干旱半干旱区坡面覆被格局的水土流失效应研究进展

植被恢复与建设是控制水土流失的重要措施.在干旱半干旱地区,植被在空间上的分布呈现离散特征,在坡面上形成的裸地-植被镶嵌和植被条带分布等覆被格局对水土流失过程具有重要影响.覆被格局与水土流失关系研究是景观生态学格局与过程研究的重要内容.从植被斑块、坡面覆被格局对水土流失的影响与耦合覆被格局与水土流失的手段和方法三个方面对国内外相关研究进行总结分析.从中发现,植被类型、层次结构和形态特征是植被斑块尺度上影响径流泥沙的关键因素;不同覆被类型的产流产沙特征与覆被格局的准确描述是研究覆被格局水土流失效应的重要基础;坡面覆被格局对水土流失的影响关键在于其改变了径流泥沙运移和汇集的连续性,应重点关注径流泥沙源汇区的连通性和空间分布在水土流失中的作用;以坡面的精确覆被制图为基础,建立基于水土流失过程的覆被格局指数和耦合格局信息的径流与侵蚀模型是定量研究覆被格局影响的有效手段.今后应加强以下研究:以动态系统的观点研究覆被格局的变化对水土流失的影响,系统理解覆被格局与水土流失之间的相互联系与反馈机制,探讨两者间动态关系随尺度变化的规律性并发展尺度上推方法;构建过程意义明确且简单实用的格局指数,将覆被格局与水土流失过程有机联系起来;发展覆被格局的动态信息与产流产沙过程相结合的水文模型,加强对格局与径流泥沙反馈系统的耦合,建立真正意义上的覆被格局-水土流失过程耦合模型.     

Region‐wide temporal and spatial variation in Caribbean reef architecture: is coral cover the whole story?

The architectural complexity of coral reefs is largely generated by reef‐building corals, yet the effects of current regional‐scale declines in coral cover on reef complexity are poorly understood. In particular, both the extent to which declines in coral cover lead to declines in complexity and the length of time it takes for reefs to collapse following coral mortality are unknown. Here we assess the extent of temporal and spatial covariation between coral cover and reef architectural complexity using a Caribbean‐wide dataset of temporally replicated estimates spanning four decades. Both coral cover and architectural complexity have declined rapidly over time, with little evidence of a time‐lag. However, annual rates of change in coral cover and complexity do not covary, and levels of complexity vary greatly among reefs with similar coral cover. These findings suggest that the stressors influencing Caribbean reefs are sufficiently severe and widespread to produce similar regional‐scale declines in coral cover and reef complexity, even though reef architectural complexity is not a direct function of coral cover at local scales. Given that architectural complexity is not a simple function of coral cover, it is important that conservation monitoring and restoration give due consideration to both architecture and coral cover. This will help ensure that the ecosystem services supported by architectural complexity, such as nutrient recycling, dissipation of wave energy, fish production and diversity, are maintained and enhanced.     

Autoradiography of Water-Soluble Materials in Plant Tissues

Samples of tissue with water-soluble substances are lyophilized and doubly-infiltrated with parlodion and paraffin. Sections are mounted on an adhesive-coated cover slip with chloroform. The reverse side of the cover slip is coated successively with a thin layer of Harleco resin and a thick layer of Kodak “Opaque”. A corner of the cover slip is broken off as a marker. The cover slip is assembled with a covering glass slide on a nuclear track plate (Kodak NTB-2) with protective coating, the section being in contact with the emulsion, and held in place during radioactive exposure. Before development, the cover slip and plate are exposed briefly to light and then disassembled. Following development of the plate and removal of the layer of resin and opaque from the cover slip, staining of the section is optional. Lining up of the section with its autoradiograph can be accurate within 1-5μ.     

A rapid survey method for the estimation of density and cover in desert plant communities

Abstract. The Log-series survey method allows rapid estimates of density and cover and is applicable for studies of perennial vegetation in arid environments. An optical rangefinder is used to determine boundaries of large circular plots. Numbers of individuals of each species within a plot are assessed; this information is used to assign species to logarithmic density classes equivalent to the logarithm base 2 of actual abundances. Each species is then assigned to a logarithmic canopy cover class, equivalent to the logarithmbase 2 of average cover per individual. Log total cover per species per plot is obtained by the addition of logarithmic density and cover classes. Percent cover per species is rapidly computed by taking the antilog of the difference between log total cover per species and log total plot area.     

Measurement of the predictive value of species lists for species cover in phytosociological samples

The method known as Analysis of Concentration (AOC) is proposed as a tool to measure the predictivity of binary data for cover data. The application of AOC to structured tables of oak forests of Central Italy has proved that binary data are more predictive for cover than cover for binary data. The ordinations produced by AOC with binary and cover data are very similar and interpretable with similar results.     

Cover Pole Design for Easy Transport,Assembly, and Field Use

ABSTRACT Cover poles, also called Robel poles, are used to measure a variety of structural vegetation attributes commonly used in wildlife and livestock management. Although cover pole dimensions, measurement criteria, and interpretation of cover pole data vary depending on measurement objectives, the technical use of cover poles is fairly consistent. Practical use of cover poles requires that they are sturdy, lightweight, and easily transported. We describe a cover pole apparatus that can be easily constructed, transported, and assembled for use in remote locations. This self-supporting cover pole enables one person to take measurements, and can be modified for use in multiple habitats and soil types.     

Hierarchical and spatially aggregated plant cover data

Most plant species are spatially aggregated and here the importance of taking the spatial variation into account when analyzing plant cover data is demonstrated in a general stochastic model where both the within-site and the among-site spatial variation of species cover data are parameterized. Using a generalised binomial distribution (or Pólya–Eggenberger distribution), where the among-site variation in mean cover is modeled by a zero-inflated beta distribution, it is possible to adequately analyze hierarchical plant cover data and link the estimates to the underlying ecological processes. The model is demonstrated in a case-study of pin-point cover data of Erica tetralix from 1148 wet heathland plots at 84 Danish sites, and it is shown that both parameter estimates and the conclusions of hypotheses testing critically depend on the correct modeling of the observed spatial variation. Finally, statistical power simulations of plant cover measurements are presented, which will be useful for planning ecological experiments and monitoring programs.     

Autoradiography of Water-Soluble Materials in Plant Tissues

Samples of tissue with water-soluble substances are lyophilized and doubly-infiltrated with parlodion and paraffin. Sections are mounted on an adhesive-coated cover slip with chloroform. The reverse side of the cover slip is coated successively with a thin layer of Harleco resin and a thick layer of Kodak “Opaque”. A corner of the cover slip is broken off as a marker. The cover slip is assembled with a covering glass slide on a nuclear track plate (Kodak NTB-2) with protective coating, the section being in contact with the emulsion, and held in place during radioactive exposure. Before development, the cover slip and plate are exposed briefly to light and then disassembled. Following development of the plate and removal of the layer of resin and opaque from the cover slip, staining of the section is optional. Lining up of the section with its autoradiograph can be accurate within 1-5μ.     

Microhabitat preferences of longear sunfish: low light intensity versus submerged cover

Both cover and light intensity are important factors in microhabitat selection of fishes, suggesting a possible hierarchical relationship. Many centrarchids have been shown to prefer low light intensities and heavy submerged cover, but adult nonbreeding longear sunfish are reported to occupy sparsely vegetated, clear, shallow water during the brightest part of the day, probably for foraging. To determine the relative importance of cover and light intensity to longear sunfish, we presented nonbreeding adults with choices between these two conditions in a series of three laboratory experiments. Longear sunfish preferred low light to high light intensity and preferred submerged cover to no cover. However, when given a choice, they preferred to occupy low light intensity conditions without cover rather than submerged cover under higher light intensity conditions. Thus, low light intensity, such as is found in greater pool depths, may be the preferred refuge from predation for longear sunfish.     

Experimental study on the effect of cover and vaccination on the survival of juvenile European rabbits

In Mediterranean ecosystems, the European rabbit is a keystone species that has declined dramatically, with profound implications for conservation and management. Predation and disease acting on juveniles are considered the likely causes. In the field, these processes are managed by removing predators, increasing cover to reduce predation risk and by vaccinating against myxomatosis. These manipulations can be costly and, when protected predators are killed, they can also be damaging to conservation interests. Our goal was to test the effectiveness of cover and vaccination on juvenile survival in two large enclosures, free of mammalian predators, by adding cover and vaccinating juveniles. Rabbit warrens were our experimental unit, with nine replicates of four treatments: control, cover, vaccination, and cover and vaccination combined. Our results showed that improved cover systematically increased juvenile rabbit survival, whereas vaccination had no clear effect and the interactive effect was negligible. Our experimental data suggest that improved cover around warrens is an effective way of increasing rabbit abundance in Mediterranean ecosystems, at least when generalist mammalian predators are scarce. In contrast the vaccination programme was of limited benefit, raising questions about its efficacy as a management tool.     

Influences of transect relocation errors on line-point estimates of plant cover

The line-point transect method has been used to estimate plant cover for about nine decades. In particular, the method is often used to determine baseline plant cover and monitor for changes in plant cover over time. In such cases, detection of change requires both the initial transect starting position and angle of orientation are exact in relocation without error. A study was conducted on influences of errors in basal cover estimates that resulted from inexact relocation and orientation of a resample transect. Simulation studies of actual field data showed that variation in plant cover estimates from relocated line-point transects increased with each source of error and combinations of these errors. Relocated transects resulted in unbiased estimates of total-plant cover only when means over all transects are used to detect changes over time. Substantial errors were observed when the mean cover of individually relocated transect was compared to its original transect.     

Maximizing feeding efficiency and minimizing time exposed to predators: a trade-off in the black-capped chickadee

Summary Animals often must feed away from protective cover, sometimes at a considerable risk of being preyed upon. Feeding at the maximum rate while away from cover may simultaneously minimize the time spent exposed to predators, but this is not always the case. Under some circumstances, carrying prey items to protective cover before they are consumed will minimize the time spent exposed to predators, whereas feeding at maximum efficiency (staying to eat prey where they are found) will actually increase the time spent exposed to predators. Whether or not there is a conflict between maximizing foraging efficiency and minimizing exposure time, depends upon the travel time to cover relative to the handling time of a prey item; short handling times and/or long travel times are associated with the no-conflict situation, whereas the conflict situation is associated with long handling times and/or short travel times to cover. Free-ranging chickadees foraging at an artificial patch at various distances from cover can distinguish between these two foraging situations. When there is no conflict, they stay and eat at the patch. Their behavior in the conflict situation indicates that they are tradingoff foraging considerations against the risk of predation. When the cost of carrying is low and the benefit gained is high, the chickadees elect to carry items to cover; they tend to stay and eat at the patch when the relative magnitudes of costs and benefits are reversed.     

Evapotranspiration (ET) covers

Evapotranspiration (ET) cover systems are increasingly being used at municipal solid waste (MSW) landfills, hazardous waste landfills, at industrial monofills, and at mine sites. Conventional cover systems use materials with low hydraulic permeability (barrier layers) to minimize the downward migration of water from the surface to the waste (percolation), ET cover systems use water balance components to minimize percolation. These cover systems rely on soil to capture and store precipitation until it is either transpired through vegetation or evaporated from the soil surface. Compared to conventional membrane or compacted clay cover systems, ET cover systems are expected to cost less to construct. They are often aesthetic because they employ naturalized vegetation, require less maintenance once the vegetative system is established, including eliminating mowing, and may require fewer repairs than a barrier system. All cover systems should consider the goals of the cover in terms of protectiveness, including the pathways of risk from contained material, the lifecycle of the containment system. The containment system needs to be protective of direct contact of people and animals with the waste, prevent surface and groundwater water pollution, and minimize release of airborne contaminants. While most containment strategies have been based on the dry tomb strategy of keeping waste dry, there are some sites where adding or allowing moisture to help decompose organic waste is the current plan. ET covers may work well in places where complete exclusion of precipitation is not needed. The U.S. EPA Alternative Cover Assessment Program (ACAP), USDOE, the Nuclear Regulatory Commission, and others have researched ET cover design and efficacy, including the history of their use, general considerations in their design, performance, monitoring, cost, current status, limitations on their use, and project specific examples. An on-line database has been developed with information about specific projects using ET covers. There are three general approaches for non-conventional cover systems to achieve approval for installation; the first is when equivalent performance to conventional final cover systems can be demonstrated directly on site. This is the approach used by the Sandia study, by most ACAP sites, and the Rocky Mountain Arsenal. A second approach is used when there are data from a site specific study such as an ACAP installation at a site that has analogous soil and climate conditions. Several sites in Colorado and Southern California have achieved approval based on data from similar sites. The third most common approach for regulatory approval is by installation of data collection systems with the agreement that the permanence of the ET cover installation is contingent on success of the cover in meeting certain performance goals. This article is intended as an introduction to the topic and is not intended to serve as guidance for design or construction, nor indicate the appropriateness of using an ET cover systems at a particular site.     

From forest to farmland: pollen‐inferred land cover change across Europe using the pseudobiomization approach

Maps of continental‐scale land cover are utilized by a range of diverse users but whilst a range of products exist that describe present and recent land cover in Europe, there are currently no datasets that describe past variations over long time‐scales. User groups with an interest in past land cover include the climate modelling community, socio‐ecological historians and earth system scientists. Europe is one of the continents with the longest histories of land conversion from forest to farmland, thus understanding land cover change in this area is globally significant. This study applies the pseudobiomization method (PBM) to 982 pollen records from across Europe, taken from the European Pollen Database (EPD) to produce a first synthesis of pan‐European land cover change for the period 9000 bp to present, in contiguous 200 year time intervals. The PBM transforms pollen proportions from each site to one of eight land cover classes (LCCs) that are directly comparable to the CORINE land cover classification. The proportion of LCCs represented in each time window provides a spatially aggregated record of land cover change for temperate and northern Europe, and for a series of case study regions (western France, the western Alps, and the Czech Republic and Slovakia). At the European scale, the impact of Neolithic food producing economies appear to be detectable from 6000 bp through reduction in broad‐leaf forests resulting from human land use activities such as forest clearance. Total forest cover at a pan‐European scale moved outside the range of previous background variability from 4000 bp onwards. From 2200 bp land cover change intensified, and the broad pattern of land cover for preindustrial Europe was established by 1000 bp . Recognizing the timing of anthropogenic land cover change in Europe will further the understanding of land cover‐climate interactions, and the origins of the modern cultural landscape.     

Simple Aids to Microscope Illumination

The following rapid but reliable method of making permanent preparations from temporary mounts has proved to be very useful.

Pollen mother-cell smears: Smeared anthers are treated hi the usual way with Belting's acetocarmine, except that the cover slip is left off. When correct differentiation is attained the stain is thoroly washed off with 50% acetic acid and the slide flooded with dioxan. This is followed by 2 changes of dioxan for 2 minutes each. A drop of Canada balsam dissolved in dioxan is added and a cover slip applied. In cases where a cover slip has been used at the acetocarmine stage it can be floated off in a staining jar of 50% acetic acid and dehydration with dioxan carried out as above.

Insect salivary gland chromosome smears: The glands are crushed under a cover slip in acetocarmine on a slide coated with dried egg albumen. After 20 minutes the area around the cover slip is flooded with 50% acetic acid and the cover slip floats loose so that it can be removed. The above described dioxan dehydrating procedure is then employed.

Squash preparations: Root tips are fixed in some suitable fixative and the Feulgen technic applied. The stained root tips can either be dehydrated by passing thru 3 changes of dioxan and mounting in dioxan-balsam where they are divided into small longitudinal sections by sharp needles, or they can be put immediately into a mixture of 1 part of 50% acetic acid to 1 part of corn syrup where shredding with needles is carried out. A cover slip is put on and separation of the cells completed by tamping or by applying pressure to the cover. This squash method is useful with anthers which are difficult to smear when in the early prophase stages of meiosis.     

Measuring fractional forest canopy element cover and openness – definitions and methodologies revisited

Canopy structural parameters are often used to give adequate representation of vegetated ecosystems for various purposes including primary productivity, climate system, water and carbon gas exchanges, and radiation extinction. Canopy structural parameters are usually described using several pseudo‐synonymous terms, often measuring different components of vegetation canopies. Standardization in the definitions has fallen short, leading to confusion of terms even in standard text books making the comparison of historic measures futile. Here we clarify concepts that have been used for fractional canopy element cover and openness measures. The fractional canopy element cover and openness concepts considered are canopy closure, canopy cover, canopy openness, crown closure, crown completeness, crown cover, crown porosity, site openness and tilt openness. New methodologies are presented to obtain large scale fractional canopy element cover and openness measures using hemispherical photography. The new methodologies and variations in definitions of fractional canopy element cover and openness concepts are demonstrated using photographic measurements in complex topography. The results indicate that both fractional canopy element cover and openness parameters can be estimated with a few point‐based measurements using hemispherical photography. Hemispherical photography is therefore less time, labour and resource intensive, as compared to point based measuring techniques of canopy element cover and openness. Most of the commonly and interchangeably used concepts of fractional canopy element cover and openness measures represent physically different structural properties of a vegetated ecosystem.     

Relationships between aboveground biomass and plant cover at two spatial scales and their determinants in northern Tibetan grasslands

The relationships between cover and AGB for the dominant and widely distributed alpine grasslands on the northern Tibetan Plateau is still not fully examined. The objectives of this study are to answer the following question: (1) How does aboveground biomass (AGB) of alpine grassland relate to plant cover at different spatial scales? (2) What are the major biotic and abiotic factors influencing on AGB–cover relationship? A community survey (species, cover, height, and abundance) was conducted within 1 m × 1 m plots in 70 sites along a precipitation gradient of 50–600 m. Ordinary linear regression was employed to examine AGB–cover relationships of both community and species levels at regional scale of entire grassland and landscape scale of alpine meadow, alpine steppe, and desert steppe. Hierarchical partitioning was employed to estimate independent contributions of biotic and abiotic factors to AGB and cover at both scales. Partial correlation analyses were used to discriminate the effects of biotic and abiotic factors on AGB–cover relationships at two spatial scales. AGB and community cover both exponentially increased along the precipitation gradient. At community level, AGB was positively and linearly correlated with cover for all grasslands except for alpine meadow. AGB was also linearly correlated with cover of species level at both regional and landscape scales. Contributions of biotic and abiotic factors to the relationship between AGB and cover significantly depended on spatial scales. Cover of cushions, forbs, legumes and sedges, species richness, MAP, and soil bulk density were important factors that influenced the AGB–cover relationship at either regional or landscape scale. This study indicated generally positive and linear relationships between AGB and cover are at both regional and landscape scales. Spatial scale may affect ranges of cover and modify the contribution of cover to AGB. AGB–cover relationships were influenced mainly by species composition of different functional groups. Therefore, in deriving AGB patterns at different spatial scales, community composition should be considered to obtain acceptable accuracy.     

Permanent Preparations from Rapid Cytological Technics

The following rapid but reliable method of making permanent preparations from temporary mounts has proved to be very useful.

Pollen mother-cell smears: Smeared anthers are treated hi the usual way with Belting's acetocarmine, except that the cover slip is left off. When correct differentiation is attained the stain is thoroly washed off with 50% acetic acid and the slide flooded with dioxan. This is followed by 2 changes of dioxan for 2 minutes each. A drop of Canada balsam dissolved in dioxan is added and a cover slip applied. In cases where a cover slip has been used at the acetocarmine stage it can be floated off in a staining jar of 50% acetic acid and dehydration with dioxan carried out as above.

Insect salivary gland chromosome smears: The glands are crushed under a cover slip in acetocarmine on a slide coated with dried egg albumen. After 20 minutes the area around the cover slip is flooded with 50% acetic acid and the cover slip floats loose so that it can be removed. The above described dioxan dehydrating procedure is then employed.

Squash preparations: Root tips are fixed in some suitable fixative and the Feulgen technic applied. The stained root tips can either be dehydrated by passing thru 3 changes of dioxan and mounting in dioxan-balsam where they are divided into small longitudinal sections by sharp needles, or they can be put immediately into a mixture of 1 part of 50% acetic acid to 1 part of corn syrup where shredding with needles is carried out. A cover slip is put on and separation of the cells completed by tamping or by applying pressure to the cover. This squash method is useful with anthers which are difficult to smear when in the early prophase stages of meiosis.     

Identifying and quantifying uncertainty and spatial disagreement in the comparison of Global Land Cover for different applications

This paper provides a methodology for comparing global land cover maps that allows for differences in legend definitions between products to be taken into account. The legends of the two maps are first reconciled by creating a legend lookup table that shows how the legends map onto one another. Where there is overlap, the specific definitions for each legend class are used to calculate the degree of overlap between legend classes. In this way, one‐to‐many mappings are accounted for unlike in most methods where the legend definitions are often forced into place. Another advantage over previous map comparison methods is that application‐specific requirements are captured using expert input, whereby the user rates the importance of disagreement between different legend classes based on the needs of the application. This user‐defined matrix in conjunction with the degree of overlap between legend classes is applied on a pixel‐by‐pixel basis to create maps of spatial disagreement and uncertainty. The user can then highlight the areas of highest thematic uncertainty and disagreement between the different land cover maps allowing for areas that require further detailed examination to be readily identified. It would also be possible for several users to input their knowledge into the process, leading to a potentially more robust comparison of land cover products. The methodology of map comparison is illustrated using different land cover products including Global Land Cover 2000 (GLC‐2000) and the MODIS land cover data set. Two diverse applications are provided including the estimation of global forest cover and monitoring of agricultural land. In the case of global forest cover, an example was provided for Columbia, which showed that the MODIS land cover map overestimates forest cover in comparison with the GLC‐2000. The agricultural example, on the other hand, served to illustrate that for Sudan, MODIS tends to underestimate crop areas while GLC‐2000 overestimates them.     

Relationships between lake macrophyte cover and lake and landscape features

We examined the ability of lake and landscape features to predict a variety of macrophyte cover metrics using 54 north temperate lakes. We quantified submersed cover, emergent cover, floating leaf cover, Eurasian watermilfoil cover and total macrophyte cover. Measured lake features included lake physio-chemical and morphometric variables and landscape features included hydrologic, catchment and land use/cover variables. Univariate regression analyses demonstrated that these macrophyte cover metrics are predicted by a wide range of predictor variables, most commonly by: Secchi disk depth, maximum or mean depth, catchment morphometry, road density and the proportion of urban or agricultural land use/cover in the riparian zone or catchment (r² = 0.06–0.46). Using a combination of lake and landscape features in multiple regressions, we were able to explain 29–55% of the variation in macrophyte cover metrics. Total macrophyte cover and submersed cover were related to Secchi disk depth and mean depth, whereas the remaining metrics were best predicted by including at least one land use/cover variable (road density, proportion local catchment agriculture land use/cover, proportion cumulative catchment urban land use/cover, or proportion riparian agriculture land use/cover). The two main conclusions from our research are: (1) that different macrophyte growth forms and species are predicted by a different suite of variables and thus should be examined separately, and (2) that anthropogenic landscape features may override patterns in natural landscape or local features and are important in predicting present-day macrophytes in lakes.