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Alpine vegetation on the Tibetan Plateau (TP) is known to be sensitive to both climate change and anthropogenic disturbance. However, the magnitude and patterns of alpine vegetation dynamics and the driving mechanisms behind their variation on the TP remains under debate. In this study, we used updated MODIS Collection 6 Normalized Difference Vegetation Index (NDVI) from the Terra satellite combined with linear regression and the Break for Additive Season and Trend model to reanalyze the spatiotemporal patterns of vegetation change on the TP during 2000–2015. We then quantified the responses of vegetation variation to climatic and anthropogenic factors by coupling climatic and human footprint datasets. Results show that growing season NDVI (GNDVI) values increased significantly overall (0.0011 year−1, p < 0.01) during 2000–2015 and that 70.37% of vegetated area on the TP (23.47% significantly with p < 0.05) exhibited greening trends with the exception of the southwest TP. However, vegetation greenness experienced trend shifts from greening to browning in half of the ecosystem zones occurred around 2010, likely induced by spatially heterogeneous temporal trends of climate variables. The vegetation changes in the northeastern and southwestern TP were water limited, the mid-eastern TP exhibited strong temperature responses, and the south of TP was driven by a combination of temperature and solar radiation. Furthermore, we found that, to some extent, anthropogenic disturbances offset climate-driven vegetation greening and aggravated vegetation browning induced by water deficit. These findings suggest that the impact of anthropogenic activities on vegetation change might not overwhelm that of climate change at the region scale.

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The root biomass distribution in alpine ecosystems (alpine meadow, alpine steppe, desert grassland and alpine desert) was investigated along a transect on the northern Tibetan Plateau in 2009. The results showed that roots were mainly concentrated in the 0-20 cm layer, and root biomass decreased exponentially with increasing soil depth. Root biomass was estimated to be 1,381.41 ± 245.29 g m -2 in the top 20 cm soil, accounting for 85% of the total root biomass. The distribution pattern of the root biomass proportion along the soil profile was similar in different alpine ecosystems. The root biomass density varied with different alpine ecosystems and the total average root biomass was 1,626.08 ± 301.76 g m -2 . Root biomass was significantly correlated with average relative humidity, annual precipitation and soil organic matter. This indicates that precipitation and soil organic matter might be crucial for plant growth in the study area, while temperature is not an important factor controlling root growth.

The influences of grazing exclusion on plant species composition and intrinsic water-use efficiency (W i) of alpine grasslands on the Northern Tibet Plateau are not well understood. We conducted a multisite transect field survey across the three main alpine ecosystems (meadow, steppe, and desert steppe) with nine pairs of grazing-excluded and adjacent open grazed pastures. Shortterm grazing exclusion (started in 2006) did not result in significant changes in nutrients or bulk density of the surface soils (0-15 cm), but it slightly changed the aboveground biomass (AGB) and coverage at both community and species levels. Community foliar d 13 C and W i differed among alpine grassland types, with values for steppe being similar to those for meadow and with desert steppe values being higher than both of these. Foliar d 13 C and inferred W i differed among the dominant species and varied negatively with precipitation and positively with temperature in 2010. These results confirm that there is an environmentally selective effect on the replacement of dominant species. There was no evident difference in foliar d 13 C or W i between grazing-excluded and open grazed sites, but there was a slight increase of AGB and coverage in grazing-excluded sites compared to open grazed ones at the species and community levels. These results indicated that grazing exclusion may have no clear influence on the physiological processes related to foliar water usage at the species level, but may have a cumulative effect on the carbon-water balance at the community level. Slight changes in linear regressions of foliar d 13 C and W i plotted across climatic gradients indicated that grasslands under grazing management might be more sensitive to regional climatic changes.

Biomass allocation is an essential concept for understanding above- vs. below-ground functions and for predicting the dynamics of community structure and ecosystem service under ongoing climate change. There is rare available knowledge of grazing effects on biomass allocation in multiple zonal alpine grassland types along climatic gradients across the Northern Tibetan Plateau. We collected the peak above- and below-ground biomass (AGB and BGB) values at 106 pairs of well-matched grazed vs. fenced sites during summers of 2010–2013, of which 33 pairs were subject to meadow, 52 to steppe and 21 to desert-steppe. The aboveground net primary productivity (ANPP) was represented by the peak AGB while the belowground net primary productivity (BNPP) was estimated from ANPP, the ratio of living vs. dead BGB, and the root turnover rate. Two-ways analyses of variance (ANOVA) and paired samples comparisons with t-test were applied to examine the effects of pasture managements (PMS, i.e., grazed vs. fenced) and zonal grassland types on both ANPP and BNPP. Allometric and isometric allocation hypotheses were also tested between logarithmically transformed ANPP and BNPP using standardized major axis (SMA) analyses across grazed, fenced and overall sites. In our study, a high community-dependency was observed to support the allometric biomass allocation hypothesis, in association with decreased ANPP and a decreasing-to-increasing BNPP proportions with increasing aridity across the Northern Tibetan Plateau. Grazing vs. fencing seemed to have a trivial effect on ANPP compared to the overwhelming influence of different zonal grassland types. Vegetation links above- and below-ground ecological functions through integrated meta-population adaptive strategies to the increasing severity of habitat conditions. Therefore, more detailed studies on functional diversity are essentially to achieve conservation and sustainability goals under ongoing climatic warming and intensifying human influences.