The Kobresia pygmaea Ecosystem Of The Tibetan Highlands: Origin, Functioning And Degradation Of The World's Largest Pastoral Alpine Ecosystem

Kobresiapastures in the eastern Tibetan highlands occupy 450000 sqm and form the world9s largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresiapygmaea, which forms a lawnwith a 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 remain largely unknown. Here we present a review on ecosystem diversity, reproduction and ecology of the key species, pasture health, cycles of carbon (C), water and nutrients, and on the paleo-environment. The methods employed include molecular analysis, grazing exclusion, measurements with micro- lysimetersand gas exchange chambers, 13C and 15N labeling, eddy-covarianceflux measurements, remote sensing and atmospheric modeling. The following combination of traits makes Kobresiapygmaea resilient and highly competitive: dwarf habit, predominantly below-ground allocation of photo-assimilates, mixed reproduction strategy with both seed production and clonal growth, and high genetic diversity. Growth of Kobresiapastures is co-limited by low rainfall during the short growing season and livestock-mediated nutrient withdrawal. Overstockinghas caused pasture degradation and soil deterioration, yet the extent remains debated. In addition, we newly describe natural auto-cyclic processes of turf erosion initiated through polygonal cracking of the turf cover, and accelerated 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 and (2) earlier diurnal formation of clouds resulting in (3) decreased surface temperatures with (4) likely consequences for atmospheric circulationon large regional and, possibly global, scales. Paleo-environmental reconstruction, in conjunction with grazing experiments, suggests that the present grazing lawnsof Kobresiapygmaea are synanthropicand may have existed since the onset of pastoralism. The traditional migratory rangeland managementwas sustainable over millennia and possibly still offers the best strategy to conserve, and possibly increase, the C stocks in the Kobresiaturf, as well as its importance for climate regulation.