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Geert van Wirdum 《Hydrobiologia》1993,265(1-3):129-153
A survey of base-rich wetlands in The Netherlands is presented. The main area of their occurrence is the low-lying Holocene part of the country, until some thousand years ago a large and coherent wetland landscape: the Holland wetland. The development of various parts of the Holland wetland into marshes, fens and bogs can be understood from hydrological relations in mire basins, as recognized in the distinction of primary, secondary and tertiary mire basin stages. Presently, the remnants of the Holland wetland are separate base-rich wetlands. The succession of their vegetation reflects various abiotic conditions and human influences. Three main developmental periods are distinguished as regards these factors. The first, geological period of mire development is seen as a post-glacial relaxation, with the inertia due to the considerable mass of wetland as a stabilizing factor. Biological “grazing” influences, as an aspect of utilization by humans, converted base-rich wetlands to whole new types in the second, historical period. Presently, mass and harvesting have decreased in importance, and actual successions in terrestrializing turbaries seem to reflect rapidly changing environmental conditions. Human control could well become the most important factor in the future development of wetland nature. The present value of open fen vegetation strongly depends on the continuation of the historical harvesting. The development of wooded fen may help to increase the mass of wetland in the future. Best results in terms of biodiversity are expected when their base state is maintained through water management. The vegetation and hydrology of floating fens in terrestrializing turbaries is treated in some more detail. Various lines and phases in the succession are distinguished. Open fen vegetation at base-rich, yet nutrient-poor sites is very rich in species threatened elsewhere. The fast acidification of certain such fens is attributed to hydrological and management factors. This acidification is illustrated in the profile of a floating raft sample. At the scale of these small fens, the elemental structure comprising base-rich fen, transitional fen and bog vegetation, is not as stable as it was in the large Holland wetland. A critical role seems to be played by the supply of bases with the water influx. The changing base state is supposed to change the nutrient cycling to such an extent that it would be correct to call this trophic excitation of the ecosystem, rather than just eutrophication. Eutrophication indicates a quantitative reaction to an increased nutrient supply, the internal system being unaltered. The drainage of fens, resulting in an increased productivity of the vegetation, provides another example of excitation, to the effect that the functional system is dramatically changed internally.  相似文献   
3.
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.  相似文献   
4.
目的测定不同生态环境中铜绿丽金龟幼虫肠道细菌产消化酶活性。方法采用平板透明圈法和分光光度计法。结果平板透明圈法测得废弃菜园和花生田中铜绿丽金龟幼虫肠道细菌产蛋白酶和淀粉酶活性差异无统计学意义(F=1.089、0.4963,P0.05),而细菌的产纤维素酶活性有显著差异,其中花生田铜绿丽金龟幼虫肠道中分离到的粘质沙雷菌产酶活性显著高于其他菌株;分光光度计测得的不同生态环境中铜绿丽金龟幼虫肠道细菌的产蛋白酶、淀粉酶和纤维素酶活性差异均有统计学意义(F=461.565、42.349、18.7673,P0.05)。从变异系数来看,分光光度计法测得的蛋白酶、淀粉酶和纤维素酶的变异程度较低,而脂肪酶测定时平板透明圈法的变异系数较低。结论平板透明圈法和分光光度计法均可用于铜绿丽金龟幼虫肠道细菌产消化酶活性测定。通过分析研究比较,分光光度计法适于测定细菌的产蛋白酶、淀粉酶和纤维素酶活性,平板透明圈法适于测定细菌的产脂肪酶活性。  相似文献   
5.
The study of natural hybrid zones can illuminate aspects of lineage divergence and speciation in morphologically cryptic taxa. We studied a hybrid zone between two highly divergent but morphologically similar lineages (south‐western and south‐eastern) of the Iberian endemic Bosca's newt (Lissotriton boscai) in SW Iberia with a multilocus dataset (microsatellites, nuclear and mitochondrial genes). STRUCTURE and NEWHYBRIDS analyses retrieved few admixed individuals, which classified as backcrosses involving parental individuals of the south‐western lineage. Our results show asymmetric introgression of mtDNA beyond the contact from this lineage into the south‐eastern lineage. Analysis of nongeographic introgression patterns revealed asymmetries in the direction of introgression, but except for mtDNA, we did not find evidence for nonconcordant introgression patterns across nuclear loci. Analysis of a 150‐km transect across the hybrid zone showed broadly coincident cline widths (ca. 3.2–27.9 km), and concordant cline centres across all markers, except for mtDNA that is displaced ca. 60 km northward. Results from ecological niche modelling show that the hybrid zone is in a climatically homogenous area with suitable habitat for the species, suggesting that contact between the two lineages is unlikely to occur further south as their distributions are currently separated by an extensive area of unfavourable habitat. Taken together, our findings suggest the genetic structure of this hybrid zone results from the interplay of historical (biogeographic) and population‐level processes. The narrowness and coincidence of genetic clines can be explained by weak selection against hybrids and reflect a degree of reproductive isolation that is consistent with cryptic speciation.  相似文献   
6.
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.  相似文献   
7.
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.  相似文献   
8.
西藏草地生态系统植被碳贮量及其空间分布格局   总被引:4,自引:0,他引:4  
在广泛收集资料的基础上,利用平均碳密度方法,估算了西藏高原草地生态系统17类草地植被的碳贮量,并分析了其空间分布格局.结果表明:(1)17类草地植被总面积为8205.194×104hm2,总碳贮量为189.367 Tg (1TgC=1012g),平均碳密度为2307.895 kgC/hm2,不同植被类型差异较大,在395.977~20471.161kgC/hm2之间波动;(2)从草地类型分布看,高寒草原和高寒草甸是西藏分布面积最大的2类草地,分布面积占西藏草地总面积的70.210%,又是西藏草地碳贮量的主要贮库,碳贮量占西藏草地总碳贮量的79.393%;(3)在空间分布格局上,随着自藏东南向西北的延伸,草地植被总碳密度逐次降低,这一水平分布格局与西藏独特的水热分布相一致;碳密度的垂直分布规律因地区而异,但各地区均以高寒草甸或高寒荒漠的低碳密度为终点,表现出"殊途同归"的特征.  相似文献   
9.
王力  卫三平  吴发启 《生态学报》2009,29(12):6578-6588
利用CoupModel模型模拟了黄土丘陵沟壑区燕沟流域刺槐(Robinia pseudoacacia)林地、辽东栎(Quercus liaotungensis)林地、荒草地、农地等7种土地类型的土壤热量状况,分析了不同植被类型的潜热通量、感热通量、土壤热通量以及植被生长对土壤热量的响应.结果表明,农地潜热通量较小,林地和荒草地潜热通量较大,各地类潜热通量季节变化规律基本一致.潜热通量是黄土丘陵区土壤-植被-大气系统能量的主要支出项,占总净辐射的72.1%~81.4%以上;感热通量变化振幅相对较小,占总净辐射的16.4%~26.4%;土壤热通量仅占总净辐射的1.4%~2.4%,但直接影响土壤温度的变化速度和变化时间.试验地各地类地表温度随季节的变化趋势均呈单峰曲线型.2~7月份0~20cm平均土壤温度随累积土壤热通量的增大而升高,9月到翌年1月份0~20cm平均土壤温度随累积土壤热通量的减小而降低,但累积土壤热通量的变化滞后于土壤温度变化.同一植被类型条件下,阳坡土壤温度年变幅显著高于阴坡.在阴坡,0cm、10cm、20cm深土壤温度年变幅农地>阴坡荒草地>阴坡辽东栎林地>阴坡刺槐林地;在阳坡,阳坡荒草地>阳坡刺槐林地>阳坡辽东栎林地.阴坡刺槐林地、阴坡荒草地和农地0~20cm土壤温度达到5℃以上的时间比阳坡刺槐林和阳坡荒草地推迟1周左右,根系开始生长活动的时间也推迟1周左右;而阴坡辽东栎林地则晚于阳坡辽东栎林地5d左右,根系开始生长活动的时间也较阳坡辽东栎林晚5d左右.出叶时间阳坡刺槐林和阳坡荒草地植物比阴坡刺槐林、阴坡荒草地和阳坡辽东栎林的早1周左右,比阴坡辽东栎林早12d左右.  相似文献   
10.
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.  相似文献   
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