Impact of river management history on the community structure,species composition and nutrient status in the Rhine alluvial hardwood forest

The present-day Rhine alluvial hardwood forest (Querco-Ulmetum minoris, Issler 24) in the upper Rhine valley (France/Germany) is comprised of three vegetation units, one still flooded by calm waters (F) and the two others unflooded, one for 30 years (UF30) (after the river canalisation) and the other for about 130 years (UF130) (after river straightening and embankment work in the middle nineteenth century). In the three stands, species composition, structure and diversity of vegetation and nutrient content of mature leaf, leaf litter and soil have been studied. Fungi (Macromycetae) were only studied in two stands (F and UF130). The intensity of nutrient recycling was exemplified by comparing the chemical composition of rainwater, flood, throughfall, mature leaf, leaf litter, soil and groundwater in two of these stands (F and UF30).The elimination of floods has caused a change in floristic composition, tree density and plant diversity. Tree density was higher in the two unflooded stands and was related to a large increase in sapling (< 6 cm dbh) density more than to a change of stem (> 6 cm dbh) density. Sapling density increased 2 times and three times in the UF30 and the UF130 respectively, whereas the stem density increased only 12% in the first stand and decreased 29% in the second one. The saprophytic macromycete communities have been supplemented with mycorrhizal species. Leaf litter production was slightly greater in the flooded (4.44 T ha^-1 yr^-1) than in the two unflooded stands ( 3.72 T ha^-1 yr^-1). Foliar N level is twice as high in the flooded stands in spite of an opposite soil status. P level decreased in soil and leaves with the duration of isolation and was at the same level in the groundwater in two stands (F and UF30). K, Mg and Ca contents of green leaf and leaf litter were high due to the geochemistry of the Rhine substrate (calcareous gravels and pebbles) and similar in all the stands studied, even though there are large inputs of these three elements by floods. Moreover we showed that the groundwater chemistry reflected the variations of nutrient inputs and thus could be a good indicator of the functioning of an alluvial ecosystem and of its change as a result of human activities. The restoration of floods in hardwood forest contributes to the preservation of alluvial vegetational structure and composition, the stimulation of biological processes and a better plant mineral nutrition and water supply.     

The effect of mollusc grazing on seedling recruitment in artificially created grassland gaps

Two experiments conducted in spring and autumn 1992 examined the effect of mollusc grazing on seedling regeneration from natural grassland seedbanks by creating artificial gaps in plots in a grassland sward. Molluscs were excluded from half the gaps by application of molluscicide. Mollusc grazing in both the spring and autumn experiment significantly reduced seedling recruitment, though the intensity of grazing was greatest in autumn. Recruitment of five species was markedly influenced by molluscicide application. In spring, plots from which molluscs were excluded contained significantly more seedlings of Chenopodium polyspermum and Ranunculus acris. In the autumn, exclusion of molluscs resulted in increased populations of R. acris, Stellaria graminea and Rumex acetosa. Cerastium holosteoides populations were greatest in autumn grazed plots. Other species, notably the grasses Holcus lanatus and Agrostis capillaris and the legume Trifolium repens were unaffected by molluscicide application. Species diversity was significantly decreased by molluscicide application in the autumn. Gap size significantly affected the recruitment of two species. Ranunculus acris populations were significantly higher in small gaps in both spring and summer, while Chenopodium recruitment in the spring was greater in small gaps. Gap size also significantly influenced the risk of mollusc attack on Ranunculus as molluscs appeared to show an aggregative feeding response in the high seedling density small gaps. Selective grazing of vulnerable seedlings by molluscs may influence the eventual relative proportions of the species present and so provide a potent mechanism in shaping community composition in grasslands.     

Untangling the roles of fire,grazing and rainfall on small mammal communities in grassland ecosystems

In grassland systems across the globe, ecologists have been attempting to understand the complex role of fire, grazing and rainfall in creating habitat heterogeneity and the consequences of anthropogenic control of these factors on ecosystem integrity and functioning. Using a South African grassland ecosystem as a model, we investigated the impact of fire and grazing pressure on small mammal communities during three differing periods of a rainfall cycle. Over 2 years, 15,203 trap nights revealed 1598 captures of 11 species (nine rodents, one macroscelid and one insectivore). Results highlighted the importance of the interplay between factors and showed that the role of fire, grazing and rainfall in determining small mammal abundance was species-dependant. While no two species were affected by the same environmental variables, grass cover or height was important to 56% of species. Considered independently, high rainfall had a positive influence on small mammal abundance and diversity, although the lag period in population response was species-specific. High grazing negatively affected overall abundance, but specifically in Mastomys coucha; fire alone had little immediate impact on small mammal diversity. Six months after the fire, vegetation cover had recovered to similar levels as unburned areas, although small mammal diversity and richness were higher in burned areas than unburned areas. Grazing levels influenced the rate of vegetation recovery. In conclusion, low-level grazing and burning can help to maintain small mammal biodiversity, if conducted under appropriate rainfall levels. A too high grazing pressure, combined with fire, and/or fire conducted under drought conditions can have a negative impact on small mammal biodiversity. To maintain small mammal diversity in grassland ecosystems, the combined effects of the previous year’s rainfall and existing population level as well as the inhibition of vegetation recovery via grazing pressure need to be taken into consideration before fire management is applied. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Three levels of floristical variation in the forests of Chaco,Argentina

Abstract. This paper analyses the floristic variation in the southern segment of the Oriental Chaco, known as the Santa Fe Forest Wedge. Floristical variation in Chaco forests is described at three levels of scale. On the coarsest scale there is a gradient of increasing floristic richness towards the north, related to a geographic temperature gradient. At an intermediate scale different types of forests are arranged according to environmental gradients correlated with topographic elevation. At a fine scale many microsites can be discerned with different micro-environments colonized differentially by species. Microsites arise as the result of site-physiographic history, development of vegetation and the interaction of both and with animals. Several dimensions of regenerationniche variation are distinguished, including canopy and soil characteristics, and propagule dispersal.     

Red spruce forest regeneration dynamics across a gradient from Acadian forest to old field in Greenwich,Prince Edward Island National Park,Canada

Red spruce forests have declined considerably throughout their range in the past decades. As agricultural fields are abandoned and land becomes available for reforestation, the possibility arises for red spruce forests to expand onto them. This study addresses the potential for red spruce forests to expand onto adjacent old fields in Greenwich, Prince Edward Island National Park, Canada. We examined red spruce distribution and abundance, plant species diversity and changes in community composition along a gradient from the interior of red spruce forests out towards the centre of adjacent old fields. Examining the patterns of red spruce distribution and abundance revealed that, where cultivation and logging have been abandoned recently in the fields and forests, regeneration is limited to the forest stands, but in the sites with older fields and forests, regeneration extends into and is more vigorous in the fields. Although species diversity varied from forest to field only for the tree and shrub layers, important changes occurred in the ground species composition. There is no evidence yet that the herbaceous species present in the forest stands will colonise the old fields. The results suggest that both environmental differences among sites and length of time since the fields were abandoned explain red spruce regeneration patterns. In order to more accurately assess the potential for red spruce regeneration in old fields, long-term monitoring of the production, dispersal and viability of red spruce seeds from adjacent forests and of the constraints to seedling establishment and survival in old fields will be needed.     

Island chain models and gradient systems

A range of island chain models in the literature are gradient systems. This implies that the asymptotic dynamics of such models is trivial: all solutions tend to stationary points. Results for one-dimensional island chains are extended to habitats with more complicated geometries and to models with competitive species.     

Ectomycorrhizal colonisation of Sitka spruce [Picea sitchensis (Bong.) Carr] seedlings in a Scottish plantation forest

 The ectomycorrhizal (ECM) colonisation of seedling Sitka spruce (Picea sitchensis) was examined in an uneven-aged plantation forest in southern Scotland. The extent of ECM colonisation of individual seedlings was 43.8–97.2%, with an overall mean of 80.3 ± 1.1%. A total of 13 ECM morphotypes were differentiated, with 1–4 ECM types colonising an individual seedling. ECM colonisation was dominated by a single species, Tylospora fibrillosa, which accounted for 72.4–97.7% of the ECM colonisation recorded, on a plot mean basis. Other ECM types appeared to be distributed very patchily, only two types (Lactarius sp. and Mycelium radicis atrovirens Melin) exceeding a mean of 10% colonisation in any one plot. No significant correlations were recorded between ECM colonisation and seedling growth, or between ECM colonisation and soil pH, loss-on-ignition, or water content. Accepted: 16 October 1997     

森林更新与空间异质性

森林更换是一个重要的生态学过程,一直是森林生态系统动态研究中的主要领域之一。森林更新受物理环境、自然干扰、人为干扰、更新树种特性、树种对干扰的反应等因素及其相互作用的影响。这些生物和非生物的因素随空间和时间而不断变化,构成了森林的空间异质性和时间异质性,使森林更新具有空间和时间上的变化特点,表现在异质性的格局和过程中。探索森林更新与空间异质性的内在规律,可揭示空间格局对更新的生态学过程的潜在作用机制。本文主要综述了近年来有关森林更新与空间异质性研究的主要内容和一些观点,分析了更新中空间异质性的来源,着重评述了空间异质性的生境及更新树种的反应、小尺度的空间异质性与更新动态、林分中光有效性的空间异质性与更新格局以及土壤和更新的空间异质性尺度的关联性等方面的研究。     

Influence of natural and regulated streamflow on survival of brown trout (Salmo trutta L.) in a chalkstream

Synopsis Numbers of age 0± trout in October, and annual survival of fish over 2 years old, were closely correlated with mean April flow (discharge) in a groundwater-fed stream. A pilot flow regulation scheme, which enhanced flows from May to September in a drought year, failed to increase survival.Hampshire Rivers Division, Southern Water Authority     

Average seasonal phytomass: a temporally independent index of herbaceous plant dominance

Diversity, similarity and dominance in plant communities are usually assessed from data collected at a single date. Herbaceous species have differing periods of growth, making phytomass or cover-weighted indices of diversity, similarity or dominance highly dependent on the timing of data collection. A new way of mathematically expressing residence time is presented. Average seasonal phytomass values can be used to better express dominance throughout a growing season. A similar approach could be used for cover or any other dynamic property of plant growth. These values could then be used in dominance-weighted indices of community similarity and diversity.Acknowledgements: J. Watkins-Williams assisted in the field and laboratory. R. L. Bayn, Jr. assisted in data processing. This study was supported in part by Grant DEB-78-05328 from the National Science Foundation to J. A. MacMahon.     

Maintaining diversity in an artificially propagated population

Populations may suffer unexpected loss or distortion of biodiversity as a consequence of strategies employed in artificial propagation programs. The Trinity River Fish Hatchery may have inadvertently experienced this while attempting to preserve diversity in a return time within a Chinook salmon population. We develop a model for this system and prove that the long-term distribution of return types converges and that it is strongly tied to the management strategy. Given estimates of heritabilities for return type and differential survival rates, an estimate of this long-term distribution can be computed easily.     

Nitrogen dynamics in the hyporheic zone of a forested stream during a small storm, Hokkaido, Japan

Water and dissolved nitrogen flows through the hyporheic zone of a 3rd-order mountain stream in Hokkaido, northern Japan were measured during a small storm in August 1997. A network of wells was established to measure water table elevations and to collect water samples to analyze dissolved nitrogen concentrations. Hydraulic conductivity and the depth to bedrock were surveyed. We parameterized the groundwater flow model, MODFLOW, to quantify subsurface flows of both stream water and soil water through the hyporheic zone. MODFLOW simulations suggest that soil water inflow from the adjacent hill slope increased by 1.7-fold during a small storm. Dissolved organic nitrogen (DON) and ammonium (NH ₄ ⁺ ) in soil water from the hill slope were the dominant nitrogen inputs to the riparian zone. DON was consumed via mineralization to NH ₄ ⁺ in the hyporheic zone. NH ₄ ⁺ was the dominant nitrogen species in the subsurface, and showed a net release during both base and storm flow. Nitrate appeared to be lost to denitrification or immobilized by microorganisms and/or vegetation in the riparian zone. Our results indicated that the riparian and hyporheic system was a net source of NH ₄ ⁺ to the stream.     

Aboveground decomposition dynamics in riparian depression and slope wetlands of central Pennsylvania

We examined two types of groundwater-fed wetlands (riparian depressions and slopes) classified using the hydrogeomorphic (HGM) system. These wetland types had previously been shown to differ hydrologically. Our first objective was to determine if HGM was a useful structuring variable when examining aboveground decomposition dynamics (rate of mass loss and rate of nitrogen loss). Our second objective was to determine what soil variables were related to any differences in aboveground decomposition dynamics we might find regardless of HGM subclass. We used the litterbag field bioassay technique, and employed a standard litter type (Phalaris arundinacea) across all wetlands. Our results indicated that HGM would not readily serve as an adequate structuring variable for aboveground decomposition in riparian depressions and slope wetlands of central Pennsylvania. Discriminant analysis and classification and regression tree (CART) modeling found soil cation exchange capacity, soil pH, soil organic matter, and soil % nitrogen to be potentially important soil variables related to mass loss, and soil % nitrogen and soil pH to be potentially important variables related to nitrogen loss rate.     

Management Effects on Bryophytes in Estonian Forests

Bryophyte species richness and composition of six different forest types and three different management rates was analysed in 36 Estonian forests. Analyses were done separately for six different bryophyte species groups: total species number, hepatic, hemerophobic, common, non-epigeic and epigeic species. The differences in species richness between unmanaged and moderately managed forests were significant for all species groups, while the differences between moderately and intensively managed forests were significant only for total species number, and numbers of common and non-epigeic species. Our results showed also that in comparison of unmanaged and moderately managed forests, management effect was significant for species richness of all species groups except non-epigeic species, while in comparison of moderately and intensely managed forests, management rate remained significant only for total species richness and richness of common species. Also the species composition was affected by forest management. Only the compositions of unmanaged forest types were found to be significantly different between each other. In conclusion, even moderate management eliminates a significant number of bryophyte species, and the species composition of different forest types become more similar.     

Linking vegetation pattern to hydrology and hydrochemistry in a montane river floodplain, the Šumava National Park, Central Europe

Cover of higher plants (in 4 × 4 m plots), groundwater table height, and water chemistry in boreholes were sampled at 43 sites along three cross-sectional transects in a flat floodplain of the Upper Vltava River in the Šumava Mountains (Šumava National Park, Czech Republic). The goal was to describe the relationships between vegetation and alluvial environment. Correlations between hydrochemical and plant community characteristics were calculated, and Canonical Correspondence Analysis (CCA) was used to express relationships between the abiotic factors and vegetation. The following characteristics were significantly correlated with the vegetation pattern: mean height of the water table, distance from the river, pH, and concentration of NH₄ and humic acids in the groundwater. Two distinct zones were distinguished in the floodplain: Zone I was under direct influence of the river, and exhibited higher pH and ammonium content in a fluctuating groundwater table. Zone II, covering more than half of the floodplain extent, was under the prevailing influence of water coming from the adjacent upland, and exhibited lower pH, higher content of humic acids, and a higher and relatively stable groundwater table. A diverse mosaic of the riparian communities, especially of tall-sedge and tall-grass marshes and alluvial meadows, was typical for the former zone, while peatland vegetation characterised the latter one. The floodplain exhibited a rather oligo- to mesotrophic status with only very local eutrophication, and harboured diverse and valuable plant communities. The protection of this floodplain should be among the priorities of the National Park authorities.