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Introduction and objectivesGood comprehension of the energy and mass cycles and their effect on climate dynamics is crucial to understanding, predicting and anticipating ecological changes due to possible future climate perturbations. Here direct and long-term flux density measurements of greenhouse gases from various ecosystems provide means to supply such fundamental knowledge. For the global water vapour and carbon cycles, however, the interactions between different spatial scales become important, where extrapolating from canopy flux density measurements to global budgets lead to practical and theoretical problems. This thesis focuses on the direct and long-term measurement of surface flux densities and interaction processes at the canopy (< 1 km scale within the framework of the Surface Layer Integration Measurements and Modelling (SLIMM) project. Furthermore, some characteristics and limitations of the scintillation technique are studied in two field experiments in New Zealand.As indicated in Chapter 1, the first objective of this project was the direct and continuous long-term measurement of the surface flux densities of radiation, momentum, heat, water vapour and carbon dioxide (CO 2 ) to study the effect of biological and climatic processes that regulate carbon dioxide exchange of this ecosystem at the canopy scale. At the same time these data were used to study the effect of plant related and environmental conditions on the interaction of carbon dioxide and water vapour exchange, to satisfy the second objective of the thesis. The third objective focussed on the prospect of obtaining both the spatial averaged sensible heat flux density and momentum flux density from scintillation measurements.Generally, a compromising point measurement of the mean horizontal wind speed or friction velocity is used to calculate the sensible heat flux density from the temperature structure parameter. By using two scintillometers at two heights, point measurements to obtain th

The study described in this thesis focuses on the understanding of growth of leaf area and biomass in wheat and sunflower under low phosphorus conditions.Chapters 2 and 3 address the question whether P-deficiency limits leaf area expansion directly by inhibiting the individual leaf expansion, or through its effects on the availability of assimilates for growth. Experimental and simulation techniques were used to find an answer to this question. It was concluded that in sunflower the effects of P on sink size and assimilate supply were important for determining plant leaf area expansion (Chapter 2). Instead, wheat responded to a low P supply mainly by reducing sink size (Chapter 3). A sensitivity analysis of the effects of different model parameters at different levels of P supply, on the simulated total leaf area, showed that the value of the phyllochron, in wheat and sunflower, and tillering in wheat were crucial in determining the total leaf area.Chapter 4 focuses on the effects of P on the generation of leaves and tillers in wheat. Here, it was shown that the emergence of main stem leaves in wheat plants grown under P-deficient conditions was delayed (higher phyllochron) as both, the rate of leaf primordia initiation in the apex, and the rate of individual leaf expansion were reduced.In Chapter 5, the effects of P and assimilate supply on tillering were studied in a field experiment. P deficiencies directly altered the normal pattern of tiller emergence by slowing down the emergence of leaves on the main stem, and by reducing the maximum rate of tiller emergence. In general terms, assimilate supply seemed to play a minor role determining tillering. Including the effects of P on the phyllochron and on the maximum tillering rate in a morphogenetic model of the generation of leaves and tillers, it was possible to explain the dynamics of tiller emergence of a field experiment.In Chapter 6, a comparative study of the response to P in wheat and sunflower is presented.

For a long time, the emphasis in silviculture in Western Europe was solely on even-aged, monospecific stands; many empirical stand-level growth models were developed and successfully used for managing such stands. In contrast, no generally accepted growth and yield approach has emerged so far for mixed forests. Moreover, the inexhaustible number of species combinations, management regimes, and site-dependent interactions make an empirical approach less suitable.In the present study, a mechanistic model was developed that simulates growth and yield in mixed forest stands. Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) and beech ( Fagus sylvatica L.) were used in this research. In the model, tree growth is dependent on radiation availability. Stand development is largely driven by competition for radiation. A spatial module was developed to investigate the effects of tree and stand characteristics on radiation interception. The study showed that in heterogeneous stands a spatial approach is needed to account for competition between trees.Growth of the trees was estimated using the radiation-use efficiency concept (RUE). Results revealed that detailed process models can be used to estimate RUE and that it is a suitable tool for (mixed) forest modelling.To describe the distribution of the dry matter growth, a separate module was developed using functional relationships between tree components: the dry matter distribution is driven by the aim to maintain structural balances within the tree. The study showed that this approach is able to reproduce the development of an individual forest tree. The approach was thus considered very suitable for modelling the effects of between-tree competition for resources on growth and development of mixed forest stands.The overall growth model, COMMIX, was applied to investigate the effects of stand composition on mixed stand productivity, using a replacement series. Analysis showed that the productivity of mixed forest stands is ge

This thesis represents detailed research on the "rice-soils-weather" system of the Ebro Delta (Spain) providing knowledge on how temperature, radiation, soil properties and farm management determine rice growth. After an introductory chapter, the findings are developed step-by-step. (i) Chapter 2 is an overview of the conditions in the study area. (ii) Chapter 3 focuses on the study of the soils creating a soil geographic database of the northern part of the Ebro Delta. Four properties were used for defining soil units; soil development, drainage status, texture and salinity. (iii)Chapters 4 and 5 deal with rice growth under weather limited conditions. Potential productions of 13000 kg ha -1were estimated, while maximum yields of 11000 kg ha -1were recorded at field level. In Chapter 4, the phenological development, the daily dry matter production and the leaf area development of the rice crop were modelled. In Chapter 5, the use of remote sensing techniques at field level for monitoring the rice crop status were tested. (iv)Chapters 6 and 7 analyze rice growth in relation to soil properties and farm management. Two soil factors were found to dominate the effects on yield: one was topsoil CEC (in strong association with clay content) with a positive effect, and the other one was soil salinity with a negative effect. High groundwater tables did not have significant effects on rice yield, except when the water had a high salt content. Four main groups of causes within the cropping status limited rice growth: potassium and zinc shortage where a strong antagonism of either factor with sodium was observed, low plant establishment where a minimum number of 160-180 plants m -2was necessary to maximize yield, and length of the growing season, especially the length of the pre-heading period in which the potential size of the crop was primarily determined.Potassium and zinc shortages in the plant were mainly induced by soil salinity. High K in saline soils did not increase K

The study described in this thesis focuses on a quantification of leaf wetness formation in paddy rice crops based on insight in the physical processes of the formation of leaf wetness. For this purpose, experimental research was conducted in a tropical paddy rice field.A shielding (nocturnal net radiative loss) experiment was designed to investigate the dependence of dew formation on nocturnal net radiative loss. A simple method was developed to estimate dew amount and duration using the nightly total net radiative loss. The shielding experiment unexpectedly provided a feasible method for experimentally distinguishing dew from guttation. For a better understanding of the characteristics of the formation of leaf wetness in paddy rice crops, warm water flooding both in the paddy rice field and in a phytotron was conducted to investigate the effects of water temperature on dew formation and guttation. The experimental data indicated an optimum paddy water temperature of 30°C for rice leaf guttation. The effect of water temperature on dew formation depended on the weather condition above the canopy, especially the thermal stratification condition, net radiation and vapour pressure deficit. A simple device, the dewball, was designed to observe dew formation and to determine its threshold value of nocturnal net radiative loss from top leaves. This was shown to be a simple and accurate method for estimating daily dew duration on top leaves. The possibility of using the mean diurnal patterns of weather variables to estimate dew formation was analysed by comparing the dew estimated using a simulation model to the measured results. This helped to understand to what extent the diurnal patterns will give as good an estimate of dew formation as when the observed hourly weather data are used. Based on the observation results on the dewball and the results of the shielding experiment, a simple approach to estimate daily dew duration on top leaves of the paddy rice crops was formu

Research topicsThe relationships between climate and both phenology and growth of some important European tree species were studied to evaluate the potential impacts of climate change on trees and forests in Europe. In order to make such assessments, insight is required on the mechanisms how climatic variables interact with plant processes. The topics addressed in this study were: (1) the modelling of phenology, (2) the consequences of climate change on spring frost damage, (3) the importance of phenotypic plasticity, (4) the importance of phenology on the effects of climate change on growth of monospecies deciduous forests, and (5) the importance of phenology on the effects of climate change on growth of mixed-species deciduous forests.Modelling phenologyTo evaluate the impacts of climate change on growth of temperate deciduous tree species, the onset and cessation of the growth must be accurately described. A review is presented on eight models predicting the date of leaf unfolding depending on temperature. These models were fitted using 57 years of observations on the date of leaf unfolding of Fagus sylvatica in The Netherlands, and used to predict 40 years of similar observations collected in Germany. As conflicting experimental evidence exist on the role of photoperiod on leaf unfolding of Fagus sylvatica, photoperiod was incorporated into each of these models.The timing of leaf unfolding could best be described by a model in which the effects of chilling temperatures (-5 to +10°C) and forcing temperatures (>0°C) operate sequentially in time, according to a triangular and logistic function, respectively. Including photoperiod reduced the predicting power of this model.Spring frost damageTwo studies presented in literature evaluate the effect of increasing winter temperature on the probability of spring frost damage to trees. However, one study predicted an increase, while the other predicted a decrease in the probability of spring frost damage. It is unclear wh

Accurate prediction of crop phenology is important not only for modelling purposes but also for crop improvement and management actions. The objective of this study was to develop a model for predicting phenological development to flowering in rice ( Oryza sativa L.).Data from the literature were analysed to evaluate a basic equation for crop development. The Beta function, commonly used as a skewed probability density function in statistics, was found to accurately describe responses of rice development rate to both temperature and photoperiod. Controlled-environment experiments were then conducted for detailed understanding of the physiological basis of photothermal responses of preflowering development in rice. Effects of day and night temperature on development to flowering were found to be different. The plants did not respond to photoperiod throughout the entire preflowering period, instead, the photoperiod- sensitive phase was sandwiched by two photoperiod-insensitive phases. It appeared that responses to both day and night temperature were stronger during the photoperiod-sensitive phase than during the two photoperiod-insensitive phases. The results also indicated that leaf appearance occurring simultaneously during the preflowering development had a different thermal response from that of the development per se .Based on the experimental results, a detailed model for photothermal responses of flowering in rice was developed, using the Beta function. The model, referred to as the three-stage Beta (3s-Beta) model, describes different photothermal responses during the photoperiod-sensitive phase and the photoperiod-insensitive phases of preflowering ontogeny. Using parameter values derived from controlled-environment experiments, the 3s-Beta model adequately predicted rice flowering dates observed in field conditions. It preformed better than several existing models over a wide range of environments.In order to assist new plant type design, the 3s-Beta model w

Yields of agricultural crops are largely determined by the weather conditions during the growing season. Weather data are therefore important input variables for crop growth simulation models. In practice, these data are accepted at their face value. This is not realistic. Like all measured values, are weather data subject to inaccuracies. Crop growth simulation models are sensitive to weather data used as input, so inaccuracies in weather data can affect the simulation results. The errors in weather data were estimated and their effects on the simulation results of a spring wheat crop growth simulation model were determined. Inaccuracies in weather data caused deviations in simulated yields of 10-15 %.In most weather data sets missing values occur and since crop growth models require daily data the values of the missing data have to be estimated. Several methods to estimate missing values were discussed and their effects on simulation results were studied. Large differences in quality of the estimation methods were found. Some of them resulted in deviations in simulated yields up to 30 %.Daily weather data are not always available and often average weather data are used instead. The effects of using average weather data on simulation results were studied for three sites in different climates. For all sites large deviations in simulation results were found.The increasing CO 2 concentration is affecting agricultural production in two ways: via a climatic change and via effects on assimilation and transpiration rates. The spring wheat model was used to study the overall effects of higher CO 2 levels on wheat yields in Western Europe. A temperature rise of 3 °C resulted in a yield decline, doubled CO 2 concentration in a yield increase and the combination of both in a yield increase of about 2 ton ha -1.

This thesis describes an investigation into the possibilities of linking X-band radar backscattering and optical remote sensing data with crop growth models for the monitoring of crop growth. The emphasis is on the usability of X-band radar data, with a detailed analysis of the main backscattering influencing factors of agricultural crops in The Netherlands.Six-years of ground-based X-band radar observations (VV and HH polarized, 10° to 80° incidence angle) were used to study the temporal radar backscattering of sugar beet, potato, wheat, barley and oats. The geometry of the crop canopy was found to be a major backscattering influencing factor, especially for the cereals. The possibilities of crop growth parameter (soil cover, biomass, height) estimation from the radar data were investigated using empirical and simple physical relationships. Except for sugar beet in the early growing season, the accuracies of parameter estimation were generally too low to be used in crop growth models.In the optical region, the accuracy of estimating the leaf area index ( LAI ) from vegetation indices was studied. In a case study for sugar beet, the LAI was fairly accurately estimated from the so-called Weighted Difference Vegetation Index ( WDVI ).Two methodologies were developed to link X-band radar and optical remote sensing data with crop growth models. In the first method, remote sensing data were used to estimate the fraction soil cover of a crop as input for a simple lightinterception growth model. This method was especially suitable for the use of optical remote sensing data. The use of X-band radar data was only feasible for sugar beet.In the second method, X-band radar and optical remote sensing data were used to initialize and re-parameterize the crop growth simulation model SUCROS (Simple and Universal Crop Growth Simulator). In six years of sugar beet observations, this method improved the simulation of canopy biomass over the use of SUCROS only. The radar and optical r

This thesis describes research done to ascertain the possibilities and limitations of the use of remote sensing observations for agriculture. The topic is defined in Chapter 1. In Chapter 2 the possible applicability of certain existing models for this study is examined. Three models are developed further in Chapter 3. Two of them describe the relation between properties of a crop and its reflective behaviour; the third is a model for soil reflection. Chapter 4 presents calculations on hypothetical crops; in Chapter 5 some calculations based on actual crop data are described.In the early years of the use of remote sensing, the agricultural applications of this technique were almost exclusively qualitative. However, the quantitative applications have been steadily increasing. These applications always require measured electromagnetic radiative energy to be translated into parameters of agricultural interest, such as coverage, leaf area index, biomass, development stage or health conditions of a crop, and in most situations the qualitative information which crop is being grown must also be determined from the observed radiation.The problem is defined in Chapter 1. The conclusion is that the optical behaviour of a crop is determined by a number of more or less independent parameters. This number generally exceeds the number of data measured. Consequently, in many situations it is very difficult to reliably estimate the quantities that cause this behaviour, if these estimations are only based on the observations. Two additional complicating factors are that the relations between the optical and the agricultural properties of a crop are surely not unequivocal, and that the reflection of a crop also depends on the spatial distribution of the incident radiation. This justifies the conclusion that it is useful to study the relations between the agricultural properties of a crop and the upward radiation, measured under different irradiance conditions.In Chapter 2, it is inve