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Universität Hamburg (2018)

Grazing impacts on vegetation patterns in the Qilian Mountains, HeiHe River Basin, NW China

Baranova, Alina Nikolaevna

Titre : Grazing impacts on vegetation patterns in the Qilian Mountains, HeiHe River Basin, NW China

Veränderungen der Vegetationsmuster auf den Weiden im Qilian Mountains, HeiHe River Basin, NW China, ausgelöst durch die Intensivierung der Beweidung

Auteur : Baranova, Alina Nikolaevna

Université de soutenance : Universität Hamburg

Grade : Doctoral degree at the Faculty of Mathematic, Informatics and Natural Sciences 2018

Résumé partiel
Domestic grazing is a wide-spread land use, known since prehistoric times. Grazing ecosystems all over the globe represent a vital source for livestock husbandry, sustaining the fragile balance between ecosystem integrity and human impact. Extended pastures are found on each of the five continents and account to 30-40 % of the total land cover of the globe. However, due to the global increase in grazing activity, many areas inevitably face the problems of soil degradation processes, maintaining watershed function and ecological integrity, i.e. problems of deteriorating rangeland condition. In arid/semi-arid grasslands, increasing grazing activity is often coupled with the process of land degradation, resulting in soil erosion and depletion of soil nutrients, and threatening floristic diversity and forage quality as well. This PhD dissertation aims to examine ongoing changes in vegetation patterns and underlying soil properties of pastures in the Qilian Mountains (NW China) on the northern edge of Tibetan Plateau, which are triggered by intensified domestic grazing in recent decades. Transhumance pastoral system is still in use in the Qilian Mountains, where mixed flocks of sheep and goats, and yaks graze on the spring/autumn and summer pastures, over an altitudinal range of 2600-3600 m a.s.l. During field work, spatially differentiated and grazing-induced changes in vegetation patterns and corresponding environmental variables were investigated. Major rankless plant communities in the study area were identified : Picea crassifolia forest (1) ; Salix gilashanica – Arctostaphylos alpina shrubland (2) ; Potentilla anserina - Geranium pratense grassland (3) ; Stellera chamaejasme shrubby grassland (4), and Stipa capillata mixed grassland (5). A transformation from more homogeneous grassland less affected by grazing, dominated by Stipa spp. and Agropyron spp., to severely degraded Stipa capillata grassland, co-dominated by Iris lactea var. chinensis and Stellera chamaejasme, was observed. Analyzing the forage quality of the spring/autumn and summer pastures, intensive grazing was found to decrease aboveground dry herbage biomass and to increase fiber content of the forages. Slightly grazing intensity was associated with the highest protein (16.3%) and the lowest fiber (51.3%) contents. The highest fiber content (59.2%) was found in the plots most disturbed by grazing. Maximum concentrations of the macro- and micronutrients were observed under low grazing intensity. However, no linearity was observed between nutritive value and grazing intensity. Along the altitudinal gradient, soil water content, carbon and nitrogen, organic matter and electric conductivity increased, while soil pH and base saturation decreased. Vegetation patterns of spring/autumn pastures showed a direct response to grazing. Degraded montane grasslands, i.e. spring/autumn pastures (2600-3000 m a.s.l), are characterized by low biomass and vegetation cover as well as low content of soil organic matter, total nitrogen and carbon. This altitudinal zone was shown to be most affected by intensive grazing and vulnerable to changes of precipitation. By contrast, we found alpine meadows, i.e., summer pastures (3000-3600 m a.s.l.) showing different indications of vegetation and soil disturbances, to be more resistant to degradation. In terms of herbage biomass and total vegetation cover, north-, north-east- and north-west-facing slopes in forest-grassland and shrubland-grassland ecotones were found to be the most productive. Although all pastures were exposed to intensive grazing, the spring/autumn pastures (2600-3000 m a.s.l.) experienced more severe degradation in terms of dry herbage biomass, total vegetation cover, chemical and physical soil properties.

Mots clés  : altitudinal gradient , pasture degradation , grazing intensity , nutritive value , soil properties

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