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Three years of eddy covariance measurements were used to characterize the seasonal and interannual variability of the CO 2 fluxes above an alpine meadow (3250 m a.s.l.) on the Qinghai-Tibetan Plateau, China. This alpine meadow was a weak sink for atmospheric CO 2 , with a net ecosystem production (NEP) of 78.5, 91.7, and 192.5 g C m À2 yr À1 in 2002, 2003, and 2004, respectively. The prominent, high NEP in 2004 resulted from the combination of high gross primary production (GPP) and low ecosystem respiration (R e ) during the growing season. The period of net absorption of CO 2 in 2004, 179 days, was 10 days longer than that in 2002 and 5 days longer than that in 2003. Moreover, the date on which the mean air temperature first exceeded 5.0 1C was 10 days earlier in 2004 (DOY110) than in 2002 or 2003. This date agrees well with that on which the green aboveground biomass (Green AGB) started to increase. The relationship between light-use efficiency and Green AGB was similar among the three years. In 2002, however, earlier senescence possibly caused low autumn GPP, and thus the annual NEP, to be lower. The low summertime R e in 2004 was apparently caused by lower soil temperatures and the relatively lower temperature dependence of R e in comparison with the other years. These results suggest that (1) the Qinghai-Tibetan Plateau plays a potentially significant role in global carbon sequestration, because alpine meadow covers about one-third of this vast plateau, and (2) the annual NEP in the alpine meadow was comprehensively controlled by the temperature environment, including its effect on biomass growth.

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Knowledge about the role of litter and dung decomposition in nutrient cycling and response to climate change and grazing in alpine ecosystems is still rudimentary. We conducted two separate studies to assess the relative role of warming and grazing on litter mass loss and on the temperature sensitivity of litter and dung mass loss. Experiments were conducted for 1-2 years under a controlled warming-grazing system and along an elevation gradient from 3200 to 3800 m. A free-air temperature enhancement system (FATE) using infrared heaters and grazing significantly increased soil temperatures (average 0.5-1.6 1C) from 0 to 40 cm depth, but neither warming nor grazing affected soil moisture except early in the growing seasons at 30 cm soil depth. Heaters caused greater soil warming at night-time compared with daytime, but grazing resulted in greater soil warming during daytime compared with night-time. Annual average values of the soil temperature at 5 cm were 3.2, 2.4 and 0.3 1C at 3200, 3600 and 3800 m, respectively. Neither warming nor grazing caused changes of litter quality for the first year of the controlled warming-grazing experiment. The effects of warming and grazing on litter mass losses were additive, increasing litter mass losses by about 19.3% and 8.3%, respectively, for the 2-year decomposition periods. The temperature sensitivity of litter mass losses was approximately 11% 1C À1 based on the controlled warming-grazing experiment. The annual cumulative litter mass loss was approximately 2.5 times that of dung along the elevation gradient. However, the temperature sensitivity (about 18% 1C À1 ) of the dung mass loss was about three times that of the litter mass loss. These results suggest greater warming at night-time compared with daytime may accelerate litter mass loss, and grazing will enhance carbon loss to atmosphere in the region through a decrease of litter biomass and an increase of dung production with an increase of stocking rate in future warmer conditions.

Understanding how flowering phenology responds to warming and cooling (i.e., symmetric or asymmetric response) is needed to predict the response of flowering phenology to future climate change that will happen with the occurrence of warm and cold years superimposed upon a long-term trend. A three-year reciprocal translocation experiment was performed along an elevation gradient from 3200 m to 3800 m in the Tibetan Plateau for six alpine plants. Transplanting to lower elevation (warming) advanced the first flowering date (FFD) and transplanting to higher elevation (cooling) had the opposite effect. The FFD of early spring flowering plants (ESF) was four times less sensitive to warming than to cooling (by À2.1 d/8C and 8.4 d/8C, respectively), while midsummer flowering plants (MSF) were about twice as sensitive to warming than to cooling (À8.0 d/8C and 4.9 d/8C, respectively). Compared with pooled warming and cooling data, warming alone significantly underpredicted 3.1 d/8C for ESF and overestimated 1.7 d/8C for MSF. These results suggest that future empirical and experimental studies should consider nonlinear temperature responses that can cause such warming-cooling asymmetries as well as differing life strategies (ESF vs. MSF) among plant species.

The uncertainties of China's gross primary productivity (GPP) estimates by global data-oriented products and ecosystem models justify a development of high-resolution data-oriented GPP dataset over China. We applied a machine learning algorithm

Questions How can we understand the limitations to plant growth at high altitudes? Our aim was to test the hypotheses that for alpine grasslands along a large altitudinal gradient in semi‐arid regions, plant growth is mainly limited by drought at low altitudes but by low temperature at high altitudes, resulting in a unimodal pattern of biomass and productivity associated with an optimal combination of temperature and precipitation. Such knowledge is important to understanding the response of alpine ecosystems to climate change. Location We conducted a 5‐yr livestock exclosure experiment along the south‐facing slope of the Nyaiqentanglha Mountains, central Tibetan Plateau. Methods We measured above‐ and below‐ground biomass, species richness, leaf δ13C and water potential, and related climate and soil variables across 42 fenced and unfenced quadrats near seven HOBO weather stations along the slope. The vegetation changed from alpine steppe‐meadow at 4390–4500 m to alpine meadow at 4600–5210 m. Results Total above‐ and below‐ground biomass across fenced and unfenced quadrats increased with increasing altitude up to 4950–5100 m, and then decreased above 5100 m. Altitudinal trends in leaf δ13C and water potential of dominant species also showed a unimodal pattern corresponding to that of vegetation biomass. Total above‐ and below‐ground biomass as well as sedge above‐ground biomass all showed a quadratic relationship with mean temperatures and the ratio of growing season precipitation (GSP) to ≥5 °C accumulated temperature (AccT; R2 = 0.83−0.88, P < 0.001). In general, above‐ and below‐ground biomass increased with increasing water availability when the GSP/AccT ratio was lower than the threshold level of 0.80–0.84, but decreased when the GSP/AccT ratio was higher than this threshold level. No significant relationship was found between residuals of above‐ground biomass and species richness after removing the effects of climate factors on both stand variables. Conclusions The results support our hypotheses, further suggesting a threshold of water limitation that is consistent with the model prediction over the Tibetan Plateau. Species richness per se appears to weakly affect community‐level productivity. The response of alpine grasslands to climate warming may vary with altitude because of altitudinal shifts in factors limiting plant growth.