Dissertations / Theses on the topic 'GPP photosynthesis by vegetation'

Rapid climate change in Arctic regions is of concern due to important feedbacks between the Arctic land surface and the global climate system. A large amount of organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated under warmer conditions additional release of CO2 could result in an accelerating feedback on global climate. The strength and direction of Arctic C cycle - climate feedbacks will depend on the growth response of vegetation; if plant growth increases some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in the future is unknown. In this thesis I explore some of the critical unknowns in current understanding of C cycle dynamics in Arctic vegetation. Quantifying gross primary productivity (GPP) over regional scales is complicated by large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use data from five Arctic sites to test the generality of a relationship between leaf area index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden, therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at higher Arctic latitudes. Arctic ecosystems are unique among biomes in the large relative contribution of bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of bryophytes to ecosystem function has been relatively understudied and they are poorly represented in terrestrial C models. I use ground based measurements in Northern Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches in early spring and summer, and develop a simple model of bryophyte GPP. Using the model I compare bryophyte GPP to that of vascular plants before, during and after the summer growing season, finding that productive bryophyte patches can contribute up to 90 % of modelled annual GPP for typical vascular plant communities at the same site, and that the relative magnitude of bryophyte GPP is greatest in spring whilst the vascular plant canopy is still developing. Understanding how GPP relates to plant growth is important in relating remotely sensed increases in Arctic ‘greenness’ to changes in plant C stocks. I use a 13C pulselabelling techniques to follow the fate of recently fixed C in mixed vascular and bryophyte vegetation, with a focus on quantifying the contribution of bryophytes to ecosystem carbon use efficiency (CUE). I show that bryophytes contribute significantly to GPP in mixed vegetation, and act to increase ecosystem CUE. I highlight the importance of including bryophytes, which do not have roots, in aboveground: belowground partitioning schemes in C models. To further explore C turnover in bryophytes, I use the results of a second 13C labelling experiment to develop a model of C turnover in two contrasting Arctic mosses (Polytrichum piliferum and Sphagnum fuscum). I find significant differences in C turnover between Polytrichum piliferum which respires or translocates about 80 % of GPP, while Sphagnum fuscum respires 60 %. This analysis is the first to explicitly model differences in C partitioning between Arctic bryophyte species. Finally, I discuss the implications of each chapter for our understanding of Arctic C dynamics, and suggest areas for further research.

Existing terrestrial carbon accounting models have mainly investigated atmosphere-vegetationsoil stocks and fluxes but have largely ignored the hydrological flux of organic carbon. It is generally assumed that biomass and soil carbon are the only relevant pools in a landscape ecosystem. However, recent findings have suggested that significant amounts of organic carbon can dissolve (dissolved organic carbon or DOC) or particulate (particulate organic carbon or POC) in water and enter the hydrological flux at the catchment scale. A significant quantity of total organic carbon (TOC) sequestered through photosynthesis may be exported from the landscape through the hydrological flux and stored in downstream stocks.¶ This thesis presents a catchment-scale case study investigation into the export of organic carbon through a river system in comparison with carbon that is produced by vegetation through photosynthesis. The Cotter River Catchment was selected as the case study. It is a forested catchment that experienced a major wildfire event in January 2003. The approach is based on an integration of a number of models. The main input data were time series of in-stream carbon measurements and remotely sensed vegetation greenness. The application of models to investigate diffuse chemical substances has dramatically increased in the past few years because of the significant role of hydrology in controlling ecosystem exchange. The research firstly discusses the use of a hydrological simulation model (IHACRES) to analyse organic carbon samples from stream and tributaries in the Cotter River Catchment case study. The IHACRES rainfall-runoff model and a regionalization method are used to estimate stream-flow for the 75 sub-catchments. The simulated streamflow data were used to calculate organic carbon loads from concentrations sampled at five locations in the catchment.¶ The gross primary productivity (GPP) of the vegetation cover in the catchment was estimated using a radiation use efficiency (RUE) model driven by MODIS TERRA data on vegetation greenness and modeled surface irradiance (RS). The relationship between total organic carbon discharged in-stream and total carbon uptake by plants was assessed using a cross-correlation analysis.¶ The IHACRES rainfall-runoff model was successfully calibrated at three gauged sites and performed well. The results of the calibration procedure were used in the regionalization method that enabled streamflow to be estimated at ungauged locations including the seven sampling sites and the 75 sub-catchment areas. The IHACRES modelling approach was found appropriate for investigating a wide range of issues related to the hydrological export of organic carbon at the catchment scale. A weekly sampling program was implemented to provide estimates of TOC, DOC and POC concentrations in the Cotter River Catchment between July 2003 and June 2004. The organic carbon load was estimated using an averaging method.¶ The rate of photosynthesis by vegetation (GPP) was successfully estimated using the radiation use efficiency model to discern general patterns of vegetation productivity at sub-catchment scales. This analysis required detailed spatial resolution of the GPP across the entire catchment area (comprising 75 sub-catchment areas) in addition to the sampling locations. Important factors that varied at the catchment scale during the sampling period July 2003 – June 2004, particularly the wildfire impacts, were also considered in this assessment. ¶ The results of the hydrologic modelling approach and terrestrial GPP outcome were compared using cross correlation and regression analysis. This comparison revealed the likely proportion of catchment GPP that contributes to in-stream hydrological flux of organic carbon. TOC Load was 0.45% of GPP and 22.5 - 25% of litter layer. As a result of this investigation and giving due consideration to the uncertainties in the approach, it can be concluded that the hydrological flux of organic carbon in a forested catchment is a function of gross primary productivity.

Global modeling of gross primary production (GPP) is a critical component of climate change research. On local scales, GPP can be assessed from measuring CO₂ exchange above the plant canopy using tower-based eddy covariance (EC) systems. The limited footprint inherent to this method however, restricts observations to relatively few discrete areas making continuous predictions of global CO₂ fluxes difficult. Recently, the advent of high resolution optical remote sensing devices has offered new possibilities to address some of the scaling issues related to GPP using remote sensing. One key component for inferring GPP spectrally is the efficiency (ε) with which plants can use absorbed photosynthetically active radiation to produce biomass. While recent years have seen progress in measuring ε using the photochemical reflectance index (PRI), little is known about the temporal and spatial requirements for up-scaling these findings continuously throughout the landscape. Satellite observations of canopy reflectance are subject to view and illumination effects induced by the bi-directional reflectance distribution function(BRDF) which can confound the desired PRI signal. Further uncertainties include dependencies of PRI on canopy structure, understorey, species composition and leaf pigment concentration. The objective of this research was to investigate the effects of these factors on PRI to facilitate the modeling of GPP in a continuous fashion. Canopy spectra were sampled over a one-year period using an automated tower-based, multi-angular spectroradiometer platform (AMSPEC), designed to sample high spectral resolution data. The wide range of illumination and viewing geometries seen by the instrument permitted comprehensive modeling of the BRDF. Isolation of physiologically induced changes in PRI yielded a high correlation (r²=0.82, p<0.05) to EC-measured ε, thereby demonstrating the capability of PRI to model ε throughout the year. The results were extrapolated to the landscape scale using airborne laser-scanning (light detection and ranging, LiDAR) and high correlations were found between remotely-sensed and EC-measured GPP (r²>0.79, p<0.05). Permanently established tower-based canopy reflectance measurements are helpful for ongoing research aimed at up-scaling ε to landscape and global scales and facilitate a better understanding of physiological cycles of vegetation and serve as a calibration tool for broader band satellite observations.

Mestrado MEDfOR - Mediterranean Forestry and Natural Resources Management - Instituto Superior de Agronomia
The Eddy covariance method is a widespread method used for measuring carbon fluxes between the atmosphere and the ecosystem. It provides a high temporal resolution of measurements, but it is restricted to an area around the tower called footprint, and other methods are usually used in combination with eddy covariance data in order to estimate carbon fluxes for larger areas. Spectral vegetation indices derived from increasingly available satellite data can be combined with eddy covariance data to estimate carbon fluxes outside of the tower footprint. Following that approach, the present study attempted to model carbon fluxes for a karst grassland in Slovenia. Three types of model were considered: (1) a linear relationship between NEE or GPP and each vegetation index, (2) a linear relationship between GPP and the product of a vegetation index with PAR, and (3) a simplified LUE model assuming a constant LUE. We compared the performance of several vegetation indices from two sources (Landsat and SPOT-Vegetation) as predictors of NEE and GPP, based on three accuracy metrics (R², RMSE and AIC). Two types of aggregation of flux data were explored, midday average fluxes and daily average fluxes. The Vapor Pressure Deficit was used to separate the growing season in two phases, a greening phase and a dry phase, which were considered separately in the modelling process, in addition to the growing season as a whole. The results showed that NDVI was the best predictor of GPP and NEE during the greening phase, whereas water related vegetation indices, namely LSWI and MNDWI were the best predictors during the dry phase, both for midday and daily aggregates. Model type 1 (linear relationship) was found to be the best in many cases. The best regression equations obtained were used to illustrate the mapping of GPP and NEE for the study area
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Considerando que estudos sobre fenologia vegetal são importantes para a compreensão do funcionamento e verificação da ocorrência de padrões no ciclo vegetativo das plantas, resultando em melhorias nas atividades de conservação e manejo, o objetivo desta pesquisa foi caracterizar a dinâmica fenológica de diferentes tipologias campestres no estado do Rio Grande do Sul (RS), a partir da relação entre a variabilidade de elementos climáticos intra e interanual e eventos em larga escala e a distribuição espaço-temporal das tipologias predominantes. A área de estudo abrangeu 10 tipologias predominantes de campo no estado do RS. A base de dados orbitais utilizada foi obtida de diferentes produtos relacionados ao estudo da vegetação do sensor MODIS (Moderate Resolution Imaging Spectroradiometer), constando os índices de vegetação NDVI (Normalized Difference Vegetation Index), EVI (Enhanced Vegetation Index) e GPP (Gross Primary Productivity). Também, foram utilizados dados meteorológicos provenientes da base TRMM (Tropical Rainfall Measuring Mission) e ERA Interim, para o período de fevereiro de 2000 a dezembro de 2014. O uso de séries temporais de dados NDVI e EVI/MODIS permitiram obter informações sobre a fenologia da vegetação campestre e a definição de padrões diretamente relacionados a variações meteorológicas. A sazonalidade da vegetação campestre apresenta cliclo anual bem marcado, com início e fim da estação de crescimento determinada pelas condições térmicas (temperatura do ar), porém alterado pela disponibilidade hídrica. A relação entre temperatura do ar e vigor vegetal apresentou maior correlação e tem influência direta sobre o início e fim da estação de crescimento (primavera e verão) A precipitação pluvial, no entanto, influencia as condições de crescimento/desenvolvimento das tipologias campestres, especialmente no verão, associado aos períodos de estiagem que tendem a ocorrer com maior frequência. Ambos os índices (EVI e NDVI) apresentam maior variabilidade durante a primavera e o verão, com diminuição da variabilidade durante o outono e inverno. A aplicação da Transformada de Ondaleta mostrou onde e quando ocorreram alterações no padrão fenológico da vegetação campestre e a Transformada Coerência apontou a intensidade (correlação) entre os índices de vegetação e a variabilidade das condições meteorológicas. O agrupamento das tipologias, com uso da técnica de Cluster, revelou seus comportamentos sazonais, sendo que a partir do índice EVI há a possibilidade de identificar diferenças entre as tipologias durante o outono e inverno, enquanto o NDVI apresentou diferença somente no inverno. As métricas fenológicas obtidas do Timesat para as imagens EVI permitiram obter dados importantes sobre o ciclo fenológico da vegetação campestre do RS, com a caracterização do padrão fenológico das tipologias predominantes. O uso de modelos para a estimativa da produtividade da vegetação campestre a partir do EVI revelou dentre as tipologias testadas que a CSR (campos de solos rasos) apresentou maior capacidade de explicar a variabilidade da produtividade dos campos por ser mais suscetível às variações meteorológicas. Os resultados obtidos permitiram confirmar a diversidade entre as tipologias campestres predominantes no RS, expressas por índices de vegetação, tanto no aspecto temporal como espacial. O uso dos índices de vegetação demonstrou potencial no monitoramento do padrão fenológico da vegetação campestre frente a variabilidade climática do RS.
Considering that studies on vegetal phenology are important to understand the mechanisms and pattern recognition on the vegetative cycle of plants, resulting in improvements in conservation and management activities, the aim of this research was to characterize the phenological dynamics of different grassland typologies in Rio Grande do Sul State (RS), based on the relationship between the variability of intra-annual and inter-annual climatic elements, large-scale events and the spatio-temporal distribution of predominant typologies . The study area included 10 predominant grassland typologies in RS state. The orbital database used was obtained from different products related to vegetation studies of MODIS sensor (Moderate Resolution Imaging Spectroradiometer), presenting the vegetation indices NDVI (Normalized Difference Vegetation Index), EVI (Enhanced Vegetation Index) and GPP (Gross Primary Productivity). Also, meteorological data from TRMM base (Tropical Rainfall Measuring Mission) and ERA Interim were used for the period of February 2000 to December 2014. The use of time series data from NDVI and EVI/MODIS led to information on grassland vegetal phenology and the definition of patterns directly related to meteorological variations. The seasonality of grassland vegetation presents a well marked annual cycle, with the beginning and the end of growing season determined by thermal conditions (air temperature) but altered by water availability. The relationship between air temperature and vegetal vigor presented a strong correlation and influences directly on the beginning and on the end of the growth season (spring and summer). The rainfall, however, influences growth/development conditions of grassland typologies, especially in summer, associated to drought periods that tend to occur more frequently Both indices (EVI and NDVI) presented a greater variability during spring and summer, with a lesser variability during fall and winter. The application of Ondaleta Transform showed where and when alterations occurred in the phenological pattern of grassland vegetation and the Coherence Transform pointed the intensity (correlation) between vegetation indices and the variability of meteorological conditions. The grouping of typologies, using the Cluster technique, revealed their seasonal behaviors, and from the EVI index there is the possibility of identifying differences between typologies during fall and winter, whereas NDVI showed differences only in winter. The phenological metrics obtained from Timesat to EVI images allowed to obtain important data on the phenological cycle of grassland vegetation of RS state, with a characterization of the phenological pattern. The use of models for estimation of productivity of grassland vegetation based on EVI revealed among the typologies tested that the CSR (shallow soils grasslands) presented greater ability to explain the variability of grasslands productivity because it is more susceptible to meteorological variations. The obtained results allowed for the confirmation of diversity among the grassland typologies predominant in RS state, expressed by vegetation indices, both in temporal and spatial aspects. The use of vegetation indices demonstrated potential on the monitoring of phenological pattern of grassland vegetation considering the climatic variability of RS state.

Tropical montane cloud forests (TMCF) are one of the most fascinating, but least understood ecosystems in the world, and the interest in the carbon (C) cycle of TMCFs with regard to carbon sequestration and storage practices has increased rapidly in recent years. One feature that prevails in all TMCFs is a decrease in aboveground net primary productivity (ANPP) and standing biomass and leaf area index (LAI) with increasing altitude, together with the stunted growth form of the trees. This thesis focuses on the input part of the TMCF C-cycle, and investigates the controlling factors on photosynthesis on a leaf, canopy, and ecosystem level in the Kosñipata valley in south east Peru, on the eastern slope of the Andes (13º11’28’’S / 71º35’24’’W). Leaf traits are known to relate to foliar C-exchange, and compared with other altitudinal transect studies of TMCFs, the studied sites had similar altitudinal trends for foliar nitrogen (N) content (though not for phosphorus) and leaf mass per area (LMA), with N content decreasing and LMA increasing with altitude. N concentrations were relatively high and LMA values relatively low, but this observed relationship was consistent with those found in global leaf trait surveys. Examining plant stoichiometry (i.e. N:P ratios), the data suggests that unlike the general hypothesis, the Kosñipata forests are not N limited, except for the study site at 2990 m a.s.l. At the 2990 m a.s.l. site, which is the focal study site of the thesis, photosynthetic parameters Vcmax (the carboxylation efficiency of the Rubisco protein) and Jmax (the electron transport efficiency) proved to be similar to those found in lowland tropical rainforest leaves when expressed on an area basis and standardised to 25 °C (55.6 ± 2.6 and 106.5 ± 5.2 mmol m-2 s-1, for Vcmax and Jmax, respectively). However, when standardised to the mean ambient TMCF temperature of 12.5 °C, both photosynthetic parameters were much lower than ambient tropical rainforest Vcmax and Jmax values. The TMCF Jmax -Vcmax relationships were steeper than found in other tropical biomes, indicating a possible adaptation to the lower light availability in TMCFs because of frequent cloud cover, or a consequence of little atmospheric evaporative demand, which is also due to the humid conditions in this forest type. Although N-Vcmax relationships were significant (P<0.05), the fit was not very strong and the relationship between nitrogen use efficiency (NUE) and Vcmax indicates that TMCF species can be regarded as a different plant functional type compared with other tropical forest types. Diurnal measurements of net photosynthesis (A), stomatal conductance (gs) and leaf water potential (Yleaf) showed that different TMCF species experienced non-contrasting diurnal patterns of Yleaf and gs in the dry season. The observed patterns suggest that some TMCF species can be classified as isohydric species, while others behave anisohydrically. Additionally, in situ gs was not very responsive to these to the range of experienced photosynthetically active radiation (PAR), vapour pressure deficit (VPD) or soil water content (SWC), leading to the conclusion that in the studied TMCF, drought stress does not play a role in C-uptake. When using the measured photosynthetic parameters for up-scaling C-uptake to stand scale with a Soil-Plant-Atmosphere model, simulated annual gross primary productivity (GPP) was 16.24 ±1.6 T C ha-1 yr-1, which is about half the GPP observed in neotropical lowland rainforests. Analyses of the modelled results showed that GPP in this TMCF is mostly controlled by temperature, PAR and leaf area index (LAI) and when increasing these three factors to values found in tropical lowland forest, GPP increased up to 75%. In addition, the modelled results indicate that hydraulic limitations on TMCF C-uptake are very unlikely under current climatic conditions. The modelled results also showed that increases in radiation as a result of less cloud cover do not translate to straightforward increases of GPP. The cloudy conditions of TMCFs, which reduced incident PAR in TMCFs, should therefore not be regarded simply as a negative control on TMCF GPP. Instead, the increase in fraction of diffuse radiation partially offsets the decrease in GPP following the reduction in PAR. Overall, the results of this study show that leaves of Andean TMCF forests have similar C-uptake capacity to tropical lowland rainforests when standardized to similar temperatures, but that for in situ C-uptake temperature, radiation and LAI are the key controls.

Moss dominated ecosystems are an important part of the global terrestrial carbon cycle. Over large areas, remote sensing can be useful to provide an improved understanding of these ecosystems. Two boreal mossess (Pleurozium and Sphagnum) were assessed using remote sensing based spectral vegetation indices for estimating biochemical capacity and photosynthetic efficiency by varying net photosynthesis rate via changes in water content. In the laboratory, changes in the normalized difference vegetation index (NDVI) and chlorophyll index coincided with declining photosynthetic capacity due to desiccation. This effect was more dramatic in Sphagnum. The photochemical reflectance index (PRI) did not vary with changes in CO2 supply as anticipated, possibly due to overriding effects of changing water content. The water band index (WBI) was strongly related to water content but this relationship showed an uncoupling in the field. Bi-directional reflectance measurements indicated what WBI was sensitive to sensor, sun, and moss surface slope angles.
xi, 110 leaves : ill. (some col.) ; 29 cm.

Bibliography: pages 162-177.
The seasonal and diurnal patterns of photosynthetic gas exchange and the water relations of seven species of the mediterranean-climate region of South Africa (fynbos) were investigated. The following species, representing the major fynbos elements, were chosen for intensive investigation: Erica plukenetii and Erica hispidula (ericoid element), Thamnochortus lucens and Askidiosperma paniculatum (restioid element), Protea laurifolia and Leucadendron salignum (proteoid element). Metrosideros angustifolia, a shrub of riparian habitats, was also studied.

A la conca Mediterrània els principals factors que afecten el rendiment dels cultius agrícoles són la sequera i el dèficit de nitrogen. És necessària la recerca de noves eines de fenotipejat per accelerar la millora genètica pel rendiment potencial i l'adaptació dels cultius a condicions limitants. Aquesta Tesi ha estudiat composició isotòpica de carboni (δ13C), oxigen (δ18O) i hidrogen (δ2H), en la seva abundància natural, de diferents cereals. Referent a la δ13C només la composició dels grans madurs van mostrar correlacions fenotípiques (negatives) consistents amb el rendiment del gra (GY) en blat dur, sobretot sota condicions d'estrès moderat (Capítol 5). Aquesta Tesi també proposa l’ús de la δ13C com una aproximació per quantificar la contribució relativa dels diferents òrgans de la planta a l'ompliment del gra (Capítols 1 i 2). L’objectiu es poder emprar la δ13C per seleccionar cereals amb una major fotosíntesi de l'espiga. En blat dur l'aproximació de la δ13C va assignar un paper més gran a l'espiga (tant de l'òrgan sencer com de les arestes) en comparació a la fulla bandera i al peduncle (que representa els assimilats que provenen de les parts inferiors de l'espiga), especialment en les varietats antigues, amb independència de les condicions de creixement. Sota bones condicions agronòmiques, la contribució de les arestes de l’espiga a l'ompliment del gra de blat fariner també va ser més important que la de la fulla bandera. Finalment aquesta aproximació basada en la δ13C de diferents parts de planta es va comparar amb altres aproximacions convencionals com són l'ombrejat o l'aplicació de l'herbicida DCMU a la planta (Capítol 3). Els tractaments d'ombrejat de l'espiga i de la tija van contribuir de forma similar a l'ompliment del gra. Per contra, l'aproximació del DCMU va assignar un paper major a la fotosíntesi de la tija, però l'aplicació de l’herbicida a la tija també va afectar l'espiga, esbiaixant el pes final dels grans. Aquesta Tesi també va estudiar l'isòtop d'oxigen tant en blat de moro (Capítol 4) com en blat dur (Capítol 5). En el blat de moro, les correlacions genotípiques entre la δ18O i el GY van ser marginals, tot i que les sedes va ser l'òrgan que millor va correlacionar amb el GY. En el blat dur les correlacions fenotípiques entre la δ18O dels grans i el GY també van ser marginals. Només va correlacionar fortament la δ18O de l'aigua de la fulla quan es van combinar els dos règims hídrics (reg suplementari i sequera). L'absència de correlacions de la δ18O podria ajudar a descartar els teixits de les plantes que són més susceptibles als processos de fraccionament post-fotosintètic. Escollir l’òrgan idoni pot ajudar a millorar l’ús de la δ18O com a eina de millora de cultius. Per últim es va comparar la δ13C i la δ18O amb la δ2H (Capítol 5). La δ2H en els grans de blat dur va correlacionar contra el GY millor que els altres dos isòtops en condicions de sequera però combinant nivells de nitrogen contrastats. També es van observar correlacions genotípiques entre la δ2H dels foto-assimilats de l'espiga contra el GY. A més, la δ2H de les fulles va correlacionar més amb la δ13C que no pas amb la δ18O, el que suggereix que la δ2H dels foto-assimilats de la fulla no només està afectada per la transpiració i la conductància estomàtica sinó també per les reaccions fotosintètiques. A més, els valors baixos de la δ2H a l’espiga comparats amb els dels grans donen suport al paper fotosintètic de l’espiga, el que recolza els resultats obtinguts en els Capítols 1, 2 i 3.
This Thesis has studied the isotope composition on its natural abundance of carbon (δ13C), oxygen (δ18O) and hydrogen (δ2H) as phenotypic traits for cereal breeding and crop adaptation to optimal and limited agronomic conditions. Regarding the δ13C, only mature grains showed consistent phenotypic correlations (negative) against grain yield in durum wheat, especially under moderate stress conditions. In addition δ13C is also proposed as a tool to quantify the relative contribution of different plant organs to grain filling. In durum wheat and bread wheat δ13C approach assigned a higher role to the ear (both whole body and awns) compared to the flag leaf and peduncle (which integrates the assimilates produced by photosynthetic organs below the ear), regardless of growing conditions. Finally, δ13C approach based on the different plant parts was compared with other conventional approaches, such as shading or herbicide DCMU application, which assigned on average a comparable contribution to the ear than the culm. This thesis also studied the δ18O in maize and durum wheat. In both crops, phenotypic correlations between δ18O and grain yield were marginal. Only δ18O of leaf water in durum wheat was strongly correlated with GY when combining two water regimes. The absence of such correlations will eventually help to understand the use of δ18O as a genotype selection tool for the adaptation of maize and other crops to drought. Finally the δ13C and δ18O were compared with δ2H in durum wheat. δ2H performed better than the other two isotopes predicting grain yield and nitrogen content under water stress but contrasting nitrogen regimes. Besides, genotypic correlations between δ2H in the ear water-soluble fraction and grain yield were observed. In addition, δ2H in the water soluble fraction of leaves was better correlated against δ13C than with δ18O, suggesting that δ2H of leaf photo-assimilated is affected not only by transpiration and stomatal conductance but also by the photosynthetic reactions. In addition, the low values observed in the δ2H in the ear compared to mature grains supported the photosynthetic role of the ear, which reinforced results obtained in other chapter of this Thesis.

Cette etude realisee a differents niveaux de perception (du peuplement a la cellule) est une contribution a une meilleure connaissance du fonctionnement et de la dynamique des landes subalpines a rhododendron ferrugineum. Dans la premiere partie, l'analyse diachronique permet de replacer la dynamique des populations de rhododendron dans un contexte global de developpement des communautes vegetales a l'etage subalpin des alpes nord-occidentales. Dans la seconde partie, l'etude demographique realisee dans une sequence pelouselande ouvertelande fermee montre que le temps necessaire pour que l'arbuste occupe plus de 80% de l'espace varie entre 150 et 300 ans. Au cours de ce developpement il adopte progressivement la reproduction vegetative, augmente son emprise sur le milieu ; certaines caracteristiques edaphiques sont modifiees, ainsi que les communautes vegetales qu'il envahit. La troisieme partie montre comment l'ericacee est capable de bien exploiter les ressources du milieu notamment de prelever et d'assimiler l'azote sous ses differentes formes minerales et de les incorporer dans des cycles internes efficients. La resorption de composes azotes agissant dans le compartiment chlorophyllien couvre environ 50% des besoins de la productivite primaire nette. Cette strategie de conservation permet a la plante d'optimiser l'utilisation des nutriments et de pallier la relative infertilite des sols sur lesquels elle se developpe. Les mesures de transpiration et de conductance montrent que le comportement stomatique des individus varie peu avec leur position microstationnelle. Par contre la masse d'eau transpiree par 1 m#2 de lande, fonction de la structure des houppiers, varie avec le degre de fermeture des populations. La quatrieme partie est une reflexion portant sur les interactions entre l'ericacee et son environnement. Des hypotheses sont emises quant a ses aptitudes a dominer la plupart des especes subalpines

Existing terrestrial carbon accounting models have mainly investigated atmosphere-vegetationsoil stocks and fluxes but have largely ignored the hydrological flux of organic carbon. It is generally assumed that biomass and soil carbon are the only relevant pools in a landscape ecosystem. However, recent findings have suggested that significant amounts of organic carbon can dissolve (dissolved organic carbon or DOC) or particulate (particulate organic carbon or POC) in water and enter the hydrological flux at the catchment scale. A significant quantity of total organic carbon (TOC) sequestered through photosynthesis may be exported from the landscape through the hydrological flux and stored in downstream stocks.¶ This thesis presents a catchment-scale case study investigation into the export of organic carbon through a river system in comparison with carbon that is produced by vegetation through photosynthesis. The Cotter River Catchment was selected as the case study. It is a forested catchment that experienced a major wildfire event in January 2003. The approach is based on an integration of a number of models. The main input data were time series of in-stream carbon measurements and remotely sensed vegetation greenness. The application of models to investigate diffuse chemical substances has dramatically increased in the past few years because of the significant role of hydrology in controlling ecosystem exchange. The research firstly discusses the use of a hydrological simulation model (IHACRES) to analyse organic carbon samples from stream and tributaries in the Cotter River Catchment case study. The IHACRES rainfall-runoff model and a regionalization method are used to estimate stream-flow for the 75 sub-catchments. The simulated streamflow data were used to calculate organic carbon loads from concentrations sampled at five locations in the catchment.¶ ...

University of Technology Sydney. Faculty of Science.
Australia’s dryland ecosystems play a critical role in regulating the climate system and considerably influence the interannual variability in global carbon cycle. However, the dynamics of dryland vegetation under climate variability and extreme events have not been as thoroughly investigated as in other ecosystems. Spaceborne solar-induced chlorophyll fluorescence (SIF) provide a fresh means to evaluate vegetation photosynthetic activity and detect vegetation stress. Considering its spatially coarse resolution, studies with reference to the application of SIF over heterogeneous dryland ecosystem are rarely reported. The main goal of this thesis is to explore the spatial and temporal dynamics of Australia’s dryland vegetation under hydro-climatic extremes using satellite-estimated fluorescence and greenness. To achieve this goal, I first utilized a strong wet pulse in 2016-2017 as well as in the 2011 big wet period as natural experiments to assess the response of major dryland biomes in central Australia. Next, I investigated the impact of a recent extreme drought on spatiotemporal variability of Australia’s dryland vegetation indicated by multi-source satellite-based SIF. Finally, I analysed the spatial pattern and seasonal variations in dryland vegetation phenology under climate variability. The results showed semiarid ecosystems to have the largest variability and were most sensitive to climate extremes. SIF derived from the Global Ozone Monitoring Experiment-2 (GOME-2) at 0.5 spatial resolution has an insufficient capacity for capturing spatiotemporal dynamics over xeric central Australia as a result of low signal level and high retrieval noise. In contrast to humid ecosystems, both SIF and enhanced vegetation index (EVI) simultaneously captured the declines of arid/semiarid plant growth from the beginning of extreme drought events at 16-day scale. SIF data retrieved from TROPOspheric Monitoring Instrument (TROPOMI) at a 0.05 spatial grid exhibits promising capability of mapping and characterizing the dynamics of heterogeneous dryland vegetation in future. This thesis highlights that the incorporation of satellite-observed greenness and fluorescence can potentially contribute to an improved understanding of dryland vegetation dynamics and can advance our ability to detect ecosystem alterations under future changing climates.

Plant breeders can select cultivars with physiological traits that confer a growth or yield advantage to individual plants. The extent to which single plant characters influence canopy performance depends on interactions between vegetation and the atmosphere and the non-linear response of physiological processes to the environment. Better understanding of the scaling of photosynthesis and water use will allow the assessment of changes to leaf scale physiological traits at the canopy scale and prediction of the response of vegetation to climate change. This thesis examines the relationship between reduced stomatal conductance and canopy scale water-use efficiency (ratio of instantaneous net canopy photosynthesis to total canopy evaporation). A multi-disciplinary research project was established with two large paddocks of wheat with cultivars of contrasting leaf-scale water-use efficiency, due to inherent differences in stomatal conductance. Intensive measurements were made of C02 and H20 fluxes at leaf and canopy scales. Different stomatal conductances at the leaf scale were reflected at the. canopy scale, although their effects on transpiration were reduced due to canopy boundary layers and soil evaporation. Comparison of scaling from leaf to canopy in the two crops was complicated by their different leaf area indices. To facilitate scaling from leaves to canopies, models of stomatal conductance, leaf photosynthesis and radiation penetration in canopies were used. A comparison of several models of conductance with field data found that using the correlation of conductance with photosynthesis was the best approach. The same model was found to work equally well at the canopy scale, using parameters derived from leaf scale data. Canopy photosynthesis was modelled with a biochemical model of leaf photosynthesis incorporated into different integration schemes. A canopy model which divided the canopy into a single layer of sunlit and shaded leaves was found to be as accurate as a multi-layer model, but simpler and allowed incorporation of within-canopy profiles of photosynthetic capacity. A big-leaf model of canopy photosynthesis was found acceptable if tuned, but the uncertainties increased when it was used to predict responses of canopies with different properties. Photosynthetic capacity, the main parameter of the canopy photosynthesis model, was found to decrease during the day under conditions of mild water stress at both the leaf and canopy scale. Combined models of photosynthesis, conductance and energy balance accurately described diurnal variation of canopy gas exchange. The model predicted that a 40% reduction in stomatal conductance would result in 36% greater leaf transpiration efficiency and 19% greater canopy transpiration efficiency (ratio of gross canopy photosynthesis to canopy transpiration) which compared favourably with field measurements, but depended on the magnitude of the conductance and wind speed. Measurements of air temperature, humidity and surface temperature along a transect across the interface between the two crops with different evaporation rates, showed that advection did occur, but that it had minimal impact on canopy fluxes. It was concluded that reduced stomatal conductance does result in reduced transpiration and better transpiration efficiency at the canopy scale, but that canopy boundary layers and greater soil evaporation reduce the benefit. In this case reduced conductance was also accompanied by greater yield, although this result depends on the availability of soil water. The models presented were an effective tool for scaling nonlinear physiological processes from leaves to canopies and provide a useful framework for assessing the impact of climate change on vegetation.

Many ecological systems follow a seasonal cycle affecting primary production, carbon flux, and vegetative gas emissions. The seasonal variation of ecological systems are both affected by and have effects upon climatic factors. A quantitative estimate of the seasonal variation of vegetation is required to characterize ecological systems and their interaction with climate. Monitoring the spaliotemporal variation of foliar biomass density (FBD) over one year will provide a quantitative estimate of the annual cycle and regional variation of photosynthetic activity. FBD is a quantitative measure of leafy material per unit of area produ\:ed by photosynthetically active vegetation. This seasonal variation in FBD is an important parameter for global and other large scale investigations of ecological, hydrological, and biogeochemical systems which require data and expertise from a variety of sources and disciplines. Therefore, FBD is potentially of great utility for ecologists, hydrologists, climatologists, and atmospheric scientists. Recent regional scale investigations of ecological systems concluded that the repetitive coverage and synoptic view of remotely sensed measurements provide data to monitor the seasonal variation of biomass. A method to estimate the seasonal variation of FBD at global scales has not been developed. The objective of this research is to develop a methodology that could be used to estimate the seasonal variation of FBD for the entire terrestrial biosphere. By coupling global satellite data, measured field data, and a vegetation classification, a model was developed to estimate the global spatiotemporal variation of FBD. Comparisons between literature estimates of FBD and estimated FBD from this model were made as a means of validation. A more specific comparison was conducted between grasslands based on work conducted in the Senegalese Sahel region in Africa. Finally, a sensitivity analysis was performed to characterize the potential propagation of error associated with the literature FBD estimates used to drive this model.
Graduation date: 1992

Tropical ecosystems play a key role in regulating the global climate and the carbon cycle thanks to the large amounts of water and carbon exchanged with the atmosphere. These biogeochemical fluxes are largely the result of high photosynthetic rates. Photosynthetic activity is highly dependent on climate and vegetation, and therefore can be easily modified along with changes in those two factors. A better understanding of what drives or alters photosynthetic activity in the tropics will lead to more accurate predictions of climate and subsequent effects on ecosystems. The seasonal pattern of photosynthetic activity is one of the main uncertainties that we still have about tropical ecosystems. However, this seasonality of tropical vegetation and its relationship to climate change and land cover is key to understanding how these ecosystems could be affected and have an effect on climate.

In this dissertation, I present three projects to improve our understanding about tropical ecosystems and how their photosynthetic activity is affected by climate and land cover change. The lack of field-based data has been one of the main limiting factors in our study of tropical ecosystems. Therefore, in these projects I extensively use remote sensing-derived data to analyze large scale and long term patterns. In the first study, I looked at the seasonal relationship between photosynthetic activity and climate, and how model simulations represent it. Vegetation in most of the tropics is either positively correlated with both water and light, or positively correlated with one of them and negatively with the other. Ecosystem models largely underestimate positive correlations with light and overestimate positive correlations with water. In the second study, I focus on the effect of land cover change in photosynthetic activity and transpiration in a highly deforested region in the Amazon. I find that land cover change decreases tropical forests photosynthetic activity and transpiration during the dry season. Also, land cover change increases the range of photosynthetic activity and transpiration in forests and shrublands. These effects are intensified with increasing land cover change. In the last project, I quantify the amount of change in evapotranspiration due to land cover change in the entire Amazon basin. Our remote sensing-derived estimates are well aligned with model predictions published in the past three decades. These results increase our confidence in climate models representation of evapotranspiration in the Amazon.

Findings from this dissertation highlight (1) the importance of the close relationship between climate and photosynthetic activity and (2) how land cover change is altering that relationship. We hope our results can build on our knowledge about tropical ecosystems and how they could change in the future. We also expect our analysis to be used for model benchmarking and tropical ecosystem monitoring.