Reconsidering the efficiency of grazing exclusion using fences on the Tibetan Plateau.

Grazing exclusion using fences is a key policy being applied by the Chinese government to rehabilitate degraded grasslands on the Tibetan Plateau (TP) and elsewhere. However, there is a limited understanding of the effects of grazing exclusion on alpine ecosystem functions and services and its impacts on herders' livelihoods. Our meta-analyses and questionnaire-based surveys revealed that grazing exclusion with fences was effective in promoting aboveground vegetation growth for up to four years in degraded alpine meadows and for up to eight years in the alpine steppes of the TP. Longer-term fencing did not bring any ecological and economic benefits. We also found that fencing hindered wildlife movement, increased grazing pressure in unfenced areas, lowered the satisfaction of herders, and rendered substantial financial costs to both regional and national governments. We recommend that traditional free grazing should be encouraged if applicable, short-term fencing (for 4-8 years) should be adopted in severely degraded grasslands, and fencing should be avoided in key wildlife habitat areas, especially the protected large mammal species.

Carbon, nitrogen, and phosphorus storage in alpine grassland ecosystems of Tibet: effects of grazing exclusion.

In recent decades, alpine grasslands have been seriously degraded on the Tibetan Plateau and grazing exclusion by fencing has been widely adopted to restore degraded grasslands since 2004. To elucidate how alpine grasslands carbon (C), nitrogen (N), and phosphorus (P) storage responds to this management strategy, three types of alpine grassland in nine counties in Tibet were selected to investigate C, N, and P storage in the environment by comparing free grazing (FG) and grazing exclusion (GE) treatments, which had run for 6-8 years. The results revealed that there were no significant differences in total ecosystem C, N, and P storage, as well as the C, N, and P stored in both total biomass and soil (0-30 cm) fractions between FG and GE grasslands. However, precipitation played a key role in controlling C, N, and P storage and distribution. With grazing exclusion, C and N stored in aboveground biomass significantly increased by 5.7 g m(-2) and 0.1 g m(-2), respectively, whereas the C and P stored in the soil surface layer (0-15 cm) significantly decreased by 862.9 g m(-2) and 13.6 g m(-2), respectively. Furthermore, the storage of the aboveground biomass C, N, and P was positively correlated with vegetation cover and negatively correlated with the biodiversity index, including Pielou evenness index, Shannon-Wiener diversity index, and Simpson dominance index. The storage of soil surface layer C, N, and P was positively correlated with soil silt content and negatively correlated with soil sand content. Our results demonstrated that grazing exclusion had no impact on total C, N, and P storage, as well as C, N, and P in both total biomass and soil (0-30 cm) fractions in the alpine grassland ecosystem. However, grazing exclusion could result in increased aboveground biomass C and N pools and decreased soil surface layer (0-15 cm) C and P pools.

Non-additive effects of litter diversity on greenhouse gas emissions from alpine steppe soil in Northern Tibet.

While litter decomposition is a fundamental ecological process, previous studies have mainly focused on the decay of single species. In this study, we conducted a litter-mixing experiment to investigate litter diversity effects on greenhouse gas (GHG) emissions from an alpine steppe soil in Northern Tibet. Significant non-additive effects of litter diversity on GHG dynamics can be detected; these non-additive effects were the result of species composition rather than species richness. Synergistic effects were frequent for CO2 and N2O emissions, as they were found to occur in 70.5% and 47.1% of total cases, respectively; antagonistic effects on CH4 uptake predominated in 60.3% of the cases examined. The degree of synergism and antagonism may be significantly impacted by litter chemical traits, such as lignin and N, lignin:N ratio, and total phenols during decomposition (P < 0.05). In addition, the relationship between chemical traits and litter-mixing effects changed over incubation time. Our study provides an opportunity to gain insight into the relationship between litter diversity and soil ecological processes. The results indicate that higher plant diversity may generally enhance CO2 and N2O emissions while inhibiting CH4 uptake; meanwhile, the direction and strength of non-additive effects appear to be related to litter chemical traits.

Effects of grazing regimes on plant traits and soil nutrients in an alpine steppe, Northern Tibetan Plateau.

Understanding the impact of grazing intensity on grassland production and soil fertility is of fundamental importance for grassland conservation and management. We thus compared three types of alpine steppe management by studying vegetation traits and soil properties in response to three levels of grazing pressure: permanent grazing (M1), seasonal grazing (M2), and grazing exclusion (M3) in the alpine steppe in Xainza County, Tibetan Plateau. The results showed that community biomass allocation did not support the isometric hypothesis under different grassland management types. Plants in M1 had less aboveground biomass but more belowground biomass in the top soil layer than those in M2 and M3, which was largely due to that root/shoot ratios of dominant plants in M1 were far greater than those in M2 and M3. The interramet distance and the tiller size of the dominant clonal plants were greater in M3 than in M1 and M2, while the resprouting from rhizome buds did not differ significantly among the three greezing regimes. Both soil bulk density and soil available nitrogen in M3 were greater than in M1 at the 15-30 cm soil depth (P = 0.05). Soil organic carbon and soil total nitrogen were greater in M3 than in M1 and M2 (P = 0.05). We conclude that the isometric hypothesis is not supported in this study and fencing is a helpful grassland management in terms of plant growth and soil nutrient retention in alpine steppe. The extreme cold, scarce precipitation and short growing period may be the causation of the unique plant and soil responses to different management regimes.

Exploring the reactivity of multicomponent photocatalysts: insight into the complex valence band of BiOBr.

The band structure of multicomponent semiconductor photocatalysts, as well as their reactivity distinction under different wavelengths of light, is still unclear. BiOBr, which is a typical multicomponent semiconductor, may have two possible valence-band structures, that is, two discrete valence bands constructed respectively from O 2p and Br 4p orbitals, or one valence band derived from the hybridization of these orbitals. In this work, aqueous photocatalytic hydroxylation is applied as the probe reaction to investigate the nature and reactions of photogenerated holes in BiOBr. Three organic compounds (microcystin-LR, aniline, and benzoic acid) with different oxidation potentials were selected as substrates. Isotope labeling (H(2)(18)O as the solvent) was used to determine the source of the O atom in the hydroxyl group of the products, which distinguishes the contribution of different hydroxylation pathways. Furthermore, a spin-trapping ESR method was used to quantify the reactive oxygen species ((.)OH and (.)OOH) formed in the reaction system. The different isotope abundances of the hydroxyl O atom of the products formed, as well as the reverse trend of the (.)OH/(.)OOH ratio with the oxidative resistance of the substrate under UV and visible irradiation, reveal that BiOBr has two separate valence bands, which have different oxidation ability and respond to UV and visible light, respectively. This study shows that the band structure of semiconductor photocatalysts can be reliably analyzed with an isotope labeling method.

Intensity and importance of competition for a grass (Festuca rubra) and a legume (Trifolium pratense) vary with environmental changes.

How plant competition varies across environmental gradients has been a long debate among ecologists. We conducted a growth chamber experiment to determine the intensity and importance of competition for plants grown in changed environmental conditions. Festuca rubra and Trifolium pratense were grown in monoculture and in two- and/or three-species mixtures under three environmental treatments. The measured competitive variations in terms of growth (height and biomass) were species-dependent. Competition intensity for Festuca increased with decreased productivity, whilst competition importance displayed a humpback response. However, significant response was detected in neither competition intensity nor importance for Trifolium. Intensity and importance of competition followed different response patterns, suggesting that they may not be correlated along an environmental gradient. The biological and physiological variables of plants play an important role to determine the interspecific competition associated with competition intensity and importance. However, the competitive feature can be modified by multiple environmental changes which may increase or hinder how competitive a plant is.

Modeling effects of temperature and precipitation on carbon characteristics and GHGs emissions in Abies fabric forest of subalpine.

Abies fabric forest in the eastern slope of Gongga mountain is one type of subalpine dark coniferous forests of southwestern China. It is located on the southeastern edge of the Qinghai-Tibet plateau and is sensitive to climatic changes. A process-oriented biogeochemical model, Forest-DNDC, was applied to simulate the effects of climatic factors, temperature and precipitation changes on carbon characteristics, and greenhouse gases (GHGs) emissions in A. fabric forest. Validation indicated that the Forest-DNDC could be used to predict carbon characteristics and GHGs emissions with reasonable accuracy. The model simulated carbon fluxes, soil carbon dynamics, soil CO2, N2O, and NO emissions with the changes of temperature and precipitation conditions. The results showed that with variation in the baseline temperature from -2 degrees C to +2 degrees C, the gross primary production (GPP) and soil organic carbon (SOC) increased, and the net primary production (NPP) and net ecosystem production (NEP) decreased because of higher respiration rate. With increasing baseline precipitation the GPP and NPP increased slightly, and the NEP and SOC showed decreasing trend. Soil CO2 emissions increased with the increase of temperature, and CO2 emissions changed little with increased baseline precipitation. With increased temperature and decreased baseline temperature, the total annual soil N2O emissions increased. With the variation of baseline temperature from -2 degrees C to +2 degrees C, the total annual soil NO emissions increased. The total annual N2O and NO emissions showed increasing trends with the increase of precipitation. The biogeochemical simulation of the typical forest indicated that temperature changes strongly affected carbon fluxes, soil carbon dynamics, and soil GHGs emissions. The precipitation was not a principal factor affecting carbon fluxes, soil carbon dynamics, and soil CO2 emissions, but changes in precipitation could exert strong effect on soil N2O and NO emissions.

Mobile carbohydrates in Himalayan treeline trees I. Evidence for carbon gain limitation but not for growth limitation.

To test whether the altitudinal distribution of trees is determined by a carbon shortage or an insufficient sugar fraction (sugar:starch ratio) in treeline trees, we studied the status of nonstructural carbohydrates (NSC) and their components (total soluble sugars and starch) in Abies fabri (Mast.) Craib and Picea balfouriana var. hirtella Rehd. et Wils. trees along three elevational gradients, ranging from lower elevations to the alpine treeline, on the eastern edge of the Tibetan Plateau. For comparison, we investigated a low-altitude species (Tsuga yunnanensis (Franch.) Pritz.) which served as a warm-climate reference because it is distributed in closed montane forests below 3100 m a.s.l. in the study area. The carbon status of T. yunnanensis responded to altitude differently from that of the treeline species. At the species level, total NSC was not consistently more abundant in treeline trees than in trees of the same species growing at lower elevations. Thus there was no consistent evidence for carbon limitation of growth in treeline trees. For the three treeline species studied (P. balfouriana and A. fabri in the Kang-Ding Valley and A. fabri in the Mo-Xi Valley), winter NSC concentrations in treeline trees were significantly lower than in lower-elevation trees of the same species, suggesting that, in winter, carbon is limited in treeline trees. However, in no case was there total overwinter depletion of NSC or its components in treeline trees. Treeline and low-altitude species had similar sugar:starch ratios of about three at their upper-elevational limits in April. We conclude that survival and growth of trees at the elevational or latitudinal climate limit depend not only on NSC concentration in perennial tissues, but also on the maintenance of an overwintering sugar:starch ratio greater than three.

[Alien species invasion in Southern China and its countermeasures].

Alien species invasion may cause serious ecological damage, resulting in ecological crisis and biodiversity comedown, and further menacing existing human environment. At present, the invasion of alien species has brought very serious damage to China's environment and society. The direct annual loss of its agro-forestry industry accounts for 574 billion RMB. Taking the alien species in Southern China as an example, this paper analyzed the bio-invasion approaches, mechanics and damages, and aiming at the characters of invaded area and concerned ecosystems and species, proposed several prevention measurements, i.e., pay attention to the phenomena of bio-invasion and to the basic researches on it; intensify the legislation and establish scientific strategies for introducing alien species; enforce the quarantine on invasive species and establish countrywide monitoring system on alien species; enhance people's defending consciousness; and strengthen international cooperation and collaboration.

[Characteristics of soil water infiltration in sub-alpine dark coniferous ecosystem of upper reaches of Yangtze River].

Dark coniferous forest is the predominant type of vegetation in the upper reaches of Yangtze River. Difference among different types of soil exists. The sand content of soil is higher and the soil texture is coarser in the early stage of forest succession. The sand content of soil decreases with the advancement of the forest succession while that of soil in Abies fabri over-mature forest is the lowest. In slope wash soil, the sand content of soil decreases with the increasing soil depth. The soil porosity and soil water-holding capacity increases and soil bulk density decreases with the advancement of forest succession and decrease of soil depth. The deeper soil depth or the smaller soil water content are, the smaller the unsaturated hydraulic conductivity of soil measured by CGA method. Moreover, the correlation of soil water content with unsaturated hydraulic conductivity of soil can be simulated by an exponential function. The saturated hydraulic conductivity of soil decreases exponentially with the increasing soil depth. The time to attain the stable infiltration rate is different among different soil depth, while the deeper the soil depth is, the longer the time needs. The variation in soil texture, soil physical properties and the high infiltration rate of soil there implicated that there are scarce surface runoff, but abundant in subsurface flow, return flow and seepage, which is the result of regulation by dark coniferous forest on hydrological processes.