The Kobresia pygmaea ecosystem of the Tibetan highlands – Origin, functioning and degradation of the world's largest pastoral alpine ecosystem

Abstract

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.

Response to reviewers (bg-2019-50)

“…Author s changes in manuscript: The end of chapter 3.3 now reads: "Furthermore, the occurrence of synanthropic taxa has been observed in the nearby Damxung valley since 8.5 cal ka BP (Schlütz et al, 2007). This corroborates the strong anthropogenic influence on the formation and restructuring of vegetation patterns in the area but leaves a time gap of almost 3 ka between the evidence from Damxung valley and Nam Co.…”

“…Furthermore, we adjusted the elevation limits in figure 4. Approximate biome elevations were taken from satellite imagery (Sentinel 2B) and herewith derived vegetation indices, field excursion and literature review (Wang and Yi, 2011;Ohtsuka et al, 2008).…”

A long-term (2005–2019) eddy covariance data set of CO<sub>2</sub> and H<sub>2</sub>O fluxes from the Tibetan alpine steppe

“…Detrending method: block averaging Time lags optimization: covariance maximization with default ±1 s Correction for air density fluctuations: application of WPL terms to fluxes (Webb et al, 1980) Spectral corrections: analytic high-pass filtering (Moncrieff et al, 2005) and analytic low-pass filtering (Moncrieff et al, 1997) this error is not evenly distributed but has a greater effect during daytime and summer when sensible heat fluxes are large. Fratini et al (2014) have shown that errors in mean concentrations leak into errors in fluxes on account of amplified or dampened estimated fluctuations.…”

“…The majority of the TP is covered by the biggest pastural system in the world, the so-called steppe-meadow ecotone, consisting of 450 000 km 2 of Kobresia (syn. Carex) pygmaea pastures and 800 000 km 2 of alpine steppe ecosystem (Miehe et al, 2011;Miehe et al, 2019). With decreasing precipitation to the west, the K. pygmaea pastures are being replaced by alpine steppe ecosystems.…”

Modeling impacts of climate change and grazing effects on plant biomass and soil organic carbon in the Qinghai–Tibetan grasslands

“…In recent decades, due to climate change and increased human disturbances, the high-altitude alpine grassland ecosystems, which are the dominant grassland vegetation type, have been severely degraded (Gao et al, 2010;Miehe et al, 2017;Qiao et al, 2015). The air temperature on the plateau has increased by 0.3 • C decade −1 , which is 3 times the global average .…”