REMOVAL OF ORGANIC AND INORGANIC MATTER FROM ANTARCTIC LAKES BY AERIAL ESCAPE OF BLUEGREEN ALGAL MATS1

Field studies on three perennially ice-covered Antarctic oasis lakes with little or no outflow disclosed a unique biological phenomenon. Benthic algal mats dominated by the prokaryotic Phormidium frigidum Fritsch and several pennate diatoms growing in shallower, more brightly illuminated areas beneath 4 to 5.5 m of ice accumulate and entrap bubbles of photosynthetically produced oxygen. Clumps of this gas-filled mat tear loose from the gravelly substrate, lift off and float to the bottom of the lake ice. Some of these floating mat pieces become frozen into newly forming ice with the onset of winter. Through the combination of ablation of ice from the upper surface and the formation of new ice from below, algal mat pieces reach the upper lake surface in 5–10 years. Here, they are lyophilized by polar winds and dispersed in at least a partially viable state. The process of mat lift-off and escape is important in removing nutrients and salts from these lakes and helps to perpetuate their oligotrophic state. Neutron activation and X-ray dispersive analyses of elements in the algal mats along with other analyses, field observations, and calculations suggest that significant quantities of organic matter, select minerals and salts are lost from the lakes annually through this process whose magnitude has not been recognized previously.     

河西走廊荒漠地区的人口和生态问题

甘肃省自然条件差,粮食自给程度低。在这一背景下,有着灌溉之利,粮食产量较高且稳定的河西走廊荒漠绿洲,便首当其冲地承受着生产更多商品粮的压力。由此必须扩大垦殖,增加用水以及由于人口增长而扩大樵采等对植被的破坏,加剧了沙漠化的发展和绿洲的衰退。本文拟对河西走廊荒漠地区的人口与生态环境问题及解决途径,做一概略讨论。     

Diel variation in the difference in air temperature between the forest edge and interior in the Usambara Mountains, Tanzania

The difference in air temperature between the forest edge and interior was simultaneously compared at ten sites at 2‐h intervals over a 229‐day period in the East and West Usambara Mountains in Tanzania. Measurements were recorded with data loggers placed 1.5 m above the ground on the shaded side of trees and positioned 1–2 and 100 m from the forest edge. The difference in air temperature between the forest edge and interior varied regularly over a 24‐h period with mean maximum difference in air temperatures of 0.85°C occurring at 15:00 h and mean minimum diel difference in air temperature of 0.02°C occurring at 21:00 h. Diel variation in the difference in air temperature between the forest edge and interior varied little by season and was unrelated to elevation.     

Mechanical properties of armchair silicene nanoribbons with edge cracks: a molecular dynamics study

Silicene has been proven to be a promising material with attractive electronic properties. During the synthesis of silicene, structural defects such as edge crack are likely to be generated and such defects in silicene have impacts on its properties. Herein, molecular dynamics simulations were performed to investigate the mechanical properties of the armchair silicene nanoribbons (ASiNRs) with edge cracks. Our results showed that the mechanical properties of the ASiNRs decrease because of the existence of edge crack. Both the pristine ASiNRs and the ASiNRs with edge cracks show brittle fracture behaviours. The crack length plays an important role in determining the critical strain and fracture strength of the ASiNRs. Moreover, we investigated the effects of strain rate and temperature on the mechanical properties of the ASiNRs with edge cracks. We observed that the increasing strain rate increases the critical strain and fracture strength while decreasing the Young’s modulus. Low-strain rates also changes the expanded directions of cracks in the ASiNRs. We also found that the increasing temperature could significantly decrease the mechanical properties of the ASiNRs with edge cracks.     

Forest landscape pattern in the KwaZulu–Natal midlands,South Africa: 50 years of change or stasis?

Abstract Understanding patterns and processes of habitat change is essential for managing and conserving forest fragments in anthropogenically altered landscapes. Digitized aerial photographs from 1944 and 1996 were examined for changes to the indigenous forest landscape in the Karkloof‐Balgowan archipelago in KwaZulu–Natal, South Africa. Attributes relating to proximate land‐use, patch shape, isolation and position in the landscape were used to determine putative causes of forest change. The total change in forest area was ?5.7% (forest covered 6739 ha in 1996). This is contrasted with previous reports for the period 1880–1940 that estimated change in total forest area of up to ?80%. Attrition was the predominant process of forest transformation between 1944 and 1996. Despite little overall change in forest area, 786 mostly small (<0.5 ha) forest patches were lost from the landscape, leaving 1277 forest patches in 1996. An increase in patch isolation, but no change in patch cohesion accompanied the changes in forest area. Ignoring patches that were eliminated, 514 patches decreased in area. This was partly a function of patch size, but the conversion of natural grassland to commercial plantation forestry in the matrix also influenced forest decline. Their small size and irregular shape caused forest patches in the region to be vulnerable to edge effects. Core area declined in a negative exponential way with increasing edge width and the total area of edge habitat exceeded that of core habitat at an edge width of only 50 m. Nevertheless, total core area decreased by only 2% (65 ha) between 1944 and 1996 because most of the eliminated patches were small and contained no core area. The large Karkloof forest (1649 ha) is a conservation priority for forest interior species, but the ecological role and biodiversity value of small forest patches should not be overlooked.     

塔里木河上游典型绿洲不同连作年限棉田土壤质量评价

开展农业生态系统土壤质量评价是保证生态农业持续高效发展的重要内容。以塔里木河上游阿拉尔垦区为典型样区,选择0a、3a、8a、12a、20a、30a等不同连作年限棉田为研究对象,综合考虑土壤物理、化学和生物学性质,分析了12个土壤理化及酶活性指标;基于因子分析和聚类分析法评价棉田间土壤质量的差异并划分等级,利用产量持续指数验证评价结果。研究表明：不同连作年限的棉田土壤的理化性质和酶活性均存在一定差异;棉田的土壤质量随年限呈先上升后下降的趋势,8a、12a棉田土壤质量相对较高,20a、30a土壤质量有退化趋势;土壤质量评价值与产量持续指数变化趋势一致,后者略显滞后效应。     

European butterfly populations vary in sensitivity to weather across their geographical ranges

Aim

The aim was to assess the sensitivity of butterfly population dynamics to variation in weather conditions across their geographical ranges, relative to sensitivity to density dependence, and determine whether sensitivity is greater towards latitudinal range margins.

Location

Europe.

Time period

1980–2014.

Major taxa studied

Butterflies.

Methods

We use long‐term (35 years) butterfly monitoring data from > 900 sites, ranging from Finland to Spain, grouping sites into 2° latitudinal bands. For 12 univoltine butterfly species with sufficient data from at least four bands, we construct population growth rate models that include density dependence, temperature and precipitation during distinct life‐cycle periods, defined to accommodate regional variation in phenology. We use partial R² values as indicators of butterfly population dynamics' sensitivity to weather and density dependence, and assess how these vary with latitudinal position within a species' distribution.

Results

Population growth rates appear uniformly sensitive to density dependence across species' geographical distributions, and sensitivity to density dependence is typically greater than sensitivity to weather. Sensitivity to weather is greatest towards range edges, with symmetry in northern and southern parts of the range. This pattern is not driven by variation in the magnitude of weather variability across the range, topographic heterogeneity, latitudinal range extent or phylogeny. Significant weather variables in population growth rate models appear evenly distributed across the life cycle and across temperature and precipitation, with substantial intraspecific variation across the geographical ranges in the associations between population dynamics and specific weather variables.

Main conclusions

Range‐edge populations appear more sensitive to changes in weather than those nearer the centre of species' distributions, but density dependence does not exhibit this pattern. Precipitation is as important as temperature in driving butterfly population dynamics. Intraspecific variation in the form and strength of sensitivity to weather suggests that there may be important geographical variation in populations' responses to climate change.     

敦煌市土地利用/覆盖变化特征及成因分析

利用ArcGIS9.2软件,结合野外调查,对敦煌市1987、1996和2007年3期Land-sat5-TM遥感影像进行解译,并分8个类型分析了土地利用/覆盖变化的特征及成因。结果表明:近20年来敦煌市呈现出植被覆盖程度不断降低、覆盖面积减小的趋势,其中耕作区、城建用地和无植被区面积净增,其他5类用地面积净减;8类用地类型之间互有转换,转移的主要方向是部分草地和无植被区转变为耕作区,水域湿地和灌木林地转变为草地,草地由高覆盖、中覆盖、低覆盖到无植被区逐次转变;除了耕作区边缘植被覆盖程度有所增加外,其他区域退化趋势明显;引起LUCC的自然因素中气温升高的影响较大,而人文因素中土地开垦、增加灌溉的影响较大。