TY - JOUR A1 - Synodinos, Alexis D. A1 - Tietjen, Britta A1 - Lohmann, Dirk A1 - Jeltsch, Florian T1 - The impact of inter-annual rainfall variability on African savannas changes with mean rainfall JF - Journal of theoretical biology N2 - Savannas are mixed tree-grass ecosystems whose dynamics are predominantly regulated by resource competition and the temporal variability in climatic and environmental factors such as rainfall and fire. Hence, increasing inter-annual rainfall variability due to climate change could have a significant impact on savannas. To investigate this, we used an ecohydrological model of stochastic differential equations and simulated African savanna dynamics along a gradient of mean annual rainfall (520–780 mm/year) for a range of inter-annual rainfall variabilities. Our simulations produced alternative states of grassland and savanna across the mean rainfall gradient. Increasing inter-annual variability had a negative effect on the savanna state under dry conditions (520 mm/year), and a positive effect under moister conditions (580–780 mm/year). The former resulted from the net negative effect of dry and wet extremes on trees. In semi-arid conditions (520 mm/year), dry extremes caused a loss of tree cover, which could not be recovered during wet extremes because of strong resource competition and the increased frequency of fires. At high mean rainfall (780 mm/year), increased variability enhanced savanna resilience. Here, resources were no longer limiting and the slow tree dynamics buffered against variability by maintaining a stable population during ‘dry’ extremes, providing the basis for growth during wet extremes. Simultaneously, high rainfall years had a weak marginal benefit on grass cover due to density-regulation and grazing. Our results suggest that the effects of the slow tree and fast grass dynamics on tree-grass interactions will become a major determinant of the savanna vegetation composition with increasing rainfall variability. KW - Rainfall variability KW - Savanna-grassland bistability KW - Stochastic differential equations KW - Coexistence mechanisms KW - Fire Y1 - 2017 U6 - https://doi.org/10.1016/j.jtbi.2017.10.019 SN - 0022-5193 SN - 1095-8541 VL - 437 SP - 92 EP - 100 PB - Elsevier Ltd. CY - London ER - TY - JOUR A1 - Müller, Eva Nora A1 - van Schaik, Loes A1 - Blume, Theresa A1 - Bronstert, Axel A1 - Carus, Jana A1 - Fleckenstein, Jan H. A1 - Fohrer, Nicola A1 - Geissler, Katja A1 - Gerke, Horst H. A1 - Gräff, Thomas A1 - Hesse, Cornelia A1 - Hildebrandt, Anke A1 - Hölker, Franz A1 - Hunke, Philip A1 - Körner, Katrin A1 - Lewandowski, Jörg A1 - Lohmann, Dirk A1 - Meinikmann, Karin A1 - Schibalski, Anett A1 - Schmalz, Britta A1 - Schröder-Esselbach, Boris A1 - Tietjen, Britta T1 - Scales, key aspects, feedbacks and challenges of ecohydrological research in Germany JF - Hydrologie und Wasserbewirtschaftung N2 - Ecohydrology analyses the interactions of biotic and abiotic aspects of our ecosystems and landscapes. It is a highly diverse discipline in terms of its thematic and methodical research foci. This article gives an overview of current German ecohydrological research approaches within plant-animal-soil-systems, meso-scale catchments and their river networks, lake systems, coastal areas and tidal rivers. It discusses their relevant spatial and temporal process scales and different types of interactions and feedback dynamics between hydrological and biotic processes and patterns. The following topics are considered key challenges: innovative analysis of the interdisciplinary scale continuum, development of dynamically coupled model systems, integrated monitoring of coupled processes at the interface and transition from basic to applied ecohydrological science to develop sustainable water and land resource management strategies under regional and global change. KW - Coastal regions KW - drylands KW - ecohydrological modelling KW - feedback KW - hyporheic zone KW - meso-scale ecosystems KW - plant-animal-soil-system KW - river networks Y1 - 2014 U6 - https://doi.org/10.5675/HyWa_2014,4_2 SN - 1439-1783 VL - 58 IS - 4 SP - 221 EP - 240 PB - Bundesanst. für Gewässerkunde CY - Koblenz ER - TY - JOUR A1 - Lohmann, Dirk A1 - Tietjen, Britta A1 - Blaum, Niels A1 - Joubert, David Francois A1 - Jeltsch, Florian T1 - Prescribed fire as a tool for managing shrub encroachment in semi-arid savanna rangelands JF - Journal of arid environments N2 - Savanna rangelands worldwide are threatened by shrub encroachment, i.e. the increase of woody plant species at the cost of perennial grasses, causing a strong decline in the productivity of domestic livestock production. Although recent studies indicate that fire might be of great importance for semi-arid and arid savanna dynamics, it is largely not applied in the management of semi-arid rangelands especially with regard to woody plant control. We used the eco-hydrological savanna model EcoHyD to simulate the effects of different fire management strategies on semi-arid savanna vegetation and to assess their longterm suitability for semi-arid rangeland management. Simulation results show that prescribed fires, timed to kill tree seedlings prevented shrub encroachment for a broad range of livestock densities while the possible maximum long-term cattle densities on the simulated semi-arid rangeland in Namibia increased by more than 30%. However, when grazing intensity was too high, fire management failed in preventing shrub encroachment. Our findings indicate that with regard to fire management a clear distinction between mesic and more arid savannas is necessary: While the frequency of fires is of relevance for mesic savannas, we recommend a fire management focussing on the timing of fire for semi-arid and arid savannas. (C) 2014 Elsevier Ltd. All rights reserved. KW - Acacia mellifera KW - Bush encroachment KW - Dry land degradation KW - Rangeland management KW - Simulation model KW - Southern Africa Y1 - 2014 U6 - https://doi.org/10.1016/j.jaridenv.2014.04.003 SN - 0140-1963 SN - 1095-922X VL - 107 SP - 49 EP - 56 PB - Elsevier CY - London ER - TY - JOUR A1 - Lohmann, Dirk A1 - Tietjen, Britta A1 - Blaum, Niels A1 - Joubert, David F. A1 - Jeltsch, Florian T1 - Shifting thresholds and changing degradation patterns: climate change effects on the simulated long-term response of a semi-arid savanna to grazing JF - Journal of applied ecology : an official journal of the British Ecological Society N2 - 1. The complex, nonlinear response of dryland systems to grazing and climatic variations is a challenge to management of these lands. Predicted climatic changes will impact the desertification of drylands under domestic livestock production. Consequently, there is an urgent need to understand the response of drylands to grazing under climate change. 2. We enhanced and parameterized an ecohydrological savanna model to assess the impacts of a range of climate change scenarios on the response of a semi-arid African savanna to grazing. We focused on the effects of temperature and CO2 level increase in combination with changes in inter- and intra-annual precipitation patterns on the long-term dynamics of three major plant functional types. 3. We found that the capacity of the savanna to sustain livestock grazing was strongly influenced by climate change. Increased mean annual precipitation and changes in intra-annual precipitation pattern have the potential to slightly increase carrying capacities of the system. In contrast, decreased precipitation, higher interannual variation and temperature increase are leading to a severe decline of carrying capacities owing to losses of the perennial grass biomass. 4. Semi-arid rangelands will be at lower risk of shrub encroachment and encroachment will be less intense under future climatic conditions. This finding holds in spite of elevated levels of atmospheric CO2 and irrespective of changes in precipitation pattern, because of the drought sensitivity of germination and establishment of encroaching species. 5. Synthesis and applications. Changes in livestock carrying capacities, both positive and negative, mainly depend on the highly uncertain future rainfall conditions. However, independent of the specific changes, shrub encroachment becomes less likely and in many cases less severe. Thus, managers of semi-arid rangelands should shift their focus from woody vegetation towards perennial grass species as indicators for rangeland degradation. Furthermore, the resulting reduced competition from woody vegetation has the potential to facilitate ecosystem restoration measures such as re-introduction of desirable plant species that are only little promising or infeasible under current climatic conditions. On a global scale, the reductions in standing biomass resulting from altered degradation dynamics of semi-arid rangelands can have negative impacts on carbon sequestration. KW - CO2 increase KW - demographic bottleneck KW - ecohydrology KW - grid-based simulation model KW - livestock KW - precipitation pattern KW - savanna resilience KW - shrub encroachment KW - soil moisture KW - sustainable rangeland management Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-2664.2012.02157.x SN - 0021-8901 VL - 49 IS - 4 SP - 814 EP - 823 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Lohmann, Dirk A1 - Guo, Tong A1 - Tietjen, Britta T1 - Zooming in on coarse plant functional types-simulated response of savanna vegetation composition in response to aridity and grazing JF - Theoretical ecology N2 - Precipitation and land use in terms of livestock grazing have been identified as two of the most important drivers structuring the vegetation composition of semi-arid and arid savannas. Savanna research on the impact of these drivers has widely applied the so-called plant functional type (PFT) approach, grouping the vegetation into two or three broad types (here called meta-PFTs): woody plants and grasses, which are sometimes divided into perennial and annual grasses. However, little is known about the response of functional traits within these coarse types towards water availability or livestock grazing. In this study, we extended an existing eco-hydrological savanna vegetation model to capture trait diversity within the three broad meta-PFTs to assess the effects of both grazing and mean annual precipitation (MAP) on trait composition along a gradient of both drivers. Our results show a complex pattern of trait responses to grazing and aridity. The response differs for the three meta-PFTs. From our findings, we derive that trait responses to grazing and aridity for perennial grasses are similar, as suggested by the convergence model for grazing and aridity. However, we also see that this only holds for simulations below a MAP of 500 mm. This combined with the finding that trait response differs between the three meta-PFTs leads to the conclusion that there is no single, universal trait or set of traits determining the response to grazing and aridity. We finally discuss how simulation models including trait variability within meta-PFTs are necessary to understand ecosystem responses to environmental drivers, both locally and globally and how this perspective will help to extend conceptual frameworks of other ecosystems to savanna research. KW - Traits KW - Dryland KW - Degradation KW - Shrub encroachment KW - Simulation KW - Eco-hydrological model KW - EcoHyD Y1 - 2018 U6 - https://doi.org/10.1007/s12080-017-0356-x SN - 1874-1738 SN - 1874-1746 VL - 11 IS - 2 SP - 161 EP - 173 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Lohmann, Dirk A1 - Falk, Thomas A1 - Geissler, Katja A1 - Blaum, Niels A1 - Jeltsch, Florian T1 - Determinants of semi-arid rangeland management in a land reform setting in Namibia JF - Journal of arid environments N2 - To assess the ecological and economic implications of the redistributive land reform in semi-arid Namibia, we investigated to what extent land reform beneficiaries adjust herd size and herd composition according to environmental (rainfall, vegetation) and economic variables (herd size, financial assets, running costs). We performed model-based role-plays with Namibian land reform beneficiaries, simulating 10 years of rangeland management. Our study revealed that the farmers surveyed mainly manage their herds according to their economic situation (herd size and account balance) but do not take environmental variability (rainfall and vegetation) into account. Further, our results indicate that, due to financial pressure, farmers are not able to apply their desired management strategies, and that owners of small farms face a higher risk of economic failure. However, farmers apply rather conservative and constant stocking rates and will thus, given the current economic limitations, likely not contribute to semi-arid savanna degradation. We conclude that land reform beneficiaries need support to be able to apply straightforward and efficient management strategies. This could be achieved by facilitating cooperation between small farming businesses and by supporting initial investment in productive cattle herds at the time of redistribution of the land. KW - Dry land degradation KW - Semi-arid savanna KW - Land reform KW - Rangeland management KW - Simulation model KW - Role-play Y1 - 2014 U6 - https://doi.org/10.1016/j.jaridenv.2013.10.005 SN - 0140-1963 SN - 1095-922X VL - 100 SP - 23 EP - 30 PB - Elsevier CY - London ER - TY - JOUR A1 - Irob, Katja A1 - Blaum, Niels A1 - Baldauf, Selina A1 - Kerger, Leon A1 - Strohbach, Ben A1 - Kanduvarisa, Angelina A1 - Lohmann, Dirk A1 - Tietjen, Britta T1 - Browsing herbivores improve the state and functioning of savannas BT - A model assessment of alternative land-use strategies JF - Ecology and evolution N2 - Changing climatic conditions and unsustainable land use are major threats to savannas worldwide. Historically, many African savannas were used intensively for livestock grazing, which contributed to widespread patterns of bush encroachment across savanna systems. To reverse bush encroachment, it has been proposed to change the cattle-dominated land use to one dominated by comparatively specialized browsers and usually native herbivores. However, the consequences for ecosystem properties and processes remain largely unclear. We used the ecohydrological, spatially explicit model EcoHyD to assess the impacts of two contrasting, herbivore land-use strategies on a Namibian savanna: grazer- versus browser-dominated herbivore communities. We varied the densities of grazers and browsers and determined the resulting composition and diversity of the plant community, total vegetation cover, soil moisture, and water use by plants. Our results showed that plant types that are less palatable to herbivores were best adapted to grazing or browsing animals in all simulated densities. Also, plant types that had a competitive advantage under limited water availability were among the dominant ones irrespective of land-use scenario. Overall, the results were in line with our expectations: under high grazer densities, we found heavy bush encroachment and the loss of the perennial grass matrix. Importantly, regardless of the density of browsers, grass cover and plant functional diversity were significantly higher in browsing scenarios. Browsing herbivores increased grass cover, and the higher total cover in turn improved water uptake by plants overall. We concluded that, in contrast to grazing-dominated land-use strategies, land-use strategies dominated by browsing herbivores, even at high herbivore densities, sustain diverse vegetation communities with high cover of perennial grasses, resulting in lower erosion risk and bolstering ecosystem services. KW - browsing KW - ecohydrology KW - land use KW - plant community KW - savanna KW - wildlife KW - management Y1 - 2022 U6 - https://doi.org/10.1002/ece3.8715 SN - 2045-7758 VL - 12 IS - 3 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Guo, Tong A1 - Weise, Hanna A1 - Fiedler, Sebastian A1 - Lohmann, Dirk A1 - Tietjen, Britta T1 - The role of landscape heterogeneity in regulating plant functional diversity under different precipitation and grazing regimes in semi-arid savannas JF - Ecological modelling : international journal on ecological modelling and engineering and systems ecolog N2 - 1. Savanna systems exhibit a high plant functional diversity. While aridity and livestock grazing intensity have been widely discussed as drivers of savanna vegetation composition, physical soil properties have received less attention. Since savannas can show local differences in soil properties, these might act as environmental filters and affect plant diversity and ecosystem functioning at the patch scale. However, research on the link between savanna vegetation diversity and ecosystem function is widely missing. 2. In this study, we aim at understanding the impact of local heterogeneity in soil conditions on plant diversity and on ecosystem functions. For this, we used the ecohydrological savanna model EcoHyD. The model simulates the fate of multiple plant functional types and their interactions with local biotic and abiotic conditions. We applied the model to a set of different landscapes under a wide range of livestock grazing and precipitation scenarios to assess the impact of local heterogeneity in soil conditions on the composition and diversity of plant functional types and on ecosystem functions. 3. Comparisons between homogeneous and heterogeneous landscapes revealed that landscape soil heterogeneity allowed for a higher functional diversity of vegetation under conditions of high competition, i.e. scenarios of low grazing stress. However, landscape heterogeneity did not have this effect under low grazing stress in combination with high mean annual precipitation. Further, landscape heterogeneity led to a higher community biomass, especially for lower rainfall conditions, but also dependent on grazing stress. Total transpiration of the plant community decreased in heterogeneous landscapes under arid conditions. 4. This study highlights that local soil conditions interact with precipitation and grazing in driving savanna vegetation. It clearly shows that vegetation diversity and resulting ecosystem functioning can be driven by landscape heterogeneity. We therefore suggest that future research on ecosystem functioning of savanna systems should focus on the links between local environmental conditions via plant functional diversity to ecosystem functioning. KW - Plant functional type KW - Trait diversity KW - Ecosystem functioning KW - Plant coexistence KW - Soil texture KW - Ecohydrological model Y1 - 2018 U6 - https://doi.org/10.1016/j.ecolmodel.2018.04.009 SN - 0304-3800 SN - 1872-7026 VL - 379 SP - 1 EP - 9 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Guo, Tong A1 - Lohmann, Dirk A1 - Ratzmann, Gregor A1 - Tietjen, Britta T1 - Response of semi-arid savanna vegetation composition towards grazing along a precipitation gradient-The effect of including plant heterogeneity into an ecohydrological savanna model JF - Ecological modelling : international journal on ecological modelling and engineering and systems ecolog N2 - Ecohydrological models of savanna rangeland systems typically aggregate plant species to very broad plant functional types (PFTs), which are characterized by their trait combinations. However, neglecting trait variability within modelled PFTs may hamper our ability to understand the effects of climate or land use change on vegetation composition and thus on ecosystem processes. In this study we extended and parameterized the ecohydrological savanna model EcoHyD, which originally considered only three broad PFTs (perennial grasses, annuals and shrubs). We defined several sub-types of perennial grasses (sub-PFTs) to assess the effect of environmental conditions on vegetation composition and ecosystem functioning. These perennial sub-PFTs are defined by altering distinct trait values based on a trade-off approach for (i) the longevity of plants and (ii) grazing-resistance. We find that increasing grazing intensity leads to a dominance of the fast-growing and short-lived perennial grass type as well as a dominance of the poorly palatable grass type. Increasing precipitation dampens the magnitude of grazing-induced shifts between perennial grass types. The diversification of perennial grass PFTs generally increases the total perennial grass cover and ecosystem water use efficiency, but does not protect the community from shrub encroachment. We thus demonstrate that including trait heterogeneity into ecosystem models will allow for an improved representation of ecosystem responses to environmental change in savannas. This will help to better assess how ecosystem functions might be impacted under future conditions. (C) 2016 Elsevier B.V. All rights reserved. KW - Plant functional types KW - Trait heterogeneity KW - Rangeland management KW - Precipitation gradient KW - Livestock KW - Ecosystem functioning Y1 - 2016 U6 - https://doi.org/10.1016/j.ecolmodel.2016.01.004 SN - 0304-3800 SN - 1872-7026 VL - 325 SP - 47 EP - 56 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Falk, Thomas A1 - Lohmann, Dirk A1 - Azebaze, Nadege T1 - Congruence of appropriation and provision in collective water provision in Central Namibia JF - International journal of the commons N2 - Achieving cooperation in natural resource management is always a challenge when incentives exist for an individual to maximise her short term benefits at the cost of a group. We study a public good social dilemma in water infrastructure provision on land reform farms in Namibia. In the context of the Namibian land reform, arbitrarily mixed groups of livestock farmers have to share the operation and maintenance of water infrastructure. Typically, water is mainly used for livestock production, and livestock numbers are subject to high fluctuations due to the given environmental conditions. Our paper assesses how alternative payment systems with differing congruence of provision and appropriation support the cooperation in the group given the ever-changing equilibria. In a first step, we conducted an exploratory overview of the social-ecological system of central Namibian land reform projects. The Social Ecological System (SES) Framework served as a guideline for this assessment (Ostrom 2009). Taking the complexity of the cooperation situation into account, in the second step we designed a role-play that is based on a social-ecological simulation model. The role-play simulates the real-life decision situations of land reform beneficiaries wherein equilibria are permanently changing. This approach helped us to not only better understand the cooperation challenges of Namibian land reform beneficiaries, but also supported stakeholders in their decision making and institution building. Our study provides evidence to support that land reform beneficiaries increase their contributions as they own more livestock and as other group members increase their payments. Nevertheless, only groups with relatively homogeneous livestock endowments manage to agree on payment rules. Interestingly, the dominant rule is an "equal payment per farmer" and not a "payment per head of livestock", though the latter would imply a higher congruence of provision and appropriation. KW - Land reform KW - Namibia KW - participatory ecological-economic modelling KW - public good KW - role play KW - savanna rangeland Y1 - 2016 U6 - https://doi.org/10.18352/ijc.583 SN - 1875-0281 VL - 10 SP - 71 EP - 118 PB - Brill CY - Urtrecht ER -