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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Limited evidence for spatial resource partitioning across temperate grassland biodiversity experiments
Barry, Kathryn E. ; Ruijven, Jasper van; Mommer, Liesje ; Bai, Yongfei ; Beierkuhnlein, Carl ; Buchmann, Nina ; Kroon, Hans de; Ebeling, Anne ; Eisenhauer, Nico ; Guimarães-Steinicke, Claudia ; Hildebrandt, Anke ; Isbell, Forest ; Milcu, Alexandru ; Neßhöver, Carsten ; Reich, Peter B. ; Roscher, Christiane ; Sauheitl, Leopold ; Scherer-Lorenzen, Michael ; Schmid, Bernhard ; Tilman, David ; Felten, Stefanie von; Weigelt, Alexandra - \ 2019
Ecology (2019). - ISSN 0012-9658
grassland - niche complementarity - niche partitioning - productivity - resource uptake - resources - standing root biomass

Locally, plant species richness supports many ecosystem functions. Yet, the mechanisms driving these often-positive biodiversity–ecosystem functioning relationships are not well understood. Spatial resource partitioning across vertical resource gradients is one of the main hypothesized causes for enhanced ecosystem functioning in more biodiverse grasslands. Spatial resource partitioning occurs if species differ in where they acquire resources and can happen both above- and belowground. However, studies investigating spatial resource partitioning in grasslands provide inconsistent evidence. We present the results of a meta-analysis of 21 data sets from experimental species-richness gradients in grasslands. We test the hypothesis that increasing spatial resource partitioning along vertical resource gradients enhances ecosystem functioning in diverse grassland plant communities above- and belowground. To test this hypothesis, we asked three questions. (1) Does species richness enhance biomass production or community resource uptake across sites? (2) Is there evidence of spatial resource partitioning as indicated by resource tracer uptake and biomass allocation above- and belowground? (3) Is evidence of spatial resource partitioning correlated with increased biomass production or community resource uptake? Although plant species richness enhanced community nitrogen and potassium uptake and biomass production above- and belowground, we found that plant communities did not meet our criteria for spatial resource partitioning, though they did invest in significantly more aboveground biomass in higher canopy layers in mixture relative to monoculture. Furthermore, the extent of spatial resource partitioning across studies was not positively correlated with either biomass production or community resource uptake. Our results suggest that spatial resource partitioning across vertical resource gradients alone does not offer a general explanation for enhanced ecosystem functioning in more diverse temperate grasslands.

Author Correction: Diversity-dependent temporal divergence of ecosystem functioning in experimental ecosystems
Guerrero-Ramírez, Nathaly R. ; Craven, Dylan ; Reich, Peter B. ; Ewel, John J. ; Isbell, Forest ; Koricheva, Julia ; Parrotta, John A. ; Auge, Harald ; Erickson, Heather E. ; Forrester, David I. ; Hector, Andy ; Joshi, Jasmin ; Montagnini, Florencia ; Palmborg, Cecilia ; Piotto, Daniel ; Potvin, Catherine ; Roscher, Christiane ; Ruijven, Jasper van; Tilman, David ; Wilsey, Brian ; Eisenhauer, Nico - \ 2019
Nature Ecology & Evolution 3 (2019). - ISSN 2397-334X - p. 1365 - 1365.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Future of Complementarity : Disentangling Causes from Consequences
Barry, Kathryn E. ; Mommer, Liesje ; Ruijven, Jasper van; Wirth, Christian ; Wright, Alexandra J. ; Bai, Yongfei ; Connolly, John ; Deyn, Gerlinde B. De; Kroon, Hans de; Isbell, Forest ; Milcu, Alexandru ; Roscher, Christiane ; Scherer-Lorenzen, Michael ; Schmid, Bernhard ; Weigelt, Alexandra - \ 2019
Trends in Ecology and Evolution 34 (2019)2. - ISSN 0169-5347 - p. 167 - 180.
Abiotic facilitation - Biodiversity - Biotic feedbacks - Complementarity - Complementarity effect - Ecosystem functioning - Plant-soil feedback - Resource partitioning - Resource tracers - Stress amelioration

Evidence suggests that biodiversity supports ecosystem functioning. Yet, the mechanisms driving this relationship remain unclear. Complementarity is one common explanation for these positive biodiversity–ecosystem functioning relationships. Yet, complementarity is often indirectly quantified as overperformance in mixture relative to monoculture (e.g., ‘complementarity effect’). This overperformance is then attributed to the intuitive idea of complementarity or, more specifically, to species resource partitioning. Locally, however, several unassociated causes may drive this overperformance. Here, we differentiate complementarity into three types of species differences that may cause enhanced ecosystem functioning in more diverse ecosystems: (i) resource partitioning, (ii) abiotic facilitation, and (iii) biotic feedbacks. We argue that disentangling these three causes is crucial for predicting the response of ecosystems to future biodiversity loss.

Plant species richness and functional groups have different effects on soil water content in a decade-long grassland experiment
Fischer, Christine ; Leimer, Sophia ; Roscher, Christiane ; Ravenek, Janneke ; Kroon, Hans de; Kreutziger, Yvonne ; Baade, Jussi ; Beßler, Holger ; Eisenhauer, Nico ; Weigelt, Alexandra ; Mommer, Liesje ; Lange, Markus ; Gleixner, Gerd ; Wilcke, Wolfgang ; Schröder, Boris ; Hildebrandt, Anke - \ 2019
Journal of Ecology 107 (2019)1. - ISSN 0022-0477 - p. 127 - 141.
biodiversity - functional groups - Jena Experiment - plant–soil–water relation - soil water content - spatial–temporal variability - species richness

The temporal and spatial dynamics of soil water are closely interlinked with terrestrial ecosystems functioning. The interaction between plant community properties such as species composition and richness and soil water mirrors fundamental ecological processes determining above-ground–below-ground feedbacks. Plant–water relations and water stress have attracted considerable attention in biodiversity experiments. Yet, although soil scientific research suggests an influence of ecosystem productivity on soil hydraulic properties, temporal changes of the soil water content and soil hydraulic properties remain largely understudied in biodiversity experiments. Thus, insights on how plant diversity—productivity relationships affect soil water are lacking. Here, we determine which factors related to plant community composition (species and functional group richness, presence of plant functional groups) and soil (organic carbon concentration) affect soil water in a long-term grassland biodiversity experiment (The Jena Experiment). Both plant species richness and the presence of particular functional groups affected soil water content, while functional group richness played no role. The effect of species richness changed from positive to negative and expanded to deeper soil with time. Shortly after establishment, increased topsoil water content was related to higher leaf area index in species-rich plots, which enhanced shading. In later years, higher species richness increased topsoil organic carbon, likely improving soil aggregation. Improved aggregation, in turn, dried topsoils in species-rich plots due to faster drainage of rainwater. Functional groups affected soil water distribution, likely due to plant traits affecting root water uptake depths, shading, or water-use efficiency. For instance, topsoils in plots containing grasses were generally drier, while plots with legumes were moister. Synthesis. Our decade-long experiment reveals that the maturation of grasslands changes the effects of plant richness from influencing soil water content through shading effects to altering soil physical characteristics in addition to modification of water uptake depth. Functional groups affected the soil water distribution by characteristic shifts of root water uptake depth, but did not enhance exploitation of the overall soil water storage. Our results reconcile previous seemingly contradictory results on the relation between grassland species diversity and soil moisture and highlight the role of vegetation composition for soil processes.

Multiple facets of biodiversity drive the diversity–stability relationship
Craven, Dylan ; Eisenhauer, Nico ; Pearse, William D. ; Hautier, Yann ; Isbell, Forest ; Roscher, Christiane ; Bahn, Michael ; Beierkuhnlein, Carl ; Bönisch, Gerhard ; Buchmann, Nina ; Byun, Chaeho ; Catford, Jane A. ; Cerabolini, Bruno E.L. ; Cornelissen, J.H.C. ; Craine, Joseph M. ; Luca, Enrica De; Ebeling, Anne ; Griffin, John N. ; Hector, Andy ; Hines, Jes ; Jentsch, Anke ; Kattge, Jens ; Kreyling, Jürgen ; Lanta, Vojtech ; Lemoine, Nathan ; Meyer, Sebastian T. ; Minden, Vanessa ; Onipchenko, Vladimir ; Polley, H.W. ; Reich, Peter B. ; Ruijven, Jasper van; Schamp, Brandon ; Smith, Melinda D. ; Soudzilovskaia, Nadejda A. ; Tilman, David ; Weigelt, Alexandra ; Wilsey, Brian ; Manning, Peter - \ 2018
Nature Ecology & Evolution 2 (2018). - ISSN 2397-334X - p. 1579 - 1587.

A substantial body of evidence has demonstrated that biodiversity stabilizes ecosystem functioning over time in grassland ecosystems. However, the relative importance of different facets of biodiversity underlying the diversity–stability relationship remains unclear. Here we use data from 39 grassland biodiversity experiments and structural equation modelling to investigate the roles of species richness, phylogenetic diversity and both the diversity and community-weighted mean of functional traits representing the ‘fast–slow’ leaf economics spectrum in driving the diversity–stability relationship. We found that high species richness and phylogenetic diversity stabilize biomass production via enhanced asynchrony in the performance of co-occurring species. Contrary to expectations, low phylogenetic diversity enhances ecosystem stability directly, albeit weakly. While the diversity of fast–slow functional traits has a weak effect on ecosystem stability, communities dominated by slow species enhance ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Our in-depth, integrative assessment of factors influencing the diversity–stability relationship demonstrates a more multicausal relationship than has been previously acknowledged.

Data from: Below-ground resource partitioning alone cannot explain the biodiversity–ecosystem function relationship: a field test using multiple tracers
Jesch, Annette ; Barry, Kathryn E. ; Ravenek, Janneke M. ; Bachmann, Dörte ; Strecker, Tanja ; Weigelt, Alexandra ; Buchmann, Nina ; Kroon, Hans de; Gessler, Arthur ; Mommer, L. ; Roscher, Christiane ; Scherer-Lorenzen, Michael - \ 2018
University of Freiburg
ecosystem function and services - Jena Experiment - Levins B - resource uptake - water uptake - stable isotopes - rare element tracers - complementarity - proportional similarity
Below-ground resource partitioning alone cannot explain the biodiversity-ecosystem function relationship : A field test using multiple tracers
Jesch, Annette ; Barry, Kathryn E. ; Ravenek, Janneke M. ; Bachmann, Dörte ; Strecker, Tanja ; Weigelt, Alexandra ; Buchmann, Nina ; Kroon, Hans de; Gessler, Arthur ; Mommer, Liesje ; Roscher, Christiane ; Scherer-Lorenzen, Michael - \ 2018
Journal of Ecology 106 (2018)5. - ISSN 0022-0477 - p. 2002 - 2018.
Complementarity - Ecosystem function - Jena Experiment - Levins B - Proportional similarity - Rare element tracers - Resource uptake - Stable isotopes
Below-ground resource partitioning is among the most prominent hypotheses for driving the positive biodiversity-ecosystem function relationship. However, experimental tests of this hypothesis in biodiversity experiments are scarce, and the available evidence is not consistent. We tested the hypothesis that resource partitioning in space, in time or in both space and time combined drives the positive effect of diversity on both plant productivity and total community resource uptake. At the community level, we predicted that total community resource uptake and biomass production above- and below-ground will increase with increased species richness or functional group richness. We predicted that, at the species level, resource partition breadth will become narrower, and that overlap between the resource partitions of different species will become smaller with increasing species richness or functional group richness. We applied multiple resource tracers (Li and Rb as potassium analogues, the water isotopologues-H2 18O and 2H2O, and 15N) in three seasons at two depths across a species and functional group richness gradient at a grassland biodiversity experiment. We used this multidimensional resource tracer study to test if plant species partition resources with increasing plant diversity across space, time or both simultaneously. At the community level, total community resource uptake of nitrogen and potassium and above- and below-ground biomass increased significantly with increasing species richness but not with increasing functional group richness. However, we found no evidence that resource partition breadth or resource partition overlap decreased with increasing species richness for any resource in space, time or both space and time combined. Synthesis. These findings indicate that below-ground resource partitioning may not drive the enhanced resource uptake or biomass production found here. Instead, other mechanisms such as facilitation, species-specific biotic feedback or above-ground resource partitioning are likely necessary for enhanced overall ecosystem function.
Diversity-dependent temporal divergence of ecosystem functioning in experimental ecosystems
Guerrero-Ramírez, Nathaly R. ; Craven, Dylan ; Reich, Peter B. ; Ewel, John J. ; Isbell, Forest ; Koricheva, Julia ; Parrotta, John A. ; Auge, Harald ; Erickson, Heather E. ; Forrester, David Ian ; Hector, Andy ; Joshi, Jasmin ; Montagnini, Florencia ; Palmborg, Cecilia ; Piotto, Daniel ; Potvin, Catherine ; Roscher, Christiane ; Ruijven, Jasper van; Tilman, David ; Wilsey, Brian ; Eisenhauer, Nico - \ 2017
Nature Ecology & Evolution 1 (2017). - ISSN 2397-334X - p. 1639 - 1642.
The effects of biodiversity on ecosystem functioning generally increase over time, but the underlying processes remain unclear. Using 26 long-term grassland and forest experimental ecosystems, we demonstrate that biodiversity–ecosystem functioning relationships strengthen mainly by greater increases in functioning in high-diversity communities in grasslands and forests. In grasslands, biodiversity effects also strengthen due to decreases in functioning in low-diversity communities. Contrasting trends across grasslands are associated with differences in soil characteristics.
Functional trait dissimilarity drives both species complementarity and competitive disparity
Wagg, Cameron ; Ebeling, Anne ; Roscher, Christiane ; Ravenek, Janneke ; Bachmann, Dörte ; Eisenhauer, Nico ; Mommer, Liesje ; Buchmann, Nina ; Hillebrand, Helmut ; Schmid, Bernhard ; Weisser, Wolfgang W. - \ 2017
Functional Ecology 31 (2017)12. - ISSN 0269-8463 - p. 2320 - 2329.
biodiversity - community ecology - competition - Jena experiment - trait-based experiment (TBE)
Niche complementarity and competitive disparity are driving mechanisms behind plant community assembly and productivity. Consequently, there is great interest in predicting species complementarity and their competitive differences from their functional traits as dissimilar species may compete less and result in more complete use of resources. Here we assessed the role of trait dissimilarities for species complementarity and competitive disparities within an experimental gradient of plant species richness and functional trait dissimilarity. Communities were assembled using three pools of grass and forb species based on a priori knowledge of traits related to (1) above- and below-ground spatial differences in resource acquisition, (2) phenological differences or (3) both. Complementarity and competitive disparities were assessed by partitioning the overyielding in mixed species communities into species complementarity and dominance effects. Community overyielding and the underlying complementarity and competitive dominance varied strongly among the three plant species pools. Overyielding and complementarity were greatest among species that were assembled based on their variation in both spatial and phenological traits. Competitive dominance was greatest when species were assembled based on spatial resource acquisition traits alone. In communities that were assembled based on species variation in only spatial or phenological traits, greater competitive dominance was predicted by greater differences in SLA and flowering initiation respectively, while greater complementarity was predicted by greater dissimilarity in leaf area and flowering senescence respectively. Greater differences in leaf area could also be linked to greater species complementarity in communities assembled based on variation in both phenological and spatial traits, but trait dissimilarity was unrelated to competitive dominance in these communities. Our results indicate that complementarity and competitive disparity among species are both driven by trait dissimilarities. However, the identity of the traits that drives the complementarity and competitive disparity depends on the trait variation among species that comprise the community. Moreover, we demonstrate that communities assembled with the greater variation in both spatial and phenological traits show the greatest complementarity among species. A plain language summary is available for this article.
Biodiversity effects on ecosystem functioning in a 15-year grassland experiment : Patterns, mechanisms, and open questions
Weisser, Wolfgang ; Roscher, Christiane ; Meyer, Sebastian T. ; Ebeling, Anne ; Luo, Guangjuan ; Allan, Eric ; Beßler, Holger ; Barnard, Romain L. ; Buchmann, Nina ; Buscot, François ; Engels, Christof ; Fischer, Christine ; Fischer, Markus ; Gessler, Arthur ; Gleixner, Gerd ; Halle, Stefan ; Hildebrandt, Anke ; Hillebrand, Helmut ; Kroon, Hans de; Lange, Markus ; Leimer, Sophia ; Roux, Xavier Le; Milcu, Alexandru ; Mommer, Liesje ; Niklaus, Pascal A. ; Oelmann, Yvonne ; Proulx, Raphael ; Roy, Jacques ; Scherber, Christoph ; Scherer-lorenzen, Michael ; Scheu, Stefan ; Tscharntke, Teja ; Wachendorf, Michael ; Wagg, Cameron ; Weigelt, Alexandra ; Wilcke, Wolfgang ; Wirth, Christian ; Schulze, Ernst Detlef ; Schmid, Bernhard ; Eisenhauer, Nico - \ 2017
Basic and Applied Ecology 23 (2017). - ISSN 1439-1791 - p. 1 - 73.
Biomass - Carbon storage - Complementarity - Multi-trophic interactions - Nutrient cycling - Selection effect
In the past two decades, a large number of studies have investigated the relationship between biodiversity and ecosystem functioning, most of which focussed on a limited set of ecosystem variables. The Jena Experiment was set up in 2002 to investigate the effects of plant diversity on element cycling and trophic interactions, using a multi-disciplinary approach. Here, we review the results of 15 years of research in the Jena Experiment, focussing on the effects of manipulating plant species richness and plant functional richness. With more than 85,000 measures taken from the plant diversity plots, the Jena Experiment has allowed answering fundamental questions important for functional biodiversity research.First, the question was how general the effect of plant species richness is, regarding the many different processes that take place in an ecosystem. About 45% of different types of ecosystem processes measured in the 'main experiment', where plant species richness ranged from 1 to 60 species, were significantly affected by plant species richness, providing strong support for the view that biodiversity is a significant driver of ecosystem functioning. Many measures were not saturating at the 60-species level, but increased linearly with the logarithm of species richness. There was, however, great variability in the strength of response among different processes. One striking pattern was that many processes, in particular belowground processes, took several years to respond to the manipulation of plant species richness, showing that biodiversity experiments have to be long-term, to distinguish trends from transitory patterns. In addition, the results from the Jena Experiment provide further evidence that diversity begets stability, for example stability against invasion of plant species, but unexpectedly some results also suggested the opposite, e.g. when plant communities experience severe perturbations or elevated resource availability. This highlights the need to revisit diversity-stability theory.Second, we explored whether individual plant species or individual plant functional groups, or biodiversity itself is more important for ecosystem functioning, in particular biomass production. We found strong effects of individual species and plant functional groups on biomass production, yet these effects mostly occurred in addition to, but not instead of, effects of plant species richness.Third, the Jena Experiment assessed the effect of diversity on multitrophic interactions. The diversity of most organisms responded positively to increases in plant species richness, and the effect was stronger for above- than for belowground organisms, and stronger for herbivores than for carnivores or detritivores. Thus, diversity begets diversity. In addition, the effect on organismic diversity was stronger than the effect on species abundances.Fourth, the Jena Experiment aimed to assess the effect of diversity on N, P and C cycling and the water balance of the plots, separating between element input into the ecosystem, element turnover, element stocks, and output from the ecosystem. While inputs were generally less affected by plant species richness, measures of element stocks, turnover and output were often positively affected by plant diversity, e.g. carbon storage strongly increased with increasing plant species richness. Variables of the N cycle responded less strongly to plant species richness than variables of the C cycle.Fifth, plant traits are often used to unravel mechanisms underlying the biodiversity-ecosystem functioning relationship. In the Jena Experiment, most investigated plant traits, both above- and belowground, were plastic and trait expression depended on plant diversity in a complex way, suggesting limitation to using database traits for linking plant traits to particular functions.Sixth, plant diversity effects on ecosystem processes are often caused by plant diversity effects on species interactions. Analyses in the Jena Experiment including structural equation modelling suggest complex interactions that changed with diversity, e.g. soil carbon storage and greenhouse gas emission were affected by changes in the composition and activity of the belowground microbial community. Manipulation experiments, in which particular organisms, e.g. belowground invertebrates, were excluded from plots in split-plot experiments, supported the important role of the biotic component for element and water fluxes.Seventh, the Jena Experiment aimed to put the results into the context of agricultural practices in managed grasslands. The effect of increasing plant species richness from 1 to 16 species on plant biomass was, in absolute terms, as strong as the effect of a more intensive grassland management, using fertiliser and increasing mowing frequency. Potential bioenergy production from high-diversity plots was similar to that of conventionally used energy crops. These results suggest that diverse 'High Nature Value Grasslands' are multifunctional and can deliver a range of ecosystem services including production-related services.A final task was to assess the importance of potential artefacts in biodiversity-ecosystem functioning relationships, caused by the weeding of the plant community to maintain plant species composition. While the effort (in hours) needed to weed a plot was often negatively related to plant species richness, species richness still affected the majority of ecosystem variables. Weeding also did not negatively affect monoculture performance; rather, monocultures deteriorated over time for a number of biological reasons, as shown in plant-soil feedback experiments.To summarize, the Jena Experiment has allowed for a comprehensive analysis of the functional role of biodiversity in an ecosystem. A main challenge for future biodiversity research is to increase our mechanistic understanding of why the magnitude of biodiversity effects differs among processes and contexts. It is likely that there will be no simple answer. For example, among the multitude of mechanisms suggested to underlie the positive plant species richness effect on biomass, some have received limited support in the Jena Experiment, such as vertical root niche partitioning. However, others could not be rejected in targeted analyses. Thus, from the current results in the Jena Experiment, it seems likely that the positive biodiversity effect results from several mechanisms acting simultaneously in more diverse communities, such as reduced pathogen attack, the presence of more plant growth promoting organisms, less seed limitation, and increased trait differences leading to complementarity in resource uptake. Distinguishing between different mechanisms requires careful testing of competing hypotheses. Biodiversity research has matured such that predictive approaches testing particular mechanisms are now possible.
Plants are less negatively affected by flooding when growing in species-rich plant communities
Wright, Alexandra J. ; Kroon, Hans de; Visser, Eric J.W. ; Buchmann, Tina ; Ebeling, Anne ; Eisenhauer, Nico ; Fischer, Christine ; Hildebrandt, Anke ; Ravenek, Janneke ; Roscher, Christiane ; Weigelt, Alexandra ; Weisser, Wolfgang ; Voesenek, Laurentius A.C.J. ; Mommer, Liesje - \ 2017
New Phytologist 213 (2017)2. - ISSN 0028-646X - p. 645 - 656.
aerenchyma - diversity - flooding traits - grasses - legumes - plant functional groups - soil air porosity - specific leaf area (SLA)

Flooding is expected to increase in frequency and severity in the future. The ecological consequences of flooding are the combined result of species-specific plant traits and ecological context. However, the majority of past flooding research has focused on individual model species under highly controlled conditions. An early summer flooding event in a grassland biodiversity experiment in Jena, Germany, provided the opportunity to assess flooding responses of 60 grassland species in monocultures and 16-species mixtures. We examined plant biomass, species-specific traits (plant height, specific leaf area (SLA), root aerenchyma, starch content) and soil porosity. We found that, on average, plant species were less negatively affected by the flood when grown in higher-diversity plots in July 2013. By September 2013, grasses were unaffected by the flood regardless of plant diversity, and legumes were severely negatively affected regardless of plant diversity. Plants with greater SLA and more root aerenchyma performed better in September. Soil porosity was higher in higher-diversity plots and had a positive effect on plant performance. As floods become more frequent and severe in the future, growing flood-sensitive plants in higher-diversity communities and in soil with greater soil aeration may attenuate the most negative effects of flooding.

Flooding disturbances increase resource availability and productivity but reduce stability in diverse plant communities
Wright, A. ; Ebeling, A. ; Kroon, H. de; Roscher, C. ; Weigelt, A. ; Buchman, N. ; Buchman, T. ; Fischer, C. ; Hacker, N. ; Hildebrandt, A. ; Mommer, L. - \ 2015
Nature Communications 6 (2015). - ISSN 2041-1723
species richness - grassland experiment - biodiversity - ecosystems - biomass
The natural world is increasingly defined by change. Within the next 100 years, rising atmospheric CO2 concentrations will continue to increase the frequency and magnitude of extreme weather events. Simultaneously, human activities are reducing global biodiversity, with current extinction rates at ~1,000 × what they were before human domination of Earth’s ecosystems. The co–occurrence of these trends may be of particular concern, as greater biological diversity could help ecosystems resist change during large perturbations. We use data from a 200–year flood event to show that when a disturbance is associated with an increase in resource availability, the opposite may occur. Flooding was associated with increases in productivity and decreases in stability, particularly in the highest diversity communities. Our results undermine the utility of the biodiversity–stability hypothesis during a large number of disturbances where resource availability increases. We propose a conceptual framework that can be widely applied during natural disturbances.
Plant species diversity affects infiltration capacity in an experimental grassland through changes in soil properties
Fischer, C. ; Tischer, J. ; Roscher, C. ; Eisenhauer, N. ; Ravenek, J. ; Gleixner, G. ; Attinger, S. ; Jensen, B. ; Kroon, H. de; Mommer, L. ; Scheu, S. ; Hildebrandt, A. - \ 2015
Plant and Soil 397 (2015)1. - ISSN 0032-079X - p. 1 - 16.
Background and aims Soil hydraulic properties drive water distribution and availability in soil. There exists limited knowledge of how plant species diversity might influence soil hydraulic properties. Methods We quantified the change in infiltration capacity affected by soil structural variables (soil bulk density, porosity and organic carbon content) along a gradient of soil texture, plant species richness (1, 2, 4, 8, 16 and 60)and functional group composition (grasses, legumes, small herbs, tall herbs). We conducted two infiltration measurement campaigns (May and October 2012) using a hood infiltrometer. Results Plant species richness significantly increased infiltration capacity in the studied grasslands. Both soil porosity (or inversely bulk density) and organic carbon played an important role in mediating the plant species richness effect. Soil texture did not correlate with infiltration capacity. In spring 2012, earthworm biomass increased infiltration capacity, but this effect could not be attributed to changes in soil structural variables. Conclusions We experimentally identified important ecological drivers of infiltration capacity, suggesting complex interactions between plant species richness, earthworms, and soil structural variables, while showing little impact of soil texture. Changes in plant species richness may thus have significant effects on soil hydraulic properties with potential consequences for surface run-off and soil erosion.
Land cover change in Europe between 1950 and 2000 determined employing aerial photography
Gerard, F. ; Bugar, G. ; Gregor, M. ; Halada, L. ; Hazeu, G.W. ; Huitu, H. ; Köhler, H.R. ; Kolar, J. ; Luque, S. ; Mücher, C.A. ; Olschofsky, K. ; Petit, S. ; Pino, J. ; Smith, G. ; Thomson, A. ; Wachowicz, M. ; Bezák, P. ; Brown, N. ; Boltiziar, M. ; Badts, E. de; Feranec, J. ; Halabuk, A. ; Manchester, S. ; Mojse, M. ; Petrovic, F. ; Pons, X. ; Roda, F. ; Roscher, M. ; Sustera, J. ; Tuominen, S. ; Wadsworth, R. ; Ziese, H. - \ 2010
Progress in Physical Geography 34 (2010)2. - ISSN 0309-1333 - p. 183 - 205.
landscapes
BIOPRESS (‘Linking Pan-European Land Cover Change to Pressures on Biodiversity’), a European Commission funded ‘Global Monitoring for Environment and Security’ project, produced land cover change information (1950—2000) for Europe from aerial photographs and tested the suitability of this for monitoring habitats and biodiversity. The methods and results related to the land cover change work are summarized. Changes in land cover were established through 73 window and 59 transect samples distributed across Europe. Although the sample size was too small and biased to fully represent the spatial variability observed in Europe, the work highlighted the importance of method consistency, the choice of nomenclature and spatial scale. The results suggest different processes are taking place in different parts of Europe: the Boreal and Alpine regions are dominated by forest management; abandonment and intensification are mainly encountered in the Mediterranean; urbanization and drainage are more characteristic of the Continental and Atlantic regions.
Landbouweconomie als beleidswetenschap : een pragmatische kritiek
Silvis, H.J. - \ 1994
Agricultural University. Promotor(en): J. de Hoogh; C.P. Veerman. - Wageningen : Silvis - ISBN 9789054852988 - 208
agrarische economie - landbouwbeleid - groene revolutie - economie - overheidsbeleid - agrarisch recht - Nederland - agricultural economics - agricultural policy - green revolution - economics - government policy - agricultural law - Netherlands
AGRICULTURAL ECONOMICS AS POLICY SCIENCE:
A PRAGMATIC CRITICISM

Introduction

This study aims to make a contribution to the discussion of how to increase the scientific quality of agricultural economics, by means of an epistemologically based assessment of the discipline. The analysis consists of two parts. Part I outlines the foundations of the study. The work begins with a pragmatic or instrumentalistic view of science, as elaborated by Meehan (1982). The criteria derived from this view for the assessment of policy-oriented empirical research are illustrated in two scientific areas that are closely tied to agricultural economics: economics and the agricultural sciences. Next, the history of agricultural economics is examined. Part II presents case studies of characteristic examples of modem, post-war agricultural economics research. For this purpose seven examples were chosen that have been presented as relevant for policymaking, are representative of their types, and that together give an accurate picture of major trends in agricultural economics research.

This summary includes a short overview of the assessment criteria used, followed by the most important findings and general conclusion of the research.

Assessment criteria

Agricultural economics exists as a specialization within the field of economics due to the shared characteristics of economic issues concerning food supply and agricultural production. These characteristics stem from agriculture's specific technology and institutional framework, and they are expressed, among other ways, in price and income formation and in the use of production factors (Horring, 1960).

To substantiate its existence, the discipline must contribute to the handling of relevant issues. Economic issues are by nature issues of choice. Questions such as which goods and services should be produced, how scarce resources should be utilized, who should be allowed to benefit from the fruits of production, and which policies should the society institutionalize to achieve its goals are of central importance.

Extensive knowledge is necessary to handle these policy issues. Reasoned choices require empirical as well as normative knowledge. Normative knowledge is considered here to be the ability to choose between various possible courses of action, and to systematically defend that choice.

One of the starting points of this research is the belief that the discipline of agricultural economics wants to and also must provide the empirical knowledge needed for policymaking. The policy concept of Meehan (1982), defined as a manual on how to achieve important goals, offers a way to further specify the knowledge that is required. The empirical requirements of policyaking are instruments with which processes can be directed. To obtain these, knowledge of causal relationships is necessary, in the sense of fixed connections between actions and effects. Such relationships are generally defined in theories. If the policy-oriented empirical sciences take their pretensions seriously, they will have to strive to develop and assess theories regarding causal relationships in important policy areas.

A generally accepted research theory that leads to results which are reliable and adequate for policyaking is not available. However, it is advisable to use the research methods that give the greatest possible chance of achieving such results. As explained in Part 1, indications for the use of such methods can be based on the assumption that theories are creatively developed from generalizations of specific experiences (induction). Generalizations are made up of concepts and observations, descriptions and predictions. Predictions can be based on theories, but also on classifications and on correlations between variables. For the development of both theories and predictions, precise and adequate empirical descriptions are needed. Descriptions, in turn, are dependent on the quality of observations and the concepts used.

Based on the above considerations, the assessment criteria for policy-oriented research are defined as follows:
- Research goals: research must aim to develop socially relevant theories that connect actions to their effects (causal relationships).
- Research methods: empiricism (descriptions and fundamental concepts, and observations) should be of central importance. Assumptions about the characteristics of people, things and their interrelationships must agree as much as possible with reality. Assertions derived from axioms cannot be challenged empirically, and are therefore not suitable for policyaking Although deduction plays an indispensable role in research, a rationalistic or postulational research strategy cannot possibly achieve the required results.
- Validation of research results: potential users should be given the opportunity to evaluate the presented theories. For this purpose, it must be apparent to what extent the theory and its limiting factors agree with existing knowledge. The description of limiting factors should make it clear what the theory exactly applies to. Also the theory should be accessible and verifiable. Enough supportive evidence should be presented to prove that the theoretical structure functions as expected when actions are based on it. Finally, the theory should address relevant side-effects.

These criteria apply to theoretical research. It cannot be expected of descriptive research that it reveals causal relationships, because it does not profess to. Considering their role in the development of theory, descriptions must be not only precise, but also adequate for the purposes for which they are eventually used.

Findings

Research goals

Critics of the economic discipline in general contend that a great deal of modem theoretical work does not relate directly to reality, that it does not attempt to develop instruments with which processes can be directed, and that it therefore cannot make a meaningful contribution to policymaking. In the agricultural sciences, however, conscious efforts have been made to describe actual processes and to develop methods for improving and increasing agricultural production. This is accomplished by influencing naturally occurring processes (for example: fertilization, cattle feeding, crop protection, improvement of machinery and buildings). 'Agricultural scientists', according to C.T. de Wit (1977, p.93), 'have long known that very little will be accomplished with a laissez faire approach'.

The agricultural sciences, thus, generally meet the criteria for research goals. This is also the case for the economic specialization within the agricultural sciences. In the history of agricultural economics there are many examples of empirical research conducted for the purpose of contributing to the solution of policy issues. For example, the physiocrats of the eighteenth century, led by F. Quesnay, conducted agricultural economic research to increase the wealth of the French monarch, and thereby also improve the wealth of the entire kingdom. The British forerunner of agricultural economics, Arthur Young, developed models of the ultimate profit-making farms. This was also one of the central objectives in the work of A.D. Thaer, the founder of the agricultural sciences. Thünen discovered that the relative attractiveness of agricultural enterprises is dependent on price ratios, which are in turn partially influenced by transport costs. Another eminent German economist who worked extensively on agriculture, W.G.F. Roscher, tried to find a business structure that would best serve the interests of society. Buchenberger looked for practical ways to improve the critical condition of agriculture, and conducted research to determine, among other things, under what circumstances price controls for agricultural products could be justified.

Modem agricultural economics, at least in the selected examples, strives to meet the following objectives:
- Agricultural Economic Research Institute LEI-DLO reporting of business results and financial status attempts to give a representative picture of developments in Dutch agriculture, while also focusing on the changes taking place in the various groups of agricultural businesses (clustered according to type, size and specialization). The LEI figures are intended for various institutions and individuals: the European Union and national government, business organizations, entrepreneurs and their advisors, and financial institutions. The reporting does not provide these users with readymade theories or instructions. However, the descriptions do serve as a basis, in agricultural economics research for example, for predictions and theories, and consequently for the making and evaluation of policies.
- The aim of Agriculture in an Unstable Economy, by Th.W. Schultz (1945), was to lay the foundations for post-war agricultural policy in the United States.. To do so, Schultz identified the circumstances and policy measures that could contribute to a successful development of agriculture in the growing, but unstable, American economy. His research is for the most part an attempt to provide empirical arguments for the concluding policy recommendations.
- Farm Prices: Myth and Reality, by W.W. Cochrane (1958), attempts to make an accessible contribution to the discussion concerning agricultural price and income policies in the United States. Cochrane uses his analysis of the price formation of agricultural products to make predictions and to provide insight into the effects of various possible policy measures. The book concludes with a plea for a supply control program.
- Agricultural Development: An International Perspective, by Y. Hayami and V.W. Ruttan (1985), is a comprehensive study that focuses on the technological and institutional developments in world agriculture. The goal of the study was to develop a complete agrarian growth theory that could serve as a guideline for agricultural policymaking in developing countries.
- The concepts PSE (Producer Subsidy Equivalent) and CSE (Consumer Subsidy Equivalent) refer to the measurement of government support of a sector. They have recently received much attention, thanks in part to the Uruguay round of GATT negotiations, in agricultural economics research concerning the extent, structure and development of agrarian protection. PSE and CSE results are supposed to reflect the politically induced income transfers between consumers, taxpayers and agricultural producers.
- Disarray in World Food Markets; A Quantative Assessment, by the Australian agricultural economists R. Tyers and K. Anderson (1992), is a recent example of a separate category of studies concerning global agricultural markets. The goal of this research was to give insight into the economic effects of protection and liberalization.
- The Economics of Agricultural Policies, by B.L. Gardner (1987) is a textbook on agricultural policy analysis. According to Gardner, the task of the agricultural economist is not to take sides in agricultural politics, but to determine the effects of policy options. The aim of his book was to develop useful methods for the economic analysis and evaluation of agricultural policies.

These examples of modem agricultural economics research are all concerned with socially relevant issues. Furthermore, nearly all of them meet the criteria that research should attempt to identify causal relationships. The LEI reporting does not claim to do this, but such reporting is a prerequisite for theoretical research and policymaking The PSE and CSE studies of OECD and USDA are also descriptive in nature, but they claim to measure in part the income effects of agricultural politics. The other studies deal explicitly with causal relationships. The question is whether, and under what circumstances, the relationships actually hold.

Research methods

Aside from being designed with good intentions, research must also be based on responsible methods if it is to make a meaningful contribution to policyaking In the agricultural sciences, observation and description of phenomena occurring under specified conditions (in the field, greenhouse, and laboratory) play a central role in research. This is much less the case in theoretical economics. As explained in Part I, Meehan and others have criticized general theoretical economics for allowing the postulational and rationalistic research strategy to dominate over the empirical. More attention is paid to axioms and mathematical manipulations than to empirical data. Such methods and assumptions form an obstacle to the emergence of theories that are suitable for policymaking.

The agricultural economics discipline, as shown in this research, has a strong empirical tradition. The gathering and interpretation of factual matter was already a major part of research for the forerunners of the profession, the physiocrats and Arthur Young. Big names in the history of the profession (including Thaer, Thünen Roscher, Buchenberger, Laur, and Taylor) are known for their original answers to the questions of what data should be gathered and how this must be done. They used comparative business research, data from bookkeeping records, agricultural surveys, bookkeeping networks, historical material, and production and price statistics.

The seven examples demonstrate the role empiricism plays in modem agricultural economics:
- The research conducted by the Agricultural Economics Research institute LEI-DLO is a textbook example of thorough descriptive economic research. It begins by typifying and giving dimension to the population of farms (as reported in the May census); thereafter a stratified sampling is made from the population. At the sample farms numerous relevant empirical data are gathered and organized. Afterwards they are translated into verifiable information using diverse statistical techniques.
- Schultz' (1945) study begins with an inventory of the concrete conditions, developments and problems that had occurred or could be expected to occur in the American agricultural economy. The descriptions and predictions are followed by theoretical analyses of the underlying factors. After evaluating the then current agricultural policies, the study concludes with advice on how particular problems could be handled by the government.
- Cochrane (1958) also focuses on the practical problems of agricultural politics. However, his book is more of an expressed opinion on which agricultural market and pricing policies should be adopted, than a report of empirical, scientific research. The theoretical discussion is rather abstract. With the help of aggregated short-term demand and supply functions, Cochrane presents an interesting but controversial analysis of historical price formation of agricultural products.
- Hayami and Ruttan (1985) place important emphasis on empiricism in their research. However, as the quality of the material used is often questionable, interpretations must be made with great caution. Furthermore, the methods that the authors use to deal with the material tend to be postulational and rationalistic. Reality is viewed using the neo- classical economic concepts of the developed induced innovation model, such as production function, demand, supply and equilibrium. But technical and institutional changes also have important socio-cultural causes and effects.
- The PSE and CSE studies quantify market price supports, measured as the actual difference between national market prices and world market prices, and subsidies that are directly financed by the government. The concepts are based on unrealistic assumptions. Therefore, caution must be taken in interpreting the results. Neither income nor trade effects of agricultural policies are measured.
- The study by Tyers and Anderson (1992) is hardly empirical at all. It does begin with factual information about developments on the world market and about agricultural polities of rich and poor countries, but the main part of the study consists of simulations derived from a mathematical model. This model is a series of equations that describe relationships between levels of production and consumption, and prices. The model is based on the period 1980-1982. It is doubtful whether the model is in fact an accurate reflection of reality. A very problematic assumption in the model is that without government interference the market mechanism in agriculture and elsewhere will lead to an optimal allocation of production factors. This assumption ignores the typical characteristics of agriculture. The idea that the market mechanism works perfectly in the rest of the economy is just as unrealistic.
- Gardner (1987) refers in his textbook on the economy of agricultural policies only occasionally to concrete conditions and problems. Instead, the numerous pages are filled with simple and more complicated partial-equilibrium models for the analysis of possible government measures. Such measures cannot be analyzed, according to Gardner, until they are reduced to a variable that can be incorporated in a supply and demand model. Reality is thus compromised on two levels. This axiom-based strategy is a typical example of rationalistic, postulational research strategies. A fundamental objection to them is that it is impossible to empirically test whether their logical conclusions agree with what actually occurs. It is for this reason that such analyses cannot be used for policymaking.

The examples illustrate that empiricism has not been forgotten in modem agricultural economics. However, modem neo-classical economics with its axioms, nominal concepts and deductive methods has played an increasingly important role in the discipline. Gardner's (1987) textbook is the best example of this, but Hayami and Ruttan (1985) and Tyers and Anderson (1992) confirm the trend. With regards to methods, these researchers are unlike Schultz (1945), who used a much more inductive approach.

Validity of research results

Causal assumptions are only defensible - and useful for policymaking - once they have been subjected to a proper test programme and have thereby been proven accurate. The completeness of the results is also very important: they can not be suitable for policymaking if they neglect to address important side-effects.

There appear to be great differences in the above mentioned areas between modem theoretical economics and the agricultural sciences. According to critics, the results of modem theoretical economics are often insufficient: the designed instruments are unsatisfactory, models or logical structures are often not appropriate, supporting evidence for assumptions is seldom offered, and there is the problem of incompleteness. The latter is an important area of concern in the agricultural sciences as well. Nevertheless, agricultural scientists have undeniably achieved impressive research results.

The history of agricultural economics demonstrates that this specialization has achieved results which are useful in policymaking. Some immediate examples are recommendations regarding the design, structure and planning of agricultural enterprises; instruments for market and price regulations and income protection, and instruments for agricultural development polities.

In the case studies of modem agricultural economics the research results are validated as follows:
- The LEI-DLO research on Dutch agriculture does not claim to reveal any causal relationships. A positive aspect of the reporting is that it provides supporting evidence for the representativeness and reliability of the results. An important limitation
of the research is that the small (part-time) farms, are not represented in the sampling. Very large agricultural firms and stock-farms are also not included in the described population.
- Schultz (1945) presents in his study empirically testable predictions and useful instruments for agricultural policymaking. The post-war years have proven that his vision of long-term development in agriculture was correct: due to a combination
of technological forces and agricultural particularities, development in agriculture coincided with great changes in the utilization of the production factors in agriculture, with severely altered relative prices, and with continuous pressure on agricul
tural incomes. Schultz's suggestions for pricing policy (forward prices at equilibrium levels) were not put into practice. In the post-war years, income-directed agricultural market and pricing policies retained its prominent position in American agricultural politics.
- Cochrane's (1958) plea for a comprehensive supply control program in agriculture is empirically less sound. Particularly weak are the assumptions that the supply control can easily be made effective, that this automatically leads to the desired price increases, and that they in turn will result in a long-term increase in agricultural incomes. These crucial , assumptions are not supported by convincing evidence.
- Hayami and Ruttan (1985) demonstrate the importance of technological and institutional innovations in agrarian development. But the validity and normative adequacy of their 'induced agricultural development model' must be brought into question. The authors admit that, in addition to changing factor scarcities, other factors (product demand, market disturbances, cultural factors, general scientific and technological innovations) can also play a role in the process of technological and institutional change. Such statements make their model irrefutable.
- The PSE and CSE calculations of OECD and USDA offer a very static insight into the degree of governmental intervention in agriculture. The concepts are in fact no more than new names given to variations on the concept nominal rate of protection (NRP). The descriptions quantitatively summarize particular conditions. This research can have very little impact on the development of sound theories and agricultural policies.
- On the basis of simulations from their model of world agricultural markets, Tyers and Anderson (1992) conclude that liberalization of agricultural markets will increase prosperity in the world. The results are derived directly from debatable assumptions, for which no supporting evidence is given. Therefore, the researchers' results and policy recommendations can not be trusted.
- Gardner (1987) recommends abstract models and formal methods for the analysis of market and price measures. His work is only indirectly related to reality. The usefulness of the methods is not demonstrated, and the theoretical contributions in the book cannot be used in policymaking

As far as validity of results in concerned, most of the examples do not meet the established criteria. In some cases an attempt is made to identify causal relationships, but supporting evidence is often lacking. Moreover, the completeness of the research results is problematic. The exception to the rule is Schultz (1945) who developed testable predictions and useful policy instruments.

Conclusion

As a policy science, the agricultural economics discipline aims to provide knowledge for policyaking Considering the economic problems faced by Dutch, European and global agriculture, this is a very worthwhile goal. The policy issues, some that have
existed for years and others that are relatively new, cover areas such as: food supply (food shortages and hunger coinciding with agricultural stockpiles); uncontrolled productivity growth; rules of conduct in international agrarian trade; alarming agricultural price and income developments; unbalanced distribution of income and wealth; declining employment opportunities; soil, water and air pollution; the diminishing quality of nature, etcetera. These policy issues are essentially important, not only to the people involved in agriculture, but also to society as a whole.

Scientific endeavour in these fields can therefore not be taken lightly. On the contrary, it brings with it a great deal of responsibility. The knowledge required to comprehensively deal with these policy issues is overwhelming. One of the starting points of this research is that agricultural economics should provide the empirical information needed to make reasoned normative choices. Namely, it should make predictions regarding future developments and theories that can connect actions with their effects (causal relationships). Due to the dynamics and complexity of society, it is a very difficult task to provide such information, but not an impossible one.

This study has demonstrated that the history of agricultural economics offers examples of high-quality policy-oriented research. It has also shown that post-war agricultural economics research provides important information about developments in the agrarian sector, and thereby creates a basis for theorizing, and for the making and evaluation of policy. The discipline also boasts a classical policy-oriented study, by Schultz (1945), that can be recommended to economists as an example worth following. There is thus some justification for the praise given to agricultural economics by Leontief (1971).

By contrast, the other examples of modem agricultural economics in this study cannot provide the scientific results needed for policyaking This judgement confirms the general view expressed by several critics (cited in Part 1), including Paarlberg, Brandow, Bonnen, Doering, Dietze, Hagedorn, Brandes, Louwes, Van den Noort, De Hoogh, Veerman and Oskam, that the quality of current agricultural economics can and must be improved.

From the described epistemological analysis of modem agricultural economics it can be concluded that the shortcomings, at least of the examples considered, are not in the goals of the research, but in the research methods and the validation of causal assumptions. Empiricism should play a much more central role in research, and assumptions about the characteristics of people, things and their interrelationships should agree as much as possible with reality. Alas, in modern agricultural economics, just as in modern general economics (including neo-institutional economics), the postulational and rationalistic research approach has become increasingly popular in the last decennia. The overwhelming trust in formal methods and nominal concepts, such as demand, supply and equilibrium, coincides with a great abstraction of real conditions and problems. As a result, the validation of causal assumptions is inadequate for the potential users of the information.

This development in agricultural economics stems from the idea that the reality to be researched can be represented as an axiomatic system. This idea in turn comes from the classical mechanistic view of the world. Such a Newtonian view of science and reality has been extremely successful in some scientific fields, but it has not been accepted as generally applicable in the natural sciences for years. The world as seen in the modem view is much more complex, less understandable, and also less predictable than in the mechanistic one. Not unrelated to this is the replacement of fundamentalism with fallibilism, according to which all knowledge is in principle fallible. The pragmatic epistemology upon which this research is based makes the same claim: only that which works is 'true'. This view is universally accepted by agricultural scientists, and also had a considerable following among classical agricultural economists.

Agricultural economics has gone through a fundamental change during the past decades. Agrarian economists nowadays lean heavily on general neo-classical microeconomics, which typically neglects the traditional policy-oriented and applicable character of the economics discipline. Colander (1992) refers to this as 'The Lost Art of Economics'. Indeed, if agricultural economists wish to contribute constructively and scientifically to the resolution of policy issues, they will have to go back to practising their trade as an art.

Surface irrigation, characteristics, design and performance.
Roscher, K. - \ 1987
Unknown Publisher - 45 p.
Economic report on weed control and silt clearance. Sudan.
Nieuwenhuyse, P. ; Roscher, K. ; Sijtsma, C. - \ 1987
Unknown Publisher - 167 p.
Design and management of tertiary units directives, Part B.
Roscher, K. - \ 1987
Unknown Publisher - 66 p.
Design and management of tertiary units directives, Part A.
Roscher, K. - \ 1987
Unknown Publisher - 30 p.
Discipline-wise contributions to regional planning. Chapter C, Hydrology
Ledeboer, H.F. ; Roscher, K. - \ 1980
In: Framework for regional planning in developing countries - p. 166 - 176.
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