Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Reviewing research priorities in weed ecology, evolution and management: a horizon scan
Neve, P. ; Barney, J.N. ; Buckley, Y. ; Cousens, R.D. ; Graham, S. ; Jordan, N.R. ; Lawton-Rauh, A. ; Liebman, M. ; Mesgaran, M.B. ; Shaw, J. ; Storkey, J. ; Baraibar, B. ; Baucom, R.S. ; Chalak, M. ; Childs, D.Z. ; Christensen, S. ; Eizenberg, H. ; Fernández-Quintanilla, C. ; French, K. ; Harsch, M. ; Heijting, S. ; Harrison, L. ; Loddo, D. ; Macel, M. ; Maczey, N. ; Merotto, A. ; Mortensen, D. ; Necajeva, J. ; Peltzer, D.A. ; Recasens, J. ; Renton, M. ; Riemens, M. ; Sønderskov, M. ; Williams, M. ; Rew, Lisa - \ 2018
Weed Research 58 (2018)4. - ISSN 0043-1737 - p. 250 - 258.
Weedy plants pose a major threat to food security, biodiversity, ecosystem services and consequently to human health and wellbeing. However, many currently used weed management approaches are increasingly unsustainable. To address this knowledge and practice gap, in June 2014, 35 weed and invasion ecologists, weed scientists, evolutionary biologists and social scientists convened a workshop to explore current and future perspectives and approaches in weed ecology and management. A horizon scanning exercise ranked a list of 124 pre‐submitted questions to identify a priority list of 30 questions. These questions are discussed under seven themed headings that represent areas for renewed and emerging focus for the disciplines of weed research and practice. The themed areas considered the need for transdisciplinarity, increased adoption of integrated weed management and agroecological approaches, better understanding of weed evolution, climate change, weed invasiveness and finally, disciplinary challenges for weed science. Almost all the challenges identified rested on the need for continued efforts to diversify and integrate agroecological, socio‐economic and technological approaches in weed management. These challenges are not newly conceived, though their continued prominence as research priorities highlights an ongoing intransigence that must be addressed through a more system‐oriented and transdisciplinary research agenda that seeks an embedded integration of public and private research approaches. This horizon scanning exercise thus set out the building blocks needed for future weed management research and practice; however, the challenge ahead is to identify effective ways in which sufficient research and implementation efforts can be directed towards these needs.
Automated assembly of species metabolomes through data submission into a public repository
Salek, Reza ; Conesa, Pablo ; Cochrane, Keeva ; Haug, Kenneth ; Williams, M. ; Kale, Namrata ; Moreno, P. ; Jayaseelan, Kalai Vanii ; Macias, Jose Ramon ; Nainala, Venkata Chandrasekhar ; Hall, R.D. ; Reed, Laura ; Viant, Mark ; Donovan, C. ; Steinbeck, Christoph - \ 2017
GigaScience 6 (2017)8. - ISSN 2047-217X - 4 p.
Following similar global efforts to exchange genomic and other biomedical data, global databases in metabolomics have now been established. MetaboLights, the first general purpose, publically available, cross-species, cross-application database in metabolomics, has become the fastest growing data repository at the European Bioinformatics Institute in terms of data volume. Here we present the automated assembly of species metabolomes in MetaboLights, a crucial reference for chemical biology, which is growing through user submissions.
Transdisciplinary weed research: new leverage on challenging weed problems?
Jordan, N. ; Schut, M. ; Graham, S. ; Barney, J.N. ; Childs, D.Z. ; Christensen, S.B. ; Cousens, R.D. ; Davis, A.S. ; Eizenberg, H. ; Ervin, D.E. ; Fernandez-Quintanilla, C. ; Harrison, L.J. ; Harsch, M.A. ; Heijting, S. ; Liebman, M. ; Loddo, D. ; Mirsky, S.B. ; Riemens, M. ; Neve, P. ; Peltzer, D.A. ; Renton, M. ; Williams, M. ; Recasens, J. ; Sønderskov, M. - \ 2016
Weed Research 56 (2016)5. - ISSN 0043-1737 - p. 345 - 358.
Transdisciplinary weed research (TWR) is a promising path to more effective management of challenging weed problems. We define TWR as an integrated process of inquiry and action that addresses complex weed problems in the context of broader efforts to improve economic, environmental and social aspects of ecosystem sustainability. TWR seeks to integrate scholarly and practical knowledge across many stakeholder groups (e.g. scientists, private sector, farmers and extension officers) and levels (e.g. local, regional and landscape). Furthermore, TWR features democratic and iterative processes of decision-making and collective action that aims to align the interests, viewpoints and agendas of a wide range of stakeholders. The fundamental rationale for TWR is that many challenging weed problems (e.g. herbicide resistance or extensive plant invasions in natural areas) are better addressed systemically, as a part of broad-based efforts to advance ecosystem sustainability, rather than as isolated problems. Addressing challenging weed problems systemically can offer important new leverage on such problems, by creating new opportunities to manage their root causes and by improving complementarity between weed management and other activities. While promising, this approach is complicated by the multidimensional, multilevel, diversely defined and unpredictable nature of ecosystem sustainability. In practice, TWR can be undertaken as a cyclic process of (i) initial problem formulation, (ii) ‘broadening’ of the problem formulation and recruitment of stakeholder participants, (iii) deliberation, negotiation and design of an action agenda for systemic change, (iv) implementation action, (v) monitoring and assessment of outcomes and (vi) reformulation of the problem situation and renegotiation of further actions. Notably, ‘purposive’ disciplines (design, humanities and arts) have central, critical and recurrent roles in this process, as do integrative analyses of relevant multidimensional and multilevel factors, via multiple natural and social science disciplines. We exemplify this process in prospect and retrospect. Importantly TWR is not a replacement for current weed research; rather, the intent is to powerfully leverage current efforts
Contribution of Fisheries and Aquaculture to Food Security and Poverty Reduction: Assessing the Current Evidence
Béné, C. ; Arthur, R. ; Norbury, H. ; Allison, E.H. ; Beveridge, M. ; Bush, S.R. ; Campling, L. ; Leschen, W. ; Little, D. ; Squires, D. ; Thilsted, S.H. ; Troell, M. ; Williams, M. - \ 2016
World Development 79 (2016). - ISSN 0305-750X - p. 177 - 196.
poverty reduction - food security and nutrition - development - fisheries - aquaculture
Following a precise evaluation protocol that was applied to a pool of 202 articles published between 2003 and 2014, this paper evaluates the existing evidence of how and to what extent capture fisheries and aquaculture contribute to improving nutrition, food security, and economic growth in developing and emergent countries. In doing so we evaluate the quality and scientific rigor of that evidence, identify the key conclusions that emerge from the literature, and assess whether these conclusions are consistent across the sources. The results of the assessment show that while some specific topics are consistently and rigorously documented, thus substantiating some of the claims found in the literature, other areas of research still lack the level of disaggregated data or an appropriate methodology to reach consistency and robust conclusions. More specifically, the analysis reveals that while fish contributes undeniably to nutrition and food security, the links between fisheries/aquaculture and poverty alleviation are complex and still unclear. In particular national and household level studies on fisheries’ contributions to poverty alleviation lack good conceptual models and produce inconsistent results. For aquaculture, national and household studies tend to focus on export value chains and use diverse approaches. They suggest some degree of poverty alleviation and possibly other positive outcomes for adopters, but these outcomes also depend on the small-scale farming contexts and on whether adoption was emergent or due to development assistance interventions. Impacts of fish trade on food security and poverty alleviation are ambiguous and confounded by a focus on international trade and a lack of consistent methods. The influences of major drivers (decentralization, climate change, demographic transition) are still insufficiently documented and therefore poorly understood. Finally the evaluation reveals that evidence-based research and policy narratives are often disconnected, with some of the strongest and long-lasting policy narratives lacking any strong and rigorous evidence-based validation. Building on these different results, this paper identifies six key gaps facing policy-makers, development practitioners, and researchers.
Pan-Arctic modelling of net ecosystem exchange of CO2
Shaver, G.R. ; Rastetter, E.B. ; Salmon, V. ; Street, L.E. ; Weg, M.J. van de; Rocha, A. ; Wijk, M.T. van; Williams, M. - \ 2013
Philosophical Transactions of the Royal Society B. Biological sciences 368 (2013)1624. - ISSN 0962-8436
leaf-area index - primary productivity - vascular plants - carbon balance - climate-change - temperature - vegetation - sensitivity - nitrogen - alaska
Net ecosystem exchange (NEE) of C varies greatly among Arctic ecosystems. Here, we show that approximately 75 per cent of this variation can be accounted for in a single regression model that predicts NEE as a function of leaf area index (LAI), air temperature and photosynthetically active radiation (PAR). The model was developed in concert with a survey of the light response of NEE in Arctic and subarctic tundras in Alaska, Greenland, Svalbard and Sweden. Model parametrizations based on data collected in one part of the Arctic can be used to predict NEE in other parts of the Arctic with accuracy similar to that of predictions based on data collected in the same site where NEE is predicted. The principal requirement for the dataset is that it should contain a sufficiently wide range of measurements of NEE at both high and low values of LAI, air temperature and PAR, to properly constrain the estimates of model parameters. Canopy N content can also be substituted for leaf area in predicting NEE, with equal or greater accuracy, but substitution of soil temperature for air temperature does not improve predictions. Overall, the results suggest a remarkable convergence in regulation of NEE in diverse ecosystem types throughout the Arctic.
Incident radiation and the allocation of nitrogen within Arctic plant canopies: implications for predicting gross primary productivity
Street, L.E. ; Shaver, G.R. ; Rastetter, E.B. ; Wijk, M.T. van; Kaye, B.A. ; Williams, M. - \ 2012
Global Change Biology 18 (2012)9. - ISSN 1354-1013 - p. 2838 - 2852.
leaf-area index - economics spectrum - air-temperature - carbon-exchange - c-3 plants - co2 flux - photosynthesis - vegetation - leaves - tundra
Arctic vegetation is characterized by high spatial variability in plant functional type (PFT) composition and gross primary productivity (P). Despite this variability, the two main drivers of P in sub-Arctic tundra are leaf area index (LT) and total foliar nitrogen (NT). LT and NT have been shown to be tightly coupled across PFTs in sub-Arctic tundra vegetation, which simplifies up-scaling by allowing quantification of the main drivers of P from remotely sensed LT. Our objective was to test the LT–NT relationship across multiple Arctic latitudes and to assess LT as a predictor of P for the pan-Arctic. Including PFT-specific parameters in models of LT–NT coupling provided only incremental improvements in model fit, but significant improvements were gained from including site-specific parameters. The degree of curvature in the LT–NT relationship, controlled by a fitted canopy nitrogen extinction co-efficient, was negatively related to average levels of diffuse radiation at a site. This is consistent with theoretical predictions of more uniform vertical canopy N distributions under diffuse light conditions. Higher latitude sites had higher average leaf N content by mass (NM), and we show for the first time that LT–NT coupling is achieved across latitudes via canopy-scale trade-offs between NM and leaf mass per unit leaf area (LM). Site-specific parameters provided small but significant improvements in models of P based on LT and moss cover. Our results suggest that differences in LT–NT coupling between sites could be used to improve pan-Arctic models of P and we provide unique evidence that prevailing radiation conditions can significantly affect N allocation over regional scales
Tools for communicating and visualising uncertainties, Deliverable, 3.3
Gerharz, L. ; Senaratne, H. ; Autermann, C. ; Truong, N.P. ; Heuvelink, G.B.M. ; Williams, M. ; Pebesma, E. ; Stasch, C. ; Cornford, D. - \ 2012
The Uncertainty Enabled Model Web - 40 p.
The Anthropocene: From Global Change to Planetary Stewardship
Steffen, W. ; Persson, A. ; Deutsch, L. ; Zalasiewicz, J. ; Williams, M. ; Richardson, K. ; Crumley, C. ; Crutzen, P. ; Folke, C. ; Gordon, L. ; Molina, M. ; Ramanathan, V. ; Rockstrom, J. ; Scheffer, M. ; Schellnhuber, H.J. ; Svedin, U. - \ 2011
Ambio 40 (2011)7. - ISSN 0044-7447 - p. 739 - 761.
safe operating space - climate system - sustainability - ecosystems - services - humanity - history - carbon - world
Over the past century, the total material wealth of humanity has been enhanced. However, in the twenty-first century, we face scarcity in critical resources, the degradation of ecosystem services, and the erosion of the planet's capability to absorb our wastes. Equity issues remain stubbornly difficult to solve. This situation is novel in its speed, its global scale and its threat to the resilience of the Earth System. The advent of the Anthropence, the time interval in which human activities now rival global geophysical processes, suggests that we need to fundamentally alter our relationship with the planet we inhabit. Many approaches could be adopted, ranging from geo-engineering solutions that purposefully manipulate parts of the Earth System to becoming active stewards of our own life support system. The Anthropocene is a reminder that the Holocene, during which complex human societies have developed, has been a stable, accommodating environment and is the only state of the Earth System that we know for sure can support contemporary society. The need to achieve effective planetary stewardship is urgent. As we go further into the Anthropocene, we risk driving the Earth System onto a trajectory toward more hostile states from which we cannot easily return.
Web Based Expert Elicitation of Uncertainties in Environmental Model Inputs
Bastin, L. ; Williams, M. ; Gosling, J.P. ; Truong, N.P. ; Cornford, D. ; Heuvelink, G.B.M. ; Achard, F. - \ 2011
Geophysical Research Abstracts 13 (2011). - ISSN 1029-7006 - p. 5384 - 5384.
Expert elicitation of input and model uncertainties
Heuvelink, G.B.M. ; Bastin, L. ; Cornford, D. ; Gossling, J.P. ; Truong, N.P. ; Williams, M. - \ 2011
The Uncertainty Enabled Model Web (Deliverable 3.1) - 61 p.
Workshop report: Proceedings of the Rank Forum on Vitamin D
Lanham-New, S.A. ; Buttriss, J.L. ; Miles, L.M. ; Ashwell, M. ; Berry, J.L. ; Boucher, B.J. ; Cashman, K.D. ; Cooper, C. ; Darling, A.L. ; Francis, R.M. ; Fraser, W.D. ; Groot, C.P.G.M. de; Hypponen, E. ; Kiely, M. ; Lamberg-Allardt, C. ; Macdonald, H.M. ; Martineau, A.R. ; Masud, T. ; Mavroeidi, A. ; Nowson, C. ; Prentice, A. ; Stone, E.M. ; Reddy, S. ; Vieth, R. ; Williams, M. - \ 2011
The British journal of nutrition 105 (2011)1. - ISSN 0007-1145 - p. 144 - 156.
The Rank Forum on Vitamin D was held on 2nd and 3rd July 2009 at the University of Surrey, Guildford, UK. The workshop consisted of a series of scene-setting presentations to address the current issues and challenges concerning vitamin D and health, and included an open discussion focusing on the identification of the concentrations of serum 25-hydroxyvitamin D (25(OH)D) (a marker of vitamin D status) that may be regarded as optimal, and the implications this process may have in the setting of future dietary reference values for vitamin D in the UK. The Forum was in agreement with the fact that it is desirable for all of the population to have a serum 25(OH)D concentration above 25 nmol/l, but it discussed some uncertainty about the strength of evidence for the need to aim for substantially higher concentrations (25(OH)D concentrations>75 nmol/l). Any discussion of 'optimal' concentration of serum 25(OH)D needs to define 'optimal' with care since it is important to consider the normal distribution of requirements and the vitamin D needs for a wide range of outcomes. Current UK reference values concentrate on the requirements of particular subgroups of the population; this differs from the approaches used in other European countries where a wider range of age groups tend to be covered. With the re-emergence of rickets and the public health burden of low vitamin D status being already apparent, there is a need for urgent action from policy makers and risk managers. The Forum highlighted concerns regarding the failure of implementation of existing strategies in the UK for achieving current vitamin D recommendations
The REFLEX project: Comparing different algorithms and implementations for the inversion of a terrestrial ecosystem model against eddy covariance data
Fox, A. ; Williams, M. ; Richardson, A.D. ; Cameron, D. ; Gove, J.H. ; Quaife, T. ; Ricciuto, D. ; Reichstein, M. ; Tomelleri, E. ; Trudinger, C.M. ; Wijk, M.T. van - \ 2009
Agricultural and Forest Meteorology 149 (2009)10. - ISSN 0168-1923 - p. 1597 - 1615.
parameter-estimation - data assimilation - carbon-dioxide - uncertainty - climate - forest - productivity - variability - simulation - feedbacks
We describe a model-data fusion (MDF) inter-comparison project (REFLEX), which compared various algorithms for estimating carbon (C) model parameters consistent with both measured carbon fluxes and states and a simple C model. Participants were provided with the model and with both synthetic net ecosystem exchange (NEE) of CO2 and leaf area index (LAI) data, generated from the model with added noise, and observed NEE and LAI data from two eddy covariance sites. Participants endeavoured to estimate model parameters and states consistent with the model for all cases over the two years for which data were provided, and generate predictions for one additional year without observations. Nine participants contributed results using Metropolis algorithms, Kalman filters and a genetic algorithm. For the synthetic data case, parameter estimates compared well with the true values. The results of the analyses indicated that parameters linked directly to gross primary production (GPP) and ecosystem respiration, such as those related to foliage allocation and turnover, or temperature sensitivity of heterotrophic respiration, were best constrained and characterised. Poorly estimated parameters were those related to the allocation to and turnover of fine root/wood pools. Estimates of confidence intervals varied among algorithms, but several algorithms successfully located the true values of annual fluxes from synthetic experiments within relatively narrow 90% confidence intervals, achieving >80% success rate and mean NEE confidence intervals
Using information theory to determine optimum pixel size and shape for ecological studies: Aggregating land surface characteristics in arctic ecosystems
Stoy, P.C. ; Williams, M. ; Spadavecchia, L. ; Bell, R.A. ; Prieto-Blanco, A. ; Evans, J.G. ; Wijk, M.T. van - \ 2009
Ecosystems 12 (2009)4. - ISSN 1432-9840 - p. 574 - 589.
leaf-area index - vegetation indexes - co2 flux - model - photosynthesis - transpiration - heterogeneity - productivity - agriculture - temperature
Quantifying vegetation structure and function is critical for modeling ecological processes, and an emerging challenge is to apply models at multiple spatial scales. Land surface heterogeneity is commonly characterized using rectangular pixels, whose length scale reflects that of remote sensing measurements or ecological models rather than the spatial scales at which vegetation structure and function varies. We investigated the 'optimum' pixel size and shape for averaging leaf area index (LAI) measurements in relatively large (85 m2 estimates on a 600 × 600-m2 grid) and small (0.04 m2 measurements on a 40 × 40-m2 grid) patches of sub-Arctic tundra near Abisko, Sweden. We define the optimum spatial averaging operator as that which preserves the information content (IC) of measured LAI, as quantified by the normalized Shannon entropy (E S,n) and Kullback¿Leibler divergence (D KL), with the minimum number of pixels. Based on our criterion, networks of Voronoi polygons created from triangulated irregular networks conditioned on hydrologic and topographic indices are often superior to rectangular shapes for averaging LAI at some, frequently larger, spatial scales. In order to demonstrate the importance of information preservation when upscaling, we apply a simple, validated ecosystem carbon flux model at the landscape level before and after spatial averaging of land surface characteristics. Aggregation errors are minimal due to the approximately linear relationship between flux and LAI, but large errors of approximately 45% accrue if the normalized difference vegetation index (NDVI) is averaged without preserving IC before conversion to LAI due to the nonlinear NDVI-LAI transfer function
Topographic controls on the leaf area index and plant functional type of a tundra ecosystem
Spadavecchia, L. ; Williams, M. ; Bell, R. ; Stoy, P.C. ; Huntley, B. ; Wijk, M.T. van - \ 2008
Journal of Ecology 96 (2008)6. - ISSN 0022-0477 - p. 1238 - 1251.
arctic ecosystems - statistical variables - principal components - soil properties - tussock tundra - global change - co2 flux - vegetation - alaska - biomass
Leaf area index (LAI) is an emergent property of vascular plants closely linked to primary production and surface energy balance. LAI can vary by an order of magnitude among Arctic tundra communities and is closely associated with plant functional type. We examined topographic controls on vegetation type and LAI distribution at two different scales in an Arctic tundra ecosystem in northern Sweden. `Micro-scale' measurements were made at 0.2-m resolution over a 40 m × 40 m domain, while `macro-scale' data were collected at approximately 10-m resolution over a 500 m × 500 m domain. Tundra LAI varied from 0.1-3.6 at the micro-scale resolution, and from 0.1-1.6 at the macro-scale resolution. The correlation between dominant vascular species and LAI at the micro-scale (r2 = 0.40) was greater than the correlation between dominant vegetation and LAI at the macro-scale (r2 = 0.14). At the macro-scale, LAI was better explained by topographic parameters and spatial auto-correlation (pseudo r2 = 0.32) than it was at the micro-scale (r2 = 0.16). Exposure and elevation were significantly but weakly correlated with LAI at the micro-scale, while on the macro-scale the most significant explanatory topographic variable was elevation (r2 = 0.12). The distribution of plant communities at both scales was significantly associated with topography. Shrub communities, dominated by Betula nana, were associated with low elevation sites at both scales, while more exposed and/or high elevation sites were dominated by cryptogams. Synthesis. Dominant vegetation, topography and LAI were linked at both scales of investigation but, for explaining LAI, topography became more important and dominant vegetation less important at the coarser scale. The explanatory power of dominant species/functional type for LAI variation was weaker at coarser scales, because communities often contained more than one functional type at 10 m resolution. The data suggest that remotely sensed topography can be combined with remotely sensed optical measurements to generate a useful tool for LAI mapping in Arctic environments.
Upscaling leaf area index in an Arctic landscape through multiscale observations
Williams, M. ; Bell, R. ; Spadavecchia, L. ; Street, L.E. ; Wijk, M.T. van - \ 2008
Global Change Biology 14 (2008)7. - ISSN 1354-1013 - p. 1517 - 1530.
wet sedge - vegetation - tundra - model - fertilization - ecosystems - dynamics - exchange - scale - ndvi
Monitoring and understanding global change requires a detailed focus on upscaling, the process for extrapolating from the site-specific scale to the smallest scale resolved in regional or global models or earth observing systems. Leaf area index (LAI) is one of the most sensitive determinants of plant production and can vary by an order of magnitude over short distances. The landscape distribution of LAI is generally determined by remote sensing of surface reflectance (e.g. normalized difference vegetation index, NDVI) but the mismatch in scales between ground and satellite measurements complicates LAI upscaling. Here, we describe a series of measurements to quantify the spatial distribution of LAI in a sub-Arctic landscape and then describe the upscaling process and its associated errors. Working from a fine-scale harvest LAI¿NDVI relationship, we collected NDVI data over a 500 m × 500 m catchment in the Swedish Arctic, at resolutions from 0.2 to 9.0 m in a nested sampling design. NDVI scaled linearly, so that NDVI at any scale was a simple average of multiple NDVI measurements taken at finer scales. The LAI¿NDVI relationship was scale invariant from 1.5 to 9.0 m resolution. Thus, a single exponential LAI¿NDVI relationship was valid at all these scales, with similar prediction errors. Vegetation patches were of a scale of 0.5 m and at measurement scales coarser than this, there was a sharp drop in LAI variance. Landsat NDVI data for the study catchment correlated significantly, but poorly, with ground-based measurements. A variety of techniques were used to construct LAI maps, including interpolation by inverse distance weighting, ordinary Kriging, External Drift Kriging using Landsat data, and direct estimation from a Landsat NDVI¿LAI calibration. All methods produced similar LAI estimates and overall errors. However, Kriging approaches also generated maps of LAI estimation error based on semivariograms. The spatial variability of this Arctic landscape was such that local measurements assimilated by Kriging approaches had a limited spatial influence. Over scales >50 m, interpolation error was of similar magnitude to the error in the Landsat NDVI calibration. The characterisation of LAI spatial error in this study is a key step towards developing spatio-temporal data assimilation systems for assessing C cycling in terrestrial ecosystems by combining models with field and remotely sensed data.
Functional convergence in regulation of net CO2 flux in heterogeneous tundra landscapes in Alaska and Sweden
Shaver, G.R. ; Street, L.E. ; Rastetter, E.B. ; Wijk, M.T. van; Williams, M. - \ 2007
Journal of Ecology 95 (2007)4. - ISSN 0022-0477 - p. 802 - 817.
kuparuk river-basin - leaf-area index - arctic ecosystems - climate-change - primary productivity - species composition - carex-bigelowii - carbon exchange - growing-season - plant biomass
1. Arctic landscapes are characterized by extreme vegetation patchiness, often with sharply defined borders between very different vegetation types. This patchiness makes it difficult to predict landscape-level C balance and its change in response to environment. 2. Here we develop a model of net CO2 flux by arctic landscapes that is independent of vegetation composition, using instead a measure of leaf area derived from NDVI (normalized-difference vegetation index). 3. Using the light response of CO2 flux (net ecosystem exchange, NEE) measured in a wide range of vegetation in arctic Alaska and Sweden, we exercise the model using various data subsets for parameter estimation and tests of predictions. 4. Overall, the model consistently explains similar to 80% of the variance in NEE knowing only the estimated leaf area index (LAI), photosynthetically active photon flux density (PPFD) and air temperature. 5. Model parameters derived from measurements made in one site or vegetation type can be used to predict NEE in other sites or vegetation types with acceptable accuracy and precision. Further improvements in model prediction may come from incorporating an estimate of moss area in addition to LAI, and from using vegetation-specific estimates of LAI. 6. The success of this model at predicting NEE independent of any information on species composition indicates a high level of convergence in canopy structure and function in the arctic landscape.
Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe
Flechard, C. ; Ambus, P. ; Skiba, U. ; Rees, R.M. ; Hensen, A. ; Amstel, A.R. van; Pol, A. van den; Soussana, J.F. ; Jones, M. ; Clifton-Brown, J.C. ; Rachi, A. ; Horvath, L. ; Neftel, A. ; Jocher, M. ; Ammann, C.R. ; Leifeld, J. ; Fuhrer, J. ; Calanca, P. ; Thalman, E. ; Pilegaard, K. ; Marco, G.S. Di; Campbell, C. ; Nemitz, E. ; Hargreaves, K.J. ; Levy, P.E. ; Ball, B. ; Jones, S.K. ; Bulk, W.C.M. van de; Groot, T. ; Blom, M. ; Domingues, R. ; Kasper, G.J. ; Allard, V. ; Ceschia, E. ; Cellier, P. ; Laville, P. ; Henault, C. ; Bizouard, F. ; Abdalla, M. ; Williams, M. ; Baronti, S. ; Berretti, F. ; Grosz, B. - \ 2007
Agriculture, Ecosystems and Environment 121 (2007)1-2. - ISSN 0167-8809 - p. 135 - 152.
n2o emissions - agricultural soils - fertilized grassland - temperate grassland - n fertilizer - denitrification - exchange - ch4 - variability - atmosphere
Soil/atmosphere exchange fluxes of nitrous oxide were monitored for a 3-year period at 10 grassland sites in eight European countries (Denmark, France, Hungary, Ireland, Italy, The Netherlands, Switzerland and United Kingdom), spanning a wide range of climatic, environmental and soil conditions. Most study sites investigated the influence of one or several management practices on N2O exchange, such as nitrogen fertilization and grazing intensity. Fluxes were measured using non-steady state chambers at most sites, and alternative measurement techniques such as eddy covariance and fast-box using tunable diode laser spectroscopy were implemented at some sites. The overall uncertainty in annual flux estimates derived from chamber measurements may be as high as 50% due to the temporal and spatial variability in fluxes, which warrants the future use of continuous measurements, if possible at the field scale. Annual emission rates were higher from intensive than from extensive grasslands, by a factor 4 if grazed (1.77 versus 0.48 kg N2O-N ha¿1 year¿1) and by a factor 3 if ungrazed (0.95 versus 0.32 kg N2O-N ha¿1 year¿1). Annual emission factors for fertilized systems were highly variable, ranging from 0.01% to 3.56%, but the mean emission factor across all sites (0.75%) was substantially lower than the IPCC default value of 1.25%. Emission factors for individual fertilization events increased with soil temperature and were generally higher for water-filled pore space values in the range 60¿90%, though precipitation onto dry soils was also shown to lead to high losses of N2O-N from applied fertilizer. An empirical, multiple regression model to predict N2O emission factors on the basis of soil temperature, moisture and rainfall is developed, explaining half of the variability in observed emission factors
What is the relationship between changes in canopy leaf area and changes in photosynthetic CO² flux in artic ecosystems?
Street, L.E. ; Shaver, G.R. ; Williams, M. ; Wijk, M.T. van - \ 2007
Journal of Ecology 95 (2007)1. - ISSN 0022-0477 - p. 139 - 150.
long-term nutrient - wet sedge tundra - tussock tundra - plant-communities - carbon storage - alaskan wet - responses - biomass - vegetation - fertilization
1 The arctic environment is highly heterogeneous in terms of plant distribution and productivity. If we are to make regional scale predictions of carbon exchange it is necessary to find robust relationships that can simplify this variability. One such potential relationship is that of leaf area to photosynthetic CO2 flux at the canopy scale. 2 In this paper we assess the effectiveness of canopy leaf area in explaining variation in gross primary productivity (GPP): (i) across different vegetation types; (ii) at various stages of leaf development; and (iii) under enhanced nutrient availability. To do this we measure net CO2 flux light response curves with a 1 × 1 m chamber, and calculate GPP at a photosynthetic photon flux density (PPFD) of 600 µmol m2 s1. 3 At a subarctic site in Sweden, we report 10-fold variation in GPP among natural vegetation types with leaf area index (LAI) values of 0.05¿2.31 m2 m2. At a site of similar latitude in Alaska we document substantially elevated rates of GPP in fertilized vegetation. 4 We can explain 80% of the observed variation in GPP in natural vegetation (including vegetation measured before deciduous leaf bud burst) by leaf area alone, when leaf area is predicted from measurements of normalized difference vegetation index (NDVI). 5 In fertilized vegetation the relative increase in leaf area between control and fertilized treatments exceeds the relative increase in GPP. This suggests that higher leaf area causes increased self-shading, or that lower leaf nitrogen per unit leaf area causes a reduction in the rate of photosynthesis. 6 The results of this study indicate that canopy leaf area is an excellent predictor of GPP in diverse low arctic tundra, across a wide range of plant functional types.
Identifying Differences in Carbon Exchange among Arctic Ecosystem Types
Williams, M. ; Street, L.E. ; Wijk, M.T. van; Shaver, G.R. - \ 2006
Ecosystems 9 (2006)2. - ISSN 1432-9840 - p. 288 - 304.
species composition - tundra ecosystems - global change - co2 exchange - water-vapor - landscape - dioxide - fluxes - energy - models
Our objective was to determine how varied is the response of C cycling to temperature and irradiance in tundra vegetation. We used a large chamber to measure C exchange at 23 locations within a small arctic catchment in Alaska during summer 2003 and 2004. At each location, we determined light response curves of C exchange using shade cloths, twice during a growing season. We used data to fit a simple photosynthesis-irradiance, respiration-temperature model, with four parameters. We used a maximum likelihood technique to determine the acceptable parameter space for each light curve, given measurement uncertainty. We then explored which sites and time periods had parameter sets in common - an indication of functional similarity. We found that seven distinct parameter sets were required to explain observed C flux responses to temperature and light variation at all sites and time periods. The variation in estimated maximum photosynthetic rate (Pmax) was strongly correlated with measurements of site leaf area index (LAI). The behavior of tussock tundra sites, the dominant vegetation of arctic tundra, could largely be described with a single parameter set, with a Pmax of 9.7 ¿mol m-2 s -1. Tussock tundra sites had, correspondingly, similar LAI (mean = 0.66). Non-tussock sites (for example, sedge and shrub tundras) had larger spatial and temporal variations in both C dynamic parameters (Pmax varying from 9.7-25.7 ¿mol m-2 s-1) and LAI (0.6-2.0). There were no clear relationships between dominant non-tussock vegetation types and a particular parameter set. Our results suggest that C dynamics of the acidic tussock tundra slopes and hilltops in northern Alaska are relatively simply described during the peak growing season. However, the foot-slopes and water tracks have more variable patterns of LAI and C exchange, not simply related to the dominant vegetation type.
Optical instruments for measuring leaf area index in low vegetation: application in arctic ecosystems
Wijk, M.T. van; Williams, M. - \ 2005
Ecological Applications 15 (2005)4. - ISSN 1051-0761 - p. 1462 - 1470.
tussock tundra - carbon-dioxide - exchange - productivity - responses - light - ndvi - lai
Leaf area index (LAI) is a powerful diagnostic of plant productivity. Despite the fact that many methods have been developed to quantify LAI, both directly and indirectly, leaf area index remains difficult to quantify accurately, owing to large spatial and temporal variability. The gap-fraction technique is widely used to estimate the LAI indirectly. However, for low-stature vegetation, the gap-fraction sensor either cannot get totally underneath the plant canopy, thereby missing part of the leaf area present, or is too close to the individual leaves of the canopy, which leads to a large distortion of the LAI estimate. We set out to develop a methodology for easy and accurate nondestructive assessment of the variability of LAI in low-stature vegetation. We developed and tested the methodology in an arctic landscape close to Abisko, Sweden. The LAI of arctic vegetation could be estimated accurately and rapidly by combining field measurements of canopy reflectance (NDVI) and light penetration through the canopy (gap-fraction analysis using a LI-COR LAI-2000). By combining the two methodologies, the limitations of each could be circumvented, and a significantly increased accuracy of the LAI estimates was obtained. The combination of an NDVI sensor for sparser vegetation and a LAI-2000 for denser vegetation could explain 81% of the variance of LAI measured by destructive harvest. We used the method to quantify the spatial variability and the associated uncertainty of leaf area index in a small catchment area
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