1,721,774 research outputs found
The effects of nitrogen availability on the response of terrestrial ecosystems to elevated carbon dioxide
Full text linkHuman activities are increasing the concentration of carbon dioxide (CO2) in the atmosphere, warming the planet. Terrestrial ecosystems currently sequester about a quarter of human CO2 emissions, slowing climate change. The principal mechanism believed to be responsible for this is an increasing rate of plant growth (that is, “CO2 fertilization”). However, the fate of this ecosystem service is uncertain, as it has been proposed that soil nitrogen (N) availability will limit plants’ capacity to continue absorbing increasing quantities of CO2. Whether N will limit the CO2 fertilization effect in the future will determine the rate at which human CO2 emissions will accumulate in the atmosphere, thereby influencing the climate. In this thesis, I have collected and synthesized the large body of information about the N limitation of CO2 fertilization, using data from experiments in which atmospheric CO2 concentration is manipulated. I have found that the hypothesis that the increase in the strength of the CO2 fertilization effect will be eliminated by restricted N availability is simplistic. Based on the experimental data available, I have found evidence supporting a mechanism by which plants under elevated CO2 can acquire additional N in exchange for carbohydrates via symbiotic fungi. Using this framework, I have quantified the magnitude of the terrestrial CO2 fertilization effect on plant biomass worldwide, and identified the areas of the global land mass that could potentially experience a greater enhancement in biomass under elevated CO2. I propose a framework and areas of further research that may help models better simulate the interactions between the carbon and nitrogen cycles under elevated CO2 using a plant- economics approach, in which nitrogen is a resource that can be acquired by plants in exchange for energy.Open Acces
Crop production and global food security in relation to climate variation: an empirical analysis
Full text linkThe challenge of meeting increasing global food demand is amplified by climate change. Crop yield is vulnerable to extreme conditions, including heatwaves, droughts and downpours, leading to widespread concern about negative effects of climate change on food security. This thesis describes a novel empirical analysis of total production, yields and harvested area data for three major crops (wheat, maize and soybean), using a unique, global, gridded agricultural time-series data set. Trend analysis is applied to changes in production, yield and harvested area of these three crops. Machine learning is used to quantify their responses to climate. A new methodology is introduced to identify “shocks”.
Results show a more complex dynamics of agricultural production than is suggested by current liter- ature. Large changes in regional production, driven by harvested area rather than yield, have been driven by policy shifts. A large “killing degree-day” sum depresses yields for some regions and crops, but enhances them in others. Heat deficits can be as deleterious as heatwaves. Shocks can be negative or positive. Production variability has increased, but major negative shocks have been few, and have not become more frequent. Production shocks have been caused as often by changes in harvested area as in yield.
These findings do not support a universal negative effect of climate change on crop production. More- over, stable global food supplies will not be assured by maximizing yields. It is equally important that farmers in different countries and environments grow a variety of crops. Climate-related risk is currently concentrated in the most productive baskets, exposing the global food supply to avoidably high risk. Increasing frequencies of climate extremes in the main producing areas only make such shocks more likely. Various measures that are not directly related to climate would help to make global food supplies more resilient.Open Acces
Principles of carbon allocation in plants and ecosystems
No full textCarbon (C) allocation refers to the processes by which plants distribute assimilated C among different compartments. While most ecosystem and land surface models explicitly represent C allocation, there is no consensus on how this should be done: treatment in many models remains rudimentary, especially when compared to more advanced representations of C assimilation. C allocation needs theoretical analysis, with predictions tested against experimental and large-scale observational data, to strengthen model foundations. This thesis develops robust semi-empirical models of C allocation of root:shoot biomass ratios (R:S), forest dynamics and biomass production efficiency (BPE) in order to explore how C partitioning is influenced by the availability of different resources. The predictors of R:S are selected based on eco-evolutionary optimality (EEO) principles. It is hypothesized that the demands of foliage production, and concomitant below-ground production to support that foliage, are satisfied with highest priority; and that any excess C (the net C profit, Pn) is allocated to stems in such a way as to maximize height growth, as a strategy for competitive fitness. The average diameter growth of a tree, and maximum tree height, in an even-aged forest are shown to be proportional to Pn. BPE quantifies the efficiency of assimilated C that is converted into structural growth. It reflects the balance between C gain by photosynthesis and C losses, principally autotrophic respiration (Ra). BPE is shown to decrease with growth temperature (Tg), stand age, soil C:N ratio, pH and sand content, and to increase with mean temperature of the coldest month—resolving a contradiction in the literature, about its apparent response to mean annual temperature—and to be greater for deciduous than evergreen woody plants. These findings contribute to an optimality-based theoretical framework for improved process-based C allocation modelling.Open Acces
Changes in productivity across northern terrestrial ecosystems
Full text linkIt is well established that the terrestrial biosphere is currently a net carbon sink, driven by increases in productivity, but the mechanisms involved remain uncertain. Observations indicating large-scale increases in productivity across northern terrestrial ecosystems include aircraft data that show an increase in the amplitude of the seasonal cycle of CO2 (ASC) between 45-90N of 56±9.8% over the last 50 years (1958-61 - 2009-11), and satellite observed greening trends of 0.25-0.5%yr 1 between 30-90N since the 1980s. I analyse output from 13 terrestrial biosphere models taking part in the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) against these observations, which together provide a powerful benchmarks for models. I also validate a simple, data-driven model for photosynthesis, the P-model, which I use to further examine changes in productivity. New approaches are also required for analysing how model output relates to real-world observations. Thus, I additionally use the metric of ecosystem light-use e fficiency (eLUE), the ratio of ecosystem productivity to the light absorbed, to distinguish structural (i.e. greening) from physiological (i.e. rate of carbon fixation) vegetation changes in models on ecosystem scales.
The MsTMIP models have good skill at representing observed greening trends and thus capture structural changes in vegetation, but they underestimate the observed increase in ASC, indicating that they underestimate changes in eLUE and productivity. The P-model, which is driven by observed greening, has a similar trend in productivity to the MsTMIP models and eLUE changes contribute only 34% to this trend. Although both the P-model and MsTMIP models show that productivity changes were driven primarily by CO2 fertilisation, this analysis suggests that models require better representation of CO2 fertilisation and other processes that can increase eLUE. Overall these results indicate that eLUE trends, larger than current models allow, are driving increases in productivity across high northern latitudes.Open Acces
Assessing links between vegetation and dust emission using Earth observation data
Full text linkMineral dust aerosol is a key component of the Earth system, affecting the planet's radiative balance, weather and biogeochemistry as well as having a significant impact on human health and economic well-being. Despite its importance, it is currently poorly represented in global climate models. As a consequence, predicting future dust loading and any associated physical processes is hampered by a high level of uncertainty. A significant factor underpinning the poor representation of dust in models arises from a lack of understanding of the processes that govern its emission at the resolution of these models.
In this thesis, a new global dataset of dust emission is derived using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue Collection 6 aerosol dataset. Robust thresholding is applied to the dataset in order to distinguish dust at uplift from aged dust and other aerosols. Threshold values are determined through a global statistical analysis of manned observations of dust at uplift. The resulting dataset compares well to other remotely sensed dust uplift datasets.
Controls on dust emission are examined by incorporating the new dust emission dataset in a generalised linear model. Candidate predictor variables that have been suggested to play a controlling role in dust emission are regridded to a common monthly 0.5 degree grid for this purpose.
For the first time, a global statistical model is used to confirm and quantify a number of expected relationships between vegetation and climate variables and dust uplift frequency. Other quantities often used in the parameterisation of dust emission, such as the presence of pans, are not found to play a key role in suppressing dust emission at the scale of the study.
The detection of dust aerosol often relies on its direct radiative effect in the longwave, a characteristic considered unique among aerosols emitted from the land surface. This assumption is interrogated in this thesis through the examination of a longwave signal from a biomass burning plume in Southern Africa. A series of sensitivity studies using a radiative transfer model are performed to establish that a dust component is not required to replicate the signal. The longwave direct radiative efficiency of the pure biomass burning aerosol case tested here is found to have a value of 4.53Wm-2 per unit aerosol optical depth. The longwave direct effect of biomass burning aerosol in the longwave has possible implications for the understanding of the aerosols's impact on climate and suggests further measurements of the optical properties in the longwave are required.Open Acces
Supplement of Holocene climates of the Iberian Peninsula: pollen-based reconstructions of changes in the west–east gradient of temperature and moisture
No full textPDF file contains: Suplemmentary Figures S1-S11 and Suplemmentary Tables S1-S2. © Author(s) 2023. CC BY 4.0 License. The copyright of individual parts of the supplement might differ from the article licence.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Primary productivity of West African tropical forests: Carbon budget of of West African tropical forests
No full textThe natural energy on Earth is primarily harnessed through photosynthesis by plants and algae, with a significant portion taking place in tropical forests. The Gross Primary Productivity (GPP) of a forest measures the amount of photosynthate it produces. The field measurement of GPP, which requires extensive fieldwork, has been attempted at Amazonia forests and Southeast Asian forests, but no field measurements have been taken for African forests until now. The thesis presents the first field quantification of GPP in West African forests.
The study also delves into several aspects of GPP, including (1) how plants allocate photosynthate to their own metabolism (autotrophic respiration) versus building plant tissue (biomass production), which is later consumed by other organisms (heterotrophic respiration). (2) Considering that the main environmental variable in West Africa is aridity, how do plant photosynthetic strategies vary from wet to dry sites? (3) The first part of the thesis found that West African forests have higher GPP than Amazonia forests. The study also explores whether global vegetation models, used for climate change projections, can account for this pattern. If not, the thesis seeks to uncover the cause of the deficiency
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