Abstract

Kobresia pastures in the eastern Tibetan highlands occupy an area of 450000 km² and form the world’s largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresia pygmaea, which forms a lawn with a very durable turf cover anchored by a felty root mat, and occurs from 3000 m to nearly 6000 m a.s.l. The existence and functioning of this unique ecosystem and its turf cover have not yet been explained against a backdrop of natural and anthropogenic factors, and thus its origin, drivers, vulnerability or resilience as well as its future development remain largely unknown. Here we present a summary of our own studies and literature data on ecosystem diversity, pasture health, cycles of carbon (C), water and nutrients, and on the palaeo-environment, as well as results from studies on the reproduction and ecology of the key species K. pygmaea. The methods employed include molecular analysis, grazing exclusion, measurements with micro-lysimeters and gas ex-change chambers, 13C and 15N labelling, eddy-covariance flux measurements, remote sens-ing and atmospheric modellsens-ing.

The following combination of traits makes Kobresia pygmaea resilient against disturbance and highly competitive under grazing: dwarf habit, predominantly below-ground allocation of photoassimilates, mixed reproduction strategy with both seed production and clonal growth, and high genetic diversity. For an unknown period Kobresia pastures have been co-limited by low rainfall during the short growing season and livestock-mediated nutrient withdrawal. Changing rangeland policies during the last 50 years, together with overstock-ing, have caused pasture degradation, yet the overall extent of this remains a matter of de-bate. In addition to the grazing-driven changes, we newly describe natural autocyclic pro-cesses of turf erosion initiated through polygonal cracking of the turf cover, and increased by soil-dwelling endemic small mammals. The major consequences of the deterioration of the vegetation cover and its turf include: (1) the release of large amounts of C and nutrients to the atmosphere, rivers and surrounding terrestrial ecosystems; (2) earlier diurnal formation of clouds resulting in decreased surface temperatures with likely consequences for atmos-pheric circulation on large regional and, possibly global, scales.

Palaeo-environmental reconstruction, in conjunction with grazing experiments, suggests that much of the present grazing lawns of Kobresia pygmaea are a synanthropic ecosystem that has developed over millennia, since the onset of pastoralism. The traditional migratory rangeland management was largely sustainable and possibly still offers the best strategy to conserve, and possibly increase, the C stocks in the Kobresia turf, as well as its importance for climate regulation.

2.1.2 Introduction

The Tibetan highlands encompass 90% of the Earth’s terrain above 4000 m and host the world’s largest pastoral alpine ecosystem: the Kobresia pastures of the south-eastern high-lands, with an area of 450000 km² (Fig. SI-1). This ecosystem is globally unique as it is: (1) dominated by a single endemic sedge species of 1 to 4 cm in height; (2) forms a golf-course like lawn, with a very durable turf cover anchored by a felty root mat; (3) occurs over an ele-vational extent of 3000 m, stretching between 3000 m (in the north-eastern highlands) to nearly 6000 m a.s.l. (on the north slope of Mt. Everest; Miehe 1989, Miehe et al. 2008b).

Figure SI-1: Kobresia pygmaea pastures of the Tibetan highlands and forest relics. After Atlas Tibet Plateau 1990, Miehe et al. 2008b, 2014, Babel et al. 2014.

The evolution and recent changes of the Kobresia ecosystem, as well as its future develop-ment, are of great importance because surface properties of the highlands have an undis-puted effect on the global climate (Cui and Graf 2009, Babel et al. 2014, Yang et al. 2014).

The whole Tibetan plateau is significant in terms of C turnover and CO2 fluxes from regional to global scales (Zhao et al. 2005, Zhang et al. 2007). Its soil stores huge amounts of carbon

(C), mounting to up to 2.5% to the global C pool (Ni 2002, Wang et al. 2002, Hafner et al.

2012). The Kobresia ecosystem in particular contributes about 1% (Batjes 1996, Wang et al.

2002), though covering only about 0.3% of the global land surface (equivalent to one-third of the whole Tibetan plateau). The livelihoods of 5 million pastoralists depend on forage re-sources from the rangelands, which sustain about 13 million yak, and 30 million goats and sheep (Wiener et al. 2003, Suttie et al. 2005). One quarter of the world’s population living in the surrounding lowlands ultimately depend on ecosystem functions of the Kobresia mats, which represent the upper catchment areas of the Huang He, Yangtze, Salween, Mekong, and partly of the Brahmaputra, rivers.

The aim of this review is therefore to summarize recent findings relating to: (1) the diversity and the distribution of plant species of the ecosystem and its palaeo-ecological background;

(2) the ecology and reproduction of the dominant species, Kobresia pygmaea (C.B.Clarke) C.B.Clarke; (3) the ecosystem’s water budget and hydrological fluxes; (4) fluxes in the carbon cycle; (5) soil properties and functions, including productivity and nutrients stocks; (6) the current extent of rangeland degradation and its causes; (7) the human impact shaping this ecosystem; (8) the current understanding of the age of this human impact. In addition, a new concept of natural autocyclic processes of turf erosion, initiated through polygonal cracking of the turf cover increased by overgrazing and facilitated by soil-dwelling endemic small mammals, will be presented.

Building on available literature, this review integrates field observations from surveys in the highlands undertaken by the first author between 1984 and 2015, and related studies on the status and dynamics of the ecosystem from grazing exclusion-experiments, obtained mainly in the northeastern montane Kobresia rangelands near Xinghai (Qinghai Province, 3440 m, 35°32’N / 99°51’E ), and in the core area of alpine Kobresia pygmaea pastures next to the

‘Kobresia pygmaea Research Station’ (Kema), now managed by the Institute of Tibetan Plat‐

eau Research, Chinese Academy of Science, southeast of Nagqu (Xizang Autonomous Region, 4450 m, 31°16’N / 92°06’E).