15 research outputs found

    Assessing water stress of desert vegetation using remote sensing : the case of the Tamarugo forest in the Atacama Desert (Northern Chile)

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    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 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

    Detecting leaf pulvinar movements on NDVI time series of desert trees: A new approach for water stress detection

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    Heliotropic leaf movement or leaf ‘solar tracking’ occurs for a wide variety of plants, including many desert species and some crops. This has an important effect on the canopy spectral reflectance as measured from satellites. For this reason, monitoring systems based on spectral vegetation indices, such as the normalized difference vegetation index (NDVI), should account for heliotropic movements when evaluating the health condition of such species. In the hyper-arid Atacama Desert, Northern Chile, we studied seasonal and diurnal variations of MODIS and Landsat NDVI time series of plantation stands of the endemic species Prosopis tamarugo Phil., subject to different levels of groundwater depletion. As solar irradiation increased during the day and also during the summer, the paraheliotropic leaves of Tamarugo moved to an erectophile position (parallel to the sun rays) making the NDVI signal to drop. This way, Tamarugo stands with no water stress showed a positive NDVI difference between morning and midday (¿NDVImo-mi) and between winter and summer (¿NDVIW-S). In this paper, we showed that the ¿NDVImo-mi of Tamarugo stands can be detected using MODIS Terra and Aqua images, and the ¿NDVIW-S using Landsat or MODIS Terra images. Because pulvinar movement is triggered by changes in cell turgor, the effects of water stress caused by groundwater depletion can be assessed and monitored using ¿NDVImo-mi and ¿NDVIW-S. For an 11-year time series without rainfall events, Landsat ¿NDVIW-S of Tamarugo stands showed a positive linear relationship with cumulative groundwater depletion. We conclude that both ¿NDVImo-mi and ¿NDVIW-S have potential to detect early water stress of paraheliotropic vegetation

    Ecología y diagnóstico de Enallodiplosis discordis (Diptera:Cecidomyiidae): Un nuevo defoliador feroz con repercusiones directas en la pérdida del bosque seco de Prosopis y los medios de vida en Perú

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    The coastal desert of Peru and Chile is home to Prosopis (Leguminosae: Mimosoideae) tree species that are exceptionally well-adapted to the hyperarid conditions and keystone in dry-forest ecosystems. From 2001 to 2018, Prosopis in Peru have suffered widespread defoliation and die-back, with consequent deforestation and collapse in pod production. This paper reports a new insect plague species of Prosopis forest in Peru: Enallodiplosis discordis Gagné 1994 (Diptera: Cecidomyiidae) as a fiercely defoliating agent contributing to widespread Prosopis mortality. An analysis of E. discordis larval taxonomy, life cycle and plague infestation, following El Niño Southern Oscillation (ENSO) 1998/99 is provided. Using distinct lines of evidence, its spread, distribution, and ecology are examined. Over two decades of fieldwork, Prosopis forest die-back and loss was observed devastating rural livelihoods and ecosystem services across lowland regions of southern (Ica), central and northern coastal Peru (Lambayeque, La Libertad, Piura). The collapse in production of Prosopis pods (algarroba, huaranga) and honey was recorded. Supplementary notes provide observations of: (i) plague development, changing land-use and climate, (ii) biological and physical control of E. discordis, (iii) the moth Melipotis aff. indomita (Lepidoptera: Noctuidae) as a concurrent defoliator of Prosopis.Las regiones desérticas costeras del Pacífico de Perú y Chile albergan especies de Prosopis (Leguminosae: Mimosoideae), árboles bien adaptados a las condiciones del desierto y con funciones clave en los ecosistemas de bosques secos. Entre el 2001 y 2017, Prosopis en Perú ha sufrido una extensiva defoliación y muerte regresiva, con la consecuente deforestación y disminución de la producción de vainas de algarrobo. Aquí, se reporta una nueva especie de insecto plaga del bosque de Prosopis en Perú: Enallodiplosis discordis Gagné 1994 (Diptera: Cecidomyiidae), una feroz especie defoliadora que contribuye a la mortalidad generalizada de Prosopis. Se proporciona un análisis de la taxonomía larvaria de E. discordis, ciclo de vida y la infestación ocurrida después de El Niño Oscilación del Sur (ENSO) 1998/99. Su dispersión, distribución y ecología es examinada utilizando distintas líneas de evidencia. Durante casi dos décadas de trabajo de campo, se observó la muerte regresiva del bosque de Prosopis devastando los medios de vida rurales y los servicios de los ecosistemas en las regiones de las tierras bajas del sur (Ica), el centro y el norte de la costa peruana (Lambayeque, La Libertad, Piura). El colapso en la producción de vainas de Prosopis (algarroba, huaranga) y miel también fue registrada. Las notas complementarias proporcionan observaciones sobre: (i) el desarrollo de la plaga y el cambio de uso de la tierra y el clima, (ii) el control biológico y físico de E. discordis, (iii) la polilla Melipotis aff. indomita (Lepidoptera: Noctuidae) como defoliador concurrente de Prosopis

    Mapping Prosopis glandulosa (mesquite) invasion in the arid environment of South African using remote sensing techniques

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    A dissertation submitted to the School of Geography, Archaeology and Environmental Studies, Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science in Environmental Sciences. Johannesburg, March 2016.Mapping Prosopis glandulosa (mesquite) invasion in the arid environment of South Africa using remote sensing techniques Mureriwa, Nyasha Abstract Decades after the first introduction of the Prosopis spp. (mesquite) to South Africa in the late 1800s for its benefits, the invasive nature of the species became apparent as its spread in regions of South Africa resulting in devastating effects to biodiversity, ecosystems and the socio-economic wellbeing of affected regions. Various control and management practices that include biological, physical, chemical and integrated methods have been tested with minimal success as compared to the rapid spread of the species. From previous studies, it has been noted that one of the reasons for the low success rates in mesquite control and management is a lack of sufficient information on the species invasion dynamic in relation to its very similar co-existing species. In order to bridge this gap in knowledge, vegetation species mapping techniques that use remote sensing methods need to be tested for the monitoring, detection and mapping of the species spread. Unlike traditional field survey methods, remote sensing techniques are better at monitoring vegetation as they can cover very large areas and are time-effective and cost-effective. Thus, the aim of this research was to examine the possibility of mapping and spectrally discriminating Prosopis glandulosa from its native co-existing species in semi-arid parts of South Africa using remote sensing methods. The specific objectives of the study were to investigate the spectral separability between Prosopis glandulosa and its co-existing species using field spectral data as well as to upscale the results to different satellites resolutions. Two machine learning algorithms (Random Forest (RF) and Support Vector Machines (SVM)) were also tested in the mapping processes. The first chapter of the study evaluated the spectral discrimination of Prosopis glandulosa from three other species (Acacia karoo, Acacia mellifera and Ziziphus mucronata) in the study area using in-situ spectroscopy in conjunction with the newly developed guided regularized random forest (GRRF) algorithm in identifying key wavelengths for multiclass classification. The GRRF algorithm was used as a method of reducing the problem of high dimensionality associated with hyperspectral data. Results showed that there was an increase in the accuracy of discrimination between the four species when the full set of 1825 wavelengths was used in classification (79.19%) as compared to the classification used by the 11 key wavelengths identified by GRRF (88.59%). Results obtained from the second chapter showed that it is possible to spatially discriminate mesquite from its co-existing acacia species and other general land-cover types at a 2 m resolution with overall accuracies of 86.59% for RF classification and 85.98% for SVM classification. The last part of the study tested the use of the more cost effective SPOT-6 imagery and the RF and SVM algorithms in mapping Prosopis glandulosa invasion and its co-existing indigenous species. The 6 m resolution analysis obtained accuracies of 78.46% for RF and 77.62% for SVM. Overall it was concluded that spatial and spectral discrimination of Prosopis glandulosa from its native co-existing species in semi-arid South Africa was possible with high accuracies through the use of (i) two high resolution, new generation sensors namely, WorldView-2 and SPOT-6; (ii) two robust classification algorithms specifically, RF and SVM and (iii) the newly developed GRRF algorithm for variable selection and reducing the high dimensionality problem associated with hyperspectral data. Some recommendations for future studies include the replication of this study on a larger scale in different invaded areas across the country as well as testing the robustness of the RF and SVM classifiers by making use of other machine learning algorithms and classification methods in species discrimination. Keywords: Prosopis glandulosa, field spectroscopy, cost effectiveness, Guided Regularised Random Forest, Support Vector Machines, Worldview-2, Spot-

    Use of laboratory and image spectroscopy to evaluate drought resistance of Scots pine and to distinguish its ecotypes

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    Cílem této práce bylo navrhnout nedestruktivní způsob měření semenáčků borovice lesní (Pinus sylvestris), zjistit, zda se v laboratorních spektrech borovic projeví stres z nedostatku vody, porovnat, zda se bude reakce borovic lišit dle ekotypu a zkoumat, zda z obrazových a laboratorních spekter bude možné odlišit dva ekotypy borovice lesní. K tomuto účelu byla zpracována obrazová hyperspektrální data semenných sadů ze srpna 2020 a byl proveden tříměsíční laboratorní experiment, v rámci něhož byl u dvouletých semenáčků borovice z náhorního a pahorkatinného ekotypu navozen stres z nedostatku vody. Analýza spektrálních dat proběhla s pomocí smíšených statistických modelů, analýzy rozptylu, analýzy hlavních komponent, natrénování klasifikátorů řízené pixelové klasifikace, vegetačních indexů a lineární regrese. Z analýz bylo zjištěno, že lze detekovat vodní stres u silně stresovaných semenáčků borovice lesní. Nejcitlivější spektrální pásma na obsah vody byla pozorována v oblasti mezi 1000-2500 nm. Počáteční reakce na stres z nedostatku vody se dle ekotypu nelišila, u náhorního ekotypu však byla pozorována rychlejší regenerace po etapě sucha. Dva ekotypy borovice lesní bylo možné s vysokou přesností odlišit jak z laboratorních, tak obrazových dat. I přes vysokou variabilitu mezi jedinci způsobenou vodním...The aim of this study was to propose a non-destructive method for measuring Pinus sylvestris seedlings, to determine whether water stress would be evident in laboratory spectra of pines, to compare whether the response of pines would differ by ecotype, and to investigate whether two ecotypes of Pinus sylvestris could be distinguished using laboratory and image spectroscopy. For these purposes, hyperspectral images of seed orchards from August 2020 were processed and a three-month laboratory experiment was conducted, in which stress from water deficit was induced in two-year-old pine seedlings from the upland and hilly ecotypes. Spectral data were analysed using mixed statistical models, analysis of variance, principal component analysis, training of supervised pixel classifiers, vegetation indices, and linear regression. Based on the analyses, it was found that water stress can be detected in severely stressed Pinus sylvestris seedlings. The most sensitive spectral bands to water content were observed in the region between 1000-2500 nm. The initial response to water stress did not differ by ecotype, but a faster recovery was observed at the upland ecotype after the period of draught. The two Pinus sylvestris ecotypes were distinguished with high accuracy from both laboratory and image spectral...Department of Applied Geoinformatics and CartographyKatedra aplikované geoinformatiky a kartografiePřírodovědecká fakultaFaculty of Scienc

    Variabilidad funcional de Prosopis pallida frente a factores climáticos y edáficos en un gradiente ambiental en la costa norte de Perú

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    Las zonas áridas del mundo representan el 36-40% de la superficie continental del planeta, contienen el 10% del stock de carbono del planeta, y son el hogar de más de 2000 millones de personas. El algarrobo (Prosopis pallida) es una especie nativa de las zonas áridas de la costa oeste de América, y en Perú representa la principal especie arbórea de los bosques secos. La costa norte del Perú es su principal zona de distribución, donde alcanza más del 60% de la cubierta forestal de la región. Esta zona es, además, uno de los puntos de mayor impacto del fenómeno El Niño (ENSO – del inglés El Niño Southern Oscillation), el cual genera una “fase seca” con largos periodos de sequía (con menos de 300 mm de precipitación anual) y una “fase húmeda” con cortos periodos húmedos (con más de 1000 mm anuales). La complejidad climática, junto con el pH alcalino y la textura arenosa del suelo, hacen de este territorio un lugar hostil para la vegetación, que requiere tanto de adaptaciones a condiciones pobres en nutrientes como de una respuesta plástica a la falta y exceso de agua. El objetivo de esta tesis es conocer la variabilidad intraespecífica de las principales poblaciones de algarrobo presentes en la región norte de Perú en respuesta a la variabilidad espacio-temporal del clima y a las características edáficas. Para ello, se seleccionaron 8 poblaciones de algarrobo a lo largo de un gradiente climático en la costa norte de Perú (región de Piura), y se realizaron experimentos de campo e invernadero donde se midieron características morfológicas y fisiológicas del algarrobo en respuesta a la variabilidad climática natural y también en condiciones controladas. Además, analizamos la variabilidad en las características físicas del suelo y la concentración de nutrientes en suelo y hoja, así como el efecto de la proximidad al árbol sobre la concentración de nutrientes en el suelo (efecto isla de fertilidad)

    Spatial characterization of vegetation diversity with satellite remote sensing in the khakea-bray transboundary aquifer

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    >Magister Scientiae - MScThere have been increasing calls to monitor Groundwater-Dependent Ecosystems (GDEs) more effectively, since they are biodiversity hotspots that provide several ecosystem services. The accurate monitoring of GDEs is an indispensable under Sustainable Development Goal (SDG) 15, because it promotes the existence of phreatophytes. It is imperative to monitoring GDEs, since their ecological significance (e.g., as biodiversity hotspots) is not well understood in most environments they exist. For example, vegetation diversity in GDEs requires routine monitoring, to conserve their biodiversity status and to preserve the ecosystem services in these environments. Such monitoring requires robust measures and techniques, particularly in arid environments threatened by groundwater over–abstraction, landcover and climate change. Although in–situ methods are reliable, they are challenging to use in extensive transboundary groundwater resources such as the Khakea-Bray Transboundary Aquifer

    Revisión y evaluación técnico-económica de los sistemas de micro - riego usados en repoblación forestal

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    El micro-riego (o riego deficitario y localizado) es conocido desde hace más de dos mil años por las culturas fenicia, romana y china, aplicado a cultivos de huerta y a árboles frutales. Desde entonces y hasta nuestros días, ha ido extendiéndose y evolucionando tanto en el sector agrícola como en el sector forestal. En este último encuentra su principal aplicación en el establecimiento de brinzales, cuando la causa mayoritaria de mortalidad de las plantas sea el estrés hídrico. Las dudas técnicas y económicas que su aplicación suscita deben ser estudiadas y respondidas. La presente tesis doctoral realiza una exhaustiva revisión bibliográfica de los distintos sistemas de micro-riego forestal existentes, clasificándolos en base a su principio hidráulico de funcionamiento y su eficiencia técnica. Seguidamente, se desarrolla un modelo matemático con el que se obtiene el umbral de marras a partir del cual el micro-riego resulta ventajoso desde el punto de vista económico frente a la tradicional reposición de planta. El modelo se informatiza y se aplica a un amplio conjunto de casos de estudio de repoblación para analizar su utilidad y determinar la sensibilidad de sus diferentes parámetros de entrada. A continuación, se estima el tamaño y evolución del mercado internacional del micro-riego. Los resultados obtenidos con el modelo permiten al repoblador adoptar una decisión razonada respecto a la conveniencia (o no) de incluir riegos de apoyo en sus proyectos. También pueden orientar al fabricante de sistemas de micro-riego para fijar una oferta de precios que resulte atractiva al selvicultor.Doctorado en Conservación y Uso Sostenible de Sistemas Forestale
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