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Continued climate warming and precipitation fluctuations are expected to further affect aboveground net primary productivity (ANPP) across alpine grasslands, with associated implications for ecosystem functions. The spatial and temporal variability of ANPP in Tibetan Plateau (TP) grasslands and its response to temperature and precipitation were investigated in this study, based on the ANPP estimated by means of ensemble analysis of multiple machine learning models. First, the response of the spatial distribution of ANPP to variations in the precipitation gradient was nonlinear and showed an S-shaped growth pattern, which could be divided into four stages (stationary, rapid growth, slower growth, and saturation) corresponding to arid (mean annual precipitation (MAP) < 250 mm), semiarid (250 mm < MAP < 450 mm), semihumid (450 mm < MAP < 650 mm) and humid (MAP > 650 mm) precipitation regimes, respectively. Second, regional precipitation regimes affected the sensitivity of ANPP's interannual variability to temperature and precipitation; ANPP is more sensitive to temperature in wetter areas than in dryer areas of the TP region. The results of our study suggest that in the region of Tibetan Plateau, the response of grassland ANPP variation to climate was constrained by the background precipitation regimes. [ABSTRACT FROM AUTHOR]

Plant leaf stoichiometry reflects its adaptation to the environment. Leaf stoichiometry variations across different environments have been extensively studied in grassland plants, but little is known about intraspecific leaf stoichiometry, especially for widely distributed species, such as Stellera chamaejasme L. We present the first study on the leaf stoichiometry of S. chamaejasme and evaluate its relationships with environmental variables. S. chamaejasme leaf and soil samples from 29 invaded sites in the two plateaus of distinct environments [the Inner Mongolian Plateau (IM) and Qinghai-Tibet Plateau (QT)] in Northern China were collected. Leaf C, N, P, and K and their stoichiometric ratios, and soil physicochemical properties were determined and compared with climate information from each sampling site. The results showed that mean leaf C, N, P, and K concentrations were 498.60, 19.95, 2.15, and 6.57 g kg−1; the average C:N, C:P, N:P, N:K and K:P ratios were 25.20, 245.57, 9.81, 3.13, and 3.21, respectively. The N:P:K-ratios in S. chamaejasme leaf might imply that its growth is restricted by K- or K+N. Moreover, the soil physicochemical properties in the S. chamaejasme-infested areas varied remarkably, and few significant correlations between S. chamaejasme leaf ecological stoichiometry and soil physicochemical properties were observed. These indicate the nutrient concentrations and stoichiometry of S. chamaejasme tend to be insensitive to variations in the soil nutrient availability, resulting in their broad distributions in China's grasslands. Besides, different homeostasis strength of the C, N, K, and their ratios in S. chamaejasme leaves across all sites were observed, which means S. chamaejasme could be more conservative in their use of nutrients improving their adaptation to diverse conditions. Moreover, the leaf C and N contents of S. chamaejasm were unaffected by any climate factors. However, the correlation between leaf P content and climate factors was significant only in IM, while the leaf K happened to be significant in QT. Besides, MAP or MAT contribution was stronger in the leaf elements than soil by using mixed effects models, which illustrated once more the relatively weak effect of the soil physicochemical properties on the leaf elements. Finally, partial least squares path modeling suggested that leaf P or K contents were affected by different mechanisms in QT and IM regions, suggesting that S. chamaejasme can adapt to changing environments by adjusting its relationships with the climate or soil factors to improve its survival opportunities in degraded grasslands. [ABSTRACT FROM AUTHOR]

Fencing is an important method for restoring and reconstructing degraded grasslands on the Tibetan Plateau. Understanding the characteristics of soil water percolation and soil dissolved organic carbon (DOC) leaching and their response to long-term fencing (10 years) could provide a scientific basis for the management of soil water resources and carbon sinks in alpine grasslands. In this study, grazing plots and plots fenced for 10 years were selected in an alpine meadow, and soil bulk density, soil organic carbon (SOC) density, vegetative carbon (VC) density, soil water percolation and DOC leaching were monitored regularly in the two treatments. The results were as follows: (1) Long-term fencing reduced the soil bulk density and improved carbon sequestration in the alpine meadow. The soil bulk density at a depth of 0-20 cm in the fenced plots was significantly lower than that of the grazing plots ( p < 0.05), and SOC density and VC density at a depth of 0-40 cm were higher than those of the grazing plots to a different degree. (2) During the non-frozen period (from May to September), total soil water leakage at a depth of 40 cm in the grazing plots was 9.6 mm, which accounted for 2.2% of the total rainfall in this period. After fencing for 10 years, soil water leakage was increased by 53.1%. (3) From May to September, total DOC leaching from the grazing plots was 34.6 gC m, and total DOC leaching from the fenced plots was increased by 55.5% compared to the grazing plots. Our results demonstrated obvious soil water percolation and DOC leaching in the alpine meadow. The DOC entered into the groundwater system with the percolation water and finally flowed into lakes and rivers, which represents an important soil carbon loss pathway in alpine meadows. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau is primarily composed of alpine grasslands. Spatial distributions of alpine grasses, however, are not well documented in this remote, highly uninhabited region. Taking advantage of the frequently observed moderate resolution imaging spectroradiometer (MODIS) images (500-m, 8-day) in 2010, this study extracted the phenological metrics of alpine grasses from the normalized difference vegetation index time series. With the Support Vector Machine, a multistep classification approach was developed to delineate alpine meadows, steppes, and desert grasses. The lakes, permanent snow, and barren/desert lands were also classified with a MODIS scene acquired in the peak growing season. With ground data collected in the field and aerial experiments in 2011, the overall accuracy reached 93% when alpine desert grasses and barren lands were not examined. In comparison with the recently published national vegetation map, the alpine grassland map in this study revealed smoother transition between alpine meadows and steppes, less alpine meadows in the southwest, and more barren/deserts in the high-cold Kunlun Mountain in the northeast. These variations better reflected climate control (e.g. precipitation) of different climatic divisions on alpine grasslands. The improved alpine grassland map could provide important base information about this cold region under the pressure of rapidly changing climate. [ABSTRACT FROM AUTHOR]