Jiangwen Fan

Effects of soil fertility level and cutting frequency on interference among Hieracium pilosella, H. praealtum, Rumex acetosella, and Festuca novae‐zelandiae

Abstract The role of ecological factors in the invasion of fescue tussock grassland by Hieracium species was investigated in a box experiment. Soil fertility and cutting frequency effects on interference between the introduced flat weeds Hieracium pilosella, H. praealtum, and Rumex acetosella and the native bunch grass Festuca novae‐zelandiae established on a denuded soil were examined using a model and technique involving stress and disturbance gradients. The weed species showed marked biomass and phenological responses to increased availability of mineral nutrients compared to F. novae‐zelandiae, which had reduced yield at the highest level of soil fertility. At high soil fertility, R. acetosella suppressed both Hieracium species and F. novae‐zelandiae, probably because it competed more effectively for light. Cutting frequency also influenced the pattern of interference between the species by effects on competition for light, vegetative spread, and partitioning of biomass to parts of the plants above an...

Effects of plateau pika (Ochotona curzoniae) on net ecosystem carbon exchange of grassland in the Three Rivers Headwaters region, Qinghai-Tibet, China

Background and aimBecause the indigenous burrowing lagomorph plateau pika (Ochotona curzoniae) is considered to have negative ecological impacts on alpine meadow steppe grasslands of the Headwaters Region of the Yellow, Yangtze and Mekong Rivers we investigated its effects on ecosystem productivity and soil properties, and especially net ecosystem carbon flux.MethodsWe measured net ecosystem CO2 exchange (NEE) and its components gross ecosystem productivity (GEP) and ecosystem respiration (ER) at peak aboveground biomass by the chamber method with reference to plant and soil characteristics of areas of alpine meadow steppe with different densities of pika burrows.ResultsHigher burrow density decreased NEE, GEP and ER. Above-ground biomass, species number, plant cover and leaf area index decreased with increasing pika density. Higher burrow density was associated with lower soil moisture and higher soil temperature. Responses of NEE were related to changes of abiotic and biotic factors affecting its two components. NEE was positively related to soil moisture, soil ammonium nitrogen, plant cover, leaf area index and above-ground biomass but was negatively correlated with higher soil nitrate nitrogen.ConclusionDecrease of NEE by plateau pika may reduce the carbon sink balance of Qinghai-Tibet plateau grassland. Such effects may be influenced by grazing pressure from domestic livestock, population levels of natural predators, and climate change.

Effects of grassland restoration programs on ecosystems in arid and semiarid China.

We explored the ecological effects of grassland restoration programs using satellite imagery and field plots sampling data and analyzing the patterns and mechanisms of land cover change and vegetation activities in arid and semiarid China during the period from 1982 to 2008. The grassland cover in the 1980s, 2000 and 2005 was compared before and after the restoration programs. The variability of net primary production (NPP) and rain use efficiency (RUE) were analyzed as indicators of vegetation productivity. Our study showed that changes in grassland cover were closely related to the relative area of farmland, with increases in grassland being caused by returning farmland to grassland and decreases being caused by reclamation for agriculture. The results of NPP and RUE measurements over the past 30 years showed systematic increases in the area of grassland in most regions, especially from 2000 to 2008. This fact was reflected by intensified vegetation activity and cannot be completely explained by the warmer and wetter climate, which suggested a contribution from restored, ungrazed grasslands. Our analysis indicates that both vegetation activity and grassland cover increased in regions in which grassland and rangeland restoration programs were implemented.

A meta-analysis of the canopy light extinction coefficient in terrestrial ecosystems

The canopy light extinction coefficient (K) is a key factor in affecting ecosystem carbon, water, and energy processes. However, K is assumed as a constant in most biogeochemical models owing to lack of in-site measurements at diverse terrestrial ecosystems. In this study, by compiling data of K measured at 88 terrestrial ecosystems, we investigated the spatiotemporal variations of this index across main ecosystem types, including grassland, cropland, shrubland, broadleaf forest, and needleleaf forest. Our results indicated that the average K of all biome types during whole growing season was 0.56. However, this value in the peak growing season was 0.49, indicating a certain degree of seasonal variation. In addition, large variations in K exist within and among the plant functional types. Cropland had the highest value of K (0.62), followed by broadleaf forest (0.59), shrubland (0.56), grassland (0.50), and needleleaf forest (0.45). No significant spatial correlation was found between K and the major environmental factors, i.e., mean annual precipitation, mean annual temperature, and leaf area index (LAI). Intra-annually, significant negative correlations between K and seasonal changes in LAI were found in the natural ecosystems. In cropland, however, the temporal relationship was site-specific. The ecosystem type specific values of K and its temporal relationship with LAI observed in this study may contribute to improved modeling of global biogeochemical cycles.

Relationships between altitudinal gradient and plant carbon isotope composition of grassland communities on the Qinghai-Tibet Plateau, China

Foliar and root carbon isotope composition (δ13C) of grassland communities on the Qinghai-Tibet Plateau, China, was obtained by the biomass weighting method and direct measurement. We investigated the characteristics and altitudinal patterns of foliar and root δ13C and determined which environmental factors influenced foliar δ13C most. Foliar δ13C of alpine steppe was significantly higher than that of alpine meadow and temperate steppe. For alpine meadow, root δ13C was significantly higher than of foliar δ13C. Foliar δ13C increased with altitude at an average rate of 0.60‰ km−1 for the whole grassland ecosystem. This rate was lower than that at species level. However, there were no significant relationships between root δ13C and altitude. Atmospheric pressure was a more important factor than temperature and precipitation in its influence on the altitudinal pattern of foliar δ13C at the community level.

Effects of an ecological conservation and restoration project in the Three-River Source Region, China

The first-stage of an ecological conservation and restoration project in the Three-River Source Region (TRSR), China, has been in progress for eight years. However, because the ecological effects of this project remain unknown, decision making for future project implementation is hindered. Thus, in this study, we developed an index system to evaluate the effects of the ecological restoration project, by integrating field observations, remote sensing, and process-based models. Effects were assessed using trend analyses of ecosystem structures and services. Results showed positive trends in the TRSR since the beginning of the project, but not yet a return to the optima of the 1970s. Specifically, while continued degradation in grassland has been initially contained, results are still far from the desired objective, ‘grassland coverage increasing by an average of 20%–40%’. In contrast, wetlands and water bodies have generally been restored, while the water conservation and water supply capacity of watersheds have increased. Indeed, the volume of water conservation achieved in the project meets the objective of a 1.32 billion m3 increase. The effects of ecological restoration inside project regions was more significant than outside, and, in addition to climate change projects, we concluded that the implementation of ecological conservation and restoration projects has substantially contributed to vegetation restoration. Nevertheless, the degradation of grasslands has not been fundamentally reversed, and to date the project has not prevented increasing soil erosion. In sum, the effects and challenges of this first-stage project highlight the necessity of continuous and long-term ecosystem conservation efforts in this region.

Relationships between C3 Plant Foliar Carbon Isotope Composition and Element Contents of Grassland Species at High Altitudes on the Qinghai-Tibet Plateau, China

Relationships of foliar carbon isotope composition (δ13C) with foliar C, N, P, K, Ca, Mg contents and their ratios of 219 C3 species leaf samples, obtained in August in 2004 to 2007 from 82 high altitude grassland sites on the Qinghai-Tibet Plateau China, were examined. This was done with reference to the proposition that foliar δ13C increases with altitude and separately for the life-form groups of graminoids, forbs and shrubs and for the genera Stipa and Kobresia. For all samples, foliar δ13C was negatively related to foliar K, P and ∑K+ Ca+ Mg, and positively correlated to foliar C, C/N and C/P. The significance of these correlations differed for the taxonomic and life-form groups. Lack of a relationship of foliar δ13C with foliar N was inconsistent with the majority of studies that have shown foliar δ13C to be positively related to foliar N due to a decrease of Ci/Ca (the ratio between intercellular and atmospheric concentration of CO2) and explained as a result of greater photosynthetic capacity at higher foliar N concentration. However this inconsistency relates to other high altitude studies that have found that photosynthetic capacity remains constant as foliar N increases. After accounting for the altitudinal relationship with foliar δ13C, of the elements only the K effect was significant and was most strongly expressed for Kobresia. It is concluded that factors critical to plant survival and growth at very high altitudes, such as low atmospheric pressure and low temperatures, may preclude expression of relationships between foliar δ13C and foliar elements that have been observed at lower altitudes.

Influence of Land Use Patterns on Evapotranspiration and Its Components in a Temperate Grassland Ecosystem

To better understand variation in response of components of ecosystem evapotranspiration (ET) to grassland use differences, we selected three typical land use patterns in a temperate steppe area: grazed steppe (G), steppe with grazers excluded (GE), and steppe cultivated to cropland (C). ET was divided into its components evaporation (E) and canopy transpiration (T) using herbicide and a chamber attached to a portable infrared gas analyzer (Li-6400). The results indicated that daily water consumption by ET in G was 3.30 kg m−2 d−1; compared with G, ET increased significantly in GE at 13.4% and showed a trend of 6.73% increase in C. Daily water consumption by E increased 24.3% in GE relative to G, and C showed 20.2% more than GE. At 0.46, E/ET in C was significantly higher than G at 0.35. Air temperature and the vapor pressure deficit were closely correlated with variation in diurnal ET, E, and T. The leaf area index (LAI) was also positively correlated with daily ET and E varied among grassland use patterns and explained variation in E/ET (81%). Thus, variation in LAI strongly influences the overall magnitude of ecosystem ET and the composition of its components under different grassland use patterns.

Patterns of Soil 15N and Total N and Their Relationships with Environmental Factors on the Qinghai-Tibetan Plateau

The patterns of soil nitrogen (N) isotope composition at large spatial and temporal scales and their relationships to environmental factors illustrate N cycle and sources of N, and are integrative indicators of the terrestrial N cycle and its response to global change. The objectives of this study were: i) to investigate the patterns of soil N content and natural abundance of N-15 (delta N-15) values in different ecosystem types and soil profiles on the Qinghai-Tibetan Plateau; ii) to examine the effects of climatic factors and soil characteristics on the patterns of soil N content and soil delta N-15 values; and iii) to test the relationship between soil delta N-15 values and soil C/N ratios across ecosystems and soil profiles. Soil profiles were sampled at 51 sites along two transects 1 875 km in length and 200 km apart and distributed in forest, meadow and steppe on the Qinghai-Tibetan Plateau. Each site was sampled every 10 cm from a soil depth of 0 to 40 cm and each sample was analyzed for soil N content and delta N-15 values. Our results indicated that soil N and delta N-15 values (0-40 cm) in meadows were much higher than in desert steppe. Soil N decreased with soil depth for each ecosystem, while variations of soil delta N-15 values along soil profiles were not statistically significant among most ecosystems but for mountain meadow, lowland meadow, and temperate steppe where soil delta N-15 values tended to increase with soil depth. The parabolic relationship between soil delta N-15 values and mean annual precipitation indicated that soil delta N-15 values increased with increasing precipitation in desert steppe up to 500 mm, and then decreased with increasing precipitation across all other ecosystems. Moreover, the parabolic relationship between delta N-15 values and mean annual temperature existed in all individual ecosystem types. Soil N and delta N-15 values (0-40 cm) increased with an increase in soil silt and clay contents. Furthermore, a threshold of C/N ratio of about 11 divided the parabolic relationship between soil delta N-15 values and soil C/N ratios into positive (C/N 11) parts, which was valid across all ecosystems and soil profiles. The large explanatory power of soil C/N ratios for soil delta N-15 values suggested that C and N concentrations, being strongly controlled by precipitation and temperature, were the primary factors determining patterns of soil delta N-15 on the Qinghai-Tibetan Plateau.

Effects of cutting at different reproductive development stages on aftermath growth of red clover (Trifolium pratense) in a subtropical montane environment

Abstract The use of red clover (Trifolium pratense) in subtropical montane areas of southern China is reviewed as a preliminary to an investigation, at Hongchiba, Wuxi County, Chongquing, of the effects on aftermath growth of cutting at different stages of reproductive development of a local cultivar ‘Wuxi’ red clover. The reproductive stages were branching, flower bud formation, flowering, and seed pod formation. The time taken to reach the defined reproductive stages after cutting, and the height of herbage and yield at cutting, became less with successive cuts. The greatest rate of dry matter accumulation was during the branching and flower bud formation stages. Cutting at the flower bud stage produced the highest total yield of 13 380 kg DM ha−1 and cutting at the pod stage the least yield of 10 480 kg DM ha−1. It is recommended that the first cut should be made about 70 days after the start of spring growth, and the second, third, and fourth harvests 50, 30, and 35 days after the previous harvests.