The Kobresia Pastures on the Tibetan Plateau: Degradation Processes and Consequences for Carbon and Nutrient Stocks
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The Kobresia Pastures on the Tibetan Plateau: Degradation Processes and Consequences for Carbon and Nutrient Stocks Dissertation to obtain the degree “Doctor rerum naturalium“ (Dr. rer. nat.) of the Georg-August-University Göttingen within the doctoral program Biodiversity and Ecology of the Georg-August University School of Science (GAUSS) submitted by Per-Marten Schleuss from Eckernförde Göttingen, October 2016 Members of the thesis committee (supervisors): 1. Prof. Dr. Yakov Kuzyakov, Soil Science of Temperate Ecosystems, Georg-August University of Göttingen 2. Prof. Dr. Georg Guggenberger, Insitute for Soil Science, Leibniz University of Hannover 3. Prof. Dr. Hermann Jungkunst, Geoecology and Physical Geography, University of Koblenz-Landau Day of the doctoral defense: 28 October 2016 Summary II Summary The Kobresia grasslands represent the world’s largest pastoral ecosystem, covering about 450,000 km² on the eastern Tibetan Plateau. The vegetation is dominated by an endemic sedge (Kobresia pygmaea), forming a lawn with a very durable, felty root mat, which occurs from 3000 to nearly 6000 m a.s.l.. The Kobresia ecosystem supports the livelihood of the Tibetan herders, for whom it is an essential grazing ground. Furthermore, it hosts tremendous terrestrial biodiversity and stores large amounts of soil organic carbon (SOC) and nutrients. This unique ecosystem has, however, slightly been investigated in relation to the backdrop of natural and anthropogenic factors that affect it. As such, its origin, drivers, vulnerability or resilience, as well as its likely future development remain largely unknown. This study focuses on the development and degradation of the Kobresia ecosystem. It is divided into four main parts, which correspond to the objectives (I. Adaptation, II. Vulnerability, III. Degradation, IV. Consequences). The first objective was to elucidate mechanisms making K. pygmaea highly competitive and the predominant plant species on the eastern Tibetan plateau. It was expected that species-specific plant traits enabled non- evolutionary adaptations to biotic and abiotic stresses such as long-term moderate grazing pressure and recurrent N and P limitation. Within the second objective it was expected that recent management intensification and a transition from a migratory to a sedentary grazing system have caused disadvantages to former adaptations and harmed this pastoral ecosystem. This mainly includes an (over)grazing-induced disturbance of the above and belowground plant trade-off in terms of photosynthetic CO2 assimilation and belowground resource uptake (i.e. nutrients and water). The third objective was a mechanistic explanation of the recurrent degradation patterns that are common across the whole ecosystem. Degradation concepts were developed to explain the formation of polygonal cracks and bare soil patches as well for crust-covered dead root mats. Finally, the fourth objective was to quantify the consequences of degradation in terms of SOC and nutrient losses, assuming that these were mainly caused by soil erosion, organic matter mineralization and leaching. Summary III This PhD thesis has identified certain mechanisms for efficient nutrient acquisition by Kobresia pygmaea that partly explain its dominance on the eastern Tibetan plateau. These include the development of a dense root network to take up nutrients very efficiently at the times and depths that they are available. Nutrients such as N and P often limit plant growth in the Kobresia ecosystem and therefore the high belowground investments are an important mechanism to overcome resource limitations. However, supporting the large root biomass (ca. 6 kg DM m-2) involves high belowground C maintenance costs that must be covered by the comparatively low amount of photosynthetically active shoots (ca. 0.3 kg DM m-2). The high root:shoot biomass ratio of 20 indicates that the efficient above- belowground trade-off might be vulnerable to increasing grazing intensities. Indeed, at the Kema research sites (alpine Kobresia pasture), (over)grazing decreased C allocation to belowground plant compartments compared to sites that had been ungrazed for three years. It means that a permanent removal of the shoot biomass leads death of the Kobresia turf, because the belowground C-costs cannot be maintained. Therefore, the large population that is dependent on livestock, the increasing stocking rates and the diminishing availability of quality grazing grounds due to degradation make it likely that degradation will be intensified in coming decades. It is estimated that about 30% of the Kobresia ecosystem has already suffered from degradation, mainly taking the form of (a) polygonal cracks and bare soil patches and (b) dead Kobresia root mats. However, the drivers and mechanisms of this degradation are not often considered. This thesis provides new mechanistic understanding of ecosystem degradation due to combined anthropogenic and natural impacts. Man-made changes (mainly via overgrazing) and their amplification by harsh environments (i.e. freeze- thaw, soil drought, and soil erosion) cause plant death, accelerate SOC mineralization, induce erosion, and increase element leaching. A literature review on degradation studies in this ecosystem revealed that these processes have caused high losses of C, N and P. For instance, on the Kema research site, up to 7.5 kg C m-2, 0.63 kg N m-2 and 0.062 kg P m-2 had been lost from the most degraded stage compared with the intact stage. In sum, it was concluded that high C and nutrient losses have far-reaching consequences for Kobresia pastures including several ecosystem functions and services from landscape to global scale. Degradation has decreased soil fertility and pasture quality on the landscape scale and therefore jeopardizes the livelihood of the Tibetan herders. The fates of lost Summary IV elements remain unknown, but it is likely, that (a) very large amounts of C are released as CO2 to the atmosphere and (b) that elements are deposited to subjacent landscape positions and rivers, polluting the Tibetan headwaters. Moreover, increased degradation affected carbon allocation and strongly reduced carbon uptake. Consequently, the best approach to maintain carbon storage, soil fertility and other important functions of this vulnerable Kobresia ecosystem is to lower livestock densities by limiting numbers, and perhaps more importantly improve mobility of livestock activities. This was the status quo for the traditional migratory rangeland management, which proved sustainable for millennia. Table of Contents V Table of Contents Summary ........................................................................................................ II Table of Contents ............................................................................................ V List of Tables ................................................................................................... X List of Figures ................................................................................................. XI Acknowledgements ....................................................................................... XV Abbreviations ............................................................................................. XVII 1. Extended Summary ..................................................................................... 1 1.1 Introduction ................................................................................................... 1 1.1.1 Global change and grassland ecosystems ............................................................... 1 1.1.2 Development and relevance of the Kobresia pastures ........................................... 2 1.1.3 Degradation of the Kobresia pastures ..................................................................... 5 1.2 The Kobresia cluster ........................................................................................ 7 1.3 Objectives ....................................................................................................... 9 1.3.1 Objective I: Adaptation of Kobresia pygmaea to biotic and abiotic stress ........... 10 1.3.2 Objective II: Vulnerability of the Kobresia pastures to land use changes ............. 10 1.3.3 Objective III: Degradation mechanisms of the Kobresia pastures ........................ 11 1.3.4 Objective IV: Consequences of degradation for carbon and nutrient storage ..... 11 1.4 Material and Methods .................................................................................. 12 1.4.1 Study sites .............................................................................................................. 12 1.4.2 Research approaches ............................................................................................. 13 1.5 General discussion and conclusions ............................................................... 15 1.5.1 Adaptations of Kobresia pygmaea to grazing pressure and N and P limitations .. 15 1.5.2 Vulnerability of Kobresia grasslands to land use change ...................................... 16 1.5.3 Degradation concepts and driving forces .............................................................. 17 1.5.3.1 Formation of polygonal cracks and bare soil patches .............................. 17 1.5.3.2 Formation of crust-covered dead root mats ............................................ 18 1.5.4 Biogeochemical consequences of degradation ..................................................... 19 1.5.4.1 C, N and P losses