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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    Assessing water stress of desert vegetation using remote sensing : the case of the Tamarugo forest in the Atacama Desert (Northern Chile)
    Chávez Oyanadel, R.O. - \ 2014
    Wageningen University. Promotor(en): Martin Herold, co-promotor(en): Jan Clevers; E. Acevedo. - Wageningen : Wageningen University - ISBN 9789462570795 - 175
    waterstress - woestijnen - vegetatie - remote sensing - bossen - waterbeheer - ecosystemen - droogte - adaptatie - chili - water stress - deserts - vegetation - remote sensing - forests - water management - ecosystems - drought - adaptation - chile

    Water stress assessment of natural vegetation plays a key role in water management of desert ecosystems. It allows scientists and managers to relate water extraction rates to changes in vegetation water condition, and consequently to define safe water extraction rates for maintaining a healthy ecosystem. Previous research has shown that optical remote sensing constitutes a powerful tool for assessing vegetation water stress due to its capability of quantitatively estimating important parameters of vegetation such as leaf area index (LAI), green canopy fraction (GCF), and canopy water content (CWC). However, the estimation of these parameters using remote sensing can be challenging in the case of desert vegetation. Desert plants have to cope with high solar irradiation and limited water. In order to maintain an adequate water balance and to avoid photoinhibition, desert plants have evolved different adaptations. A common one is heliotropism or ‘solar tracking’, an ability of many desert species to move their leaves to avoid facing direct high solar irradiation levels during the day and season. This adaptation (paraheliotropism) can have an important effect on the canopy spectral reflectance measured by satellites as well as on vegetation indices such as the normalized difference vegetation index (NDVI). In this thesis, I propose a remote sensing based approach to assess water stress of desert vegetation, exemplified in the case of the Tamarugo (Prosopis tamarugo Phil) tree in the Atacama Desert (Northern Chile), a ‘solar tracker’ species, which is threatened by groundwater overexploitation.

    In the first chapter of this thesis (general introduction), I explained the motivation of the PhD project and elaborated four research questions, which are later discussed in chapters 2, 3, 4, and 5. The thesis concluded with chapter 6, where I provide a synthesis of the main results, general conclusions and a final reflection and outlook.

    In the second chapter, I studied the effects of water stress on Tamarugo plants under laboratory conditions and modelled the light-canopy interaction using the Soil-Leaf-Canopy radiative transfer model. I described for the first time pulvinar movement of Tamarugo and quantified its effects on canopy spectral reflectance with and without stress. I showed that different spectral indices have potential to assess water stress of Tamarugo by means of LAI and CWC. In the third chapter, I measured the effects of pulvinar movements on canopy reflectance for Tamarugos under field conditions and used high spatial resolution images to assess water stress at the tree level. I developed an automated process to first identify single trees and delineate their crowns, and secondly, to estimate LAI and GCF using spectral vegetation indices. These indices (NDVI and chlorophyll red-edge index) were negatively correlated to diurnal values of solar irradiation as a consequence of leaf pulvinar movements. For this reason, higher values of both vegetation indices are expected to occur in the morning and in winter (low solar radiation) than at midday or summer.

    In the fourth chapter I studied the effects of diurnal pulvinar movements on NDVI time series from the MODIS-Terra satellite (acquired in the morning) and the MODIS-Aqua satellite (acquired at midday) for the period 2003-2012 and the seasonal effects of pulvinar movements on NDVI time series of Landsat images for the period 1998-2012 for Tamarugo areas with and without water stress. NDVI values measured by MODIS-Terra (morning) were higher than the NDVI values measured by MODIS-Aqua (afternoon) and the difference between the two, the ΔNDVImo-mi, showed good potential as water stress indicator. In a similar way, I observed a strong seasonal effect on the Landsat NDVI signal, attributed to pulvinar movements, and the difference between winter and summer, the ΔNDVIW-S, also showed good potential for detecting and quantifying water stress. The ΔNDVImo-mi, the ΔNDVIW-S and the NDVI itself measured systematically in winter time (NDVIW) were negatively correlated with in situ groundwater depth measurements.

    In chapter five I used a dense NDVI time series of Landsat images for the period 1989-2013, combined with high spatial resolution satellite imagery and hydrogeological records, to provide a quantitative assessment of the water status of Tamarugo vegetation after 50 years of increasing groundwater extraction. The results showed that the NDVIW and ΔNDVIW-S of the Tamarugo vegetation declined 19% and 51%, respectively, as groundwater depleted (3 meters on average) for the period 1989-2013. Both variables were negatively correlated to groundwater depth both temporally and spatially. About 730.000 Tamarugo trees remained in the study area by 2011, from which 5.2% showed a GCF<0.25 which is associated to severe water stress. Based on this spatio-temporal analysis, I suggest that the survival of Tamarugo trees is limited to a maximum groundwater depth of 20 meters.

    The main conclusions of this PhD thesis are summarized as follows:

    Heliotropism or leaf ‘solar tracking’, a common adaptation among desert plants, has an important impact on canopy spectral reflectance. As shown in the case of the Tamarugo trees, widely used vegetation indices such as the NDVI were negatively correlated to solar irradiation (the stimulus for leaf solar tracking), showing a distinct diurnal and seasonal cycle.An early symptom of water stress in paraheliotropic plants (leaves facing away the sun) is the decline of the amplitude of the diurnal and seasonal NDVI cycles. Thus, remote sensing estimations of this amplitude (e.g. the NDVI difference between winter and summer or the difference between midday and morning) can be used to detect and map early water stress of paraheliotropic vegetation.At the tree level, very high spatial resolution images combined with object based image analysis and in-situ data provided accurate estimations of the water status of small desert vegetation features, such as isolated trees. For monitoring purposes, careful consideration of the time during the day and the season at which the images are taken needs to be taken to avoid misleading interpretations.Time series analysis of historical satellite images combined with very high spatial resolution images and hydrogeological records can provide a quantitative spatio-temporal assessment of the effects of long-term groundwater extraction on desert vegetation.
    On the propagation of drought : how climate and catchment characteristics influence hydrological drought development and recovery
    Loon, A.F. van - \ 2013
    Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Henny van Lanen. - [S.l.] : s.n. - ISBN 9789461735010 - 196
    droogte - waterstress - waterbeheer - hydrologie - klimaat - hydrologie van stroomgebieden - stroomgebieden - modellen - drought - water stress - water management - hydrology - climate - catchment hydrology - watersheds - models

    Drought is a severe natural disaster resulting in high economic loss and huge ecological and societal impacts. In this thesis drought is defined as a period of below-normal water availability in precipitation (meteorological drought), soil moisture (soil moisture drought), or groundwater and discharge (hydrological drought), caused by natural variability in climate. Drought propagation is the change of the drought signal as it moves from anomalous meteorological conditions to a hydrological drought through the terrestrial part of the hydrological cycle. The objective of this PhD research is to investigate the processes underlying drought propagation and their relation with climate and catchment characteristics, both on the catchment scale and on the global scale.
    The catchment-scale studies are based on five headwater catchments in Europe with contrasting climate and catchment characteristics. In one of these case study areas, anthropogenic influence on the water system was significant, resulting in severe water scarcity. As I only study natural processes in this thesis, there was a need to separate drought (as defined in this thesis) from human-induced water scarcity in this case study area. I proposed an observation-modelling framework that consists of a hydrological model to simulate the ‘naturalised’ situation and an anomaly analysis method to quantify drought and water scarcity events. Both the time series and the anomaly characteristics of the ‘disturbed’ and ‘naturalised’ situation were compared to quantify human and natural influences on the hydrological system.
    After simulation of hydrometeorological variables of all case study areas with a conceptual hydrological model and drought identification with the variable threshold level method, time series and characteristics of drought events were analysed. I classified the drought events into six hydrological drought types that are the result of the interplay of temperature, precipitation, evapotranspiration and storage in different seasons. The most common hydrological drought type develops as a result of a rainfall deficit. However, in the development of the most severe hydrological drought events temperature and storage-related processes play an important role, for example through a lack of recovery of the drought.
    As I aimed to investigate drought propagation also on larger scales, I tested an ensemble mean of a number of large-scale models (both land-surface models and global hydrological models) on their ability to reproduce the drought propagation processes found in the case study areas. The large-scale models did simulate general aspects of drought propagation (e.g. fewer and longer drought events in discharge than in precipitation), but the above-mentioned effects of temperature and storage-related processes were only partly reproduced. In the large-scale model ensemble, daily runoff reacted almost immediately to changes in precipitation, resulting in important deficiencies in drought simulation in cold and semi-arid climates and regions with large storage. For the time being, this limits the use of large-scale models for the study of processes underlying drought propagation on a global scale.
    Consequently, I used a synthetic conceptual hydrological model to study drought propagation on the global scale. I focused on climate control by isolating forcing effects from effects of catchment properties. The drought characteristics (duration and deficit combined) of both soil moisture and subsurface discharge exhibited strongly non-linear patterns in seasonal climates. The non-linear effects in soil moisture drought were caused by the fact that the development of soil moisture droughts in warm seasonal climates is limited by the wilting point. Hydrological droughts in both warm and cold seasonal climates showed a strong increase of deficit with duration due to a lack of recovery in the dry season or snow season, respectively. This effect was strongest in cold seasonal climates, which indicates that for the development and recovery of within-year hydrological drought temperature is an important factor.
    The overall conclusion of this research is that, although drought is a complex, nonlinear phenomenon with drought characteristics varying with climate type and catchment characteristics, generic patterns can be derived that reflect the different hydrological processes underlying drought propagation. These processes result in different hydrological drought types that are shown to play a role both on the catchment scale and on the global scale. The non-linear effects of snow and storage-related processes on drought are not incorporated sufficiently in the currently-used large-scale models and drought indices. Possible future steps include more focus on catchment control, in particular the representation of storage, and the role of temperature and evapotranspiration. Additionally, the findings of this research can be applied to hydrological drought forecasting, prediction in ungauged basins, and prediction under global change.

    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.

    Coping with drought for food security in Tigray, Ethiopia
    Berhe, A.A. - \ 2011
    Wageningen University. Promotor(en): Leo Stroosnijder, co-promotor(en): Saskia Keesstra. - [S.l.] : S.n. - ISBN 9789085859253 - 173
    landbouwklimatologie - droogte - waterstress - irrigatie - regen - gewassen - hordeum vulgare - gerst - eragrostis tef - mulches - opbrengsten - ethiopië - agroclimatology - drought - water stress - irrigation - rain - crops - hordeum vulgare - barley - eragrostis tef - mulches - yields - ethiopia

    Agricultural production in drylands suffers from drought and in sub-Saharan Africa, only about 15% of the terrestrial rainwater is used by plants for food, fodder and fiber. Drought was the major cause of crop failure and food insecurity for the last three decades in the semi-arid northern Ethiopia. Information from field experiments, farmers and FAO’s AquaCrop model were used to understand and evaluate characteristics of the local climate in relation to drought and its mitigation. Farmers often have a different perception of drought than scientists. This gap was solved with an improved analysis of rainfall data that focuses on drought. Drought in semi-arid north Ethiopia can be due to (1) a too short growing season due to late start and/or early cessation of rains, (2) a too long dry spell at critical growth stage or (3) due to total failure of rain. Different types of drought have different coping strategies. Coping strategies for drought are adaption of sowing date, in-situ water conservation and water harvesting for deficient irrigation. Physical water conservation structures need to account for extreme variations in rainfall typical for most drylands. Ridging is most promising while mulching is no longer promoted. Agronomic measures such as the use of quick maturing and drought resistant crops are often more attainable than adoption of water conservation practices. Crop failure is often more due to water scarcity than to lack of fertility. However, when agricultural production increases, water and nutrients often alternate as production limiting factors. The strong water-nutrient synergy in drylands requires precision fertilization and plant density adjustment. Farmers also have a different opinion about climate change; farmers’ climate change has more to do with desertification. Experimental results showed that drought alleviation strategies have the potential to reduce the risk of crop failure and drought in the semi-arid northern Ethiopia.

    Tree recruitment in West African dry woodlands : the interactive effects of climate, soil, fire and grazing
    Biaou, S.S.H. - \ 2009
    Wageningen University. Promotor(en): Frits Mohren, co-promotor(en): Frank Sterck; Milena Holmgren Urba. - [S.l.] : S.n. - ISBN 9789085853183 - 182
    bosgebieden - verjonging - begrazing - bodem - brandgevolgen - soortenrijkdom - bodemvruchtbaarheid - bossen - waterstress - populatie-aanwas - west-afrika - benin - woodlands - regeneration - grazing - soil - fire effects - species richness - soil fertility - forests - water stress - recruitment - west africa - benin
    Hydrological drought. Processes and estimation methods for streamflow and groundwater
    Tallaksen, L. ; Lanen, H.A.J. van - \ 2004
    Amsterdam : Elsevier (Developments in water science 48) - ISBN 9780444516886 - 579
    droogte - droog klimaat - waterbehoefte - waterontbering - waterstress - hydrologie - grondwater - rivierafvoer - schatting - methodologie - drought - arid climate - water requirements - water deprivation - water stress - hydrology - groundwater - stream flow - estimation - methodology
    Hydrological drought is a textbook for university students, practising hydrologists and researchers. The main scope of this book is to provide the reader with a comprehensive review of processes and estimation methods for streamflow and groundwater drought. It includes a qualitative conceptual understanding of drought features and processes, a detailed presentation of estimation methods and tools, practical examples and key aspects of operational practice. The methods are demonstrated using sample data sets and tools that are provided on the accompanying CD. The drought phenomenon and its diversity across the world are illustrated using a global set of daily streamflow series, whereas regional and local aspects of drought are studied using a combination of hydrological time series and catchment information. The book concludes with human impacts, ecological issues and examples of procedures for designing and operating water resources schemes. The majority of the examples are taken from regions where the rivers run most of the year. The material presented ranges from well established knowledge and analysing methods to recent developments in drought research. Its nature varies accordingly, from a more traditional textbook with its clear overview to that of a research paper, which introduces new approaches and methodologies for drought analysis.
    Voorspellen van droogtestress bij aardappelplanten uit reflectiemetingen aan het gewas : relatie tussen bodemvocht en ratios van pieken en reflectiemetingen
    Vereijken, P.F.G. - \ 2002
    Wageningen : Plant Research International (Nota / Plant Research International 167) - 10
    solanum tuberosum - aardappelen - waterstress - plant-water relaties - lichtrelaties - reflectie - optische eigenschappen - bodemwater - methodologie - statistische analyse - solanum tuberosum - potatoes - water stress - plant water relations - light relations - reflection - optical properties - soil water - methodology - statistical analysis
    Magnetic resonance imaging of plants: plant water status and drought stress response
    Weerd-Meulenkamp, L. van der - \ 2002
    Wageningen University. Promotor(en): T.J. Schaafsma; H. van As. - S.l. : S.n. - ISBN 9789058087072 - 119
    planten - waterbalans - droogte - droogteresistentie - waterstress - plant-water relaties - membraanpermeabiliteit - celgroei - kernmagnetische resonantie - microscopie - computersimulatie - plantenfysiologie - plants - water balance - drought - drought resistance - water stress - plant water relations - membrane permeability - cell growth - nuclear magnetic resonance - microscopy - computer simulation - plant physiology

    This Thesis presents an approach for the study of plant water balance during drought stress, using a combination of in vivo NMR experiments and computer simulations. The ultimate aim is the interpretation of the NMR parameters in terms of physiologically relevant characteristics, such as cell dimensions and membrane permeability. Especially the latter has raised a growing interest in plant science, and up to now the measurement of this parameter in vivo was limited to single cells and short experiment time spans.

    NMR microscopy of plants yields information on various levels of organisation. The NMR images provide clear anatomical details, which have been used to monitor the response of stem growth rates to osmotic stress. On the tissue and cell levels, the NMR parameters T 2 (transverse spin relaxation time) and D app (apparent diffusion coefficient) provide information on the physical and chemical properties. Correct quantitative values for T 2 and D app are crucial for a useful interpretation. Therefore, Chapter 2 evaluates the accuracy of different fitting procedures.

    The physical and chemical properties can vary considerably between and within different tissues, cells, and intracellular compartments, resulting in distinctly different relaxation and diffusion characteristics for these compartments. The interpretation of these parameters is not straightforward. A numerical model of restricted diffusion and relaxation behaviour was therefore developed, based on Fick's second law of diffusion (Chapter 3). This model expands previous one-dimensional models to a two-dimensional space, consisting of multiple concentric cylindrical compartments, separated by membranes. Numerical simulation experiments using this model demonstrate the importance of modelling two-dimensional diffusion in relation to the effects of spatial restrictions, and spin exchange between the different compartments.

    This model has been applied to investigate the effects of diffusive exchange on the transverse spin relaxation times, the apparent diffusion coefficients, and the NMR signal amplitudes of water in plant cells (Chapter 4). For different multi-compartment model systems a Pulsed Field Gradient Multiple Spin Echo (PFG-MSE) experiment was simulated, and intrinsic physiological parameters, i.e. the bulk diffusion constant, the cell radius and the membrane permeability were afterwards extracted using common theoretical models. The results justify the use of these models to interpret the in vivo experiments, since meaningful diffusion constants, cell radii and membrane permeabilities can be extracted for a large range of conditions. This is still true if not all conditions of the theory are known or met, e.g . for intact plants.

    Chapters 5 and 6 study the effect of mild osmotic stress on maize and pearl millet by in vivo1H NMR microscopy, and water uptake measurements. Single NMR parameter images of (i) the water content, (ii) the transverse relaxation time ( T 2 ) and (iii) the apparent diffusion coefficient ( D app ) were used to follow the water status of the stem apical region during osmotic stress. The results are interpreted using the multi-compartment model (Chapter 4), tailored to suit plant cells. For this particular case, an equation was derived to describe the relation between the observed T 2 , the cell dimensions, the bulk T 2 , and the membrane permeability, based on the Brownstein & Tarr theory. Experimentally determined T 2 values of non-stressed stem tissue are indeed correlated to the cell dimensions, which is in agreement with the derived equation. The T 2 of maize cells is higher than the T 2 of equally sized millet cells, implying that the membrane permeability of the latter is higher.

    The growth rate was strongly inhibited by mild stress in both species, even though the water uptake was only mildly affected. During stress, there are hardly any changes in water content or T 2 of the stem region of maize. In contrast, the apical tissue of pearl millet showed a ~30% decrease of T 2 within 48 hours of stress, whereas the water content and D app hardly changed. This decrease in T 2 can be caused by a decreasing cell radius, a decreasing bulk T 2 , and/or an increasing membrane permeability for water. To distinguish between these contributions, additional scanning electron microscopy was used, showing no apparent changes in cell size. Transverse spin relaxation measurements of a wide range of sugar solutions showed only very small effects of osmotic adjustment on the bulk T 2 . Together, these results point to an increase in membrane permeability during stress. This conclusion is confirmed by numerical simulations of the plant cell model, which showed that only an increasing membrane permeability yields a similar combination of water content, T 2 , and D app values during stress.

    Under severe osmotic stress, the effects on the plant water balance are naturally larger (Chapter 7). During stress, no significant changes occurred in the maize stem, though the leaves wilted, and the plant died after two days of stress. Pearl millet showed again changes in T 2 , especially in the secondary shoots, which were more pronounced than during mild stress. Furthermore, the stem tissue shrunk, implying that the cell dimensions changed; the secondary shoots showed far less decrease in water content, however. Despite these changes, the plants recovered once stress was relieved. In the framework of the plant cell model, the decreasing T 2 is interpreted as the result of a combination of decreasing cell size and increasing membrane permeability. The latter can result in a higher tissue conductance, thereby facilitating water re-allocation to young, expanding tissues to prevent irreparable damage.

    The combination of experimental data and simulations as presented in this Thesis has proven to be an effective tool to link NMR information to physiology (Chapter 8). This approach promises to be of great use to plant science, and to NMR microscopy in general.

    Mixed cropping of barley (Hordeum vulgare) and wheat (Triticum aestivum) landraces in the central highlands of Eritrea
    Woldeamlak, A. - \ 2001
    Wageningen University. Promotor(en): P.C. Struik; D. Ghebreselassie. - S.l. : S.n. - ISBN 9789058083357 - 220
    hordeum vulgare - triticum aestivum - teeltsystemen - gemengde teelt - tussenteelt - concurrentie tussen planten - planteninteractie - gebruiksefficiëntie - waterstress - gerst - tarwe - eritrea - hordeum vulgare - triticum aestivum - cropping systems - mixed cropping - intercropping - plant competition - plant interaction - use efficiency - water stress - barley - wheat - eritrea

    A common cropping system in the central highlands of Eritrea is mixed cropping of barley (Hordeum vulgare) and wheat (Triticum aestivum); it is called hanfetz (Tigrigna word). Mixtures may give higher yield, better yield stability, better food quality and more animal feed. Factors affecting the productivity of mixtures include genotype combination, crop density and component crop ratio.

    Grain yields differed significantly among genotype combinations in certain years. A combination of Ardu 12/60 + Kenya + Mana gave high mean grain yield (2009 kg ha-1) and a relative yield advantage (RYT= 1.57) of 57% increase in grain yield over the sole crops. Harvest index, biomass, stand cover and thousand grain weight were well correlated with yield. Wheat plants were first suppressed by barley but later on grew taller than barley. The potential yield increase for mixtures over barley sole cropping may be associated with the relative height and higher light use (efficiency). Some genotype combinations also showed reasonable yield and resistance to stress with a drought susceptibility index < 1 such as IAR 485 + Mana (DSI=0.565).

    Crop ratios of 100/50 (2275 kg ha-1) and 100/25 (2241 kg ha-1) were the best in grain yield when averaged over the three basic crop densities (100% = 100, 200 and 300 plants m-2) and years. Barley showed greater competitive ability than wheat. For barley the intra-specific competition was more important than the inter-specific competition. In such studies, yield advantage can be either due to the density effect or complementary use of resources. The drawback of the additive design is that yield advantage may be partly due to increased density. However, niche differentiation showed that mixtures shared resources efficiently and the yield advantage was the result of complementary use of resources. In the study under drought stress, an additive ratio (higher density) did not result in higher total yield compared to that in replacement series. The niche differentiation in both years under drought stress also showed that the yield advantage was due to complementary use of resources among the crop species.

    Stability analysis of barley and wheat mixtures on yield data from a large set of experiments showed that mixed cropping was significantly more stable than barley and wheat sole cropping. Some of the genotype combinations such as Ardu 12/60 + Kenya + Mana and Ardu 12/60 + Mana were more stable than others.

    The most promising genotype combinations and crop ratios obtained from this study have to be verified on farm and demonstrated to farmers before the technology is released for use.

    Vochtindicatie van grasvegetaties in relatie tot de vochttoestand van de bodem
    Jansen, P.C. ; Runhaar, J. ; Witte, J.P.M. ; Dam, J.C. van - \ 2000
    Wageningen : Alterra (Alterra-rapport 57)
    vegetatie - natuurlijke graslanden - bodemwater - plant-water relaties - waterstress - nederland - ecohydrologie - vegetation - natural grasslands - soil water - plant water relations - water stress - netherlands - ecohydrology
    Om effecten van het waterbeheer op de vegetatie te kunnen voorspellen is inzicht in de relatie tussen de vegetatie en de waterhuishouding van de standplaats noodzakelijk. Het onderzoek richt zich op de relatie tussen vochthuishouding en soortensamenstelling in vochtige tot droge omstandigheden. Verschillende gangbare maten voor de vochtindicatie worden gerelateerd aan verschillende berekende maten voor de vochttoestand van de bodem.
    Root water uptake under non-uniform transient salinity and water stress
    Homaee, M. - \ 1999
    Agricultural University. Promotor(en): R.A. Feddes; C. Dirksen. - S.l. : Homaee - ISBN 9789058080226 - 173
    wateropname (planten) - wortels - zoutgehalte - waterstress - water uptake - roots - salinity - water stress

    The study described in this thesis focuses on the quantitative understanding of water uptake by roots under separate and combined salinity and water stresses. The major difficulty in solving Richards' equation stems from the lack of a sink term function that adequately describes root water uptake. From the existing microscopic and macroscopic sink term functions, the empirical macroscopic approach was chosen because it requires the least number of parameters whose values can readily be determined. All existing reduction functions as well as those newly developed in this study are used in the macroscopic model and tested against experimental data. The experimentally obtained data are used to derive the parameter values needed for the simulation model HYSWASOR. The experiments cover root water uptake by alfalfa under salinity stress, water stress, and combined salinity and water stress. This order is followed with the analysis of the data and the simulation.

    Under salinity stress , both experimental and simulated results indicate that the well-known linear crop response function can be used as a reduction function. The parameter values available in the literature for different reduction functions cannot provide acceptable agreement with the experimental data. When experimentally derived parameters are used in the simulation model, the agreement becomes much closer, but calibration is still needed. The parameter values obtained by calibration differ slightly from the experiments, because the experimentally derived parameter values are based upon mean soil solution salinity. Both experimental and simulation results indicate that different salinity reduction functions can provide almost the same results if the parameter values are well specified. For practical use the linear reduction function with the least number of parameters appears to be adequate.

    Under water stress , all existing reduction functions as well as the one developed in this study are tested on the experimental data. Since the trend of the experimental relative transpiration versus mean soil water pressure head is nonlinear, the linear reduction function cannot fit the data. The existing nonlinear reduction functions can fit only half of the data range satisfactorily. The best agreement is obtained with the newly developednonlinear two-threshold reduction function .

    The parameter values obtained by calibration differ only slightly from those of the experiments. Soil water pressure head heterogeneity over the root zone does not play an important role in water uptake. The roots appear to take up water from the relatively wetter parts of the root zone to compensate for the water deficit in the drier parts. On the first day after irrigation both relative transpiration and relative leaf water head are almost the same for the stressed and non-stressed plants. While the simulated transpiration agrees closely with the experimental data, the main reason for the discrepancy between the simulated and actual water contents appears to be water uptake during the night.

    Under combined water and salinity stress , the additive and multiplicative reduction functions are first tested against the experimental data and then inserted in the simulation model. A new combination reduction function is introduced that differs conceptually from the additive and multiplicative functions. Both the experimental and simulated results show that the newly proposed model fits the data best, while the worst results are obtained with the simple additive model.

    Early detection of water stress in sub-humid climates
    Stanghellini, C. ; Lorenzi, F. De; Bosma, A.H. - \ 1992
    Wageningen : IMAG-DLO (Rapport / IMAG - DLO 92-9) - ISBN 9789054060277 - 29
    omloop - gewassen - droogte - groei - vochtigheid - vloeistoffen (liquids) - lolium - sapstroom - waterstress - waterbehoefte - circulation - crops - drought - growth - humidity - liquids - lolium - sap flow - water stress - water requirements
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