Browsing by Subject "Land cover changes"

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Sensitivity of land-atmosphere coupling strength in dependence of land cover and atmospheric thermodynamics over Europe
(2023) Jach, Lisa; Wulfmeyer, Volker
Biogeophysical feedbacks between the land surface and the atmosphere have been identified to heavily control the climate system. Land-atmosphere (L-A) coupling strength is a concept to quantify the feedback processes. However, the quantification is still subject to uncertainties, in particular, in the context of land surface influences on local convective precipitation. On the one hand, feedback processes are the result of a chain of complex interactions between various components in the L-A system all exhibiting spatiotemporal variability. On the other hand, L-A coupling strength is not a directly measurable quantity. It can be assessed with different scientific approaches, which makes the quantification dependent on the methodology and the availability of suitable data sets. The aim of this doctoral thesis is to investigate the impact of changes in the vegetation cover and the atmospheric thermodynamic conditions on the long-term coupling signal between the land surface and the triggering of deep moist convection during the European summer. The ‘convective triggering potential – low-level humidity index’ framework, which is a commonly used L-A coupling metric, classifies a day in favor for L-A coupling or not, based on the prevailing thermodynamic conditions in the atmosphere. The daily classifications are used to measure the frequency of days with favorable conditions during the study period, and to identify regions with high frequencies of favorable conditions as coupling hot spots. The framework is applied to model output from regional climate model (RCM) simulations with WRF-NoahMP with diverging land cover conducted over the historical period 1986-2015 for the Euro-CORDEX domain. Impacts of changes in vegetation cover are analyzed by comparing the L-A coupling strength from two sensitivity experiments with idealized extreme land use and land cover changes (LULCCs) against a simulation with realistic land cover. A posteriori modifications to the temperature and moisture output fields of the simulation with realistic land cover were implemented to analyze impacts of systematic changes in the atmospheric thermodynamic conditions. A potential coupling hot spot with predominantly positive feedbacks was identified over Eastern Europe. In Southern Europe and Europe’s coastal areas, the coupling is regularly inhibited by very dry, very wet or stable conditions in the atmosphere. The location of the hot spot appeared insensitive to LULCCs and changes in the thermodynamic conditions. None of the sensitivity tests within a realistic range of temperature and moisture modifications for a recent climate period, led to a disappearance of the hot spot or to overcome the causes for inhibiting coupling in the respective areas in summer. Nevertheless, the experiments demonstrated also considerable variance of the coupling strength within the hot spot region. LULCCs changed the turbulent heat fluxes from the land surface, and thus the atmospheric boundary layer (ABL) heating and moistening. This impacted the boundary layer development of each day. It also caused changes in the average thermodynamic characteristics during the study period, which changed the frequency of favorable pre-conditioning for convection triggering and enhanced the variance in the coupling strength in the hot spot. Both effects were identified to influence the land surface control on the occurrence of convective precipitation. Furthermore, the sensitivity tests with a posteriori modifications revealed uncertainties in the predominant atmospheric response to differently wet surfaces around the Black Sea, shown by a disagreement in the predominant coupling pathway between the modification cases. The findings further indicate uncertainty in whether the hot spot expands over Central Europe, as the feedback signal was sensitive to changes in temperature and moisture. Additionally, the model has a warm and dry bias in this area, which suggests an overestimation of the humidity deficit. The large humidity deficit, in turn, was the inhibiting factor for a high frequency of occurrence of favorable pre-conditions for deep moist convection. The analyses reveal a sensitivity of the L-A coupling strength and atmospheric response to the prevailing land surface and atmospheric conditions in the hot spot. This highlights the need to consider both the land surface state and its impact on L-A coupling strength with respect to predictions of convective precipitation events in strongly coupled regions (and periods). Given that L-A coupling provides predictive skill for climate projections and seasonal forecasts, improved understanding about causes of variability in L-A coupling strength is crucial for improvements therein.
The influence of land use and cover changes on the pastoral rangeland systems of southern Ethiopia : how much woody cover is enough?
(2014) Mohammed, Hasen Yusuf; Treydte, Anna C.
The Borana rangelands in southern Ethiopia are facing deterioration caused by intensification of grazing and woody plant encroachment, resulting in marked reductions in pastoral production. This process affects the food security and livelihoods of the Borana pastoral people negatively. Woody plant encroachment might result in an increase in carbon (C) storage in these rangelands, which represents an important aspect for climate change mitigation potentials. However, it is unclear how much C is currently stored in the above-and belowground vegetation biomass and in the soils of these rangeland ecosystems and how grazing intensity and woody cover influence soil or ecosystem C-stocks. The research work presented in this thesis aimed at developing tools to estimate the aboveground woody biomass C stocks. It describes the structure of semiarid savanna vegetation in different grazing regimes at various levels of woody encroachment, examines changes in woody plant encroachment, and provides field-based quantification methods and tools to derive site-based estimates of above- and belowground C pools. The thesis also aimed at assessing the influence of grazing on herbaceous above- and belowground biomass C stocks, soil organic carbon (SOC) and total soil nitrogen (TSN) to estimate possible increases in ecosystem C stocks by long term reduction of grazing intensity (e.g., low livestock density and seasonal grazing) at various levels of woody plant encroachment. A long-term temporal satellite imagery over the last 37 years and GIS mapping aided by ground truthing was used to investigate vegetation cover changes. In the field, data was collected to analyze vegetation attributes such as composition and structure under different grazing regimes and woody encroachment sites. Herbaceous species were destructively harvested to quantify the biomass and C stocks in the herbaceous vegetation community. Allometric tree biomass models were developed by destructively harvesting eight woody species to indirectly quantify the woody biomass and C stocks. Total soil nitrogen and SOC stocks in the different grazing management systems and woody encroachment levels were assessed from soil cores collected within 0- 40 cm soil depth. The performance of allometric biomass models as expressed as a goodness of fit (adj r2) depended on the species and biomass components estimated. The allometric models were highly accurate for large woody species such as A. mellifera, A. bussei, and A. etabaica. The most important single models predictor variable identified was stem basal circumference for tall shrubs with more or less open canopy structure. Meanwhile, for tall shrubs with closed and umbrella-like canopy structures, pairs of canopy volume and stem basal circumferences were more reliable predictors. It was further shown that, by using canopy volume as a standalone predictor variable, biomass can still be accurately predicted for shrubs whose growth form comprise discrete canopy clumps with multiple stems (e.g., A. oerfota). Vegetation cover analysis using temporal Landsat imageries from 1976 to 2012 revealed that areas covered by shrub and tree savanna (open savanna types) in the 1970s declined from 45% to 9%, while heavily encroached areas (bushland thickets and bushed savanna) increased from 22% to 61% during this time interval. The abundance of total and the regenerative woody plants (< 1 m height) were high in lower woody encroachment sites but significantly reduced at heavily woody encroachment sites. At all levels of woody encroachment enclosures significantly increased total woody plant density, especially the proportion of woody plants in < 1 m height size class compared to the open rangelands. Estimated total aboveground biomass C stocks varied significantly between woody encroachments levels, with total aboveground biomass C stocks ranging from 2 Mg ha-1 in the low encroachment site to 9 Mg ha-1 in heavy encroachment sites. Enclosures significantly raised the herbaceous biomass C stocks, with enclosures containing 50% more herbaceous aboveground biomass C stocks than openly grazed land. However, the response of herbaceous aboveground biomass C stocks to grazing was also strongly influenced by the woody encroachment characteristics including woody density, canopy cover, species composition and other specific traits of woody species. Mean total SOC stock in the 0 - 40 cm soil depth ranged from 30 Mg ha-1 in the openly grazed soils at the high woody encroachment site to 81 Mg ha-1 in the enclosure soils at the low encroachment site ha-1. Soil OC and TSN did not differ in the enclosure at heavily encroached sites but were two times as high in enclosures compared to openly grazed soils at low encroached sites. Soil OC was positively related to TSN and soil cation exchange capacity (CEC), but negatively to sand content. Contrary to expectations, SOC stocks did not uniformly follow the pattern of increasing aboveground biomass C stocks with increasing woody encroachment. Rather, it seemed to be influenced by variations in soil characteristics across the Borana rangelands. The study highlights the influence of woody encroachment and reduction of grazing pressure on ecosystem C stocks. The allometric models developed by this study can serve as a tool for future biomass and C sequestration studies in semiarid regions of east Africa. The information presented on the ecosystem C stocks by this thesis could help integrate the effects of grazing and vegetation cover dynamics on the rangeland C storage. An understanding of these interactions are deemed necessary to develop a sound rangeland management policy that can link the C storage potential of the rangelands to global climate change mitigation and adaptation strategies through establishing a viable mechanism of payment for ecosystem services.