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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Ecosysteemdiensten van bomen en groen in de stad
Hiemstra, J.A. - \ 2017
Wageningen : Groen Kennisnet
bomen - planten - biodiversiteit - ecosystemen - klimaat - luchtkwaliteit - regenwateropvang - stedelijke gebieden - trees - plants - biodiversity - ecosystems - climate - air quality - water harvesting - urban areas
Groen kost geld en de baten zijn slecht tastbaar. Het project 'Ecosysteemdiensten van boomkwekerijproducten' ontwikkelt instrumenten om de voordelen van groen beter te laten meewegen bij het nemen van beslissingen. Voordelen: verkoeling, luchtzuivering, waterberging en beleefbare biodiversiteit. In eerste instantie voor de productgroep bomen, later ook voor andere productgroepen.
Chobe district integrated land use plan
Sluis, Theo van der; Cassidy, Lin ; Brooks, Chris ; Wolski, Piotr ; VanderPost, Cornelis ; Wit, Piet ; Henkens, Rene ; Eupen, Michiel van; Mosepele, K. ; Maruapula, O. ; Veenendaal, Elmar - \ 2017
Wageningen : Wageningen Environmental Research (Wageningen Environmental Research rapport 2813) - 181
land use - sustainability - tourism - ecosystems - savannas - botswana - landgebruik - duurzaamheid (sustainability) - toerisme - ecosystemen - savannen
Biotic interactions and trait-based ecosystem functioning in soil
Sechi, Valentina - \ 2017
University. Promotor(en): Lijbert Brussaard, co-promotor(en): Ron de Goede; C. Mulder; M. Rutgers. - Wageningen : Wageningen University - ISBN 9789463431316 - 176
soil - ecosystems - plant-animal interactions - interactions - soil biology - soil quality - grasslands - collembola - diversity - bodem - ecosystemen - plant-dier interacties - interacties - bodembiologie - bodemkwaliteit - graslanden - diversiteit

Recent emphasis on ecosystem services as a framework to evaluate ecosystems and to promote their sustainable use has drawn attention to how organisms contribute to the delivery of services. Soil attributes and biotic interactions play important roles in ecological processes (e.g. soil formation, nutrient turnover, carbon sequestration and transformation) and, consequently, in the related delivery of ecosystem services.

Therefore, understanding how soil organisms interact and how they respond to environmental conditions is fundamental to preserve soil functioning and provide a meaningful assessment of ecosystem services. Functional traits determine individual responses to pressures and their effects on ecosystem functioning hence, investigating soil ecosystems from a trait-based perspective offers an interesting opportunity to link the functional responses of the organisms to environmental pressures and to give insight into how the entire community influences ecological processes.

The main objective of this thesis is to develop and to test concepts for a trait-driven quantification of ecosystem services through the assessment of the effects of land management on soil processes. In particular, it focuses on exploring the potential of a trait-based approach in identifying and better understanding the response of the soil biota to environmental pressures and analyses the responses of soil organisms in terms of changes in functional trait distribution and trophic interactions.

This work shows that approaches taking the whole soil community into consideration are more suitable to give insight into the effect of anthropogenic pressure on ecosystem functioning than approaches based on single taxonomic groups. Moreover, performing combined analysis (e.g. analysing body-mass distribution and trophic grouping) helps to better identify community response to environmental pressure.

A clear methodology for the next step, i.e. quantification of ecosystem services, is still lacking due to the current difficulties to link and quantify the effect of anthropogenic pressure to ecosystem functioning in soil. For this reason, it is essential that methods analysed in this thesis will be further explored under different environmental pressures to enable the development of tools to be used at the interface of science and society for sustainable development.

Draagkracht voor schelpdieren : definities, indices en case studie
Smaal, Aad - \ 2017
Yerseke : Wageningen Marine Research (Wageningen Marine Research rapport C023/17) - 26
schaaldieren - ecosystemen - draagkracht - gevalsanalyse - visserijbeheer - natuurbeheer - oosterschelde - shellfish - ecosystems - carrying capacity - case studies - fishery management - nature management - eastern scheldt
North Sea reefs : benthic biodiversity of artificial and rocky reefs in the southern North Sea
Coolen, Joop Waltherus Petrus - \ 2017
University. Promotor(en): S. Degraer. - Wageningen : Wageningen University - ISBN 9789463430876 - 199
coral reefs - benthos - biodiversity - north sea - marine ecology - marine organisms - ecosystems - koraalriffen - biodiversiteit - noordzee - mariene ecologie - zee-organismen - ecosystemen

The objective of the research presented in this thesis was to understand the patterns of benthic biodiversity on reefs in the North Sea. To gain this understanding, I studied which species are present on natural and artificial reefs, what environmental and biotic variables influence the presence and absence of a selection of these species and one of the possible pathways by which these species may colonise the reefs studied; the stepping stone effect. The first goal was to increase the available knowledge on which species are present at reefs. The second goal was to understand the patterns observed in the variation of species at these reefs. The third goal was to evaluate whether Mytilus edulis utilises offshore artificial structures as stepping stone to colonise very far offshore locations. The final goal was to assess the impact artificial reefs have on the benthic biodiversity of the North Sea. To attain these goals, natural and artificial reefs were sampled using diver operated airlift samplers and box corers. To evaluate the effects generalised linear and additive models were created. When available, additional data from other sources were also used.

The following conclusions are drawn: Lanice conchilega is an ecosystem engineer creating intermediate sand-reef systems. When rocky reefs are present on a sandy bottom, local biodiversity is doubled. Distributions of the native Caprella linearis and the invasive Caprella mutica showed a significant difference, demonstrating that C. linearis’ habitat preference does not fully overlap with that of C. mutica. Thus, the native and alien Caprellids are likely to be able to co-exist in the North Sea. M. edulis presence increases habitat heterogeneity, which increases species richness. Mytilus edulis uses offshore structures as stepping stones to colonise locations in the North Sea that cannot be reached in a single generation.

Depth, location effect and habitat type influence the species composition on North Sea reefs most strongly. The relation between depth and species richness on artificial reefs is non-linear, with a maximum at intermediate depths. Although substrates with mixed surface orientation (i.a. rocks) hold the most species rich communities, biodiversity is also strongly influenced by M. edulis and Psammechinus miliaris. When artificial reefs are to be colonised by communities that are similar to natural reefs, their structures should resemble natural reefs as much as possible.

Van chemie naar ecologie: niet straks maar nu!
Vet, Emely de - \ 2016
plant protection - pesticides - biological control - golf - agriculture - agri-environment schemes - ecosystems - biodiversity
'Van chemie naar ecologie. Niet straks maar nú!'
Vet, Emely de - \ 2016
golf - pesticides - agri-environment schemes - biological control - agriculture - plant protection - sports grounds - golf green soils - ecosystems - biodiversity - golf courses
Youtube registratie van de presentatie 'Van chemie naar ecologie. Niet straks maar nú!'
A study to identify alternatives for the use of chemical amenities in turfgrass systems
Struik, Paul - \ 2016
sports grounds - golf courses - golf - grass sward - chemical control - biological control - integrated pest management - ecosystems
Assessing biodiversity change in scenario studies : introducing a decision support tool for analysing the impact of nature policy
Pouwels, Rogier ; Bilt, Willem van der; Hinsberg, Arjen van; Knegt, Bart de; Reijnen, Rien ; Verboom, Jana ; Jones-Walters, Lawrence - \ 2016
Wageningen : Wettelijke Onderzoekstaken Natuur & Milieu (WOt-paper 39) - 16
biodiversity - policy - decision support systems - habitats - ecosystems - biodiversiteit - beleid - beslissingsondersteunende systemen - ecosystemen
Biodiversity conservation is firmly established on the
political agenda. Nested goals and targets for biodiversity
have therefore been formulated and agreed at global,
regional, national and sub-national levels in order to halt
and reverse its decline. In order to measure progress in
relation to the delivery of such targets, policymakers have
a range of tools and indicators that allow them to monitor
and evaluate the effect of their policies, instruments and
associated actions. In terms of the policy cycle, evaluation
should result in the further modification and refinement of
policy instruments towards improved delivery in the
future.
Voortgang realisatie nationaal natuurbeleid : technische achtergronden van een aantal indicatoren uit de digitale Balans van de Leefomgeving 2016
Sanders, M.E. ; Wamelink, G.W.W. ; Wegman, R.M.A. ; Clement, J. - \ 2016
Wageningen : Wettelijke Onderzoekstaken Natuur & Milieu (WOt-technical report 79) - 74
ecologische hoofdstructuur - ecologie - ecosystemen - verdroging (milieu) - verzuring - depositie - natuurbeleid - ecological network - ecology - ecosystems - groundwater depletion - acidification - deposition - nature conservation policy
The Dutch government is, together with its partners, taking measures to create a coherent network ofprotected nature areas and to improve environmental conditions. This in order to halt the decline in the areaof natural habitat and biodiversity and to improve their conservation status. The Government wants to stayinformed on the progress of this policy. The Netherlands Environmental Assessment Agency (PBL) has selectedindicators that should provide answers to the question: ‘What is the progress of the policy measures taken,especially for realising the nature network, improving the nature quality and the environmental conditions aswell?’ The selected indicators have been updated and analysed in order to assess this progress. This reportdescribes the results of the policy measures taken on the basis of the indicators, the technical setting of thedata and methods used to bring these indicators up to date and the reliability and acceptability of it
Making eco logic and models work : an integrative approach to lake ecosystem modelling
Kuiper, Jan Jurjen - \ 2016
University. Promotor(en): W.M. Mooij, co-promotor(en): J.H. Janse; Jeroen de Klein. - Wageningen : Wageningen University - ISBN 9789462579446 - 192
ecology - models - ecosystems - modeling - aquatic ecology - water management - water quality - databases - ecologie - modellen - ecosystemen - modelleren - aquatische ecologie - waterbeheer - waterkwaliteit - databanken

Dynamical ecosystem models are important tools that can help ecologists understand complex systems, and turn understanding into predictions of how these systems respond to external changes. This thesis revolves around PCLake, an integrated ecosystem model of shallow lakes that is used by both scientists and water quality managers to understand and predict eutrophication effects in shallow lake ecosystems. Shallow lakes provide some of the clearest examples of alternative stable states in natural systems. PCLake can be used to calculate the critical nutrient loading, that is, the nutrient loading where an abrupt regime shift occurs from a clear aquatic plant dominated state to a turbid phytoplankton dominated state, or vice versa. Four different aspects of modelling with PCLake are addressed in this thesis: (1) making the model better accessible for the modelling community, (2) improving the model, (3) developing scientific theory, and (4) exploring new applications for water quality management.

Following a general introduction to the thesis in chapter 1, the Database Approach To Modelling (DATM) is introduced in chapter 2. DATM is invented to make dynamic models more accessible. The idea of DATM is that the mathematical equations of a model are stored in a database independently of program language and software specific information. From the database, the information can be automatically translated, augmented and compiled into working model code of various different modelling frameworks (software programs).

In chapter 3 the weak link between ecosystem models and real ecosystems is discussed in relation to model calibration and improvement. In a previous stage PCLake has been calibrated using data of more than 40 lakes to obtain a best overall fit, which has greatly increased the scope of the model by making it suitable for more generalized studies on temperate shallow lakes. However, because of this calibration, adding missing functional components to the model at a later stage does not automatically increase the validity of the model, as it may bring the model ‘out of balance’. This is exemplified by adding filter feeding zoobenthos to PCLake, which were previously ignored.

In chapter 4, the relation between food-web theory and alternative stable states theory is scrutinized. Both theoretical paradigms are highly influential in modern ecology as they help scientists understand how stability emerges in complex natural ecosystems. Unfortunately, they developed independently and it is largely unclear how the resilience of a food web relates to the stability of the complete ecosystem. For this study PCLake was used as a virtual reality from which ‘empirical’ information is sampled to parameterize a food web model, following traditional food web methods. This allowed calculating the stability of the food web along a gradient of environmental change, knowing that the complete ecosystem shows a regime shift once the critical nutrient loading is exceeded.

In chapter 5 the question is asked to what extent models of a different form can be used to describe the same natural phenomenon, and hence, how these models can be used for a better understanding of such natural phenomena. Using three classical extensions of the famous Lotka-Volterra equations, which unlike PCLake can be fully mathematically understood, we analyze the consequence of changing a system with a sophisticated functional response term (e.g. Holling type II or III) into a system with a simpler functional response term while maintaining equilibrium densities and material fluxes. These results give new insight into when empirical data can be linked to mathematical models to estimate the stability properties of real ecosystems.

Although PCLake is predominantly applied in the context of ecosystem restoration of turbid phytoplankton dominated lakes, chapter 6 focusses on the clear water state after the reestablishment of aquatic plant dominance as occured. Dense stands of aquatic plants easily cause nuisance, and hence the removal of aquatic plants is an emerging management issue. Yet, because aquatic plants play an important role in stabilizing the clear water state, the removal of plant biomass can potentially trigger a critical transition back to the turbid water state. Currently there is only limited empirical and theoretical understanding of how harvesting of aquatic plants affects ecosystem functioning, which frustrates effective and efficient ecosystem management. With PCLake the impact of harvesting is evaluated, in terms of reducing nuisance and ecosystem stability, for a wide range of external nutrient loadings, mowing intensities and timings. Additionally, the model is used to estimate how much phosphorus is removed from the system during harvesting.

In chapter 7 I discuss the added value of taking an integrative approach to modelling, and discuss the integrated nature of the studies presented in this thesis. It’s also important to note that these studies were part of a larger research project with the overall aim of increasing the usefulness and the validity of PCLake and its twin model PCDitch, and to enhance the confidence in the models among water quality managers. A synopsis of the overarching collaborative research project on PCLake and PCDitch is presented in chapter 8.

Waarheen met het Nederlandse bos?
Hekhuis, Harrie ; Ouden, J. den - \ 2016
Vakblad Natuur Bos Landschap 13 (2016)127. - ISSN 1572-7610 - p. 4 - 7.
bosbeheer - houtachtige planten - gebruikswaarde - nederland - klimaatverandering - ecosystemen - bostypen - recreatie - zandgronden - oppervlakte (areaal) - forest administration - woody plants - use value - netherlands - climatic change - ecosystems - forest types - recreation - sandy soils - acreage
Het Nederlandse bos is een populair park met jaarlijks honderden miljoenen bezoeken. Het bos is een belangrijke pijler van de Nederlandse natuur met bijvoorbeeld veel Natura 2000-habitats, nieuwe wildernissen en een bron van hernieuwbare grondstoffen. Welke kennis en beheer zijn nodig om aan al deze functies tegemoet te blijven komen? Wij zijn ons als bosbeheerders, beleidsmakers, onderzoekers en opleiders onvoldoende bewust van de forse uitdagingen waar het Nederlandse bosbeheer voor staat!
Roots in the tundra : relations between climate warming and root biomass and implications for vegetation change and carbon dynamics
Wang, Peng - \ 2016
University. Promotor(en): Frank Berendse, co-promotor(en): Monique Heijmans; Liesje Mommer. - Wageningen : Wageningen University - ISBN 9789462578609 - 168
roots - biomass - climatic change - vegetation - carbon - global warming - tundra - ecosystems - decomposition - siberia - wortels - biomassa - klimaatverandering - vegetatie - koolstof - opwarming van de aarde - toendra - ecosystemen - decompositie - siberië

Global climate has been warming up for the last decades and it will continue in this century. The Arctic is the part of the globe that warms fastest and is more sensitive to climate warming. Aboveground productivity of Arctic tundra has been shown to increase in response to warmer climates. However, belowground responses of tundra vegetation are still unclear. As the major part of plant biomass in tundra lies belowground, it is pivotal to investigate changes in the belowground parts of tundra vegetation for our understanding of climate warming effects on tundra ecosystems.

To get a general idea of how belowground plant biomass may change in a warmer climate, we synthesized published data on the belowground biomass of tundra vegetation across a broad gradient of mean annual air temperature from −20 to 0 °C. We found that aboveground biomass of tundra biomass indeed increases with mean annual temperature as well as summer air temperature, while belowground biomass did not show a significant relationship with temperature. The increases in the aboveground biomass were significantly larger than belowground biomass, resulting in reduced below/above ratios at higher temperatures. The shifted biomass allocation with temperature can influence the carbon dynamics of tundra ecosystems. Future tundra studies need to focus more on the species or functional type composition of belowground biomass and species or functional type specific belowground responses to climate warming.

To determine the seasonal changes and vertical distribution of root biomass of different plant functional types, we sampled roots at a Siberian tundra site in the early and late growing season, from vegetation types dominated by graminoids and shrubs respectively. We distinguished the roots of graminoids and shrubs, and found that shrub roots grew earlier and shallower than graminoid roots, which enables shrubs to gain advantage over graminoids at the early growing season when nutrient pulses occur during snowmelt and soil thaw. The deeper roots of graminoids can help them to be more competitive if climate warming induces more nutrient release in the deeper soil.

In a soil thawing and fertilization experiment, we further investigated the effects of increased thawing depth and nutrient supply in the upper soil, which can be the consequences of climate warming, on root biomass and its vertical distribution. In this study we distinguished between the roots of grasses, sedges, deciduous shrubs and evergreen shrubs. The study was done in a moist tussock tundra site with similar abundance of the different plant functional types. We found that only sedges benefited from the increased thawing depth, probably through their deepest root distribution among the four functional types, while the shrubs, which were shallower-rooted, benefited from the increased nutrient availability in the upper soil. The deep-rooted grasses had the highest plasticity in vertical root distribution, which enabled them also to benefit greatly from the fertilization. Our results show that tundra plants with different rooting strategies can show different responses to climate warming dependent on the relative warming impacts on the nutrient supply in shallow and deeper soil layers. This insight can help to predict future tundra vegetation dynamics.

The carbon balance of tundra ecosystems also depends on the decomposition of plant litter, particularly the root litter, which may account for a larger part of annual litter input than leaf litter in tundra ecosystems. Vegetation shifts also change litter quality which ultimately influences carbon dynamics. To investigate the differences in the decomposition of leaves and roots of graminoids and shrubs, we performed a litter transplant experiment. We found that although the decomposability of leaf litter did not differ between the graminoid and shrub, root decomposability might be lower for the shrub. However, this cannot be extrapolated to the overall decomposition in different vegetation types, as these different plant communities differ in rooting depths. We also found evidence of home-field advantage in the decomposition in Arctic tundra, and we show that the early stage of litter decomposition at our research site could be driven by the phosphorus concentration of the litter. To get a full understanding of the carbon balance of tundra ecosystems, much more efforts are needed to quantify litter input and decomposition.

In this thesis we show that belowground parts, which account for a major part of plant biomass in tundra, can show a different response to climate warming from aboveground parts. Belowground responses to climate warming can have crucial impacts on the competitive balance between tundra plants, and consequently result in vegetation shifts in tundra. Such shifts in species composition can have large effects on carbon dynamics through altered input and decomposability of plant litter, particularly root litter.

Optimising land use in Central Kalimantan Province, Indonesia : modelling ecosystem benefits and land use dynamics
Suwarno, Aritta - \ 2016
University. Promotor(en): Rik Leemans; Lars Hein, co-promotor(en): Hans-Peter Weikard. - Wageningen : Wageningen University - ISBN 9789462578739 - 156
land use - deforestation - decentralization - ecosystem services - ecosystems - forest policy - forests - modeling - kalimantan - indonesia - landgebruik - ontbossing - decentralisatie - ecosysteemdiensten - ecosystemen - bosbeleid - bossen - modelleren - indonesië

The rising global population has increased the demand for food, renewable energy and other materials. Yet at the same time to meet this demand requires land and the amount of available land is finite. Considering the importance of land and ecosystems in providing benefits for human, I conducted four independent research on the socio-economic and biophysical aspects of ecosystem service, in Central Kalimantan Province, Indonesia. The first three independent studies were used to understand ecosystem management under decentralised forest governance in Indonesia and its influence on ecosystems, ESs and the benefits that different beneficiaries receive. The fourth study combines the outcomes from the previous three studies to assess and select the most appropriate areas for conservation and community development. As the results, I show the applicability of the ecosystem services concept and land-use modelling in optimising land-use under certain decentralised forest governance systems. My thesis’ results were obtained through the application of methods and steps that integrated a comprehensive set of qualitative and quantitative analyses to support land-use optimisation in the Kapuas Protected Forest Management Unit. My results can inform decision makers on the options of land-use optimisation and the consequences of their management decisions regarding land-use intensification, nature conservation and local economic conditions. I show how land-use optimisation provides an important step in preventing further land degradation and ecosystem loss.

Nat zandlandschap van de 21e eeuw : kennisagenda
Schouten, M.G.C. ; Jansen, A.A.M. ; Tweel-Groot, Loekie van - \ 2016
Landschap : tijdschrift voor Landschapsecologie en Milieukunde 33 (2016)2. - ISSN 0169-6300 - p. 118 - 121.
natuurbeheer - natuurgebieden - biodiversiteit - zandgronden - duurzame ontwikkeling - landschapsecologie - kennismanagement - soortendiversiteit - ecotypen - systeemanalyse - ecosystemen - toegepast onderzoek - bodems van waterrijke gebieden - nature management - natural areas - biodiversity - sandy soils - sustainable development - landscape ecology - knowledge management - species diversity - ecotypes - systems analysis - ecosystems - applied research - wetland soils
Hoe ziet een duurzaam en biodivers nat zandlandschap van de 21e eeuw eruit en hoe ontwikkelen we dat? Dat landschap zal ongetwijfeld een ander zijn dan dat van de 19e en de eerste helft van de 20e eeuw, maar de uitdaging is de totale soortenrijkdom hierin weer voldoende plaats te bieden. Dit artikel biedt een overzicht van de kennis die daartoe ontwikkeld moet worden.
Bedrijven en hun impact op en afhankelijkheid van natuurlijk kapitaal
Smits, M.J.W. ; Bos, E.J. ; Heide, C.M. van der; Selnes, T. ; Vogelzang, T.A. - \ 2016
Wageningen : LEI Wageningen UR (Rapport / LEI Wageningen UR 2016-060) - ISBN 9789462578395 - 27 p.
natuurlijke hulpbronnen - duurzaamheid (sustainability) - ecosystemen - dienstensector - bedrijven - organisaties - natural resources - sustainability - ecosystems - services - businesses - organizations
What tools and data do companies use to measure their impact on natural capital, and what are the gaps in terms of instruments ? The main findings, based on interviews are: i) companies mainly work with LCA ii) companies with many products prefer to work with labels, iii) there is a need for standardisation at the sector level, iv) availability of data at field level is a bottleneck, v) nonfrontrunners could be more involved in natural capital.
Costs of seabed protection on the Frisian Front and Central Oyster Grounds for the Dutch fishing sector : addendum to LEI report 2015-145
Oostenbrugge, J.A.E. van; Turenhout, M.N.J. ; Hamon, K.G. - \ 2016
Wageningen : LEI Wageningen UR (Memorandum / LEI Wageningen UR 2016-062) - 29 p.
cost benefit analysis - fisheries - aquatic ecosystems - protection - benthos - ecosystems - oyster culture - north sea - kosten-batenanalyse - visserij - aquatische ecosystemen - bescherming - ecosystemen - oesterteelt - noordzee
This memorandum provides an estimation of the costs for four variant closures for the protection of the benthic ecosystem on the Frisian Front and the Central Oyster Grounds for the Dutch fishing sector in addition to the cost-benefit analysis carried out in Van Oostenbrugge et al. (2015). The two preferential variants lead to similar costs for the fisheries sector, whereas the costs of the two alternative combinations are either 20% higher or lower.
Simplifying complexity : reflections on ecosystems, psychiatric disorders and biogeochemical pathways
Leemput, I.A. van de - \ 2016
University. Promotor(en): Marten Scheffer; Egbert van Nes. - Wageningen : Wageningen University - ISBN 9789462577930 - 211 p.
ecosystems - models - biogeochemistry - nitrogen - coral reefs - climate - aquatic ecology - ecosystemen - modellen - biogeochemie - stikstof - koraalriffen - klimaat - aquatische ecologie
The facilitative role of trees in tree-grass interactions in savannas
Priyadarshini, K.V.R. - \ 2016
University. Promotor(en): Herbert Prins; Steven Bie, co-promotor(en): Ignas Heitkonig. - Wageningen : Wageningen University - ISBN 9789462577008 - 134 p.
plant interaction - trees - grasses - savannas - ecosystems - ecology - planteninteractie - bomen - grassen - savannen - ecosystemen - ecologie

Terrestrial ecosystems support a high plant diversity where different plant types coexist. However, the mechanisms that support plant coexistence are not entirely clear. Savanna ecosystems that are nutrient and water limited are characterized by a unique ecological feature: the coexistence of trees and grasses. Tree-grass interactions in savannas are typically viewed as being competitive and are based on the Gaussian principle of niche or habitat differentiation. Trees and grasses are reported to suppress the growth of each other and the interactions are viewed as competition. However, tree-grass mixtures persist in a range of rainfall conditions in savannas. This study examined tree-grass interactions to understand the ecological processes that may sustain tree-grass coexistence in dry savannas (< 800 mm of rainfall) of southern Africa. Water and nitrogen resource-use patterns of trees and grasses were investigated and the effects of competition between trees and grasses on resource storage in perennial grasses were examined. An ecological perspective of the role of trees in two human land-use types in African drylands is provided and the functionality of trees in these land-use types was reviewed.

Seasonality of plant available water imposes intense water limitation to plants in savannas. Yet, trees and grasses coexist. The water relations between trees and grasses are poorly understood. In Chapter 2, the principal water-sources for trees and grasses in different seasons were identified using the natural variation in H and O stable isotope composition of source waters. Seasonal differences in the stable isotope composition of water in trees and grasses indicated that there was water-source use partitioning as well as overlap. Trees and grasses used water from the topsoil after rainfall indicating overlap of water-sources. Trees shifted to groundwater or subsoil water when there was no water in the topsoil, indicating partitioning of water-use. Grasses always used water from the topsoil. By labelling deep-soil (2.5 m depth) with a deuterium tracer, hydraulic-redistribution in all the studied tree species and water transfer to grasses via the topsoil was confirmed. However, this occurred only in the dry-season. Results indicated possible shifts in tree-grass interactions during different periods of the year. Furthermore, dry-season hydraulic-redistribution indicated potential facilitation affects by trees to their understory grasses.

A key question in savanna ecology is how trees and grasses coexist under nitrogen limitation. In Chapter 3, the sources of nitrogen for trees and grasses in a semi-arid savanna were investigated using natural abundance of foliar δ15N and nitrogen content. 15N tracer additions were used additionally to investigate the redistribution of subsoil nitrogen by trees to grasses. Foliar δ15N values were consistent with trees and grasses using mycorrhiza-supplied nitrogen in all seasons and a switch to microbially-fixed nitrogen during the wet season. Based on seasonal variation in mineralization rates in the Kruger Park region, the use of mineralized soil nitrogen by trees and grasses seemed highly unlikely. The foliar δ15N values were similar for all the studied tree species differing in the potential for nitrogen-fixation consistent with the absence of nodules indicating the lack of rhizobially fixed nitrogen. The tracer experiment showed that nitrogen was redistributed by trees to understory grasses in all seasons. Redistribution of nitrogen by trees and subsequent uptake of this tree redistributed nitrogen by grasses from the topsoil was independent of water redistribution. Although there was overlap of nitrogen sources between trees and grasses, dependence on biological sources of nitrogen coupled with redistribution of sub-soil nitrogen by trees could be contributing to the co-existence of trees and grasses in semi-arid savannas.

An important plant response to competition and resource limitation is an increase in root reserves. In Chapter 4, the root characteristics of perennial grasses in the presence and absence of trees as a proxy of competition in South African savannas in three sites that differed in rainfall were investigated. The hypothesis on which this investigation was based was that competition from trees and water limitation will result in increased storage in roots of grasses under trees. However, no significant effect of variation in rainfall of the different study locations on root characteristics of grasses were found. Furthermore, most root characteristics were not significantly influenced by tree presence with the exception of nitrogen-content. The root nitrogen content showed an increase with rainfall and tree presence through potentially higher mineralization rates and nitrogen availability in the under-tree canopy environment. The study sites occurred in the drier rainfall range in South Africa. Therefore, it is likely that trees and grasses in these dry savannas might have a positive relationship conforming to the stress-gradient hypothesis. Alternatively, grasses and trees might be using complementary water and nutritional resources.

The mix of trees and grasses is critical for the functioning of the savanna biome, which supports a large fraction of the human population and sustains the highest densities and diversities of herbivores in the world. Both, increases and decreases in tree densities have been reported from savannas globally, which are attributed to human activities and climate change. Changes in tree densities could drastically impact ecosystem functioning and lead to land degradation and large economic losses. Consequently, the sustainable and heterogeneous nature of various savanna land-use types is compromised. In Chapter 5, the significant role of trees in dry savannas (< 800mm rainfall) based on nutrient and water-redistribution capabilities of savanna trees is illustrated. An ecological perspective of the role of trees in two human land-use types in African drylands: agroforests and rangelands which include silvo-pastoral systems and mixed-game-livestock farming systems, is provided. The causes for the loss of trees in these land-use types is evaluated and the role of trees for better land and sustainable natural resource management is highlighted.

Chapter 6 synthesises the conclusions of all the preceding chapters highlighting the importance of facilitative interactions in tree-grass coexistence in savannas that are mostly overlooked. A simple Gaussian model of niche or habitat differentiation may not be a holistic and functional explanation of plant coexistence but rather the role of biotic interactions that include symbionts, parasites, or predators that will influence not only the competitive ability of plants but also facilitation, may be more pragmatic. Plant-plant interactions are complex and a multitrophic approach may be necessary to understand the functioning of these interactions and their roles in ecosystems.

Biodiversiteit werkt
Schaminee, J.H.J. ; Dekker, J.N.M. ; Dijk, J. van; Dorland, Esther ; Groenewoud-Groot, M. van het; Tooren, B.F. van - \ 2016
Landschap : tijdschrift voor Landschapsecologie en Milieukunde 33 (2016)1. - ISSN 0169-6300 - p. 4 - 7.
biodiversiteit - landbouwkundig onderzoek - kennisvalorisatie - ecosystemen - natuurbeheer - ruimtelijke ordening - overheidsbeleid - biodiversity - agricultural research - knowledge exploitation - ecosystems - nature management - physical planning - government policy
In 2010 is het Onderzoeksprogramma Biodiversiteit werkt gestart. Zoals uit de titel al spreekt, moet de vergaarde kennis een doorwerking krijgen in beleid en praktijk. Toepasbaarheid is een belangrijk aandachtspunt. In dit nummer van LANDSCHAP worden de (voorlopige) onderzoeksresultaten gepresenteerd. Tevens komen enkele stakeholders aan het woord over die toepasbaarheid en geeft NWO haar visie daarop. In deze inleiding beschrijven we de context en opzet van Biodiversiteit werkt.
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