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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    Community ecology of Neotropical ticks, hosts, and associated pathogens
    Esser, Helen J. - \ 2017
    Wageningen University. Promotor(en): H.H.T. Prins; F.J.J.M. Bongers, co-promotor(en): P.A. Jansen. - Wageningen : Wageningen University - ISBN 9789463436908 - 200
    metastigmata - host specificity - host parasite relationships - biodiversity - species diversity - pathogens - size - community ecology - tickborne diseases - panama - tropics - hosts - metastigmata - gastheerspecificiteit - gastheer parasiet relaties - biodiversiteit - soortendiversiteit - pathogenen - grootte - gemeenschapsecologie - ziekten overgebracht door teken - panama - tropen - gastheren (dieren, mensen, planten)

    The ongoing loss of global biodiversity is unprecedented in both magnitude and pace, raising urgent questions as to how this loss will affect ecosystem functioning and human well-being. Control of infectious diseases has been proposed as an important ecosystem service that is likely to be affected by biodiversity loss. A negative relationship between biodiversity and disease risk could offer a win-win situation for nature conservation and human health. However, the generality of this relationship remains the subject of contentious debate. The aim of this thesis was to contribute to a better understanding of the interactions between ticks and their vertebrate hosts in a biodiversity hotspot, and how loss of biodiversity affects these interactions and ultimately, tick-borne disease risk. My study was unique in that I simultaneously considered and directly assessed broader communities of Neotropical wildlife, ticks, and tick-borne pathogens across an anthropogenic disturbance gradient.

    Determining whether and how biodiversity loss affects tick-borne disease risk in tropical forests requires a thorough understanding of tick-host associations, which are a function of tick-host specificity as well as host biological and ecological traits. In chapter 2, I therefore quantified the degree to which adult ticks are host-specific in my study region: Panama. Using quantitative network analyses and phylogenetic tools with null model comparisons, I found that the adult life stages of most tick species were specific to a limited number of host species that were phylogenetically closely related. In Chapter 4 I showed that species assemblages of adult ticks became increasingly diverse on larger-bodied host species, indicating that adult ticks in Panama tend to select for large reproduction hosts.

    In contrast to adult ticks, understanding the ecological interactions between immature ticks and their hosts in the tropics has long been hampered by a lack of morphological identification keys. Therefore, in Chapter 3, I describe the development of a DNA barcode reference library for the molecular identification of larvae and nymphs. This reference library was highly effective in species-level identification of immature ticks collected from birds (Chapter 3) and small mammals (Chapter 4 and 6). Several avian ecological traits were positively associated with tick parasitism, but the potential role of wild birds in tick-borne disease transmission seems to be limited in Panama. Immature ticks did not show any specificity to particular bird species or avian ecological traits (Chapter 3), and species assemblages of immatures ticks were equally diverse across a large number of host taxa (Chapter 4). This suggests that larvae and nymphs may feed more opportunistically than their adult counterparts.

    High host specificity in adult ticks implies high susceptibility to tick-host coextinction, even if immature ticks feed opportunistically. In chapter 5, I tested this hypothesis by surveying tick and vertebrate host communities across a forest fragmentation gradient. Forest fragments consisted of previously connected islands and peninsulas in the Panama Canal and ranged 1000-fold in size. Abundance and species richness of ticks was positively related to that of wildlife, which in turn was related to the size of the forest fragment. Specialist tick species were only present in fragments where their specific reproduction hosts were captured by camera traps. Further, less diverse tick communities were dominated by a generalist tick species. These results indicate that loss of wildlife had cascading effects on tick communities through local host-parasite coextinction.

    In Chapter 6, I studied how communities of wildlife, ticks, and tick-borne microbes changed along a more ‘typical’ disturbance gradient, in which forest fragments were embedded in an agricultural and sub-urban landscape, rather than surrounded by water. I found that wildlife community disassembly either diluted, amplified, or had no effect on infection prevalence in ticks, depending on the pathogen and degree of disturbance. However, hyperabundance of medium- to large-sized frugivores and herbivores (important reproduction hosts for adult ticks) in sites that lacked apex predators was related to exponential increases in tick density, negating any effect of reduced pathogen prevalence. Moreover, high tick species richness in these sites was related to high microbial and pathogen richness. High parasite diversity is thus a source of infectious diseases. When medium- to large-sized frugivores and herbivores also disappeared, densities of infected ticks declined, suggesting a non-linear relationship between biodiversity loss and tick-borne disease risk, in which initial loss of apex predators increases disease risk, but further loss of species decreases disease risk again.

    In this thesis, I have quantified host-feeding relationships of adult and immature Neotropical ticks, many of which (in the case of larvae and nymphs) were largely unknown. I have shown that adult ticks tend to be highly host-specific, particularly to larger-bodied vertebrates, whereas immature ticks appear to have broader host-use patterns. I found that ticks are susceptible to local host-tick coextirpation, and that the relationship between biodiversity loss and tick-borne disease risk is non-linear. My results emphasize the importance of directly assessing host community composition and suggest that the presence of specific (reproduction) hosts are a more important factor than species richness per se for tick population and tick-borne disease dynamics.

    The contribution of sustainable trade to the conservation of natural capital
    Oorschot, M. van; Wentink, Carsten ; Kok, Marcel ; Beukering, P. ; Kuik, O. ; Drunen, M. van; Berg, J. van den; Ingram, V.J. ; Judge, L.O. ; Arets, E.J.M.M. ; Veneklaas, E.J. - \ 2016
    The Hague : PBL: Netherlands Environmental Assessment Agency (PBL publication 1700) - 96
    certification - sustainability - cost benefit analysis - resource conservation - natural resources - tropics - ecosystem services - biobased economy - certificering - duurzaamheid (sustainability) - kosten-batenanalyse - hulpbronnenbehoud - natuurlijke hulpbronnen - tropen - ecosysteemdiensten - biobased economy
    PBL Netherlands Environmental Assessment Agency has conducted a study into the potential impact of certified sustainable production on natural capital and the related ecosystem goods and services. Forests are a well-known example of natural capital; they are valuable to society, among other things because they store large amounts of carbon. The performed cost-benefit analyses show that certified resource production has several societal benefits, such as reductions in environmental pollution, soil erosion and health damage. However, for resource producers, the financial returns of more sustainable production methods are often limited. The uneven distribution of costs and benefits over public and private actors forms a barrier to any further scale up of sustainable production. Thus, there is a need for additional solutions, besides certifying trade to help conserve ecosystems elsewhere in the world.
    Integrating ecosystem services into the tropical commodity value chain : cocoa, soy and palm oil
    Berg, J. van den; Ingram, V.J. ; Judge, L.O. ; Arets, E.J.M.M. - \ 2014
    Wageningen : Statutory Research Tasks Unit for Nature & the Environment (WOT Natuur & Milieu) (WOt-technical report 6) - 101
    ecosysteemdiensten - theobroma cacao - cacao - glycine soja - palmoliën - waardeketenanalyse - basisproducten - duurzaamheid (sustainability) - tropen - innovaties - ecosystem services - theobroma cacao - cocoa - glycine soja - palm oils - value chain analysis - commodities - sustainability - tropics - innovations
    This technical report explores the governance options available to the Dutch government to promote the sustainable use and maintenance of ecosystem services in tropical commodity value chains with Dutch links. It examines how ecosystem services can be given a more explicit role in public and market mechanisms, using the cocoa, soy, palm oil and timber chains as case studies. The document presents a discourse analysis of the way Dutch policies and practice address ecosystem services, updating the report of a forerunner study on the timber chain (Van den Berg et al. 2013). The discourse analysis indicates that the term ecosystem services still lacks a clear definition in Dutch policy, with ecosystem services largely being seen as an economic issue, which can be solved by market drive, voluntary and multi-actor value chain based solutions. The report presents results of a detailed examination of specific cases of innovation in sustainability initiatives and payments for ecosystem services projects in the cocoa chain, the Round Table for Responsible Soy (RTRS) and the Round Table for Sustainable Palm Oil (RSPO). Lessons learnt from the analysis of these cases include the need to simplify what is meant by ecosystem services – for example using the term natural capital - to make it more appealing and intuitive, particularly for business. More evidence is needed on the impact of certification and how it maintains or enhances ecosystem services. Internationally agreed impact indicators are also recommended. The array of available certification schemes could be harmonised. A mix of policy instruments appears to offer more scope for the government, using market based ‘carrots’ and incentive-based ‘sticks’ (such as tax incentives and pilot projects) to stimulate new partnerships and initiatives. Challenges include giving ecosystem services an explicit role in policysupported innovations, and engaging with all value chain stakeholders, particularly community and consumer organisations.
    Environmental and physiological drivers of tree growth : a pan-tropical study of stable isotopes in tree rings
    Sleen, J.P. van der - \ 2014
    Wageningen University. Promotor(en): Pieter Zuidema; Frans Bongers; Niels Anten. - Wageningen : Wageningen University - ISBN 9789461739544 - 174
    bomen - groei - plantenfysiologie - jaarringen - milieueffect - isotopen - tropische bossen - tropen - trees - growth - plant physiology - growth rings - environmental impact - isotopes - tropical forests - tropics

    Forests in the wet tropics harbour an incredible biodiversity, provide many ecosystem services and regulate climatic conditions on regional scales. Tropical forests are also a major component of the global carbon cycle, storing 25% of the total terrestrial carbon and accounting for a third of net primary production. This means that changes in forest structure and forest cover in the wet tropics will not only affect biodiversity and ecosystem services, but also have implications for the global carbon cycle and – as a result – may speed up or slow down global warming. Deforestation rates are still high in the tropics and have profoundly affected the extent of forests in many countries. Additionally, there are indications that undisturbed and pristine tropical forests are changing. The most notable changes found by the monitoring of permanent forest plots are an increase of tree growth and forest biomass per unit of surface area over the last decades. If this is indeed the case, it would entail that the world’s tropical forests are potentially absorbing a significant fraction of human caused CO2emissions and as such are mitigating global warming. However, increased tree growth and forest biomass have not been found in all studies. Furthermore, it is unknown whether the observed changes in intact forests are part of a long-term change, or merely reflect decadal scale fluctuations. These uncertainties lead to an ongoing debate on whether tree growth and forest biomass have increased in tropical forests and – if so – to what extent. In addition, there is also a scientific discussion on the factor(s) that could underlie the potential changes in tree growth and forest biomass. Possibly, they are caused by an internal driver, like the lasting effect of large scale disturbances in the past, or by external drivers. Possible external factors affecting tropical forest dynamics are (1) climate change (temperature and precipitation), (2) increased nutrient depositions and (3) increased atmospheric CO2concentration.

    In this thesis, I investigated the environmental changes that could have formed the basis for changes in tropical tree growth. I used two relatively new tools in tropical forest ecology: tree-ring measurements and stable isotope analyses. Tree-ring widths were measured to obtain long-term information on tree growth. Stable isotopes in the wood of tree rings were analysed to provide information on the environmental and physiological drivers of tree growth changes. This thesis is part of a larger project on the long-term changes in intact forests in the wet tropics (the TroFoClim project, led by Pieter Zuidema) and also includes the PhD theses of Mart Vlam and Peter Groenendijk. In this project, ~1400 trees of 15 species were examined that were collected in three forest sites distributed across the tropics (in Bolivia, Cameroon and Thailand).

    For the assessment of long-term changes in growth and stable isotopes, it is important to understand shorter term fluctuations due to forest dynamics (i.e. gap formation), because these interfere with changes on a longer temporal scale. The formation of a gap in a closed canopy forest, after the death of a tree, can cause considerable environmental changes in the surrounding area, e.g. in light, nutrient and water availability. This can strongly affect the growth rates of the remaining trees. However, in most studies the environmental drivers of changes in tree growth after gap formation are not considered. In CHAPTER 2 I measured carbon isotope discrimination (Δ13C) in annual growth rings of Peltogyne cf.heterophylla, from a moist forest in North-eastern Bolivia, and evaluated the environmental drivers of growth responses after gap formation. Growth and Δ13C was compared between the seven years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10m) to single tree-fall gaps; the other half grew more than 40 m away from gaps (control trees). I found that increased growth was mainly associated with decreased Δ13C suggesting that this response was driven by increased light availability and not by improved water availability. Interestingly, most small trees did not show a growth stimulation after gap formation. This result was hypothesized to be caused by an increased drought stress. However, the measurement of Δ13C showed that increased water stress is unlikely the cause for the absence of increased growth, but rather suggested that light conditions had not improved after gap formation. These results show that combining growth rates with changes in Δ13Cis a valuable tool to better understand the causes of temporal variation in tree growth.

    An important potential driver of long-term changes in tree growth is climate change, e.g. global warming and altered annual precipitation. To understand the effect of climate change on tree growth, the availability of reliable data on historical climate is off course crucial. For the study areas in Bolivia and Thailand, previous studies have investigated the occurrence of temporal trends in temperature and precipitation. For the study area in Cameroon however, as well as for West and Central Africa in general, the availability of instrumental climate data is very restricted. This limits the possibility to relate past climatic variation to changes in tree growth and calls for proxies that allow reconstruction of past climatic conditions. In CHAPTER 3 I assessed the potential use of stable isotopes of oxygen (δ18O) in tree rings as a tool for the reconstruction of precipitation in tropical Africa. I measured δ18O in tree rings of five large Entandrophragmautiletreesfrom North-western Cameroon. A significant negative correlation was found between annual tree-ring δ18O values (averaged over the five individuals) and annual precipitation amount during 1930-2009 in large areas of West and Central Africa. I also found tree-ring δ18O to track sea surface temperatures (SST) in the Gulf of Guinea (1930-2009). These two results are related because rainfall variabilityin West and Central Africa is profoundly influenced by the SST of the tropical AtlanticOcean. Thus a high SST in the Gulf of Guinea is associated with high precipitation over large parts of West and Central Africa and recorded in tree rings by a relatively low δ18O value. On the other hand, dry years when SST is low, are recorded by relatively high tree-ring δ18O values. I also found a significant long-term increase of tree-ring δ18O values. This trend seems to be caused by lowered precipitation from 1970 to 1990 (the Sahel drought period). From 1860 to 1970, no significant long-term trend was observed in tree-ring δ18O values, suggesting no substantial change in precipitation amount occurred over this period.

    Another potential driver of altered tree growth and biomass in intact tropical forests is the increase of anthropogenic nutrient depositions, especially nitrogen. The deposition of nitrogen has likely risen due to an increased industrialization and use of artificial N fertilizers in most tropical countries. Nitrogen can stimulate plant growth, as is well known from the positive effect of N fertilizer application on crop yields. Previous studies have shown that the stable isotope ratio of nitrogen (δ15N) increased during the last decennia in the wood of trees from Brazil and Thailand as well as in tree leaves from Panama. This increased δ15N has been interpreted as a signal that tropical nitrogen cycles have become more ‘open’ and ‘leaky’ during the last decades in response to increased anthropogenic nitrogen depositions. The underlying mechanism is that high rates of nitrogen deposition and high ambient nitrogen availability lead to an increased nitrification. This process can cause a gradual 15N-enrichment of soil nitrogen. In CHAPTER 4 I analysed changes in tree-ring δ15N values of 400 trees of six species from the three study sites. In the trees from Cameroon no long-term change of tree-ring δ15N values was found (1850-2005), even though NH3and NOxemissions seem to have increased strongly around the study area since 1970. Possibly, the very high precipitation at that site causes the local nitrogen cycle to be already very ‘leaky’, limiting the effect of additional nitrogen input on the δ15N signature of soil nitrogen. Alternatively, nitrogen input in this forest might be much lower than reconstructed NH3and NOxemissions suggest. For the study site in Bolivia, no significant change of tree-ring δ15N values was found (1875-2005), which is in line with the expected result for areas with a low anthropogenic nitrogen input. I found a marginally significant increased of δ15N values since 1950 in trees from Thailand, which confirms previous observations. This points to an effect of increased anthropogenic nitrogen deposition, which could have stimulated photosynthetic rates, if indeed nitrogen was limiting tree growth.

    The most often hypothesized factor to cause a long-term increase of tree growth is the rise of atmospheric CO2. Since the onset of the industrial revolution (~1850) global atmospheric CO2concentration has increased by 40%. Elevated CO2can directly affect plants by increasing the activity, as well as the efficiency, of the CO2fixing enzyme rubisco and thereby increase photosynthetic rates. Potentially more important in plant communities subjected to periods of limited water availability (like a dry season) is that elevated CO2 can cause a reduction of stomatal conductance, which lowers evapo-transpiration and hence reduces water losses.This increases water-use efficiency (i.e. the amount of carbon gained through photosynthesis divided by the amount of water loss through transpiration) and might allow plants to extend their growth season and/or increase their photosynthetic activity during the hottest hours of the day when water-stress might be severe. Elevated atmospheric CO2is thus a very likely candidate to have stimulated tropical tree growth (also referred to as CO2fertilization), provided at least that plant growth was either carbon or water limited. In CHAPTER 5 I tested the CO2fertilization hypothesis by analysing growth-ring data of 1100 trees from the three study sites. The measurement of tree-ring widths allowed an assessment of historical growth rates, whereas stable carbon isotopes (δ13C) in the wood of the trees were used to obtain an estimate of the CO2concentration in the intercellular spaces in leaves (Ci) and of water-use efficiency (intrinsic water-use efficiency; iWUE). I used a sampling method that controls for ontogenetic (i.e. size developmental) changes in growth and δ13C. With this method, trees were compared across a fixed diameter (i.e. same ontogenetic stage). I chose two diameters: 8 cm (referring to small understorey trees) and 27 cm (referring to larger canopy trees). A mixed-effect model revealeda highly significant and exponential increase of Ciat each of the three sites, and in both understorey and canopy trees. Over the last 150 years Ciincreased by 43% and 53% for understorey and canopy trees respectively. Yet, the rate of increase in Ciwas consistently lower than that of atmospheric CO2. This ‘active’ response to elevated atmospheric CO2resulted in a significant and large increase of iWUE. Over the last 150 years, iWUE increased by 30% and 35% for understorey and canopy trees.A long-term increase of iWUE indicates either a proportional increase of net photosynthesis and/or a decrease of stomatal conductance and thus transpiration, both of which could have stimulated biomass growth. However, I found no increase of tree growth over the last 150 years in any of the sites. Although there are several possible explanations for these findings, I argue that it is most likely that tropical tree growth is generally not limited by water and carbon, but by a persistent nutrient limitation (e.g. of phosphates) and that this has prevented tropical trees to use the extra CO2for an acceleration of growth.

    In this thesis I have studied environmental and physiological drivers of tree growth changes. I found evidence of decreased precipitation over the last decades at the study site in Cameroon (CHAPTER 3), a changed nitrogen cycle at the study site in Thailand (CHAPTER 4) and an overall change in the physiology of all tree species studied (increased iWUE; CHAPTER 5). One of the main findings of this thesis is however, that these changes have not led to a net change of tree growth over the last 150 years (CHAPTER 5). This is an important finding that could have two major implications. Firstly, the absence of a long-term growth stimulation suggests that the increase of iWUE is mainly driven by a reduced stomatal conductance, which likely leads to a reduced evaporative water loss. If trees across the tropics are reducing evapo-transpiration, this will change affect hydrological cycles, e.g. leading to a lower humidity, higher air temperatures and a reduced precipitation. Secondly, the absence of a growth stimulation over the last 150 years suggests that the carbon sink capacity of tropical forests is currently overestimated, e.g. by Dynamic Global Vegetation Models, which assume strong CO2fertilization effects and as such a high capacity of tropical forests to mitigate global warming. I anticipate that the planned Free Air Concentration Enrichment (FACE) experiments in the tropics will shed light on the reasons why increased CO2does not stimulate the growth rates of tropical trees. Furthermore, I argue that combining tree-ring measurements and stable isotope analyses together with permanent plot research is the most promising way to increase our understanding of the changes in tropical forests.

    Interest grows for Dutch mid-tech and low-tech greenhouse technology : A greenhouse to suit all tropical conditions (interview with Anne Elings)
    Kierkels, T. ; Elings, A. - \ 2014
    In Greenhouses : the international magazine for greenhouse growers 3 (2014)2. - ISSN 2215-0633 - p. 28 - 29.
    glastuinbouw - kassen - bouwtechnologie - bouwconstructie - teelt onder bescherming - tropen - ontwerp - maleisië - materialen - aangepaste technologie - greenhouse horticulture - greenhouses - construction technology - building construction - protected cultivation - tropics - design - malaysia - materials - appropriate technology
    The Netherlands sets the standard for high-tech greenhouses worldwide. But increasingly suppliers are looking too at possibilities within the mid-tech and even the low-tech market segments. The Dutch government is supporting demonstration projects, for example in Mexico, East Africa and Malaysia. Technically it’s all going well.
    Nederland krijgt belangstelling voor mid tech en low tech kassenbouw (interview met Anne Elings)
    Kierkels, T. ; Elings, A. - \ 2013
    Onder Glas 10 (2013)10. - p. 34 - 35.
    glastuinbouw - kassen - bouwtechnologie - bouwconstructie - teelt onder bescherming - tropen - ontwerp - maleisië - materialen - aangepaste technologie - greenhouse horticulture - greenhouses - construction technology - building construction - protected cultivation - tropics - design - malaysia - materials - appropriate technology
    Nederland zet de toon in high tech kassenbouw over de hele wereld. Maar in toenemende mate kijken de toeleveringsbedrijven ook naar mogelijkheden binnen het mid tech en zelfs het low tech segment. De Nederlandse overheid ondersteunt demonstratieprojecten in bijvoorbeeld Mexico, Oost-Afrika en Maleisië. Technisch lukt het allemaal goed.
    Runoff, discharge and flood occurrence in a poorly gauged tropical basin : the Mahakam River, Kalimantan
    Hidayat, H. - \ 2013
    Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Ton Hoitink. - S.l. : s.n. - ISBN 9789461737434 - 114
    oppervlakkige afvoer - afvoer - overstromingen - monitoring - tropen - modellen - rivieren - cartografie - voorspelling - kalimantan - indonesië - runoff - discharge - floods - monitoring - tropics - models - rivers - mapping - prediction - kalimantan - indonesia

    Tidal rivers and lowland wetlands present a transition region where the interests of hydrologists and physical oceanographers overlap. Physical oceanographers tend to simplify river hydrology, by often assuming a constant river discharge when studying estuarine dynamics. Hydrologists, in turn, generally ignore the direct or indirect effects of tides in water level and discharge records. This thesis aims to improve methods to monitor, model and predict discharge dynamics in tidal rivers and lowland wetlands, by focussing on the central and lower reaches of the River Mahakam (East Kalimantan, Indonesia), and the surrounding lakes area. The 980-km long river drains an area of about 77100 km2 between 2°N - 1°S and 113°E - 118°E. Due to its very mild bottom slope, a significant tidal influence occurs in this river. The middle reach of the river is located in a subsiding basin, parts of which are below mean sealevel, featuring peat swamps and about thirty lakes connected to the river via tie channels.

    Upstream of the lakes area, at about 300 km from the river mouth, an acoustic Doppler current profiler (H-ADCP) has been horizontally deployed at a station near the city of Melak (Chapter 2). The H-ADCP profiles of velocity are converted to discharge adopting a new calibration methodology. The obtained time-series of discharge show the tidal signal is clearly visible during low flow conditions. Besides tidal signatures, the discharge series show influences by variable backwater effects from the lakes, tributaries and floodplain ponds. The discharge rate at the station exceeds 3250 m3s-1 with a hysteretic behaviour. For a specific river stage, the discharge range can be as high as 2000 m3s-1. Analysis of alternative types of rating curves shows this is far beyond what can be explained from kinematic wave dynamics. Apart from backwater effects, the large variation of discharge for a specified river stage can be explained by river-tide interaction, impacting discharge variation especially in the fortnightly frequency band.

    A second H-ADCP station has been setup in the lower reach of the Mahakam, near the city of Samarinda, where the tidal discharge amplitude generally exceeds the discharge related to runoff (Chapter 3). Conventional rating curve techniques are inappropriate to model river discharge at this tidally influenced station. As an alternative, an artificial neural network (ANN) model is developed to investigate the degree to which tidal river discharge at Samarinda station can be predicted from an array of level gauge measurements along the tidal river, and from tidal level predictions at sea. The ANN-based model produces a good discharge estimation, as established from a consistent performance during both the training and the validation periods, showing the discharges can be predicted from water levels only, once that a trained model is available. The ANN models perform well in predicting discharges up to two days in advance.

    Chapter 4 addresses the role of backwater effects and tidal influences on discharge time-series used to calibrate a rainfall-runoff model. The HBV rainfall-runoff model is implemented for the Mahakam sub-catchment upstream of Melak (25700 km2). In a first approach, the model is calibrated using a discharge series derived from the H-ADCP measurements from Melak station. In a second approach, discharge estimates derived from a rating curve are used to calibrate the model. Adopting the first approach, a comparatively low model efficiency is obtained, which is attributed to the backwater and tidal effects that are not captured in the model. The second approach produces a relatively higher model efficiency, since the rating curve filters the backwater effects out of the discharge series. Seasonal variation of terms in the water balance is not affected by the choice for one of the two calibration strategies, which shows that backwaters do not have a systematic seasonal effect on the river discharge.

    To allow for investigation of the causes of backwater effects, satellite radar remote sensing is employed to monitor water levels in wetlands (Chapter 5). A series of Phased Array L-band Synthetic Aperture Radar (PALSAR) images is used to observe the dynamics of the Mahakam River floodplain. To analyze radar backscatter behavior for different land cover types, several regions of interest are selected, based on land cover classes. Medium shrub, high shrub, fern/grass, and degraded forest are found to be sensitive to flooding, whereas peat forest, riverine forest and tree plantation backscatter signatures only slightly change with flood inundation. An analysis of the relationship between radar backscatter and water levels is carried out. For lakes and shrub covered peatland, for which the range of water level variation is high, a good water level-backscatter correlation is obtained. In peat forest covered peatland, subject to a small range of water level variation, water level-backscatter correlations are poor, limiting the ability to obtain a floodplain-wide water surface topography from the radar images.

    Chapter 6 continues to investigate the degree in which satellite radar remote sensing can serve to distinguish between dry areas and wetlands, which is a difficult task in densely vegetated areas such as peat domes. Flood extent and flood occurrence information are successfully extracted from a series of radar images of the middle Mahakam lowland area. A fully inundated region is easily recognized from a dark signature on radar images. Open water flood occurrence is mapped using a threshold value taken from radar backscatter of the permanently inundated areas. Radar backscatter intensity analysis of the vegetated floodplain area reveals consistently higher backscatter values, indicating flood inundation under forest canopy. Those observations are used to establish thresholds for flood occurrence mapping in the vegetated area. An all-encompassing flood occurrence map is obtained by combining the flood occurrence maps for areas with and without vegetation.

    Chapter 7 synthesizes the findings from the previous chapters. It is concluded that the backwater effects and subtle tidal influences may prevent the option to predict river discharge using rating curves, which can best be interpreted as a stage-runoff relationship. H-ADCPs offer a promising alternative to monitor river discharge. For a tidal river, an ANN model can be used as a tool for data gap filling in an H-ADCP based discharge series, or even to derive discharge estimates solely from water levels and water level predictions. Discharge can be predicted several time-steps ahead, allowing water managers to take measures based on forecasts. The stage-runoff relationship derived from a continuous series of H-ADCP based discharge estimates may be expected to be much more accurate than a similar rating curve derived from a small number of boat surveys. The flood occurrence map derived from PALSAR images can offer a detailed insight into the hydroperiod, the period in which a soil area is waterlogged, and flood extent of the lowland area, illustrating the added value of radar remote sensing to wetland hydrological studies. In future work, radar-based floodplain observations may serve to calibrate hydrodynamic models simulating the processes of flooding and emptying of the lakes area.

    Linking land-use intensification, plant communities, and ecosystem processes in lowland Bolivia
    Carreno Rocabado, I.G. - \ 2013
    Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Lourens Poorter; Marielos Pena Claros. - [S.l.] : s.n. - ISBN 9789461735331 - 163
    landgebruik - intensivering - plantengemeenschappen - ecosystemen - laaglandgebieden - biodiversiteit - tropen - soortendiversiteit - strooisel - decompositie - ecologie - bolivia - land use - intensification - plant communities - ecosystems - lowland areas - biodiversity - tropics - species diversity - litter (plant) - decomposition - ecology - bolivia

    Land-use intensification (LUI) is one of the main global drivers of biodiversity loss with negative impact on ecosystem processes and the services that societies derive from the ecosystems. The effect of LUI on ecosystem processes can be direct through changes in environmental conditions and indirect through changes in plant community. In this dissertation I explored the mechanisms through which land-use intensification affects plant community assembly and ecosystem processes in the Bolivian lowland tropics. Specifically I evaluated: 1) how plant communities respond to LUI via plant response traits, 2) the effects of plant communities on decomposition via their effect traits, and 3) the relative importance of direct and indirect pathways in explaining LUI effects on ecosystem processes.

    I used two gradients of LUI, a long gradient, including five common and contrasting land use types (mature forest, logged forest, secondary forest, agricultural land, and pastureland), and a short gradient of disturbance intensity represented by four experimental treatments in managed forest (unlogged forest, and forest subject to one of three levels of logging intensity and application of silvicultural treatments). Plant community response and effect were evaluated based on species diversity and functional properties. I measured for the most dominant species 12 functional traits and 14 litter traits.

    Both gradients of LUI affected functional properties of the plant communities. An increase in LUI shifted plant communities from species characterized by slow growth and slow returns on resource investment (conservative species), toward species characterized by fast growth and fast returns on resources investment (acquisitive species). However, communities with an intermediate position along the LUI gradient (i.e., secondary forests) showed dominance of conservative species mainly due to land use management (abundance of palm species due to frequent burning). Along the short gradient of LUI demographic processes mediated the changes plant communities. With and increase in disturbance caused by logging and silvicultural treatments, there was an increased recruitment of individuals with more acquisitive trait values. Moreover, the response of functional diversity differed between both LUI gradients. Whereas functional diversity decreased along the long LUI gradient, it did not change along the short LUI gradient. Communities with an intermediate position along the long LUI gradient showed higher functional diversity than communities at the extremes of the gradient. Whereas both environmental and management filters drove changes in plant communities along a long LUI gradient, changes along a short LUI gradient were mainly driven by environmental filters.

    LUI affected litter decomposition through changes in environmental conditions and through changes in plant communities. With an increase in LUI decompositionpotential (measured as mass loss of standard litter incubated in all land use types) decreased. Since soil properties only weakly affected decomposition, other factors were probably the main drivers of the direct effects of LUI on decomposition potential. With increasing LUI the litter decomposability increased due to changes in litter quality produced by plant communities; litter from mature- and logged forest had low decomposability, litter from secondary forest had an intermediate decomposability, and litter from agricultural land and pastureland had high decomposability. Functional traits, such as leaf N concentration, specific leaf area and leaf chlorophyll content, were good and positive predictors of decomposition rate. Although experimentally litter quality explained more variation in decomposition rate across the long LUI gradient (48%) than environmental site characteristics (17%), the actual decomposition rate (in-situ decomposition of litter community into its own land use type) was site-dependent, and determined by both drivers that partlycompensated each other. Thus, litter with high decomposability (litter from pastureland) incubated in the land use type with low decomposition potential (pastureland plot) had generally a similar decomposition rate as litter with low decomposability (litter from mature forest) incubated in the land use type with high decomposition potential (mature forest plot).

    Tropical ecosystems are not only very diverse in species, they are also diverse in their responses to human disturbance. I concluded that LUI has important effects on plant community properties and ecosystem processes. These effects, however, contrast with some predictions of current ecological theory. High intensification of land use does not necessarily lead to low plant functional diversity, and less favourable environmental conditions for decomposition do not necessarily lead to low decomposition rates. Instead, the multiple factors related with management decisions at local scales cause a large heterogeneity of ecosystem responses. Consequently, depending on the management decisions taken, the negative effect of LUI could be mitigated.

    Review of tropical reservoirs and their fisheries : the cases of Lake Nasser, Lake Volta and Indo-Gangetic Basin reservoir
    Zwieten, P.A.M. van; Bene, C. ; Kolding, J. ; Brummett, R. ; Valbo-Jorgensen, J. - \ 2011
    Rome : Food and Agriculture Organization of the United Nations (FAO fisheries and aquaculture technical paper 557) - 148
    reservoirs - visserij - meren - tropen - india - egypte - ghana - fisheries - lakes - tropics - egypt
    Mycorrhizal symbiosis and seedling performance of the frankincense tree (Boswellia papyrifera)
    Hizikias, E.B. - \ 2011
    Wageningen University. Promotor(en): Frans Bongers; Thomas Kuijper, co-promotor(en): Frank Sterck. - [S.l.] : S.n. - ISBN 9789085859635 - 141
    boswellia - mycorrhizae - symbiose - zaailingen - vesiculair-arbusculaire mycorrhizae - waterbeschikbaarheid - waterstress - tropen - ethiopië - boswellia - mycorrhizas - symbiosis - seedlings - vesicular arbuscular mycorrhizas - water availability - water stress - tropics - ethiopia

    Arid areas are characterized by a seasonal climate with a long dry period. In such stressful

    environment, resource availability is driven by longterm and shorterm rainfall pulses.

    Arbuscular Mycorrhizal (AM) fungi enhance access to moisture and nutrients and thereby

    influence plant performance. In this dissertation I applied field observations and

    greenhouse experiments to address four questions: 1) What are the major environmental

    factors influencing AM incidence in the Boswellia-dominated dry deciduous woodlands?

    2) How do Boswellia seedlings respond when they are exposed to AM fungi and water

    pulses? 3) How do AM fungi, water deficit and soil fertility influence the growth and gas

    exchange of Boswellia and Acacia seedlings? 4) Does the AM symbiosis influence

    competition between Acacia and Boswellia seedlings at different water pulse levels?

    The present study showed that almost all woodland plants in northern Ethiopia are

    colonized by AM fungi. Root colonization levels in dry and wet seasons demonstrated that

    in the sites with the harshest conditions, AM plants and fungi respond to pulsed resource

    availability by temporally disconnecting carbon gain by the plant and carbon expenditure

    by the fungus. Consequently, we studied below-ground processes in conferring adaptation

    to highly pulsed resources in Boswellia seedlings. The strong interactive AM fungi and

    water pulse showed that mycorrhizal Boswellia benefits from drought pulses during the

    short rainy season. Boswellia acquires carbon and water after rain events and store

    probably carbon and water in coarse roots, suggesting conservative strategy. From this

    observation we carried out an experiment to test whether other trees (Acacias) than

    Boswellia in this habitat also show this conservative acquisition strategy, or whether more

    acquisitive strategies may also be beneficial under such climates.

    My study show that acquisitive and conservative species both benefit from the AM

    symbiosis, but that the acquisitive Acacias mainly benefit at higher water availability,

    whereas the conservative Boswellia benefits at water or nutrient-stressed conditions. I also

    investigate on how mycorrhiza and water availability affect competition between plants

    with different resource acquisition strategies in these drylands. Seedlings of Boswellia are

    competitively inferior to seedlings of Acacia, and neither the presence of AM fungi nor a

    stronger water limitation (through pulsing) affected this outcome.

    A demonstration greenhouse for Malaysian horticulture: Trip report February 2011
    Elings, A. ; Blomne Sopov, M. - \ 2011
    Wageningen : Wageningen UR Glastuinbouw (rapporten GTB 1067) - 27
    glastuinbouw - kastechniek - kassen - tropen - maleisië - greenhouse horticulture - greenhouse technology - greenhouses - tropics - malaysia
    Inventory of P-Olsen data in the ISRIC-WISE soil database for use with QUEFTS
    Batjes, N.H. - \ 2010
    Wageningen : ISRIC - World Soil Information (Report / ISRIC-World Soil Information 2010/06) - 25
    bodemchemie - fosfor - bodem ph - organische koolstof - organische stikstof - bodemprofielen - databanken - tropen - soil chemistry - phosphorus - soil ph - organic carbon - organic nitrogen - soil profiles - databases - tropics
    Bossentransities in de tropen: van ontbossing naar bosuitbreiding
    Wiersum, K.F. ; Pater, C. de - \ 2010
    Ecologie en ontwikkeling 18 (2010)80. - ISSN 0928-6470 - p. 42 - 44.
    bossen - landgebruik - oppervlakte (areaal) - bebossing - tropen - forests - land use - acreage - afforestation - tropics
    Jaarlijks neemt het totale areaal tropisch bos af, maar in sommige tropische landen (Costa Rica, India, Vietnam) neemt het bosareaal juist toe. Dit duidt erop dat tropische ontbossing gestopt kan worden en geen automatisch gevolg is van bevolkingsgroei en toenemende bosgebruik. Het begrip bossentransitie verwijst naar de omslag van ontbossing naar bosuitbreiding. Hoewel de omslag zeker geen algemene trend is, is er toch verandering in zicht. Dit artikel geeft voornaamste drijvende krachten achter bosuitbreiding
    Capacity building Malaysian greenhouse horticulture
    Elings, A. - \ 2009
    [Wageningen] : Wageningen UR Greenhouse Horticulture
    kassen - glastuinbouw - kastechniek - gewasteelt - tropen - maleisië - greenhouses - greenhouse horticulture - greenhouse technology - crop management - tropics - malaysia
    A demonstration greenhouse is being realized at Serdang in Malaysia, under the responsibility of the Department of Agriculture (DoA). The general goal is to demonstrate good horticultural practices. This is very valuable, and forms the nucleus of future horticultural developments. Focus crops are chilli, tomato, cucumber and melon. Once the greenhouses have been constructed, crops will be planted, installations will be fine,tuned, staff will gain experience, and interaction with growers will commence.
    Sustainable potato production: guidelines for developing countries
    Lutaladio, N. ; Ortiz, O. ; Haverkort, A.J. ; Caldiz, D.O. - \ 2009
    Rome : FAO - ISBN 9789251064092 - 94
    solanum tuberosum - aardappelen - gewasproductie - agronomie - landbouwplantenteelt - plantenziektekunde - duurzaamheid (sustainability) - ontwikkelingslanden - tropen - subtropen - solanum tuberosum - potatoes - crop production - agronomy - crop husbandry - plant pathology - sustainability - developing countries - tropics - subtropics
    During the International Year of the Potato, celebrated in 2008, FAO and CIP helped forge partnerships worldwide to address critical aspects of sustainable potato production. This technical guide collates that experience to review technical, socio-economic, policy and institutional factors that currently constrain increased potato production and productivity in tropical and subtropical countries. It presents Good Agriculture Practices relevant to potato production, and indicators and recommendations for action in key areas, from the utilization of potato biodiversity and improvements in seed systems, to soil management, insect pest and disease control and opportunities for value addition. It outlines a new policy and research agenda for the potato subsector that aims at making a real contribution to the eradication of hunger and poverty.
    Forest management certification in the tropics: an evaluation of its ecological, economical and social impacts
    Peña-Claros, M. ; Blommerde, S. ; Bongers, F. - \ 2009
    Wageningen : Wageningen University and Research Centre - ISBN 9789080434547 - 31
    bosbedrijfsvoering - certificering - economische impact - sociale gevolgen - bosbouw - tropen - boswaardebepaling - forest management - certification - economic impact - social impact - forestry - tropics - forest valuation
    Assessing the progress made: an evaluation of forest management certification in the tropics
    Peña-Claros, M. ; Blommerde, S. ; Bongers, F. - \ 2009
    Wageningen : WUR (Tropical resource management papers = Documents sur la gestion des ressources tropicales 95) - ISBN 9789085856962 - 72
    bosbedrijfsvoering - certificering - tropen - forest management - certification - tropics
    Dutch organizations involved in the financing of forests and nature in tropical countries : a directory
    Bade, T. ; Wensing, D. ; Enzerink, R. ; Wageningen International, - \ 2009
    [Wageningen] : Wageningen University and Research Centre - 121
    bossen - bosbouw - natuurbescherming - nederland - adresboeken - tropen - financieren - forests - forestry - nature conservation - netherlands - directories - tropics - financing
    Sustainable Land Management in the Tropics : Explaining the Miracle
    Burger, C.P.J. ; Zaal, F. - \ 2009
    Farnham, England : Ashgate (International Land Management Series ) - ISBN 9780754644552 - 226
    grondbeheer - klimaatverandering - erosie - tropen - bodembescherming - waterbescherming - afrika ten zuiden van de sahara - plattelandsontwikkeling - landgebruik - ontwikkelingseconomie - duurzame ontwikkeling - economische aspecten - land management - climatic change - erosion - tropics - soil conservation - water conservation - africa south of sahara - rural development - land use - development economics - sustainable development - economic aspects
    Bringing together case studies from Kenya, Benin, Cameroon and the Philippines, this volume provides a multidisciplinary overview of the economics of natural resource management in Tropical regions, at household and village level. By comparing a wide array of climatic and economic conditions, it examines the effect of location and access to the market - as well as the importance of national policies - have on soil and water conservation. The book not only analyzes the benefits of soil and water conservation based on econometric studies, but also assesses the costs involved. In doing so it challenges commonly held assumptions about poorer community's ability to finance such measures.
    Options for Greenhouse Horticulture in Malaysia : trip report December 2008
    Elings, A. ; Campen, J.B. - \ 2008
    Wageningen : Wageningen UR Greenhouse Horticulture - 17
    kassen - glastuinbouw - tropen - maleisië - greenhouses - greenhouse horticulture - tropics - malaysia
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