D.; Crill, P. M.; Davidson, E. A.; Drake, J. E.; Frey, S. D.; Hadley, Julian; Lavine, M.; Melillo, J. M.; Munger, J. W.; Nadelhoffer, K. J.; Nicoll, L.; Ollinger, S. V.; Savage, K. E.; Steudler, P. A.; Tang, J.; Varner, R. K.; Wofsy, Steven; Foster, David; Finzi, A. C.
    To better understand how forest management, phenology, vegetation type, and actual and simulated climatic change affect seasonal and inter-annual variations in soil respiration (R\(_{s}\)), we analyzed more than 100,000 individual measurements of soil respiration from 23 studies conducted over 22 years at the Harvard Forest in Petersham, Massachusetts, USA. We also used 24 site-years of eddy-covariance measurements from two Harvard Forest sites to examine the relationship between soil and ecosystem respiration (R\(_{e}\)). R\(_{s}\) was highly variable at all spatial (respiration collar to forest stand) and temporal (minutes to years) scales of measurement. The response of R\(_{s}\) to experimental manipulations mimicking aspects of global change or aimed at partitioning R\(_{s}\) into component fluxes ranged from −70% to +52%. The response appears to arise from variations in substrate availability induced by changes in the size of soil C pools and of belowground C fluxes or in environmental conditions. In some cases (e.g., logging, warming), the effect of experimental manipulations on R\(_{s}\) was transient, but in other cases the time series were not long enough to rule out long-term changes in respiration rates. Inter-annual variations in weather and phenology induced variation among annual R\(_{s}\) estimates of a magnitude similar to that of other drivers of global change (i.e., invasive insects, forest management practices, N deposition). At both eddy-covariance sites, aboveground respiration dominated R\(_{e}\) early in the growing season, whereas belowground respiration dominated later. Unusual aboveground respiration patterns—high apparent rates of respiration during winter and very low rates in mid-to-late summer—at the Environmental Measurement Site suggest either bias in R\(_{s}\) and R\(_{e}\) estimates caused by differences in the spatial scale of processes influencing fluxes, or that additional research on the hard-to-measure fluxes (e.g., wintertime R\(_{s}\), unaccounted losses of CO\(_{2}\) from eddy covariance sites), daytime and nighttime canopy respiration and its impacts on estimates of R\(_{e}\), and independent measurements of flux partitioning (e.g., aboveground plant respiration, isotopic partitioning) may yield insight into the unusually high and low fluxes. Overall, however, this data-rich analysis identifies important seasonal and experimental variations in R\(_{s}\) and R\(_{e}\) and in the partitioning of R\(_{e}\) above- vs. belowground.
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    Analytic Webs Support the Synthesis of Ecological Data Sets
    (Ecological Society of America, 2006) Ellison, Aaron; Osterweil, Leon; Hadley, Julian; Wise, Alexander; Boose, Emery; Clarke, Lori; Foster, David; Hanson, Allen; Jensen, David Michael; Kuzeja, Paul; Riseman, Edward; Schultz, Howard
    A wide variety of data sets produced by individual investigators are now synthesized to address ecological questions that span a range of spatial and temporal scales. It is important to facilitate such syntheses so that ‘‘consumers’’ of data sets can be confident that both input data sets and synthetic products are reliable. Necessary documentation to ensure the reliability and validation of data sets includes both familiar descriptive metadata and formal documentation of the scientific processes used (i.e., process metadata) to produce usable data sets from collections of raw data. Such documentation is complex and difficult to construct, so it is important to help ‘‘producers’’ create reliable data sets and to facilitate their creation of required metadata. We describe a formal representation, an ‘‘analytic web,’’ that aids both producers and consumers of data sets by providing complete and precise definitions of scientific processes used to process raw and derived data sets. The formalisms used to define analytic webs are adaptations of those used in software engineering, and they provide a novel and effective support system for both the synthesis and the validation of ecological data sets. We illustrate the utility of an analytic web as an aid to producing synthetic data sets through a worked example: the synthesis of long-term measurements of whole-ecosystem carbon exchange. Analytic webs are also useful validation aids for consumers because they support the concurrent construction of a complete, Internet-accessible audit trail of the analytic processes used in the synthesis of the data sets. Finally we describe our early efforts to evaluate these ideas through the use of a prototype software tool, SciWalker. We indicate how this tool has been used to create analytic webs tailored to specific data-set synthesis and validation activities, and suggest extensions to it that will support additional forms of validation. The process metadata created by SciWalker is readily adapted for inclusion in Ecological Metadata Language (EML) files.
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    Linking Flux Network Measurements to Continental Scale Simulations: Ecosystem Carbon Dioxide Exchange Capacity under Non-Water-Stressed Conditions
    (Blackwell Science, 2007) Geyer, Ralf; Wang, Quan; Tenhunen, John; Falge, Eva; Vogel, Chris; Heinesch, Bernard; Gruenwald, Thomas; Hadley, Julian; Aurela, Mika; Saigusa, Nobuko; Owen, Katherine E.; Pilegaard, Kim; Verma, Shashi; Vesala, Timo; Reichstein, Markus; Arain, Altaf; Ammann, Christof; Moureaux, Christine; Aubinet, Marc; Xiao, Xiangming; Chojnicki, Bogdan; Bernhofer, Christian; Moors, Eddy; Stoy, Paul; Knohl, Alexander; Hollinger, David; Granier, Andre; Kutsch, Werner; Lohila, Annalea; Meyers, Tilden
    This paper examines long-term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non-water-stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light-class carboxylase-based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process-based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange.