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Moderate Resolution Imaging Spectroradiometer (MODIS) continuously monitors gross primary production (GPP), which is an extremely important component of carbon cycling, at the global scale. Uncertainties about MODIS GPP limit our ability to accurately quantify GPP at the regional scales. The Collection 6 MODIS/Terra and MODIS/Aqua GPP products (i.e. MOD17A2H and MYD17A2H) were compared with the estimated GPP (GPPEC) by eddy covariance measurements in an alpine meadow in the Northern Tibetan Plateau during three consecutive growing seasons of 2005-2007. The Collection 6 MODIS/Terra and MODIS/Aqua fractional photosynthetically active radiation (FPAR) products (i.e. MOD15A2H and MYD15A2H) were also validated. The MOD17A2H and MYD17A2H products tended to overestimate GPPEC by 2.17% and 7.35% in 2005-2007, respectively, although these differences were not significant. The MOD15A2H and MYD15A2H products also tended to overestimate ground-based FPAR (FPARG) by 20.31% and 24.73% in 2005-2007, respectively. The overestimation of FPAR resulted in about 17.51-23.97% overestimation of GPPEC. The default maximum light-use efficiency (εmax) of 0.86 g C MJ-1 only underestimated the ground-based εmax (0.88 g C MJ-1) by 2.27%, which in turn resulted in about 2.13-2.72% underestimation of GPPEC. The meteorology data errors only caused about 0.48-1.06% underestimation of GPPEC. Therefore, although MODIS Collection 6 GPP had a very high accuracy, the input parameters had relative greater errors in the alpine meadow of the Northern Tibetan Plateau. The differences between MODIS GPP and GPPEC mainly resulted from FPAR, followed by εmax and meteorological data. [ABSTRACT FROM AUTHOR]

Alpine treeline ecotones are harsh environment for tree establishment due to low temperature. Tree establishment at treelines requires favorable climate, suitable microsites, and viable seeds. But most researches have been addressed treeline microclimate and its effects on tree regeneration, the knowledge of seed quantity and quality and its controls on seedling recruitment were limited. We measured seed rain, soil seed bank, seed germination rate and seedling recruitment in natural forests in combination with seed transplanting manipulation to evaluate the controls of seed quantity and quality on seedling recruitment of Abies georgei var. smithii (smith fir) along altitudinal gradient, with focus on treeline ecotone in the Sygera Mountains, southeastern Tibetan Plateau. Both seed quantity and seed quality of smith fir decreased with increasing altitude and was thereby associated with decline in seed germination rate. Seed quantity and seedling recruitment were better in north-facing slope than in south slope. The treeline ecotone above 4200 m appeared as the threshold altitude to sharply decrease seed quality and seedling recruitment. The emergence and overwintering rates of transplanting seeds from 3600−3800 m also went down remarkably above 4200 m at north-facing slope. It also underpins the fact that treeline ecotone is the bottleneck of seedling recruitment. Our results suggest that seed quantity and quality are the principal limitation of treeline upward advance. This study also provides evidence to support stable treeline position in southeastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) holds ten thousands of alpine glaciers in mid-latitude. They have shrunk with an accelerating retreat rate recently. We applied a distributed temperature-index massbalance model developed by Regine Hock, and coupled with a volume-area scaling method to Xiao Dongkemadi Glacier (XDG) in the central TP, to assess its response to climate change. The result shows the simulated mass balance is in a good agreement with observations ( R=0.75, p<0.001) during the period of 1989-2012. The simulated mean annual mass balance (-213 mm w.e.) is close to the observation (-233 mm w.e.), indicating the model can be used to estimate the glacier variation in the future. Then the model was forced by the output of RegCM4 under the climate scenarios RCP4.5 and RCP8.5 from 2013 to 2050. The simulated terminus elevation of the glacier will rise from 5454 m a.s.l. in 2013 to 5533 m a.s.l. (RCP4.5) and 5543 m a.s.l. (RCP8.5) in 2050. XDG will lose its volume with an increasing rate of 600-700 m a1 during the period of 1989-2050, indicating the melting water will enhance the river runoff. But for the long term, the contribution to the river runoff will decrease for the shrinkage of glacier scale. [ABSTRACT FROM AUTHOR]

Variations in the fractions of biomass allocated to functional components are widely considered as plant responses to resource availability for grassland plants. Observations indicated shoots isometrically relates to roots at the community level but allometrically at the species level in Tibetan alpine grasslands. These differences may result from the specific complementarity of functional groups between functional components, such as leaf, root, stem and reproductive organ. To test the component complementary responses to regional moisture variation, we conducted a multi-site transect survey to measure plant individual size and component biomass fractions of common species belonging to the functional groups: forbs, grasses, legumes and sedges on the Northern Tibetan Plateau in peak growing season in 2010. Along the mean annual precipitation (MAP) gradient, we sampled 70 species, in which 20 are in alpine meadows, 20 in alpine steppes, 15 in alpine desert-steppes and 15 in alpine deserts, respectively. Our results showed that the size of alpine plants is small with individual biomass mostly lower than 1.0 g. Plants keep relative conservative component fractions across alpine grasslands at the individual level. However, the complementary responses between functional components to moisture variations specifically differ among functional groups. These results indicate that functional group diversity may be an effective tool for scaling biomass allocation patterns from individual up to community level. Therefore, it is necessary and valuable to perform intensive and systematic studies on identification and differentiation the influences of compositional changes in functional groups on ecosystem primary services and processes. [ABSTRACT FROM AUTHOR]