What was done
In an article published in Quaternary Science Reviews, Herzschuh et al. (2011), as they describe it, "critically review possible driving forces for early- to mid-Holocene vegetation shifts on the upper Tibetan Plateau (including precipitation, growing season length, radiation, human impact) with particular emphasis on changing CO2 concentrations to better predict future environmental change and impacts on the Tibetan Plateau in a rapidly changing world."

What was learned
The four researchers report that "numerous pollen records from across the upper Tibetan Plateau indicate that Kobresia-dominated high-alpine meadow invaded alpine steppes during the mid- to late-Holocene." But they say that their investigation, which employed a pollen-moisture transfer function, suggested that "this marked vegetation change cannot be satisfactorily explained by climate change." In addition, they indicate that "a literature review did not reveal convincing evidence for any widespread human impact on mid-Holocene vegetation." Thus, they propose that the reconstructed vegetation changes were likely the result of "a response to Holocene CO2 concentration changes," with values rising from approximately 260 ppm in the early Holocene to near-present-day values on the order of 375 ppm.

This conclusion is based on four lines of evidence: the fact that "high-elevation vegetation is particularly sensitive to CO2 changes due to lowered CO2 partial pressure," the fact that "water conservation of steppe vegetation in response to experimental CO2 enrichment was of the same order of magnitude as inferred from mid- to late-Holodcene Tibetan pollen records," the fact that "modern remote sensing-aided vegetation monitoring of the Central Tibetan Plateau yielded an increase in biomass, most probably as a response to modern CO2 increase," even in spite of "increasing land-use by herding," as well as the fact that "experimental CO2 fertilization of dry grassland and desert vegetation performed in several regions world-wide has stimulated plant growth directly through enhanced photosynthesis and indirectly through enhanced water-use efficiency (Morgan et al., 2004)."

What it means
As the air's CO2 content continues to rise, we should see even further enhancement in the productivity of the Tibetan Plateau's vegetation, as well as that of similar high-elevation regions around the globe, which should be a huge positive development for those particular parts of the planet.

Reference
Morgan, J.A., Pataki, D.E., Korner, C., Clark, H., Del Grosso, S.J., Grunzweig, J.M., Knapp, A.K., Mosier, A.R., Newton, P.C.D., Niklaus, P.A., Nippert, J.B., Nowak, R.S., Parton, W.J., Polley, H.W. and Shaw, M.R. 2004. Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2. Oecologia 140: 11-25.