Because of heterogeneous topographies, high-mountain areas could harbor a significant pool of cryptic forest refugia (glacial microrefugia unrecognized by palaeodata), which, as a result of poor accessibility, have been largely overlooked. The juniper forests of the southern Tibetan Plateau, with one of the highest tree lines worldwide, are ideal for assessing the potential of high-mountain areas to harbor glacial refugia. Genetic evidence for Last Glacial Maximum (LGM) endurance of these microrefugia is presented using paternally inherited chloroplast markers. Five-hundred and ninety individuals from 102 populations of the Juniperus tibetica complex were sequenced at three polymorphic chloroplast regions. Significant interpopulation differentiation and phylogeographic structure were detected (G(ST) = 0.49, N(ST) = 0.72, N(ST) > G(ST), P < 0.01), indicating limited among-population gene flow. Of 62 haplotypes recovered, 40 were restricted to single populations. These private haplotypes and overall degrees of diversity were evenly spread among plateau and edge populations, strongly supporting the existence of LGM microrefugia throughout the present distribution range, partly well above 3500 m. These results mark the highest LGM tree lines known, illustrating the potential significance of high-mountain areas for glacial refugia. Furthermore, as the close vicinity of orographic rear-edge and leading-edge populations potentially allows gene flow, surviving populations could preserve the complete spectrum of rear-edge and leading-edge adaptations.

Summary1. Sustainable management of rangelands will become increasingly important as the climate changes, yet rangeland dynamics are still a challenge to dryland ecologists because degradation patterns are difficult to sample and interpret. There are contradictions between remote sensing-based studies and field-based analyses, for which long-term data are almost non-existent. In the rangelands of North Senegal, remote sensing studies have not revealed any extensive degradation during the past three decades. The present study used a 27-year series of field data from the area to assess the impact of grazing on rangeland degradability. 2. Rainfall, standing crop and floristic data from North Senegal were analysed to quantify the effects of rainfall patterns and grazing on plant composition and the overall rain use efficiency. Monitoring plots of 1 ha comprised five ungrazed and 19 grazed plots with two different grazing treatments. Standing crop was sampled annually at the peak of biomass development. Data were analysed with mixed effect models. 3. Changes in herbaceous production were mainly caused by fluctuations in rainfall, whereas the grazing intensity had a long-term effect, interacting with precipitation dynamics. During the first and drier phase, the rainfall variability masked the grazing influence, whereas during the second phase with above-average rainfall, grazing treatments differed significantly, indicating rangeland degradation. 4. The patterns of productivity and floristic composition followed predominant non-equilibrium dynamics during the first phase (rainfall variability 40%), whereas gradual changes especially in species composition represented characteristics of equilibrium systems during the second phase (rainfall variability 23%). Thus, the study supports the existence of shifts between periods of nonequilibrium conditions and those more typical of equilibrium systems. 5. Synthesis and applications. Our 27 years field study, carried out with the aim of assessing the non-degradability of Sahelian rangelands, revealed long-term degradation trends linked to grazing intensity. Longer observation periods provide an increasing probability of including 'equilibrium phases' that allow the identification of long-term degradation processes. Consequently, both rangeland research and management policies demand monitoring periods that are long enough to account for long-term trends. The grazing experiment in this study has shown that degradation processes are reversible, but long-term exclosure and ranching with fixed stocking rates are less suitable for rangeland amelioration than moderate, production-adjusted grazing regimes mimicking traditional nomadic systems.

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This paper provides information about the distribution, structure, and ecology of the world's largest alpine ecosystem, the Kobresia pygmaea pastures in the southeastern Tibetan plateau. The environmental importance of these Cyperaceae mats derives from the extremely firm turf, which protects large surfaces against erosion, including the headwaters of the Huang He, Yangtze, Mekong, Salween, and Brahmaputra. The emphasis of the present article is on the climate-driven evolution and recent dynamics of these mats under the grazing impact of small mammals and livestock. Considering pedological analyses, radiocarbon datings, and results from exclosure experiments, we hypothesize that the majority of K. pygmaea mats are human-induced and replace forests, scrub, and taller grasslands. At present, the carrying capacity is increasingly exceeded, and reinforced settlement of nomads threatens this ecosystem especially in its drier part, where small mammals become strong competitors with livestock and the removal of the turf is irreversible. Examples of rehabilitation measures are given.

With 450,000 kmKobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000-6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf.

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