Herbivore exclusion stabilizes alpine grassland biomass production across spatial scales.

There is growing evidence that land-use management practices such as livestock grazing can strongly impact the local diversity, functioning, and stability of grassland communities. However, whether these impacts depend on environmental condition and propagate to larger spatial scales remains unclear. Using an 8-year grassland exclosure experiment conducted at nine sites in the Tibetan Plateau covering a large precipitation gradient, we found that herbivore exclusion increased the temporal stability of alpine grassland biomass production at both the local and larger (site) spatial scales. Higher local community stability was attributed to greater stability of dominant species, whereas higher stability at the larger scale was linked to higher spatial asynchrony of productivity among local communities. Additionally, sites with higher mean annual precipitation had lower dominant species stability and lower grassland stability at both the spatial scales considered. Our study provides novel evidence that livestock grazing can impair grassland stability across spatial scales and climatic gradients.

Geo-Distribution Patterns of Soil Fungal Community of Pennisetum flaccidum in Tibet.

Pennisetum flaccidum can be used as a pioneer species for the restoration of degraded grasslands and as a high-quality forage for local yak and sheep in alpine regions. The geographical distribution pattern of soil fungal community can modify that of P. flaccidum. A field survey along 32 sampling sites was conducted to explore the geo-distribution patterns of soil fungal community of P. flaccidum in Tibet. Soil fungal species, phylogenetic and function diversity generally had a closer correlation with longitude/elevation than latitude. The geo-distribution patterns of soil fungal species, phylogenetic and function diversity varied with soil depth. Soil fungal species, phylogenetic and function diversity had dissimilar geo-distribution patterns. Precipitation had stronger impacts on total abundance, species α-diversity, phylogenetic α-diversity, and function ß-diversity than temperature for both topsoil (0-10 cm depth) and subtopsoil (10-20 cm depth). Furthermore, precipitation had stronger impacts on function α-diversity for topsoil, species ß-diversity for topsoil, and phylogenetic ß-diversity for subtopsoil than temperature. The combination of species, phylogenetic and function diversity can better reflect geo-distribution patterns of soil fungal community. Compared to global warming, the impact of precipitation change on the variation in soil fungal community of P. flaccidum should be given more attention.

Asymmetrical warming of growing/non-growing season increases soil respiration during growing season in an alpine meadow.

Soil respiration (Rs) is an important carbon flux in the global carbon cycle, and understanding the influence of global warming on Rs is critical for precise prediction future climate change. Actually, global warming is expected to be seasonally asymmetric, however, it is still unclear on the response of Rs to asymmetrical warming of growing/non-growing season in alpine regions. In this study, an experiment with asymmetrical warming of growing/non-growing season (including three treatments, CK: control; GLNG: warming magnitude of growing season lower than non-growing season; GHNG: warming magnitude of growing season higher than non-growing season) was performed in an alpine meadow of the Northern Tibet since June 2015. The 'GLNG' and 'GHNG' treatments increased mean Rs by 71.22% (1.89 µmol CO2 m-2 s-1) and 34.32% (0.91 µmol CO2 m-2 s-1) during growing season in 2019, respectively. However, the 'GLNG' and 'GHNG' treatments did not significantly affect mean Rs during growing season in 2015, 2016, 2017 and 2018, respectively. The variation coefficient of growing season mean Rs was 32.95% under the CK treatment in 2015-2019. Therefore, warming may have a lagging effect on Rs. The warming scene with a greater warming during non-growing season may have a stronger effect on Rs than the warming scene with a greater warming during growing season. Inter-annual variation of Rs may be greater than the warming effect on Rs in alpine meadows.

Effects of experimental warming and increased precipitation on soil respiration in an alpine meadow in the Northern Tibetan Plateau.

Uncertainty on the response of soil respiration (Rs) to warming and increased precipitation on the Tibetan Plateau can limit our ability to predict how alpine ecosystems will respond to future climate change. Based on a warming (control, low- and high-level) and increased precipitation (control, low- and high-level) experiment, the response of Rs to experimental warming and increased precipitation was examined in an alpine meadow in the Northern Tibetan Plateau from 2014 to 2017. The low-level warming increased soil temperature (Ts) by 1.19°C and decreased soil moisture (SM) by 0.02m3m-3, whereas the high-level warming increased Ts by 2.88°C and decreased SM by 0.04m3m-3 over the four growing seasons in 2014-2017. The low- and high-level increased precipitation did not affect Ts, but increased SM by 0.02m3m-3 and 0.04m3m-3, respectively, over the four growing seasons in 2014-2017. No significant main and interactive effects of experimental warming and increased precipitation on Rs were observed over the four growing seasons in 2014-2017. In contrast, there was a significant inter-annual variation of Rs in 2014-2017. There was a marginally significant quadratic relationship between the effect of experimental warming on Rs and warming magnitude. There was a negligible difference of Rs between the low- and high-level increased precipitation over the four growing seasons in 2014-2017 and Rs also showed a quadratic relationship with precipitation. Therefore, experimental warming and increased precipitation did not change Rs and Rs responded nonlinearly to experimental warming and increased precipitation in the alpine meadow in the Northern Tibetan Plateau. Growing season precipitation may play a more important role than experimental warming and increased precipitation in affecting Rs in the alpine meadow in the Northern Tibetan Plateau.

Response of soil respiration to experimental warming in a highland barley of the Tibet.

Highland barley is an important dominant crop in the Tibet and the croplands of the Tibet are experiencing obvious climatic warming. However, information about how soil respiration will respond to climatic warming in the highland barley system is still lacking. A field warming experiment using infrared heaters with two warming magnitudes was conducted in a highland barley system of the Tibet in May 2014. Five daily cycles of soil respiration was measured using a CO2 flux system (Li-8100, Li-COR Biosciences, Lincoln, NE, USA) during the period from early June to early September in 2014. The high and low experimental warming significantly increased soil temperature by 1.98 and 1.52 °C over the whole study period, respectively. The high experimental warming significantly decreased soil moisture. Soil respiration and its temperature sensitivity did not significantly change under both the high and low experimental warming. The response of soil respiration to experimental warming did not linearly correlate with warming magnitudes because a greater experimental warming resulted in a higher soil drying. Our findings suggested that clarifying the response of soil CO2 production and its temperature sensitivity to climatic warming need consider water availability in the highland barley system of the Tibet.

Nutrient Enrichment Mediates the Relationships of Soil Microbial Respiration with Climatic Factors in an Alpine Meadow.

Quantifying the effects of nutrient additions on soil microbial respiration (R m) and its contribution to soil respiration (R s) are of great importance for accurate assessment ecosystem carbon (C) flux. Nitrogen (N) addition either alone (coded as LN and HN) or in combination with phosphorus (P) (coded as LN + P and HN + P) were manipulated in a semiarid alpine meadow on the Tibetan Plateau since 2008. Either LN or HN did not affect R m, while LN + P enhanced R m during peak growing periods, but HN + P did not affect R m. Nutrient addition also significantly affected R m /R s, and the correlations of R m /R s with climatic factors varied with years. Soil water content (Sw) was the main factor controlling the variations of R m /R s. During the years with large rainfall variations, R m /R s was negatively correlated with Sw, while, in years with even rainfall, R m/R s was positively correlated with Sw. Meanwhile, in N + P treatments the controlling effects of climatic factors on R m /R s were more significant than those in CK. Our results indicate that the sensitivity of soil microbes to climatic factors is regulated by nutrient enrichment. The divergent effects of Sw on R m /R s suggest that precipitation distribution patterns are key factors controlling soil microbial activities and ecosystem C fluxes in semiarid alpine meadow ecosystems.

Spatial and climatic patterns of the relative abundance of poisonous vs. non-poisonous plants across the Northern Tibetan Plateau.

It is the most serious challenge to promote degraded grassland recovery currently facing the developing Tibetan Autonomous Region. We conducted field surveys of 75 grazing sites between 2009 and 2012 across the Northern Tibetan Plateau and described the spatial and climatic patterns of the occurrence of poisonous plants. Our results showed lower ratios of species richness (SprRatio), coverage (CovRatio), and biomass (BioRatio) of non-poisonous vs. poisonous plants in the semi-arid alpine steppe zone, where the growing season precipitation (GSP) is between 250 and 350 mm; however, this result is in contrast to the relatively wetter meadow (GSP >350 mm) and much drier desert-steppe (GSP <250 mm) communities. Results from generalized additive models (GAMs) further confirmed that precipitation is primarily responsible for the initially decreasing and then increasing tendency of compositional ratios of non-poisonous to poisonous species. The wide confidence bands at GSP <250 mm indicated that precipitation is not an effective indicator for predicting compositional changes in desert-steppe communities. When mean annual livestock grazing pressure was incorporated into the optimal GAMs, the model performance improved: the Akaike information criterion (AIC) decreased by 1.20 for SprRatio and 3.09 for BioRatio, and the deviance explained (R (2)) increased by 6.0% for SprRatio and 3.6% for BioRatio. Therefore, more detailed information on grazing disturbance (timing, frequency, and density) should be collected to disentangle the relative contribution of climate change and grazing activities to changes in community assembly and ecological functions of alpine grasslands on the Northern Tibetan Plateau.

Response of soil C and N, dissolved organic C and N, and inorganic N to short-term experimental warming in an Alpine meadow on the Tibetan Plateau.

Although alpine meadows of Tibet are expected to be strongly affected by climatic warming, it remains unclear how soil organic C (SOC), total N (TN), ammonium N (NH4 (+)-N) , nitrate N (NO3 (+)-N), and dissolved organic C (DOC) and N (DON) respond to warming. This study aims to investigate the responses of these C and N pools to short-term experimental warming in an alpine meadow of Tibet. A warming experiment using open top chambers was conducted in an alpine meadow at three elevations (i.e., a low (4313 m), mid-(4513 m), and high (4693 m) elevation) in May 2010. Topsoil (0-20 cm depth) samples were collected in July-September 2011. Experimental warming increased soil temperature by ~1-1.4°C but decreased soil moisture by ~0.04 m(3) m(-3). Experimental warming had little effects on SOC, TN, DOC, and DON, which may be related to lower warming magnitude, the short period of warming treatment, and experimental warming-induced soil drying by decreasing soil microbial activity. Experimental warming decreased significantly inorganic N at the two lower elevations,but had negligible effect at the high elevation. Our findings suggested that the effects of short-term experimental warming on SOC, TN and dissolved organic matter were insignificant, only affecting inorganic forms.

Response of soil respiration to grazing in an alpine meadow at three elevations in tibet.

Alpine meadows are one major type of pastureland on the Tibetan Plateau. However, few studies have evaluated the response of soil respiration (R(s)) to grazing along an elevation gradient in an alpine meadow on the Tibetan Plateau. Here three fenced enclosures were established in an alpine meadow at three elevations (i.e., 4313 m, 4513 m, and 4693 m) in July 2008. We measured R s inside and outside the three fenced enclosures in July-September, 2010-2011. Topsoil (0-20 cm) samples were gathered in July, August, and September, 2011. There were no significant differences for R s , dissolved organic C (DOC), and belowground root biomass (BGB) between the grazed and ungrazed soils. Soil respiration was positively correlated with soil organic C (SOC), microbial biomass (MBC), DOC, and BGB. In addition, both R s and BGB increased with total N(TN), the ratio of SOC to TN, ammonium NH4 ⁺-H4⁺-N), and the ratio of NH4⁺-N to nitrate N. Our findings suggested that the negligible response of R s to grazing could be directly attributed to that of respiration substrate and that soil N may indirectly affect R(s) by its effect on BGB.

Responses of ecosystem CO 2 fluxes to short-term experimental warming and nitrogen enrichment in an Alpine meadow, northern Tibet Plateau.

Over the past decades, the Tibetan Plateau has experienced pronounced warming, yet the extent to which warming will affect alpine ecosystems depends on how warming interacts with other influential global change factors, such as nitrogen (N) deposition. A long-term warming and N manipulation experiment was established to investigate the interactive effects of warming and N deposition on alpine meadow. Open-top chambers were used to simulate warming. N addition, warming, N addition × warming, and a control were set up. In OTCs, daytime air and soil temperature were warmed by 2.0°C and 1.6°C above ambient conditions, but soil moisture was decreased by 4.95 m(3) m(-3). N addition enhanced ecosystem respiration (Reco); nevertheless, warming significantly decreased Reco. The decline of Reco resulting from warming was cancelled out by N addition in late growing season. Our results suggested that N addition enhanced Reco by increasing soil N availability and plant production, whereas warming decreased Reco through lowering soil moisture, soil N supply potential, and suppression of plant activity. Furthermore, season-specific responses of Reco indicated that warming and N deposition caused by future global change may have complicated influence on carbon cycles in alpine ecosystems.

[Soil N/P ratio distribution characteristics of alpine grassland ecosystem in Qinghai-Tibet Plateau].

The distribution characteristics of soil N/P ratio in alpine grassland ecosystem of Qinghai-Tibet Plateau were surveyed by field investigation and laboratory analysis. Horizontally, soil N/ P ratio was generally higher in west and lower in east in a manner of staggered patch distribution, with higher N/P ratios mainly centralized in the hinterland of northern part of Tibet Plateau and in the lake basin area of the northern foot of Himalayas. Significant differences in soil N/P ratio were observed among grassland types and natural transects. Vertically, the distribution of N/P ratio along the soil profile from aboveground to underground among different grass types could be categorized into five patterns, including low-high-low-high, low-high-low, low-high, high-low-high-low, and high-low-high. The N/P ratio showed a significant positive correlation with soil bulk density at 0-20 cm depth, soil water content at 20-30 cm depth, contents of soil available K and total nitrogen, respectively. However, it showed significant negative correlation with soil bulk density at 20-30 cm depth, contents of soil available P and total P, respectively.

Changes in individual plant traits and biomass allocation in alpine meadow with elevation variation on the Qinghai-Tibetan Plateau.

Plant traits and individual plant biomass allocation of 57 perennial herbaceous species, belonging to three common functional groups (forbs, grasses and sedges) at subalpine (3700 m ASL), alpine (4300 m ASL) and subnival (≥5000 m ASL) sites were examined to test the hypothesis that at high altitudes, plants reduce the proportion of aboveground parts and allocate more biomass to belowground parts, especially storage organs, as altitude increases, so as to geminate and resist environmental stress. However, results indicate that some divergence in biomass allocation exists among organs. With increasing altitude, the mean fractions of total biomass allocated to aboveground parts decreased. The mean fractions of total biomass allocation to storage organs at the subalpine site (7% ± 2% S.E.) were distinct from those at the alpine (23% ± 6%) and subnival (21% ± 6%) sites, while the proportions of green leaves at all altitudes remained almost constant. At 4300 m and 5000 m, the mean fractions of flower stems decreased by 45% and 41%, respectively, while fine roots increased by 86% and 102%, respectively. Specific leaf areas and leaf areas of forbs and grasses deceased with rising elevation, while sedges showed opposite trends. For all three functional groups, leaf area ratio and leaf area root mass ratio decreased, while fine root biomass increased at higher altitudes. Biomass allocation patterns of alpine plants were characterized by a reduction in aboveground reproductive organs and enlargement of fine roots, while the proportion of leaves remained stable. It was beneficial for high altitude plants to compensate carbon gain and nutrient uptake under low temperature and limited nutrients by stabilizing biomass investment to photosynthetic structures and increasing the absorption surface area of fine roots. In contrast to forbs and grasses that had high mycorrhizal infection, sedges had higher single leaf area and more root fraction, especially fine roots.

[Rainffall interception model of forest canopy: a preliminary study].

Canopy interception is an important hydrological process in forest ecosystem, and its modelling is of significance to understand and estimate the rainfall interception by the canopy. In this paper, a canopy rainfall interception model was established by dividing a rain incident into a set of short period, calculating the rainfall distribution intercepted by the canopy, and educing the process of the rain incident. This model considered the effects of the dryness of canopy and trunk on the evaporation from wet canopy and trunk during one rain incident, and introduced two factors, leaf area index (LAI) and surface area of trunk per unit area of ground (SAI), when computing the evaporation. The application of the model to simulate the rainfall interception process in a Larix principis-rupprechtii plantation in Guyuan, south Ningxia Hui autonomous region of China showed that the simulated and measured throughfall were identical, and the absolute deviation between simulated and measured results was within +/- 1 mm. But, when the precipitation was smaller than 6 mm, the simulated throughfall was lower than the measured one. The values of simulated stemflow were lower than the measured values, and the relative deviation between simulated and measured values was smaller when the precipitation was larger. The process of throughfall in the forest was also simulated, with the results fitted well to the measured one.

[Clonal growth of stoloniferous herb Potentilla anserina on degraded and non-degraded alpine meadow soil].

The clonal growth of Potentilla anserina on degraded and non-degraded Kobresia humilis meadow soil was studied by a transplanting experiment in the field. No significant differences in numbers of stolons, height, and leave size per mother ramet were observed between the two soils, but the numbers of leaves per mother ramet, length and width of stolon, and spacer length were significantly different. There were more leaves per mother ramet, longer stolon and spacer, and wider stolon on degraded soil, where available nutrient was poor than in non-degraded soil. Under degraded alpine meadow soil condition, the clonal plant species might produce much more photosynthetic product to support stolon growth by the increase of leave numbers per mother ramet, and the longer and wider stolon could intensify the foraging ability of the mother ramet that would benefit to daughter ramet. Both mother and daughter ramet of Potentilla anserina invested much more biomass to their underground part (root system) in non-degraded soil to increase the survival rate of daughter ramet.

[Analysis of formation causes of grassland degradation in Maduo County in the source region of Yellow River].

Natural and artificial factors of grassland degradation in Maduo County in source region of Yellow River were analyzed in this paper. The results showed that overgrazing was the major reason for grassland degradation. Compared with historical average, annual temperature and precipitation in Maduo had an obvious rise since the end of 1980s. Temperature rise concentrated in winter, and precipitation rise concentrated in spring and summer. Therefore, the change in water and heat condition was advantageous to grass growth there. Maduo was a county totally dependent on stock raising, and enlarging livestock population was the only choice for economic development. Together with the pressure of population growth and local Tibetan's traditional value, the number of livestock had been increased continuously, and finally reached summit in the end of 1970s. Since then, the grassland had been kept in the state of overgrazing and degradation, which also created favorable condition for the invasion of pikas, and thus sped up the process of degradation.