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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    Does input of rich litter facilitate tree growth? : growth and climate growth response of oak (Quercus robur) growing in the neighborhood of black cherry (Prunus serotina)
    Haas, Josephine ; Sass Klaassen, Ute ; Akhmetzyanov, Linar ; Ouden, Jan den - \ 2020
    Wageningen : Wageningen University and Research Forest Ecology and Management group - 45
    Atmospheric nitrogen deposition and former land use have led to widespread acidification of forest soils and disturbed nutrient balances, which has been linked to reduced forest vitality and tree growth. The admixture of rich-litter tree species in forests dominated by poor-litter species that further accelerate soil acidification may alleviate the impact of soil acidification on poor sandy soils. Admixture of litter with high base cation content has a positive impact on topsoil pH, as well as base saturation and may potentially affect growth and vitality of trees. To date, the effect of introducing rich-litter species on the long-term growth of co-occurring tree species has not been systematically studied. In this study, we aimed to evaluate the effect of the rich-litter species Prunus serotina on the growth of the poor-litter tree species Quercus robur by means of dendrochronology. We hypothesized that the presence of Prunus will – through soil amelioration - positively affect both the growth and the drought susceptibility of Quercus. We investigated the growth of co-occurring Quercus and Prunus trees in six forest sites with sandy, poor soils in the Netherlands and Germany. We compared tree-ring patterns of oaks growing next to cherry trees (influenced), oaks growing next to other oaks (uninfluenced) and the cherry trees. Tree-ring widths were analyzed on (a) average annual growth variation and basal area increment; (b) the climate sensitivity of the annual growth; (c) the growth response to selected drought years; and (d) the tree-individual growth variation. For most analyses, influenced and uninfluenced oaks showed little differences. Individual growth variation between oaks did show a division between influenced and uninfluenced trees in some sites. However, this division seemed to be linked to spatial separation rather than rich-litter effects. Overall, this study indicated that there is no clear evidence for a facilitating effect of prunus on the growth of oak. It is likely that the studied sites were too poor to capture an effect of the improved soil conditions on the growth of oak, or that competition for water may have overruled any direct soil effect by prunus. We conclude that this study did not provide evidence the rich litter species Prunus serotina does positively affect the growth of Quercus robur on the studied acidic poor sandy soils.---Atmosferische stikstofdepositie en voormalig landgebruik hebben geleid tot een sterke verzuring van bosbodems en verstoorde nutriëntenbalansen, wat in verband wordt gebracht met een verminderde vitaliteit en groei van bomen. Bovendien worden de bossen gedomineerd door boomsoorten met slecht verteerbaar strooisel, waardoor de verzuring van de bodem verder versnelt. Om de impact van bodemverzuring op arme zandgronden te verzachten wordt nu gewerkt aan het (her)introduceren van boomsoorten met goed verteerbaar strooisel in bossen. Bijmenging van strooisel met een hoog gehalte aan basische kationen (met name calcium, kalium, magnesium) heeft een positieve invloed op de pH van de bovengrond, evenals op de base-verzadiging en kan mogelijk de groei en vitaliteit van bomen positief beïnvloeden. Tot op heden is het effect van het introduceren van rijk-strooiselsoorten op de groei van bomen nauwelijks bestudeerd. In deze studie is het effect bestudeerd van de aanwezigheid van de rijk-strooiselsoort Amerikaanse vogelkers (Prunus serotina) op de groei van de zomereik (Quercus robur) met behulp van jaarringanalyse. Onze hypothese was dat de aanwezigheid van Prunus - door bodemverbetering - zowel de groei als de droogtegevoeligheid van Quercus positief zal beïnvloeden. We onderzochten de groei van naast elkaar voorkomende Quercus en Prunus bomen in zes bosgebieden op arme droge zandgronden in Nederland en Duitsland. We vergeleken de jaarringpatronen van eiken die naast vogelkersen groeiden (‘beïnvloed’) met die van eiken die te midden van andere eiken groeiden (‘niet beïnvloed’) en met die van de vogelkersen. Jaarringbreedtes werden geanalyseerd op (a) de gemiddelde jaarlijkse groeivariatie en grondvlakbijgroei; (b) de klimaatgevoeligheid van de jaarlijkse groei; (c) de groeirespons op geselecteerde droogtejaren; en (d) de patronen in groeivariatie in individuele bomen. De meeste analyses toonden aan dat er weinig verschillen zijn in groei tussen beïnvloede en niet-beïnvloede eiken. Op sommige locaties werd wel een verschil gevonden in de jaarlijkse fluctuaties in jaarringbreedtes tussen individuele beïnvloede en niet-beïnvloede eiken, maar dit leek eerder verband te houden met de ruimtelijke scheiding tussen de twee groepen op de betreffende locaties dan met rijk-strooiseleffecten. Over het algemeen geeft deze studie aan dat er geen duidelijk bewijs is voor een faciliterend effect van vogelkers op de groei van eiken. Mogelijk waren de bestudeerde locaties te arm om een effect van de verbeterde bodemgesteldheid op de groei van eiken te veroorzaken ef was vogelkers te kort aanwezig om een duidelijke invloed te hebben op de standplaatskwaliteit. Het is ook mogelijk dat mogelijk positieve effecten van bijmenging met vogelkers werden gemaskeerd door negatieve effecten als gevolg van concurrentie om water. We concluderen dat deze studie geen bewijs leverde dat de rijk-strooiselsoort Prunus serotina een positieve invloed heeft op de groei van Quercus robur op de bestudeerde zure arme zandgronden.
    Kan uitstel van houtoogst bijdragen aan CO2-mitigatie?
    Ouden, Jan den; Schelhaas, Mart-Jan ; Clerkx, Sandra ; Waal, Rein de; Lerink, Bas - \ 2020
    Wageningen : Wageningen Environmental Research (Wageningen Environmental Research rapport 2994) - 81
    Om na te gaan welk effect uitstel van oogst in bossen als beheermaatregel heeft in de klimaatmitigatie, zijn in 2018 en 2019 heropnamen gemaakt van de bosstructuur van onbeheerde bossen van douglas, lariks, beuk en grove den in bosreservaten. Hierbij is de bovengrondse bijgroei over een periode van dertig jaar gemeten, zijn koolstofbepalingen gedaan van de strooisellagen en bovengrond en zijn verteringssnelheden van dood hout bepaald. Hiermee is inzicht verkregen in de koolstofvastlegging wanneer oogst voor meerdere decennia wordt uitgesteld en vergeleken met beheerde bossen. Met de verzamelde informatie over de bijgroei van de bovengrondse biomassa in de bosreservaten, is met behulp van het model EFISCEN Space de koolstof in de biomassa gesimuleerd, opgeschaald naar het hele Nederlandse bos. Met het model CO2FIX zijn de koolstofvoorraad en de koolstofstromen in bosbiomassa, bodem en houtproducten gekwantificeerd.---In order to assess the role of postponing harvest in forests as a management option for climate mitigation, remeasurements were taken in 2018 and 2019 in unmanaged forest reserves containing douglas fir, larch, beech and Scots pine. This yielded information on the above-ground increment over a period of 30 years, carbon content in the litter layer and topsoil, and decomposition rates of dead wood. This produced insights in the carbon sequestration when timber harvesting is postponed for several decades and this was compared to managed forests. Based on the collected information on the increment of above-ground biomass in the forest reserves, the model EFISCEN Space was used to simulate carbon dynamics in the forest biomass in forest reserves, and the effects scaled up to the entire Dutch forest area. The carbon stocks and fluxes in forest biomass, trees and wood products were quantified using the model CO2FIX.
    Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe
    Streel, Géraud de; Ammer, Christian ; Annighöfer, Peter ; Barbeito, Ignacio ; Bielak, Kamil ; Bravo-Oviedo, Andrés ; Brazaitis, Gediminas ; Buraczyk, Włodzimierz ; Collet, Catherine ; Hurt, Vaclav ; Kurylyak, Viktor ; Ouden, Jan den; Pach, Maciej ; Pretzsch, Hans ; Skrzyszewski, Jerzy ; Sramek, Vit ; Stankevičiūtė, Jolanta ; Strelcova, Katarina ; Svoboda, Miroslav ; Verheyen, Kris ; Zlatanov, Tzvetan ; Ponette, Quentin - \ 2020
    Forest Ecology and Management 479 (2020). - ISSN 0378-1127
    Complementarity - Fagus sylvatica L. - Foliar nutrition - Pinus sylvestris L. - Species mixture

    Tree species-mixing has been suggested as one option to counteract the adverse effects of global change on tree mineral nutrition, yet the effect of mixing on nutrient availability remains poorly documented. We therefore analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and ilr balances (isometric log transformed ratios between elements or groups of elements) for 261 European beech and 248 Scots pine trees from 15 sites, each consisting of one beech-pine mixed stand and the respective monocultures, across a gradient of environmental conditions in Europe. We hypothesized an overall positive effect of mixing on tree foliar nutrient content, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models and multivariate linear regression models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether the species-mixing effect was related to site fertility. The nutrient composition of beech leaves and pine needles differed significantly for all ilr balances. For both species, significant mixing effects were detected for some nutrients and ilr balances; those effects, however, could not be consistently related to contrasted nutrient composition between species. For most nutrients and ilr balances, the mixing effect was influenced by the site nutritional status, but the pattern differed from expectation: absence or minor differences between monocultures and mixtures at the lower end of the chemical fertility gradient, and maximum differences in rich soils. The contrasting foliar nutrient composition of pine and beech trees and the site nutrient status only partly explained the mixing effects on tree mineral nutrition. Our results claim for a better understanding of nutrient-related mechanisms associated with complementarity and points towards the need to further expand the existing frameworks to account for the multivariate nature of tree nutrition.

    Litter share and clay content determine soil restoration effects of rich litter tree species in forests on acidified sandy soils
    Desie, Ellen ; Vancampenhout, Karen ; Berg, Leon van den; Nyssen, Bart ; Weijters, Maaike ; Ouden, Jan den; Muys, Bart - \ 2020
    Forest Ecology and Management 474 (2020). - ISSN 0378-1127
    Black cherry - Clay content - Litter quality - Nutrient cycling - Soil acidification - Soil restoration

    Many West-European forests are located on degraded and acidified soils. Soil acidification has resulted in hampered ecosystem functioning and lower delivery of ecosystem services. Forest management, particularly the choice of tree species, can accelerate or counteract soil acidification by the quality of litter input. The positive impact of so called ‘rich litter’ on the soil nutrient status and belowground ecosystem functioning has already been evidenced in common gardens. Here, we evaluate the effect of the rich litter species black cherry (Prunus serotina Ehrh.) in mixed forest stands dominated by pedunculate oak (Quercus robur L.). We study the effects using a replicated set-up of 10 established forest stands (age 30 to 90) in Belgium, the Netherlands and Germany along an edaphic gradient in sandy soils on Pleistocene aeolian deposits. We hypothesize that black cherry has a positive effect on the soil nutrient status and aim to answer the following research questions: (i) does admixture of black cherry increase soil pH and base saturation? (ii) what proportion of rich litter admixture is needed in a poor litter matrix to observe significant improvement of the soil nutrient status? and (iii) does the magnitude of the rich litter effect interact with initial soil properties? The results of this study indicate that admixture of black cherry enhances the forest floor turnover and enriches topsoil chemical conditions significantly. Thickness of the litter layer decreases from a mean of 7 cm under oak to a mean of 4.5 cm under cherry and correspondingly base saturation increases to a maximum of 25%, NO3 concentration to 26 mg/mg and organic matter content to 8%. However, large shares of rich litter admixture (>30% basal area) are needed to improve topsoil conditions. Moreover, we find that rich litter effects are more pronounced on sandy soils with higher fine particle (loam + clay) content. This suggests that the actual impact of restoration efforts in acidified forest soils is a product of the trinity “litter quality – litter share – site quality”.

    Effecten van hoefdieren op Natura 2000-boshabitattypen op de Veluwe
    Ouden, Jan den; Lammertsma, Denise ; Jansman, Hugh - \ 2020
    Wageningen : Wageningen Environmental Research (Wageningen Environmental Research rapport 3013) - 89
    De provincie Gelderland heeft gevraagd om nader inzicht te krijgen in de relatie tussen de populatiegroottes van hoefdieren en de kwaliteit van de belangrijkste Natura 2000-boshabitattypen op de Veluwe: de Beuken-eikenbossen met hulst (H9120) en de Oude eikenbossen (H9190). Kernvragen richtten zich op de maximale aantallen van verschillende hoefdiersoorten waarbij bosverjonging mogelijk blijft en welk wildbeheer gewenst is om Natura 2000-doelen in stand te houden. Het rapport bespreekt, op basis van een literatuurstudie, de invloed van hoefdieren op bosverjonging en het voorkomen van populaties (bijzondere) kwaliteitssoorten in de habitats. De nadruk ligt hierbij vooral op het edelhert, ree en wild zwijn. Het wildbeheer wordt uitgewerkt aan de hand van een aantal scenario’s, waarna aanbevelingen worden gedaan om de heersende wilddruk op de boshabitats te verlagen. Een belangrijke aanbeveling daarbij is om bij het vaststellen van de benodigde mate van ingrijpen in de populatiegroottes van hoefdieren, en bij het beoordelen van de effectiviteit van uitgevoerde maatregelen, zich niet alleen te baseren op aantalsschattingen, maar vooral te evalueren op basis van monitoring van ecologische effecten.
    Above- and Below-ground Cascading Effects of Wild Ungulates in Temperate Forests
    Ramirez, Ignacio ; Jansen, Patrick A. ; Ouden, Jan den; Moktan, Laxmi ; Herdoiza, Natalie ; Poorter, Lourens - \ 2020
    Ecosystems (2020). - ISSN 1432-9840
    decomposition - invertebrate - litter - mineralization - regeneration - rodent - soil quality - temperate forest - Ungulate

    Ungulates have become abundant in many temperate forests, shifting tree species composition by browsing and altering soil physical conditions by trampling. Whether these effects cascade down to other trophic levels and ecosystem processes is poorly understood. Here, we assess the paths through which ungulates have cascading effects on other trophic levels (regeneration, litter, invertebrates, rodents and organic matter decomposition). We compared ungulate effects by comparing 15 response variables related to different trophic levels between paired fenced and unfenced plots in twelve temperate forest sites across the Netherlands, and used pathway analysis model to identify the (in)direct pathways through which ungulates have influenced these variables. We found that plots with ungulates (that is, unfenced) compared to plots without (that is, fenced) had lower litter depth, sapling diversity, sapling density, rodent activity, macro-invertebrate biomass, decomposition rate of tea bags, pine and birch litter and higher soil compaction. These findings were used in a path analysis to establish potential causal relationships, which showed that ungulate presence: decreased sapling density, which indirectly decreased rodent activity; decreased litter depth, which indirectly reduced invertebrate diversity; increased soil compaction, which also decreased invertebrate diversity. Soil pH decreased invertebrate biomass, which also increased nitrogen mineralization. Yet, we did not find cascading effects of ungulates on decomposition rates. Importantly, an increase in ungulate abundance strengthens the cascading effects in this system. Our results suggest that ungulates can trigger cascading effects on lower trophic levels, yet decomposition and mineralization rates are resilient to ungulate browsing and trampling. Therefore, temperate forests conservation could benefit by limiting ungulate abundance.

    Assessment of workflow feature selection on forest LAI prediction with sentinel-2A MSI, landsat 7 ETM+ and Landsat 8 OLI
    Brede, Benjamin ; Verrelst, Jochem ; Gastellu-Etchegorry, Jean Philippe ; Clevers, Jan G.P.W. ; Goudzwaard, Leo ; Ouden, Jan den; Verbesselt, Jan ; Herold, Martin - \ 2020
    Remote Sensing 12 (2020)6. - ISSN 2072-4292
    Discrete anisotropic radiative transfer (DART) model - Forest - Leaf area index (LAI) - Machine learning - Sentinel-2 - Vegetation radiative transfer model

    The European Space Agency (ESA)'s Sentinel-2A (S2A) mission is providing time series that allow the characterisation of dynamic vegetation, especially when combined with the National Aeronautics and Space Administration (NASA)/United States Geological Survey (USGS) Landsat 7 (L7) and Landsat 8 (L8) missions. Hybrid retrieval workflows combining non-parametric Machine Learning Regression Algorithms (MLRAs) and vegetation Radiative Transfer Models (RTMs) were proposed as fast and accurate methods to infer biophysical parameters such as Leaf Area Index (LAI) from these data streams. However, the exact design of optimal retrieval workflows is rarely discussed. In this study, the impact of five retrieval workflow features on LAI prediction performance of MultiSpectral Instrument (MSI), Enhanced Thematic Mapper Plus (ETM+) and Operational Land Imager (OLI) observations was analysed over a Dutch beech forest site for a one-year period. The retrieval workflow features were the (1) addition of prior knowledge of leaf chemistry (two alternatives), (2) the choice of RTM (two alternatives), (3) the addition of Gaussian noise to RTM produced training data (four and five alternatives), (4) possibility of using Sun Zenith Angle (SZA) as an additional MLRA training feature (two alternatives), and (5) the choice of MLRA (six alternatives). The featureswere varied in a full grid resulting in 960 inversionmodels in order to find the overall impact on performance as well as possible interactions among the features. A combination of a Terrestrial Laser Scanning (TLS) time series with litter-trap derived LAI served as independent validation. The addition of absolute noise had the most significant impact on prediction performance. It improved the median prediction RootMean Square Error (RMSE) by 1.08m2m-2 when 5% noise was added compared to inversions with 0% absolute noise. The choice of the MLRA was second most important in terms of median prediction performance, which differed by 0.52m2m-2 between the best and worst model. The best inversion model achieved an RMSE of 0.91m2m-2 and explained 84.9% of the variance of the reference time series. The results underline the need to explicitly describe the used noise model in future studies. Similar studies should be conducted in other study areas, both forest and crop systems, in order to test the noise model as an integral part of hybrid retrieval workflows.

    Towards a new approach for dendroprovenancing pines in the Mediterranean Iberian Peninsula
    Akhmetzyanov, Linar ; Sánchez-Salguero, Raúl ; García-González, Ignacio ; Buras, Allan ; Dominguez-Delmás, Marta ; Mohren, Frits ; Ouden, Jan den; Sass-Klaassen, Ute - \ 2020
    Dendrochronologia 60 (2020). - ISSN 1125-7865
    Blue intensity - Drought - Elevational gradient - Pinus nigra - Pinus sylvestris - SPEI

    Dendroprovenancing studies frequently use site chronologies to identify the origin of archaeological and historical timber. However, radial growth (tree-ring width, TRW) of tree species is influenced by both local and regional climate scales. Here we investigate how the use of annually-resolved Blue Intensity (BI) measurements can enhance dendroprovenancing precision of black pine (Pinus nigra Arn.) and Scots pine (P. sylvestris L.) on the Iberian Peninsula. Principal Component Gradient Analyses (PCGA) was used to assess geographical patterns of annual variation in different TRW and BI proxies of pine trees from two mountain ranges in the Central System and Andalusia in Spain. Local climate-growth relationships were quantified to identify underlying causes of identified groups with diverse growth patterns. Two distinct elevational groups were observed when performing PCGA on latewood BI time series with the response to summer drought as the main factor causing the differences. Both P. nigra and P. sylvestris BI time series were found to be more related to summer drought at low-elevation sites showing an increase in sensitivity at lower latitudes. PCGA of TRW time series allowed to discriminate between trees from Andalusia and Central System within the elevation groups. February and October temperatures were found to be the main climatic factors causing the differences in TRW time series among the high- elevation sites, whereas for low-elevation trees it was the average winter temperature influencing TRW. A subsequent leave-one-out analyses confirmed that including latewood BI time series improves the precision of dendroprovenancing of pine wood in the Iberian Peninsula.

    Light availability and land-use history drive biodiversity and functional changes in forest herb layer communities
    Depauw, Leen ; Perring, Michael P. ; Landuyt, Dries ; Maes, Sybryn L. ; Blondeel, Haben ; Lombaerde, Emiel De; Brūmelis, Guntis ; Brunet, Jörg ; Closset-Kopp, Déborah ; Czerepko, Janusz ; Decocq, Guillaume ; Ouden, Jan den; Gawryś, Radosław ; Härdtle, Werner ; Hédl, Radim ; Heinken, Thilo ; Heinrichs, Steffi ; Jaroszewicz, Bogdan ; Kopecký, Martin ; Liepiņa, Ilze ; Macek, Martin ; Máliš, František ; Schmidt, Wolfgang ; Smart, Simon M. ; Ujházy, Karol ; Wulf, Monika ; Verheyen, Kris - \ 2020
    Journal of Ecology 108 (2020)4. - ISSN 0022-0477 - p. 1411 - 1425.
    atmospheric depositions - biodiversity measures - climate change - forest canopy features - functional signature - post-agricultural forests - resurvey

    A central challenge of today's ecological research is predicting how ecosystems will develop under future global change. Accurate predictions are complicated by (a) simultaneous effects of different drivers, such as climate change, nitrogen deposition and management changes; and (b) legacy effects from previous land use. We tested whether herb layer biodiversity (i.e. richness, Shannon diversity and evenness) and functional (i.e. herb cover, specific leaf area [SLA] and plant height) responses to environmental change drivers depended on land-use history. We used resurvey data from 192 plots across nineteen European temperate forest regions, with large spatial variability in environmental change factors. We tested for interactions between land-use history, distinguishing ancient and recent (i.e. post-agricultural) forests and four drivers: temperature, nitrogen deposition, and aridity at the regional scale and light dynamics at the plot-scale. Land-use history significantly modulated global change effects on the functional signature of the herb layer (i.e. cover, SLA and plant height). Light availability was the main environmental driver of change interacting with land-use history. We found greater herb cover and plant height decreases and SLA increases with decreasing light availability in ancient than in recent forests. Furthermore, we found greater decreases in herb cover with increased nitrogen deposition in ancient forests, whereas warming had the strongest decreasing effect on the herb cover in recent forests. Interactive effects between land-use history and global change on biodiversity were not found, but species evenness increased more in ancient than in recent forests. Synthesis. Our results demonstrate that land-use history should not be overlooked when predicting forest herb layer responses to global change. Moreover, we found that herb layer composition in semi-natural deciduous forests is mainly controlled by local canopy characteristics, regulating light levels at the forest floor, and much less by environmental changes at the regional scale (here: warming, nitrogen deposition and aridity). The observed disconnect between biodiversity and functional herb layer responses to environmental changes demonstrates the importance of assessing both types of responses to increase our understanding of the possible impact of global change on the herb layer.

    Plant functional trait response to environmental drivers across European temperate forest understorey communities
    Maes, S.L. ; Perring, M.P. ; Depauw, L. ; Bernhardt-Römermann, M. ; Blondeel, H. ; Brūmelis, G. ; Brunet, J. ; Decocq, G. ; Ouden, J. den; Govaert, S. ; Härdtle, W. ; Hédl, R. ; Heinken, T. ; Heinrichs, S. ; Hertzog, L. ; Jaroszewicz, B. ; Kirby, K. ; Kopecký, M. ; Landuyt, D. ; Máliš, F. ; Vanneste, T. ; Wulf, M. ; Verheyen, K. - \ 2020
    Plant Biology 22 (2020)3. - ISSN 1435-8603 - p. 410 - 424.
    Global environmental change - ground vegetation - herbaceous layer - plant–soil relations - regeneration - resource acquisition

    Functional traits respond to environmental drivers, hence evaluating trait-environment relationships across spatial environmental gradients can help to understand how multiple drivers influence plant communities. Global-change drivers such as changes in atmospheric nitrogen deposition occur worldwide, but affect community trait distributions at the local scale, where resources (e.g. light availability) and conditions (e.g. soil pH) also influence plant communities. We investigate how multiple environmental drivers affect community trait responses related to resource acquisition (plant height, specific leaf area (SLA), woodiness, and mycorrhizal status) and regeneration (seed mass, lateral spread) of European temperate deciduous forest understoreys. We sampled understorey communities and derived trait responses across spatial gradients of global-change drivers (temperature, precipitation, nitrogen deposition, and past land use), while integrating in-situ plot measurements on resources and conditions (soil type, Olsen phosphorus (P), Ellenberg soil moisture, light, litter mass, and litter quality). Among the global-change drivers, mean annual temperature strongly influenced traits related to resource acquisition. Higher temperatures were associated with taller understoreys producing leaves with lower SLA, and a higher proportional cover of woody and obligate mycorrhizal (OM) species. Communities in plots with higher Ellenberg soil moisture content had smaller seeds and lower proportional cover of woody and OM species. Finally, plots with thicker litter layers hosted taller understoreys with larger seeds and a higher proportional cover of OM species. Our findings suggest potential community shifts in temperate forest understoreys with global warming, and highlight the importance of local resources and conditions as well as global-change drivers for community trait variation.

    Drivers of above-ground understorey biomass and nutrient stocks in temperate deciduous forests
    Landuyt, Dries ; Maes, Sybryn L. ; Depauw, Leen ; Ampoorter, Evy ; Blondeel, Haben ; Perring, Michael P. ; Brūmelis, Guntis ; Brunet, Jörg ; Decocq, Guillaume ; Ouden, Jan den; Härdtle, Werner ; Hédl, Radim ; Heinken, Thilo ; Heinrichs, Steffi ; Jaroszewicz, Bogdan ; Kirby, Keith J. ; Kopecký, Martin ; Máliš, František ; Wulf, Monika ; Verheyen, Kris - \ 2020
    Journal of Ecology 108 (2020)3. - ISSN 0022-0477 - p. 982 - 997.
    ecosystem functioning - ground layer - herb layer - PhytoCalc - piecewise SEM - productivity

    The understorey in temperate forests can play an important functional role, depending on its biomass and functional characteristics. While it is known that local soil and stand characteristics largely determine the biomass of the understorey, less is known about the role of global change. Global change can directly affect understorey biomass, but also indirectly by modifying the overstorey, local resource availability and growing conditions at the forest floor. In this observational study across Europe, we aim at disentangling the impact of global-change drivers on understorey biomass and nutrient stocks, from the impact of overstorey characteristics and local site conditions. Using piecewise structural equation modelling, we determine the main drivers of understorey biomass and nutrient stocks in these forests and examine potential direct and indirect effects of global-change drivers. Tree cover, tree litter quality and differences in former land use were the main drivers of understorey biomass and nutrient stocks, via their influence on understorey light and nitrogen availability and soil acidity. Other global-change drivers, including climate and nitrogen deposition, had similar indirect effects, but these were either weak or only affecting nutrient concentrations, not stocks. Synthesis. We found that direct effects of global-change drivers on understorey biomass and nutrient stocks were absent. The indirect effects of global change, through influencing resource availability and growing conditions at the forest floor, were found to be less important than the effects of overstorey cover and composition. These results suggest that understorey biomass and nutrient stocks might respond less to global change in the presence of a dense overstorey, highlighting the buffering role of the overstorey in temperate forests.

    Litter quality and the law of the most limiting: Opportunities for restoring nutrient cycles in acidified forest soils
    Desie, Ellen ; Vancampenhout, Karen ; Nyssen, Bart ; Berg, Leon van den; Weijters, Maaike ; Duinen, Gert Jan van; Ouden, Jan den; Meerbeek, Koenraad Van; Muys, Bart - \ 2020
    Science of the Total Environment 699 (2020). - ISSN 0048-9697
    Base cations - C/N ratio - Litter quality - N deposition - Nutrient cycling - Rich litter species

    The adverse effects of soil acidification are extensive and may result in hampered ecosystem functioning. Admixture of tree species with nutrient rich litter has been proposed to restore acidified forest soils and improve forest vitality, productivity and resilience. However, it is common belief that litter effects are insufficiently functional for restoration of poorly buffered sandy soils. Therefore we examined the effect of leaf litter on the forest floor, soil chemistry and soil biota in temperate forest stands along a range of sandy soil types in Belgium, the Netherlands and Germany. Specifically, we address: i) Which tree litter properties contribute most to the mitigation of soil acidification effects and ii) Do rich litter species have the potential to improve the belowground nutrient status of poorly buffered, sandy soils? Our analysis using structural equation modelling shows that litter base cation concentration is the decisive trait for the dominating soil buffering mechanism in forests that are heavily influenced by atmospheric nitrogen (N) deposition. This is in contrast with studies in which leaf litter quality is summarized by C/N ratio. We suggest that the concept of rich litter is context dependent and should consider Liebig's law of the most limiting: if N is not limiting in the ecosystem, litter C/N becomes of low importance, while base cations (calcium, magnesium, potassium) become determining. We further find that on poorly buffered soils, tree species with rich litter induce fast nutrient cycling, sustain higher earthworm biomass and keep topsoil base saturation above a threshold of 30%. Hence, rich litter can trigger a regime shift to the exchange buffer domain in sandy soils. This highlights that admixing tree species with litter rich in base cations is a promising measure to remediate soil properties on acidified sandy soils that receive, or have received, high inputs of N via deposition.

    Data from: Patterns of nitrogen-fixing tree abundance in forests across Asia and America
    Menge, Duncan N.L. ; Chisholm, Ryan A. ; Davies, Stuart J. ; Abu Salim, Kamariah ; Allen, David ; Alvarez, Mauricio ; Bourg, Norm ; Brockelman, Warren Y. ; Bunyavejchewin, Sarayudh ; Butt, Nathalie ; Ouden, Jan den; Jansen, Patrick - \ 2019
    Dryad
    Determinants of plant community diversity and structure - Forest - Smithsonian ForestGEO - legume - symbiosis - nutrient limitation - nitrogen fixation
    Symbiotic nitrogen (N)‐fixing trees can provide large quantities of new N to ecosystems, but only if they are sufficiently abundant. The overall abundance and latitudinal abundance distributions of N‐fixing trees are well characterised in the Americas, but less well outside the Americas. Here, we characterised the abundance of N‐fixing trees in a network of forest plots spanning five continents, ~5,000 tree species and ~4 million trees. The majority of the plots (86%) were in America or Asia. In addition, we examined whether the observed pattern of abundance of N‐fixing trees was correlated with mean annual temperature and precipitation. Outside the tropics, N‐fixing trees were consistently rare in the forest plots we examined. Within the tropics, N‐fixing trees were abundant in American but not Asian forest plots (~7% versus ~1% of basal area and stems). This disparity was not explained by mean annual temperature or precipitation. Our finding of low N‐fixing tree abundance in the Asian tropics casts some doubt on recent high estimates of N fixation rates in this region, which do not account for disparities in N‐fixing tree abundance between the Asian and American tropics. Synthesis. Inputs of nitrogen to forests depend on symbiotic nitrogen fixation, which is constrained by the abundance of N‐fixing trees. By analysing a large dataset of ~4 million trees, we found that N‐fixing trees were consistently rare in the Asian tropics as well as across higher latitudes in Asia, America and Europe. The rarity of N‐fixing trees in the Asian tropics compared with the American tropics might stem from lower intrinsic N limitation in Asian tropical forests, although direct support for any mechanism is lacking. The paucity of N‐fixing trees throughout Asian forests suggests that N inputs to the Asian tropics might be lower than previously thought.
    Data from: Seasonal drivers of understorey temperature buffering in temperate deciduous forests across Europe
    Zellweger, Florian ; Coomes, David A. ; Lenoir, Jonathan ; Depauw, Leen ; Maes, Sybryn L. ; Kirby, Keith J. ; Brunet, Jörg ; Kopecký, Martin ; Máliš, František ; Schmidt, Wolfgang ; Heinrichs, Steffi ; Ouden, Jan den; Jaroszewicz, Bogdan ; Buyse, Gauthier ; Spicher, Fabien ; Wulf, Monika ; Verheyen, Kris ; Frenne, Pieter De - \ 2019
    Dryad
    macroclimate - Temperature Buffering - Global warming - Canopy Density - climate change - Forest Structure and Composition - understorey - Microclimate
    New tools for timber provenancing
    Akhmetzyanov, Linar - \ 2019
    Wageningen University. Promotor(en): G.M.J. Mohren; I. García González, co-promotor(en): U.G.W. Sass-Klaassen; J. den Ouden. - Wageningen : Wageningen University - ISBN 9789463951227 - 159

    Trees are affected by a set of spatially and/or temporarily varying biotic and abiotic factors. The effect of these factors is reflected in the width and structure of tree rings, which are widely used to study species ecology and their responses to climate. Moreover, tree-ring width time series are also used for dating and identification of the geographical origin of archaeological or historical timber – dendroprovenancing. The latter is usually done by statistical matching of tree-ring width sequence of a given timber to a network of regional and local chronologies from the same species or genera. However, despite multiple successful applications, this method also has clear pitfalls, such as lack of long reference tree-ring width chronologies from areas with intensive logging history; or possible strong teleconnections between reference chronologies over large distances leading to coarse-scale results. Therefore, to overcome these limitations and to improve dendroprovenancing results, other wood-anatomical features or chemical characteristics have to be used.

    The potential of additional variables retrieved from the wood has been recently tested and showed promising results for timber tracking, e.g. its chemical composition or DNA. However, until now time series of xylem-anatomical features have been largely ignored in timber-provenancing studies. This is most likely due to the time-consuming and labour-intensive data acquisition and challenging preparation of archaeological timber for precise wood-anatomical measurements. But with recent improvements in wood- preparation techniques and image-analysis software, it became possible to acquire a sufficient amount of data derived from various xylem-anatomical features in an efficient way.

    This thesis aims to evaluate the value of xylem-anatomical features as well as of archaeological DNA (aDNA) in addition to TRW for timber-provenancing studies. In the context of the ForSEAdiscovery project, the potential of vessel size of ring-porous oak, blue intensity variables derived from pine trees, and aDNA extracted from historical oak timber to enhance dendroprovenancing precision was assessed. Moreover, a conceptually new method, i.e. based on individual time-series rather than on average chronologies, was applied. This was done by means of Principal Component Gradient Analyses (PCGA). A description is provided for each of the chapters on their research and findings below.

    To assess the potential of oak earlywood vessels for timber provenancing, samples from nine oak stands in Northern Spain (Cantabria and the Basque country) were collected (Chapter 2). From this set of samples, vessel size and latewood-width time series were created. Based on variation in latewood-width time series, it was possible to differentiate between Cantabrian and Basque forest stands, i.e. in East-West direction. The difference in response to the summer temperature was found to be the main factor leading to such a differentiation. At the same time, variation in vessels size enabled the grouping of trees from the Basque country according to the continentality gradient, i.e. North-South direction, based on a difference in response of vessel size to winter and spring temperatures. These results suggest that the approach of combining latewood width with vessel size leads to a higher precision dendroprovenancing and pinpointing the origin of oak timber on a finer scale. Leave-one-out analyses confirmed this conclusion.

    In Chapter 3, the potential of wood-density related Blue Intensity (BI) variables in addition to TRW for dendroprovenancing pine timber in drought limited areas was evaluated. The network of the BI and TRW time series was created from six pine forests from Central Spain and Southern Spain. PCGA of the derived time series revealed a grouping of trees according to their elevation category based on the BI time series. However, it failed to group trees according to their geographical provenance both for the TRW and BI series. Trees were correctly assigned to their origin based on TRW, but only within the predefined by the BI elevation groups. Based on these results, it is possible to conclude that a multi-variable approach comprising of BI and TRW assists in enhancing pine-timber dendroprovenancing in dry areas.

    In Chapter 4, the potential of extracting and using aDNA for oak-timber provenancing on different scales was evaluated. Thirty samples from historical buildings from Spain, Latvia and Denmark were analysed. Two different extraction protocols in two genetic laboratories were tested. Furthermore, two different haplotype identification methods were applied. From 60% of the samples, at least one marker showed the presence of the aDNA with a varying percentage per extraction protocol. An existing haplotype distribution map was used to identify the potential source area of material from two study cases. The results suggest that genetic analyses have a strong potential for pinpointing timber origin, though until now only in combination with the TRW based method. Improvements in DNA extraction from degraded wood and amplification protocols are essential for future applications in dendroprovenancing studies.

    In Chapter 5, the main outcomes of each of the core chapters are discussed and contextualized in a broader perspective of dendroprovenancing. The potential of oak earlywood vessels in solving actual timber provenancing challenges in other regions in Europe is discussed. The added value of using the individual tree approach was tested and its strong potential in improving timber provenancing is confirmed. Based on the results derived from the core chapters, and from other studies conducted in the framework of the ForSEAdiscovery project on the same samples, a decision tree was created in order to facilitate oak and pine timber provenancing in the region.

    In conclusion, the multivariable approach has demonstrated a strong potential to enhance the precision of timber provenancing. However, for a successful application, specific climatic gradients were required for both genera (Chapters 2 & 3). Therefore, the decision on a suitable approach should be based on the specific provenancing conditions and should be comprised of multiple variables.

    Beslisboom Amerikaanse vogelkers : Bestrijden, uitfaseren, integreren en bos weerbaar maken
    Nyssen, B. ; Koopmans, G. ; Ouden, J. den - \ 2019
    Vakblad Natuur Bos Landschap 16 (2019)157. - ISSN 1572-7610 - p. 31 - 35.
    Gereedschapskist Klimaatslim Bos- en Natuurbeheer: handvatten voor beheerders
    Lerink, Bas ; Schelhaas, M. ; Nabuurs, G.J. ; Ouden, J. den; Boosten, Martijn ; Kremers, Julian - \ 2019
    Vakblad Natuur Bos Landschap 16 (2019)157. - ISSN 1572-7610 - p. 13 - 14.
    Gereedschapskist klimaatslim bos- en natuurbeheer biedt handvatten
    Lerink, Bas ; Schelhaas, M. ; Nabuurs, G.J. ; Ouden, J. den; Boosten, Martijn ; Kremers, Jasprina - \ 2019
    De Landeigenaar (2019)oktober. - ISSN 0166-5839 - p. 28 - 29.
    Wat is nodig om de doelen van het Klimaatakkoord te halen?
    Wat kan of moet een beheerder doen, wat kost dat en hoeveel CO2
    wordt er vastgelegd? Om deze vragen te kunnen beantwoorden is de
    online Gereedschapskist Klimaatslim Bos- en Natuurbeheer beschikbaar.
    Wild ungulates as forest engineers
    Ramírez Chiriboga, Juan Ignacio - \ 2019
    Wageningen University. Promotor(en): L. Poorter, co-promotor(en): J. den Ouden; P.A. Jansen. - Wageningen : Wageningen University - ISBN 9789463951074 - 159

    Earth’s surface is changing rapidly due to anthropogenic disturbance and thus threatening the conservation of entire biomes: from deserts to tropical rainforests and from oceans to mountains. Within this spectrum, forests are one of the biomes which has suffered the most due to the exploitation of natural resources and the expansion of the urban environment into natural landscapes. Specially the case of temperate forests which offer favourable conditions for a variety of economic activities, such as: food and timber production but as well for a blooming human life. The high concentration of humans in temperate forests has led to high indices of land conversion and fragmentation, threatening the conservation of the unique biodiversity harboured in these forests. For example, the loss of apex predators has led to the overabundance of wild ungulates in most of the temperate forests, increasing the top-down control that these animals have over vegetation.

    The top-down control of wild ungulates can influence several forest attributes such as forest diversity, structure and functioning. However, these effects are highly dependent on ungulate density, successional forest stage and the ungulate assemblage composition. Yet, there is a limited amount of field and experimental studies that elaborate on these important factors, mainly because there is a lack of long-term exclosures and a lack of studies that have evaluated the effect of a complete ungulate density gradient on forests in the same region. This thesis aims to evaluate the effects of wild ungulates on temperate forests at different scales. That is, how do wild ungulates affect forests at different 1) spatial scales (from areas with low to areas with high ungulate abundance)?, 2) temporal scales (from short-term to long-term effects)?, and 3) hierarchical scales (from trophic levels of primary producers to detritivores)? To tackle these questions, robust methods were designed in this thesis, such as a chronosequence of forest succession in presence and absence of ungulates, camera-traps paired to vegetation plots across a landscape, a global literature review on the effects of ungulate density on vegetation and litter bags placed inside and outside ungulate exclosures. Next, a description is provided for each of the chapters on their research and findings.

    To evaluate how ungulates affect different temperate forest attributes, a global quantitative literature review was conducted in chapter two. This meta-analysis provided a better understanding on how ungulate density interacts with forests at a global scale and identifies critical thresholds in ungulate density and tipping points in forests when the effect of ungulate density switches from neutral to negative in forests. From a total of 164 studies, ungulate density averaged 23.6 km-2 across studies. Ungulate density had negative effects on forest regeneration, structure and functioning in 70% of the evaluated cases. The dose-response curves had a sigmoidal shape. Critical tipping points, where ungulates started to have a negative effect on forest regeneration, were found at an ungulate metabolic weight density of 115 kg km-2 for forest regeneration, 141 kg km-2 for forest structure, and 251 kg km-2 for forest functioning, which is roughly equivalent to 10, 13 and 23 roe deer km-2. Forest regeneration was most sensitive to immediate browsing and trampling impacts of small seedlings, while forest functioning was least sensitive because of time lags. Yet, these effects may build-up over time.

    Chapter three on the short-term evaluated the shape of the dose-response for ten sites across at the Veluwe, the Netherlands, using approx. 210 camera points paired with vegetation plots. Five of the eleven forest variables measured were related to deer utilization level. With increasing red deer utilization, there was a decrease in litter depth. With increasing fallow deer utilization, there was a decrease in sapling richness. With increasing roe deer utilization, there was a decrease in sapling richness and diversity and shrub cover and an increase of sapling stem density. The dose-response curve between deer utilization and different forest attributes followed a curvilinear response, large changes at low followed by small changes at high utilization level, yet the exact shape of the curve can vary according to (a)biotic factors from each study location. Considering that the slopes of the responses were quite slight, it is possible to conclude that the influence that deer have on temperate forest structure and diversity is limited. Yet, these relationships may be different in a long-term study.

    To provide a better understanding of how forest succession proceeds in a situation of chronic browsing and trampling, chapter four evaluated the long-term effect of ungulates on temperate forests. A chronosequence approach was used, in which 17 paired fenced and unfenced plots were compared, ranging in age from 1 to 33 years since their establishment at the Veluwe. In fenced plots, where ungulates were excluded, there was a reduced understory vegetation cover and an increased canopy cover, tree species richness, tree Shannon diversity and litter layer compared to unfenced plots. In fenced compared to unfenced plots, woody vegetation developed with palatable broadleaved species such as Betula pendula, Betula pubescens, Prunus serotina, and Quercus robur. These results suggest that current ungulate densities in this system have pronounced long-term effects on forest structure, composition and litter depth, implying that ungulates can slow down natural succession of temperate forests, from light demanding to shade tolerant tree species. Yet, it remains unclear whether these long-term effects on vegetation trigger cascading effects on lower trophic levels.

    In chapter five, the potential cascading effects promoted by ungulates on temperate forests were evaluated. Using a network of twelve paired fenced and unfenced plots established across the Veluwe, soil quality, litter depth, forest regeneration, soil macro-invertebrates and decomposition rates were evaluated. Plots with ungulates had significantly higher soil compaction, but lower litter depth, tree diversity and density, rodent activity, invertebrate biomass and litter decomposition rate compared to plots with ungulates. Furthermore, ungulates presence: decreased sapling density which indirectly decreased rodent activity; decreased litter depth which indirectly reduced macro-invertebrate diversity and mineralization rates of magnesium and nitrogen in pine and oak litter, and increased soil compaction which decreased invertebrate diversity and mineralization rates of magnesium and nitrogen in pine and oak litter. If the system continues in the same direction, in the future, it will no longer sustain a diverse plant community due to nutrient limitation in soil.

    In chapter six, the main outcomes of each of the chapters were discussed and contextualized in a broader perspective of temperate forest ecology and plant-animal interactions. Although the literature review (Ch. 2) and the long-term effect chapter (Ch. 4) reported a strong top-down relationship between ungulates and different forest attributes, the short-term effect chapter (Ch. 3) and the cascading effect chapter (Ch. 5) only reported slight relationships. These results illustrate that current ungulate densities have a gradual shaping effect on forest composition, structure and functioning of these Dutch temperate forests; although, ungulates have a smaller effect on forest functioning than (a)biotic factors like soil pH, light availability and forest type (Ch. 3 and 4).

    Coming back to the main research question, ungulates are important disturbance agents that create, maintain or arrest forest dynamics. According to this thesis, ungulates mostly maintain forests properties, although they also arrest forest succession and in few cases, create opportunities for the establishment of new species. Spatially, forest dose-response to ungulate abundance is curvilinear, with large changes at low and small changes at high abundance. Temporally, ungulates have steeper relationships with forest vegetation at long-term scale compared to short-term because effects on vegetation accumulate in time. Hierarchically, ungulates cascade-down their effects to other trophic levels and ecosystem processes. Ungulate relationships with forests are difficult to understand due to the system’s complexity; yet, when their interactions occur at the extreme of the scales, their effects on forests can be irreversible.

    Beslisboom Amerikaanse vogelkers : Bestrijden, uitfaseren, integreren en bos weerbaar maken
    Nyssen, B. ; Koopmans, B. ; Ouden, J. den - \ 2019
    Provincie Gelderland - 55 p.
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