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The timing of the fruit-set stage (i.e., start and end of fruit set) is crucial in a plant's life cycle, but its response to temperature change is still unclear. We investigated the timing of seven phenological events, including fruit-set dates during 3 yr for six alpine plants transplanted to warmer (approximately +3.5°C in soils) and cooler (approximately −3.5°C in soils) locations along an altitudinal gradient in the Tibetan area. We found that fruit-set dates remained relatively stable under both warming and cooling during the 3-yr transplant experiment. Three earlier phenological events (emergence of first leaf, first bud set, and first flowering) and two later phenological events (first leaf coloring and complete leaf coloring) were earlier by 4.8-8.2 d/°C and later by 3.2-7.1 d/°C in response to warming. Conversely, cooling delayed the three earlier events by 3.8-6.9 d/°C and advanced the two later events by 3.2-8.1 d/°C for all plant species. The timing of the first and/or last fruit-set dates, however, did not change significantly compared to earlier and later phenological events. Statistical analyses also showed that the dates of fruit set were not significantly correlated or had lower correlations with changes of soil temperature relative to the earlier and later phenological events. Alpine plants may thus acclimate to changes in temperature for their fruiting function by maintaining relatively stable timings of fruit set compared with other phenological events to maximize the success of seed maturation and dispersal in response to short-term warming or cooling. [ABSTRACT FROM AUTHOR]

There is a great uncertainty about the effect of land use change on grassland ecosystem in the Tibetan Plateau. Net ecosystem carbon exchange (NEE) was measured for native alpine meadow with winter grazing (NAM), abandoned cropland/pasture (APL), perennial Elymus nutans (PEN), and annual oat pasture (AO) on the Tibetan plateau, during the growing seasons in 2009 and 2010 using a transparent chamber technique ( Licor-6400). AO significantly decreased annual average NEE by 21.6, 23.7, and 15.7% compared to PEN, NAM, and APL during the growing season in 2010. Compared to PEN, NAM, and APL, AO significantly decreased average ecosystem respiration (R) by 21.1, 52.3, and 39.3%, respectively, during the growing season in 2009. Soil moisture and total aboveground and belowground biomass together explained 39.6% of NEE variation and 71% in gross primary productivity variation. Soil moisture and belowground biomass explained about 83.1% of the R variation. Our results indicated that it is possible to convert APL to PEN in the region because it could result in a higher NEE together with higher forage production compared to AO. [ABSTRACT FROM AUTHOR]

Grassland soil organic carbon (SOC) is sensitive to anthropogenic activities. Increased anthropogenic disturbance related to overgrazing has led to widespread alpine grassland degradation on the Tibetan Plateau. The degraded grasslands are considered to have great potential for carbon sequestration after adoption of improved management practices. Here, we calibrated and employed the Century model to investigate the effects of overgrazing and improved managements on the SOC dynamics in alpine meadows. We calibrated the Century model against plant productivity at the Haibei Research Station. SOC stocks for validation were obtained in 2009-2010 from degraded alpine meadows in two communes. We found that Century model can successfully capture grassland SOC changes. Overall, our simulation suggests that degraded alpine meadow SOC significantly increased with the advent of restoration management from 2011 to 2030. Carbon sequestration rates ranged between 0.04 Mg C ha-1 yr-1 in lightly degraded winter grazing grasslands and 2.0 Mg C ha-1 yr-1 in moderately degraded summer grazing grasslands. Our modelling work also predicts that improve management in degraded Tibetan grasslands will contribute to an annual carbon sink of 0.022-0.059 Pg C yr-1. These results imply that restoration of degraded grasslands in the Tibetan Plateau has great potential for soil carbon sequestration to mitigate greenhouse gases. [ABSTRACT FROM AUTHOR]

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 −2.1 d/°C and 8.4 d/°C, respectively), while midsummer flowering plants (MSF) were about twice as sensitive to warming than to cooling (−8.0 d/°C and 4.9 d/°C, respectively). Compared with pooled warming and cooling data, warming alone significantly underpredicted 3.1 d/°C for ESF and overestimated 1.7 d/°C 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. [ABSTRACT FROM AUTHOR]

Little research has been conducted on how to balance plant production and soil respiration (Rs) under seasonal grazing patterns in alpine meadows. Our results from 2009 to 2012 showed that warm season grazing ( WG) from June to September significantly increased aboveground net primary production compared with no-grazing ( NG), except in 2010, and compared with cold season grazing ( CG) except in 2012, while there were no significant differences between NG and CG except in 2009. In both WG and CG treatments, grazing increased root biomass at 0-40 cm depth compared with NG, except in 2011. WG and CG only significantly increased seasonal Rs in 2009. Daily Rs was mainly affected by soil temperature, which explained 40-49% of the variation in daily Rs for all grazing treatments. Seasonal Rs from July to September was significantly influenced by soil temperature and root biomass, which explained 55% of the variation in seasonal Rs for all grazing treatments. Therefore, relative to NG, regardless of WG and CG, moderate grazing significantly increased plant production and had little influence on soil respiration in this alpine region. [ABSTRACT FROM AUTHOR]

Extensive research has focused on the temperature sensitivity of microbial respiration in tundra and alpine meadows. However, the response of microbial respiration to thermal stress in alpine steppe soils, which have less organic matter and greater aeration, has received less attention. We investigated spatial and temporal variations in microbial respiration using an incubation experiment under different temperature (0, 15 and 30°C) and different percentages of water-holding capacity (WHC) (50 and 100%) conditions in the alpine steppe, and using a subsequent cooling experiment determined the 'thermal stress' of soil microorganisms in response to increased temperature in the alpine steppe ecosystem. Microbial respiration rates decreased with increasing temperature at both 50 and 100% WHC for three sampling locations. Thermal stress of soil microorganisms under increased temperatures was found in the alpine steppe because subsequent cooling led to an increase in microbial respiration rates. Soil moisture did not affect microbial respiration, but the temperature sensitivity (Q10) of microbial respiration varied with the interactive effect between soil moisture and sampling location. Our findings suggest that the response of microbial respiration to high temperature may not be always positive as bacteria may experience thermal stress in the alpine steppe. Therefore, it is necessary to include thermal stress responses in alpine steppe models, as they may represent an important negative feedback effect on microbial respiration. [ABSTRACT FROM AUTHOR]