The relationship between water velocity,energetic costs,and microhabitat use in four North American stream fishes

Freshwater crayfish are key members of aquatic communities due to their large size and abundance. Although most commonly regarded as herbivores and detritivores, they are also selective predators. The crayfish plague fungus Aphanomyces astaci (Schikora) led to the elimination of a stock of white-clawed crayfish, Austropotamobius pallipes (Lereboullet) from Lough Lene, Co. Westmeath, in 1987. Samples taken of the flora and benthic communities of two Irish lakes, one (Lough Bane) formerly containing crayfish and the other (Lough Lene) immediately following a plague outbreak, were compared to similar samples taken a year later and ecological shifts were noted and compared to laboratory feeding results. Over time, Chara strands increased in mean length, and molluscs became more abundant.     

A general framework to describe the alteration of natural tree species composition as an indicator of forest naturalness

The alteration of natural tree species composition is defined as the deviation of the current tree species composition from that of the natural state. It can be used as a measure of human influence on forest vegetation, and thus as an indicator of the naturalness of forest vegetation. The aim of the study was to develop a standard procedure for estimating the alteration of natural tree species composition, to explain factors driving alteration and to examine its significance for susceptibility of forest stands to natural disturbances. The alteration of natural tree species composition was estimated for the Dinaric region (5556 km², Slovenia) by the Robič Index of Dissimilarity (RID), ranging from 0 (completely natural) to 100 (completely altered). The index was calculated on the compartment level (24 ha each on average) with data on current and potential natural forest vegetation. The influence of human activities on tree species alteration was examined by using topographic and accessibility variables. The susceptibility of forest stands to natural disturbances was analysed with data on sanitary felling. In the study area, the natural tree species composition of forest stands is moderately preserved; the average value of RID was 50.05, ranging from 1.76 to 100, and the coefficient of variation was 0.49. The alteration of the natural tree species composition of forest stands is primarily the result of forest management and past land use, conditioned either by topography or accessibility of forests. The degree of alteration of tree species composition decreased along the gradients of rockiness, inclination and elevation. A greater degree of alteration appeared on the slopes of intermediate and south facing aspects than on north facing slopes, and in areas that were closer to the forest edge. A higher level of alteration significantly increases the susceptibility of forest stands to natural disturbances. The procedure represents a novel approach in modelling the alteration (naturalness) of tree species composition of forest vegetation. It is applicable at different spatial scales and fosters an understanding of the patterns of tree species composition under the influence of human activity across forest landscapes.     

Vegetation,fire and soil feedbacks of dynamic boundaries between rainforest,savanna and grassland

At fine spatial scales, savanna‐rainforest‐grassland boundary dynamics are thought to be mediated by the interplay between fire, vegetation and soil feedbacks. These processes were investigated by quantifying tree species composition, the light environment, quantities and flammability of fuels, bark thickness, and soil conditions across stable and dynamic rainforest boundaries that adjoin grassland and eucalypt savanna in the highlands of the Bunya Mountains, southeast Queensland, Australia. The size class distribution of savanna and rainforest stems was indicative of the encroachment of rainforest species into savanna and grassland. Increasing dominance of rainforest trees corresponds to an increase in woody canopy cover, the dominance of litter fuels (woody debris and leaf), and decline in grass occurrence. There is marked difference in litter and grass fuel flammability and this result is largely an influence of strongly dissimilar fuel bulk densities. Relative bark thickness, a measure of stem fire resistance, was found to be generally greater in savanna species when compared to that of rainforest species, with notable exceptions being the conifers Araucaria bidwillii and Araucaria cunninghamii. A transect study of soil nutrients across one dynamic rainforest – grassland boundary indicated the mass of carbon and nitrogen, but not phosphorus, increased across the successional gradient. Soil carbon turnover time is shortest in stable rainforest, intermediate in dynamic rainforest and longest in grassland highlighting nutrient cycling differentiation. We conclude that the general absence of fire in the Bunya Mountains, due to a divergence from traditional Aboriginal burning practices, has allowed for the encroachment of fire‐sensitive rainforest species into the flammable biomes of this landscape. Rainforest invasion is likely to have reduced fire risk via changes to fuel composition and microclimatic conditions, and this feedback will be reinforced by altered nutrient cycling. The mechanics of the feedbacks here identified are discussed in terms of landscape change theory.     

Classifying aquatic macrophytes as indicators of eutrophication in European lakes

Aquatic macrophytes are one of the biological quality elements in the Water Framework Directive (WFD) for which status assessments must be defined. We tested two methods to classify macrophyte species and their response to eutrophication pressure: one based on percentiles of occurrence along a phosphorous gradient and another based on trophic ranking of species using Canonical Correspondence Analyses in the ranking procedure. The methods were tested at Europe-wide, regional and national scale as well as by alkalinity category, using 1,147 lakes from 12 European states. The grouping of species as sensitive, tolerant or indifferent to eutrophication was evaluated for some taxa, such as the sensitive Chara spp. and the large isoetids, by analysing the (non-linear) response curve along a phosphorous gradient. These thresholds revealed in these response curves can be used to set boundaries among different ecological status classes. In total 48 taxa out of 114 taxa were classified identically regardless of dataset or classification method. These taxa can be considered the most consistent and reliable indicators of sensitivity or tolerance to eutrophication at European scale. Although the general response of well known indicator species seems to hold, there are many species that were evaluated differently according to the database selection and classification methods. This hampers a Europe-wide comparison of classified species lists as used for the status assessment within the WFD implementation process.     

Strategies to restore floodplain vegetation after abandonment of human activities

Restoring the flooding‐related disturbance regime by removing and setting back flood defenses (channel widening) is the most efficient strategy for recovering riparian plant communities in floodplains formerly impacted by human activities such as agriculture, mining, and forestry. Removing flood defenses is generally not socially accepted, and alternative recovery strategies must be explored. We assessed vegetation establishment on 33 sites in the floodplains of the Middle Ebro River and three of its tributaries (NE Spain) where restoration approaches applied in the last 20 years include channel widening, floodplain excavation, and abandonment of agriculture, gravel extraction, and hybrid poplar plantations, with and without plantation of native species. Using analysis of similarity and ordinations, we found that channel widening led to plant communities closely resembling those found on natural gravel bars, including new recruits of keystone tree species. Excavation of the floodplain as the restoration approach resulted in pioneer, non‐strictly riparian communities. Abandonment of agricultural land or clearing of poplar plantations resulted in alternative stable states predominating, regardless of time elapsed since restoration and whether poles of native species were planted. However, forest‐like communities relatively similar to mature, natural riparian references were attainable when hybrid poplars were allowed to resprout after clear‐cutting, or after the human activities were abandoned and trees were not cut. Combining channel widening and assisted revegetation where flood defenses cannot be altered may partially reproduce a mosaic of habitats typical of natural floodplains.     

物候模型研究进展

近年来随着全球气候变暖,物候提前,物候学的研究越来越受到人们的关注.通过建立物候模型使物候期的预知成为可能,从而为生产实践活动提供依据和指导.本文探讨了物候模型研究的意义,总结了影响植物和昆虫物候的温度、水分、光和养分等主要环境因子的作用.根据国内外物候模型的研究现状,重点介绍了作物、树木、植被和昆虫4类物候模型的研究内容和进展.作物物候模型注重生理生态过程;树木物候模型以统计方法为主,但近期也有尝试将激素水平作为物候的决定因素;植被物候模型以遥感技术的应用为发展趋势;昆虫物候模型则进一步对发育起点的确定和对温度因子的修正,GIS的引入将昆虫物候模型的应用范围扩大.最后指出了目前物候模型研究中存在的问题.     

Legacy effects of top–down disturbances on woody plant species composition in semi‐arid systems

Savanna vegetation is controlled by bottom‐up (e.g. soil and rainfall) and top–down (e.g. fire and herbivory) factors, all of which have an effect on biodiversity. Little is known about the relative contribution of these factors to biodiversity, particularly the long‐term effects of top–down disturbance on patterns of woody plant composition. The aim of this study was to identify if various degrees of disturbance regimes create distinct woody species community assemblages. Data were collected over 1820 plots across Kruger National Park, South Africa. Woody species were identified and categorized into one of three height classes: shrub (0.75–2.5 m), brush (2.5–5.5 m), and tree (>5.5 m). Species richness and composition were calculated for each site and height class. A combination of long‐term fire and elephant density data were used to delineate areas with varying degrees of top–down disturbance (i.e. low, medium and high). Using these degrees of disturbance, species composition was identified and community assemblages constructed according to each disturbance regime. Our results suggest that areas with similar disturbance regimes have similar species composition. Shrub composition was mainly responsive to the number of fires between the years 1941–1990, while tree composition was more responsive to elephant disturbance. A few dominant species were found equally under all degrees of disturbance at all height classes, while others were more regularly found under specific disturbance regimes at particular height classes. This study highlights that while species richness does not appear to be influenced by long‐term, top–down disturbance regimes, species community composition may be responsive to these disturbances. Most species and structural classes persisted across all disturbance regimes, but the long‐term effects of top–down disturbances can influence compositional and structural biodiversity. This information provides context for management policies related to artificial water provision, elephants and fire.