Field measurement of the sheltering effect of vegetation on erodible land surfaces

Natural vegetation on erodible land surfaces, such as the loose sandy soils found in the southwestern United States and in Soviet Central Asia, absorbs part of the wind momentum flux (stress) and thus protects the erodible soil to a degree that depends on the geometry of plant distribution and profile. The sheltering effect of natural plants may be expressed as the ratio, R, of threshold friction velocity for the bare soil (determined in the laboratory or in specially prepared areas of bare soil in the field) to that for the naturally vegetated surface. We used new automated instrumentation to detect erosion thresholds in locations where erosion events are widely separated in time. Measured values of R were low for our most vegetated sites and nearer unity for the sparsely vegetated site.     

基于ICA和DWT的数字水印算法研究

研究了小波变换域在低频子带嵌入数字水印的效果、水印嵌入因子对宿主图像质量和提取水印图像质量的影响,研究表明：存在嵌入因子阈值T,在嵌入因子小于T时提取的水印图像有较好的质量,是一种非常有效的数字水印算法.     

Threshold responses of songbirds to long-term timber management on an active industrial forest

Forest managers often seek to balance economic benefits from timber harvesting with maintenance of habitat for wildlife, ecosystem function, and human uses. Most research on the relationship between avian abundance and active timber management has been short-term, lasting one to two years, creating the need to investigate long-term avian responses and to identify harvest thresholds when a small change in habitat results in a disproportionate response in relative abundance and nest success. Our objectives were to identify trends in relative abundance and nest success and to identify landscape-scale disturbance thresholds for avian species and habitat guilds in response to a variety of harvest treatments (clear-cuts, heavy and light partial harvests) over 14 years. We conducted point counts and monitored nests at an industrial forest in the central Appalachians of West Virginia during 1996-1998, 2001-2003, and 2007-2009. Early successional species increased in relative abundance across all three time periods, whereas interior-edge and forest-interior guilds peaked in relative abundance mid-study after which the forest-interior guild declined. Of 41 species with >10 detections, four (10%) declined significantly, 13 (32%) increased significantly (only three species among all periods), and 9 (22%) peaked in abundance mid-study (over the entire study period, four species had no significant change in abundance, four declined, and one increased). Based on piecewise linear models, forest-interior and interior-edge guilds’ relative abundance harvest thresholds were 28% total harvests (all harvests combined), 10% clear-cut harvests, and 18% light partial harvests, after which abundances declined. Harvest thresholds for the early successional guild were 42% total harvests, 11% clear-cut harvest, and 10% light partial harvests, and relative abundances increased after surpassing thresholds albeit at a reduced rate of increase after the clear-cut threshold. Threshold confidence intervals for individual species overlapped their guild threshold intervals 91% of the time. Even though relative abundance of most species (80%) did not decline as the area affected by timber management increased, implementing management at or below our approximate forest-interior and interior-edge harvest thresholds would reduce the number of declining species by half, maintain higher relative abundances of four species with a net decline in abundance but that peaked in abundance mid-study, and maintain higher relative abundances of ten additional species. In contrast, this management strategy also would prevent the increase in relative abundance of seven species and limit the increase in abundance of three species that increased throughout the study.     

Gully erosion and environmental change: importance and research needs

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales.Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates.In this respect, various research questions can be identified. The most important ones are:

What is the contribution of gully erosion to overall soil loss and sediment production at various temporal and spatial scales and under different climatic and land use conditions?

What are appropriate measuring techniques for monitoring and experimental studies of the initiation and development of various gully types at various temporal and spatial scales?

Can we identify critical thresholds for the initiation, development and infilling of gullies in different environments in terms of flow hydraulics, rain, topography, soils and land use?

How does gully erosion interact with hydrological processes as well as with other soil degradation processes?

What are appropriate models of gully erosion, capable of predicting (a) erosion rates at various temporal and spatial scales and (b) the impact of gully development on hydrology, sediment yield and landscape evolution?

What are efficient gully prevention and gully control measures? What can be learned from failures and successes of gully erosion control programmes?

These questions need to be answered first if we want to improve our insights into the impacts of environmental change on gully erosion. This paper highlights some of these issues by reviewing recent examples taken from various environments.     

Synthesis: Thresholds in conservation and management

This paper is a brief overview of some of the key issues which have emerged from the preceding set of papers on ecological thresholds. These include:

•

Whether threshold relationships are common and widespread.

•

The potential for large variations in the use and application of the threshold concept to lead to adverse conservation outcomes, particularly when overly simplistic levels of vegetation cover are specified by policy makers and land managers.

•

The inherent multi-variate nature of landscape processes and responses of individual species and assemblages that creates variability in datasets. This may lead to a limited ability to make accurate predictions from threshold relationships, even when those relationships are highly statistically significant.

We believe that although the threshold concept is an appealing one and there is some empirical evidence to support it in some landscapes, it is not free of problems and a concerted research effort on the topic is needed. This is particularly important if it is to have value for robustly underpinning applied landscape management practices without unintentionally having negative impacts on rates of species loss or the loss of particular species.     

Effects of historical and present fragmentation on plant species diversity in semi-natural grasslands in Swedish rural landscapes

Habitat loss and fragmentation of natural and semi-natural habitats are considered as major threats to plant species richness. Recently several studies have pinpointed the need to analyse past landscape patterns to understand effects of fragmentation, as the response to landscape change may be slow in many organisms, plants in particular. We compared species richness in continuously grazed and abandoned grasslands in different commonplace rural landscapes in Sweden, and analysed effects of isolation and area in three time-steps (100 and 50 years ago and today). Old cadastral maps and aerial photographs were used to analyse past and present landscape patterns in 25 sites. Two plant diversity measures were investigated; total species richness and species density. During the last 100 years grassland area and connectivity have been reduced by about 90%. Present-day habitat area was positively related to total species richness in both habitats. There was also a relationship to habitat area 50 years ago for continuously grazed grasslands. Only present management was related to species density: continuously grazed grasslands had the highest species density. There were no relationships between grassland connectivity, present or past, and any diversity measure. We conclude that landscape history is not directly important for present-day plant diversity patterns in ordinary landscapes, although past grassland management is a prerequisite for the grassland habitats that can be found there today. It is important that studies are conducted, not only in very diverse landscapes, but also in managed landscapes in order to assess the effects of fragmentation on species.     

四川盆周山地杉木人工林衰退与铝毒害阈值的探讨

根据四川盆周山地杉木人工林调查分析数据,借鉴国内外森林土壤酸化研究方法,在立地分类基础上,确定二代杉木人工纯林作为研究对象,选择根际土壤ｐＨ 值（ＫＣｌ） 、Ａｌ３＋ 、Ｃａ２＋ ／Ａｌ３ ＋ 和凋落物Ｃａ２ ＋／（Ｃａ２ ＋ ＋Ｆｅ３＋ ＋ Ａｌ３＋ ） 作为参数,探讨杉木人工林衰退与铝毒害阈值。研究结果表明,有的林地立地质量差,树种与立地不相适应,导致杉木人工林生产力下降;另有的林地,虽然立地条件较好,但是已出现铝毒害,导致杉木人工林衰退。铝毒害阈值是ｐＨ≤４－１８ ,Ａｌ３ ＋ ≥３１－６６ ｍｇ／ｋｇ,Ｃａ２ ＋ ／Ａｌ３＋ ≤１－８０９ ｍｏｌ／ｍｏｌ 和Ｃａ２＋ ／（Ｃａ２ ＋ ＋ Ｆｅ３ ＋ ＋ Ａｌ３＋ ）≤０－５５ ｍｏｌ／ｍｏｌ。     

不同花生品种对卡那霉素的敏感性研究

以花生胚小叶为外植体,观察花生胚小叶在添加不同浓度卡那霉素的分化培养基上的生长状况,研究不同花生品种对卡那霉素的敏感性,以确定各品种的卡那霉素临界浓度。结果表明,在供试品种中,D16对卡那霉素最敏感,其临界浓度为100 mg/L;花育22对卡那霉素的耐受性最高,其临界浓度为200 mg/L;J11及鲁花11的卡那霉素临界浓度均为150 mg/L。     

Regional differences in patterns of fish species loss with changing land use

Human-centered land use causes ecosystem degradation and loss of biodiversity in streams, yet such processes may be highly heterogeneous among species and regions. We quantified fish species sensitivity to several classes of land use and determined if trends varied among three geoclimatic regions of Maryland, USA. Our approach compares the cumulative frequency distribution (CFD) of sites where a species was expected to occur to the CFD of those where it was collected to estimate the species-specific maximum degree of land use tolerated and the minimal amount that may induce an impact. Fishes were most vulnerable to urban land use gradients, though trends were region-specific with heightened sensitivity in the Piedmont (64-72% of species negatively affected) relative to the Coastal Plain (45-52% negatively affected). Species found in both regions exhibited greater vulnerability to urbanization in the Piedmont. Responses to agricultural land use were less severe though trends were region-specific as well with the least sensitive in the Piedmont and the greatest negative impact in the Highlands region. In many cases, reduced population abundances were detected at lower levels of land use relative to the range where the species was apparently extirpated. Our results demonstrate that population reduction and species loss due to land use change differs substantially among regions, even within a single species. Greater sensitivity observed in abundance-weighted analyses implies that further loss of fish diversity may be impending in impacted streams. Our results have direct application to prioritize conservation resources and forecast future declines in species distributions and biodiversity.     

Observed and modelled distributions of channel and gully heads—with examples from SE Spain and Belgium

In soil erosion by overland flow, sediment discharge is driven by discharge and gradient against the resistance of the soil and vegetation. The form of the relationship, which is taken as a sum of power law terms, may then be inferred from several data sources: (1) ephemeral gully head location, (2) permanent channel head location, (3) slope profile form, (4) consistency with evolutionary models and (5) erosion plot data. The paper focuses on (1) to (4), in order to understand how far these data constrain the form of the dependence on discharge and gradient. Field data from mapping a set of ephemeral gully heads after storms show a weak inverse relationship between channel head gradient and catchment area, which flattens towards an upper threshold gradient of about 25%. These relationships are compared with those theoretically obtained for a range of sediment transport laws which have been previously used, and which are analysed to forecast both overall slope profile form and the location of stream heads. The theory predicts that the location of ephemeral gully heads produced in a single storm will differ from the long-term average stream head location. There is a fairly good relationship between theoretical and observed locations, associated with the form of the sediment transport law. For a single event, the location depends on storm magnitude which cannot be well defined from the data sets, creating a spread of data with an envelope curve defined for the largest events. The empirically acceptable sediment transport equations have also been used to analyse families of slope profiles in a small area, and used within a landscape evolution model to generate landscapes which are compatible with those observed in the field, and provide confirmation of the long-term viability of the expressions obtained to forecast the direction, style and rates of landscape evolution in response to land use or climate drivers.