| 1. |
- Berntell, Ellen, 1989-
(författare)
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Understanding West African Monsoon Variability : Insights from Paleoclimate Modelling of Past Warm Climates
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Sahel, a water-vulnerable region in West Africa, relies heavily on rainfed agriculture. The region experienced pronounced droughts during the 20th Century, emphasising the socio-economic importance of understanding the drivers of the rainfall variability. However, future rainfall projections remain uncertain due to the complex nature of the West African Monsoon (WAM), which is influenced by internal climate variability, external forcing, and feedback processes. Limited observational records in West Africa and the need for longer time series further complicate the understanding of these drivers. This thesis uses paleoclimate modelling to investigate internal and external drivers of monsoon variability in West Africa across four distinct periods. Our study confirms that atmosphere-only model simulations can capture the observed multidecadal rainfall variability in the 20th Century, even though reanalyses struggle to reproduce the correct timing. Analysis of a last millennium simulation using the Earth System Model EC-Earth3 identified two drivers of multidecadal rainfall variability, accounting for 90% of the total co-variability between the West African rainfall and Atlantic sea surface temperatures (SSTs). This finding strengthens our understanding of SST-WAM relationships observed during the 20th Century. An ensemble of climate model simulations (PlioMIP2) shows that high CO2 levels and a different paleogeography during the mid-Pliocene Warm Period led to increased rainfall and a strengthened WAM. Our study emphasised vegetation's crucial role in enhancing the monsoon in past climates. However, simulations forced with prescribed vegetation only capture a one-directional forcing. A mid-Holocene simulation using an Earth System Model with dynamic vegetation revealed that vegetation feedbacks strengthen the WAM response to external orbital forcing but are insufficient to shift the monsoon northward or increase vegetation cover over the Sahara. These results reveal a dry bias and under-representation of simulated vegetation compared to proxy records, highlighting the importance of model development and the need for additional feedback processes in driving an enhanced, northward WAM and extending vegetation to the Sahara. Overall, this thesis advances our understanding of the drivers of West African monsoon variability and provides valuable insights for improving future rainfall projections in this vulnerable region.
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| 2. |
- Kapás, Rozália E., 1988-
(författare)
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Grassland restoration : Connectivity, plant community change and cows
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ecological significance of semi-natural grasslands is high because these habitats provide a home for a diverse flora and fauna and support a range of associated ecosystem services. Due to large-scale land-use changes the extent of grassland habitat has declined. Hence, restoration efforts to mitigate grassland losses are now being prioritized across the globe and there is an increasing need to understand the drivers behind the recovery of degraded habitats. Since many restoration initiatives rely on spontaneous dispersal of plant species from sources at both local- and landscape-scales, community assembly is influenced by a range of factors which interact both over time and across spatial scales. Given this complexity over the scales, gaps remain in our understanding of how post-restoration management can be designed to facilitate the effective dispersal and establishment of target species in restored grasslands.In this thesis, I examined colonization patterns in Swedish grasslands by comparing plant communities in both ancient and restored grasslands and under contrasting management regimes. At small spatial scales and over the short-term following restoration, I investigated species recruitment sources and their relative contribution to colonization and regeneration. At larger temporal and spatial scales, I examined how the composition of vegetation and seed bank communities is determined by local environmental factors together with distance to species pool and presence of grazing livestock.I found that species mostly colonized spatially from local species sources through seed rain. The seed bank contributed to species colonization to a greater extent in ancient grasslands than in restored grasslands. Management through livestock grazing and shorter distance to species pool were positively associated with the number of species found in grasslands. Grazing livestock facilitated target species establishment into restored sites and the differences between ancient and restored grassland communities were smaller when grazing was active, highlighting that ancient sites can provide a source of colonizing species for restored sites. I found that plant species associated with former land-use declined immediately following restoration, while the occurrence of target species generally increased. However, there was a high initial stochasticity in the establishment of the target species and communities in restored grasslands were still distinct from continuously managed sites even after several decades.These results demonstrate that species presence in nearby ancient grasslands and potential dispersal from the local seed sources drives species colonization in restored and ancient grasslands. Livestock grazing was shown to be an important driver of grassland recovery, either by mediating spatial dispersal of seeds or improving site conditions for establishment. Target species are able to accumulate in the vegetation and seed bank over the long-term, as restored grasslands age. This accumulation will ensure an increasing resilience of grassland communities against future disturbances or changes in the climate. However this depends upon active management, for example by implementing management plans which include continued livestock grazing, and upon conserving remaining ancient grasslands as hotspots for biodiversity within the wider landscape.
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| 3. |
- Maneas, Georgios, 1980-
(författare)
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Towards co-management of Gialova Lagoon : A Natura 2000 coastal wetland in Messinia, Greece
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The management of Natura 2000 sites is considered as the cornerstone for the conservation and restoration of biodiversity within Europe. However, protected ecosystems provide a plethora of benefits to local societies, and support the local economy. Thus, to seek solutions for complex environmental issues within Natura 2000 sties it is imperative to approach the site of concern as a connected social-ecological system, and to strengthen the participation of stakeholders in decision-making following a co-management approach.Gialova Lagoon wetland, in Messinia, Greece represents an example of Natura 2000 site which needs to be managed. The overall aim of the PhD thesis was to assess the problem of lagoon salinization, and provide policy recommendations for wetland restoration and management of associated freshwater resources under a changing climate. The thesis has followed a social-ecological approach, by integrating DPSIR framework with participatory Systems Dynamic modelling and the concept of ecosystem services. Knowledge gaps about major social and ecological components were assessed by applying a variety of methods, namely (a) field monitoring and observations, (b) GIS analyses, (c) consultation with stakeholders, (d) modelling and scenarios.The thesis results suggested that past human interventions had multi-fold effects on the Gialova Lagoon wetland, namely hydrology alteration, ecosystem fragmentation, loss and transformation of natural habitats. Furthermore, the combined effects of alterations in hydrology and climate change have led to increased salinity in the wetland over time. These alterations had profound implications on wetland ecosystem services such as the diversity of habitats and waterbirds and the provision of fish. Under contemporary hydrological connectivity and on-going climatic conditions, the mean annual salinity of the lagoon has increased from approximately 35 g/L during the period 2016-2018 to approximately 40 g/L during the period 2021-2023 indicating a salinization increase of approximately 1 g/L per year. To identify restoration alternatives, the work under the PhD thesis has engaged scientists with local stakeholders from the sectors of agriculture, fishing, tourism, and public administration, in a co-management approach. The end product, an SDM (Systems-Dynamics model) co-created with stakeholders, was suitable for exploring scenarios for salinity regulation and management of associated freshwater resources, under a changing climate (RCP 4.5). The derived management suggestions, namely restoration of the connectivity with the surrounding freshwater bodies (river, artesian springs) and between habitats (e.g., lagoon-marshes), could result in the de-salinization of the lagoon within a 10-year period, and could be applied within the Natura 2000 framework as they consider social and ecological needs (e.g. enhancement of biodiversity and fish production). However, under current abstraction rates for irrigation and municipal water-supply, there is a high risk of groundwater scarcity during years with dry conditions, and thus investments in water-saving technologies (e.g. smart irrigation) should be promoted to ensure adequate water availability for restoration, and enhanced resilience of the local economy against groundwater scarcity.
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| 4. |
- Mas e Braga, Martim, 1991-
(författare)
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Modelling ice surface elevation changes in Dronning Maud Land, East Antarctica : Bridging the gap between in-situ and numerical model reconstructions
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ice sheets are an active component of Earth's climate system. Their topography influences atmospheric circulation and changes in their volume alters freshwater fluxes to the oceans, affecting ocean water masses, atmospheric carbon uptake, and global sea level. Sea-level rise has a marked societal impact, and thus ice sheet models are indispensable tools to predict it. To increase confidence on sea-level rise projections, it is necessary that ice sheet models accurately represent the relevant processes governing ice sheet dynamics. Given the fact that ice sheets respond to geological-scale changes in Earth's system, it is necessary that their performance is compared with in-situ data of past geological periods, which are discrete in space and time. One useful constraint used for validating model results is past ice surface elevation, which is reconstructed based on rock samples taken from nunataks (mountain summits that pierce through the ice sheet surface). However, two main problems prevent reliable comparisons of past ice surface elevations between model and empirical results. First, data-model comparisons are hindered by the fact that most large-scale ice sheet models capture neither the timing nor the magnitude of ice thinning reconstructed for the last deglaciation. Second, the complex subglacial topography of regions where nunataks are present is also reflected on the ice sheet surface, through pronounced elevation gradients. As a result, the choice of a reference point on the present-day ice sheet, which can be subjective, is a significant source of uncertainty when computing thickness-change estimates. In this thesis, I aim to reconstruct changes in ice sheet geometry over Dronning Maud Land (DML, East Antarctica) during periods that were warmer and colder than present, and the climate drivers behind such changes. I assess whether the comparison between empirical and model results can be improved by resolving local features in ice sheet models, and by using data and models in an iterative way (using data to constrain the model, and models to interpret the data). The results of this thesis demonstrate that ice flow in areas of complex topography is poorly resolved in continental-scale ice sheet models and requires modelling in high resolution to match results from empirical constraints. High-resolution ice-sheet models, in turn, show that accurate ice sheet surface elevation reconstructions from empirical data require systematic sampling and definition of reference points over the modern ice sheet surface. Moreover, a consistent reconstruction of regional ice-thickness changes needs both empirical and ice sheet model results. Based on constrained models and empirical datasets, the ice sheet in DML responds to an interplay between sea level, ocean warming, surface mass balance, and subglacial topography. Samples from nunataks mainly reflect local ice surface elevation changes, potentially missing catchment-scale (regional) changes. Accurately determining regional changes using high-resolution modelling plays a significant role when interpreting the evolution of ice streams. Hence, the work presented here highlights that accurately reconstructing past ice sheet geometry is an effort that can only be truly successful if field scientists and ice sheet modellers work in tandem, at experiment-design, sampling, and result-interpretation stages.
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| 5. |
- Page, Jessica Faye, 1991-
(författare)
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Sustainable Urban and Regional Development and Related Ecosystem Services and Water-Climate Interactions
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To accommodate a growing global population while mitigating climate change, urban areas must grow while minimising environmental impacts. To achieve this, a city must be treated as a complex socio-ecological system in which many actors and subsystems act in unclear and unpredictable ways. This thesis explores the workings and interactions of this complex socio-ecological system by assessing how urban and regional planning and policy decisions affect the contributions of cities to climate change, and whether appropriate planning and policy tools can minimise these contributions. Computer models were developed to investigate and couple planning and policy decisions and their potential impacts on the environment, particularly in terms of greenhouse gas (GHG) emissions to the atmosphere. The models were then employed for generation of scientific knowledge and for converting this knowledge into practical planning tools and recommendations.Methods used in developing models that reflect complex urban systems included cooperation with experienced county planners to improve model accuracy; coupling of sub-system models in a socio-ecological framework for scenario analysis of the outcomes of planning and policy decisions in terms of GHG emissions; systems breakdown analysis of green-blue contributions to the urban carbon cycle; and modelling to identify how these contributions could be harnessed to reduce net urban emissions. The main study area was Stockholm County, Sweden, with later extension of the modelling approach to 54 major European cities. Cooperation with Stockholm County planners during model development resulted in an improved tool for scientific research that was also suited to practical planning, increasing the potential for knowledge developed through scientific research to be applied in reality. Scenario analysis of policies for Stockholm County revealed that zoning reduced the extra GHG emissions associated with necessary urban growth by 72% compared with a baseline scenario. Analysis of the urban carbon cycle in Stockholm County showed that vegetative carbon sequestration helped offset GHG emissions locally, but that re-emissions via surface waters compromised the potential to reach ‘net-zero’ emissions from Stockholm County. However, climate action goals for Stockholm could still be achieved if its ambitious emissions reduction plans are realised and if the current sequestration capacity of Stockholm County’s many green areas can be maintained in coming decades. Extensive modelling of urban emissions in multiple European cities showed potential for green-space sequestration and revealed that nature-based solutions (NbS) applied at city scale could help reduce urban emissions. Incorporation of NbS into climate action plans for these cities would maximise the associated GHG emissions reduction and increase the likelihood of the cities achieving their climate action goals. In conclusion, the climate change impacts of future urban expansion could be mitigated by incorporating planning and policy tools such as zoning, protection of green-blue spaces and NbS into whole-system urban and regional development plans. This could bring cities closer to achieving truly sustainable urban development.
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| 6. |
- Roth, Nina, 1984-
(författare)
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Grasslands in a changing climate : Summer drought and winter warming effects on grassland vegetation
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Grasslands harbour a high biodiversity of both plants and animals, and they provide many ecosystem services such as fodder production, pollination, and carbon storage. Climate change is likely to alter grassland ecosystems, with the effects varying according to the exact nature and timing of changes. Hence, understanding of seasonal climate change effects on grasslands and how negative impacts can be reduced is important to maintain biodiversity and to ensure continued delivery of ecosystem services.In this thesis I explored how seasonally specific aspects of climate change, i.e. summer drought and winter warming, affect aboveground plant biomass, plant community composition, and floral resources for pollinating insects. Moreover, I aimed to outline ways to mitigate potential negative climate change effects by adapting conventional grazing and mowing regimes and/or by applying soil amendments (i.e. compost) as a novel management method. Soil amendments have been suggested as a method to increase carbon sequestration and they might mitigate negative drought effects. However, there is no empirical evidence of how European grassland ecosystems would be affected should such measures be applied.A literature review of climate change studies revealed that the terms ‘wetter’ and ‘drier’ can be defined by a variety of hydroclimatic variables, or are not defined at all, making it difficult to synthesise climate change effects on ecosystems and societies. In two in-situ experiments I investigated the effects of summer drought (using rain-out shelters), soil amendments and mowing on four Swedish grasslands, and the effects of winter warming (using open-top chambers) and sheep grazing on three British Upland grasslands. The experimental summer drought caused a non-significant decline in aboveground plant biomass (i.e. fodder production), plant species diversity, and floral resources. Applying soil amendments increased aboveground plant biomass and floral resources (in yearly mown plots), but these positive effects were reduced under drought. There were signs of negative soil amendment effects on legumes. Winter warming led to an increase in graminoid biomass and a decrease in bryophyte biomass. Sheep grazing buffered the growth of a competitive species under winter warming but had only minor effects overall.My thesis emphasizes that it is important to clearly define terms like ‘wetter’ and ‘drier’ when studying effects of climate change on ecosystems, since clarifying the effects of climate across habitats and management interventions will require the synthesis of results across a range of experimental and observational systems. My field experiments indicate that even relatively small climatic changes affect grassland plant biomass and biodiversity, and that these effects depend on the season and grassland site in question. Furthermore, soil amendments have mainly positive effects on the grassland vegetation, indicating that they have potential for broad-scale application as a method to increase carbon sequestration. Given that my experiments were set up in-situ in grasslands and the treatments were rather mild and realistic in magnitude according to local climate change predictions, the observed vegetation changes within only three years are quite remarkable. They therefore highlight the need for detailed empirical and mechanistic understanding of how climate change processes are likely to affect grassland ecosystems.
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| 7. |
- Åhlén, Imenne, 1991-
(författare)
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Ecosystem services of wetlands and wetlandscapes under hydro-climatic change : Impacts of water flow and inundation patterns
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wetlands provide ecosystem services valuable for human society and are therefore often considered as nature based solution to different environmental problems. However, with centuries of wetland degradation due to anthropogenic pressures, such as agricultural expansion and forest industry, as well as pressures from climate change, there are large challenges for sustainable wetland management. Thus, for wetland protection and restoration practices to be successful, a deepened understanding on the actual mechanisms controlling wetland functions is required. Understanding how wetlands are connected, with and influenced, by their surrounding environment is also needed. Although most pressures experienced by wetlands operate on scales beyond the individual wetland scale, relatively few studies have thus far addressed large-scale functions and ecosystem service provision from hydrologically interconnected wetlands at the scale of wetlandscapes (i.e., the wetlands’ aggregated hydrological catchments in the landscape). The aim of this thesis is to investigate ecosystem service delivery from wetlands and wetlandscapes under hydro-climatic changes, considering 25 different wetlandscapes located in four different climate zones of the world. The thesis also systematically quantifies ecohydrological characteristics important for ecosystem service delivery and biodiversity support of wetlands and wetlandscapes in the Norrström Drainage basin located near Stockholm, Sweden. Conducted hydro-climatic analyses showed that impacts of climate change on wetlandscapes cannot be fully understood from average changes in climatic variables of the climate zones within which the wetlandscapes are located. This may be due to the fact that wetlands are not randomly and evenly distributed within climate zones, but may be located in areas subject to stronger climatic changes than regional means. In addition, anthropogenic pressures were on average shown to have higher impacts on runoff in wetlandscapes in comparison to climate change. The pressures however showed relatively large variability between different wetlandscapes, which needs to be considered in mitigation strategies against wetland degradation and deterioration. Similarly, regarding wetlandscape ecohydrological characteristics, results indicated that there are variability between wetlandscapes of different sizes, where larger wetlandscapes showed features that can support ecosystem services to larger degree than small wetlandscapes. Large spatial variability in wetland ecohydrological characteristics was also seen within a wetlandscape. For instance, water storage dynamics and buffering capacity varied depending on the position of the wetland in the landscape. These differences in hydrological conditions were shown to result in different inundation dynamics between wetlands, which for instance also showed to impact insect community composition.Overall, this thesis shows that assessments of wetland ecosystem services need to be addressed using a wetlandscape approach, combined with actual on site hydrological measurements. The approach used in this thesis could help decrease uncertainties related to the impacts of hydro-climatic changes and anthropogenic pressures on wetlands and wetlandscapes, supporting location-specific wetland management strategies related to creation, restoration and sustainable use of wetlands and their ecosystems.
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