Evidence for the grazing hypothesis: Grazing reduces phytoplankton responses of the HNLC ecosystem to iron enrichment in the western subarctic pacific (SEEDS II)

A mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (∼18 mg chl m⁻³) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m⁻³, with no prominent diatom bloom. Photosynthetic competence in terms of F _v/F _m for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (<2 μm) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 μM in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton (copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community towards the end of the experiment.     

THERMALLY DRIVEN SORPTION,DESORPTION, AND MOISTURE MIGRATION IN THE ACTIVE LAYER IN CENTRAL ALASKA

Combined observations of hourly soil temperature and electric potential, the latter converted to a relative index of soil-water solute concentration, yield information on the physical chemistry of near-surface frost effects. Solute concentration near the descending 0° C isotherm in the refreezing active layer above permafrost is divided into three distinct zones: (1) an ion-enriched zone in the unfrozen active layer that precedes the penetrating freezing front; (2) an ion-purified desorbed zone at the freezing front that is the source region of the downward-expelled ions and water; and (3) a hydrologically isolated subfreezing zone of enhanced solute concentration located above the freezing isotherm. High-frequency fluctuations superimposed on these general patterns are traceable to vapor migration driven by surface thermal fluctuations. These effects diminish at temperatures below about -0.4° C, as permeability decreases with soil-ice formation. The combined temperature-solute concentration time series is used to develop sorption curves for the frozen organic and mineral soils, and indicates that approximately half of the pore water present in the mineral soil at -0.4° C had not been converted to ice at -6° C. Gradual soil desiccation over winter appears to result from outward vapor diffusion, possibly through soil cracks. [Key words: Alaska, active layer, frozen ground, soil temperature, soil water, permafrost.]     

RESTORATION OF SHEEP TRANSHUMANCE IN THE EBRO VALLEY,ARAGON, SPAIN*

For centuries, sheep transhumance has connected the Spanish highlands with the lowlands, creating a livestock system with important social, economic, cultural, and landscape effects. About 5 million sheep were involved in transhumant movements in the mid‐1700s, but transhumance started to decay in the nineteenth century and almost disappeared in the 1970s and 1980s. In the 1980s, transhumant sheep did not number more than 25,000 in the study area; at present the number exceeds 134,000, with herders practicing both descending and ascending transhumance. In this article we provide information not only on current numbers of transhumant sheep but also on the origins and destinations of the flocks. We also confirm that the reputation of transhumance, once regarded as quaint and antiquated, has been enhanced by the ecosystem services it provides and by the cultural and heritage continuities it renders.     

农牧交错带典型偏农区土地利用变化及其原因分析——以奈曼旗尧勒甸子村为例

中国北方农牧交错带东部农业以种植业为主。土地利用变化对农业的可持续发展具有深刻的影响。尧勒甸子村是农牧交错带典型的偏农区,50a来土地利用发生很大变化。其变化表现出明显的阶段性,20世纪80年代前后明显不同。本文主要分析了农业用地和沙漠化土地的变化,并从国家政策、土地产权制度、人口、牲畜、系统结构等方面的变化探讨了其土地利用变化的原因。     

Small-scale vegetation dynamics in semi-arid steppe in Inner Mongolia

Small-scale vegetation dynamics were followed for ecotones and in uniform stands inArtemisia-dominated steppe vegetation under grazing and when recovering from heavy grazing. Species composition was followed annually for 5 years in 1 m²and 0·25 m²plots for (1) presence–absence; (2) density; and (3) biomass.More rapid vegetation dynamics, in terms of change of type of vegetation and distance moved in DCA species space between sampling occasions, were observed in smaller plots and in early seral stages, where species were few and alpha diversity low. Only the plots recently protected from grazing showed a directional vegetation change; those protected for more than 3 years and those under continued grazing showed, at this scale, non-directional dynamics.     

海洋中型浮游动物的选择性摄食对浮游植物群落的控制

海洋中型浮游动物的选择性摄食很大程度上影响着浮游植物种群的变化,控制着海洋初级生产力的节律、规模和归宿。从海洋中型浮游动物选择性摄食对有害藻华的控制、中型浮游动物的选择性摄食机制、中型浮游动物选择性摄食的研究方法和中型浮游动物的选择性摄食模型四个方面探讨了中型浮游动物选择性摄食对控制浮游植物种群演替的贡献,为进一步预测和控制有害藻华发生提供科学依据。     

Soil and vegetation seasonal changes in the grazing Andean Mountain grasslands

Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna（Vicugna vicugna） and alpaca（Lama pacos）. The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined： total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands.     

Impact of Grazing Exclusion on the Surface Heat Balance in North Tibet

The grazing exclusion program used by the Tibetan government to protect the ecological environment has changed the vegetation and impacted the surface heat balance in North Tibet. However, little information is available to describe the in?uences of the current grazing exclusion program on local surface heat balance. This study uses the records of fenced grassland patch locations to identify the impact of grazing exclusion on surface heat balance in North Tibet. The records of fenced grassland patch locations, including the longitude, latitude, and elevation of the vertices of each fenced patch (polygon shapes), were provided by the agriculture and animal husbandry bureaus of the counties where the patches were located. ArcGIS 10.2 was used to create polygon shapes based on patch location records. Based on satellite data and the surface heat balance system determined by the model, values for changes in land surface temperature (LST), albedo and evapotranspiration (ET) induced by grazing exclusion were obtained. All of these can influence surface heat balance and alter the fluctuation of LST in the northern Tibetan Plateau. The LST trends for day and night showed an asymmetric diurnal variation, with a larger magnitude of warming in the day than cooling at night. The maximum decrease in absorbed shortwave of LST (-0.5 - -0.4 ℃ per decade) occurred in the central region, while the minimum decrease (-0.2 - -0.1 ℃ per decade) occurred in the eastern region. The decreased latent heat lead to the LST increased maximum (>1 ℃ per decade) occurred in the central region, The eastern region increased at a rate of 0.2-0.5 ℃ per decade, while the minimum increase (0-0.1 ℃ per decade) occurred in the northwestern region.     

TREE INVASION IN MEADOWS OF LASSEN VOLCANIC NATIONAL PARK,CALIFORNIA*

The causes of tree invasion into meadows in Lassen Volcanic National Park were investigated by dating invasive trees and associating age structure patterns with regional variations in fire history, grazing, and climate change. Massive invasion was associated primarily with cessation of livestock grazing and burning between 1905 and 1933, but climatic change may also have contributed. Maintenance of meadows may require continued use of unnatural processes due to uncertainties about pre-European fire regimes and other resource considerations.     

Diets of mammalian herbivores in Australian arid, hilly shrublands: seasonal effects on overlap between euros (hill kangaroos), sheep and feral goats, and on dietary niche breadths and electivities

The diets of euros or hill kangaroos (Macropus robustus), domestic sheep (Ovis aries) and feral goats (Capra hircus) in hilly, shrub rangeland in southern Australia were examined in a 12-year study. Levels of dietary overlap between these herbivores, their foraging in relation to resource availability, and the potential for competition in different conditions were also examined. The diet of euros was based around grasses. In dry seasons some shrubs were also eaten but in severe drought grass formed more than 80% of their diet. While grass was important to sheep in wetter conditions, they ate much shrub in dry conditions. Feral goats had broad diets but their preference for browse was high. In drought, euros had only a modest dietary overlap with sheep and goats. Data for dietary niche breadths and electivities pointed to only limited competition between the herbivores.