Below-ground biomass in healthy and impaired salt marshes

Twelve salt marshes in south Louisiana (USA) were classified as either impaired or healthy before a summer sample collection of above- and below-ground biomass and determination of sediment accretion rates. The above-ground biomass of plant tissues was the same at both impaired and healthy salt marshes and was not a good predictor of marsh health. However, below-ground root biomass in the upper 30cm was much lower in the impaired marshes compared to the healthy marshes. Compromises to root production apparently occur before there is an obvious consequence to the above-ground biomass, which may quickly collapse before remedial action can be taken. The subsequent change in vertical position of the marsh surface may be equivalent to many years of accretion, and be irreversible within decades without considerable effort. These results are consistent with the hypothesis that it is the plants below-ground accumulation of organic matter, not inorganic matter that governs the maintenance of salt marsh ecosystem in the vertical plane. Reversing the precursor conditions leading to marsh stress before the collapse of the above-ground biomass occurs is therefore a prudent management objective and could be easier than restoration.     

Bird community composition in an actively managed savanna reserve, importance of vegetation structure and vegetation composition

The mosaic of trees, shrubs and open grassland in mesic African savannas is highly dynamic and strongly influenced by mammal herbivory and fire. We investigated the bird fauna in four different savanna habitats to help assess the impacts of vegetation change on this component of faunal diversity. Birds were censused, plant species were identified and vegetation structure was measured in four different vegetation types (Acacia nilotica woodland, Acacia nigrescens woodland, broadleaf thicket and open grassland) in the Hluhluwe-Umfolozi Park in northern KwaZulu Natal, South Africa. Multivariate ordination analyses were used to determine the relative importance of vegetation structure and floristic composition in defining bird assemblages. The bird communities of the grasslands, the acacia woodlands, and the broadleaf woodlands were clearly separated on the first axis of the detrended canonical correspondence analysis (DCCA). Canopy cover and foliage height diversity (FHD) were strongly correlated with the first axis of DCCA, possibly reflecting a secondary successional series from grassland to woodland, known as bush encroachment. Floristic composition (based on presence–absence data only) seemed to be less important for bird community composition than vegetation structure. The results indicate that changes in vegetation structure, caused by bush encroachment, could cause concomitant changes in bird community composition.     

Restoration of salt marshes in the Netherlands

The conquest of land from the sea has been a long tradition in the Netherlands. When salt marshes were high enough, they were embanked when it was economically feasible, and transformed into intensively exploited agricultural land. This resulted in the transformation of halophytic communities to glycophytic communities. Often as an alternative, a low levee, a summerdike was built, which greatly reduced the flooding frequency of the landward summerpolder, hence creating a sedimentation deficit therein. Such summerpolders now cover 1200 ha in the Netherlands, 2100 ha in NW-Germany and small areas in England. Due to continuous embankments, the present salt-marsh area is relatively small with respect to the tidal basins. Discussions have been started how to increase the salt-marsh area. Two options will be discussed, firstly de-embankment of summerpolders and maintenance of the protective seawall, secondly increase of the effects of saline seepage behind the seawall by top soil removal. Both options include the restoration of salt-marsh communities (target communities) in intensively agriculturally exploited sites that have been salt marshes before. From the few examples abroad and experiments it is discussed (1) to which extent the sedimentation deficit in summerpolders could be compensated for, (2) if the soil seed bank is likely to contribute to re-establishment of salt-marsh communities, (3) if the dispersal of propagules of halophytic plants will be possible by hydrochory when the summerdike is breached, (4) to what extent is dispersal by endozoochory through waterfowl important in case re-establishment in a saline seepage area behind the seawall without open connection to the sea is envisaged. Two case studies of de-embanked summerpolders in the Netherlands revealed that the sedimentation deficit can be counteracted by rapid sedimentation, provided enough transport is possible from the foreshore. Dispersal by incoming tidal water from the nearby salt-marsh source area into the target area is possible for many salt-marsh plant species. The rate of success seems to depend on the relative position of source area and target area. A case study in a saline seepage area after top soil removal in the Netherlands, showed that the number of viable seeds dispersed by droppings from waterfowl is limited. Hence the possibilities for restoration of inland halophytic plant communities seem much lower than after de-embankment of summerpolders.     

Comparison of physical characteristics between created and natural estuarine marshes in Galveston Bay,Texas

Five natural and ten created Spartinaalternifloramarshes in the Lower Galveston BaySystem were compared to determine if there weresignificantly different physical characteristicsassociated with each type of marsh. The saltmarsheswere compared on the basis of microhabitats,length-width ratio, area-perimeter ratio, marsh-wateredge ratio, total size of S. alternifloraplantcommunities, fetch distances, angle of exposure,orientation, and elevation. All physicalmeasurements, except for elevation, were obtained fromphotography analyzed with the use of a GeographicInformation System with digital image processingcapabilities. Differences existed between natural andcreated marshes. The natural marsh sites in this studywere characterized by highly undulant marsh-wateredges, island-like S. alternifloraplant stands,concave shorelines, and low elevations. Createdmarshes were characterized by relatively smoothmarsh-water edges, an unbroken shoreline morphology,convex to straight shoreline configurations, andelevations on the edge and inner portions of the marshhigher than those of natural marshes. The lowelevations of the natural marsh appear to be due tocoastal subsidence in the Galveston Bay area alongwith rising sea level. Reticulated marshes andundulant shorelines appear to be caused by consequentdrowning of the natural marshes. High elevations insome of the created marshes are related to erosion ofthe low elevation marsh or deposition of coarsesediments at the marsh-water edge.     

Patterns of vegetation and grasshopper community composition

Summary A study was conducted to evaluate differences in rangeland grasshopper communities over environmental gradients in Gallatin Valley, Montana, USA. The concept of habitat type (Daubenmire 1966) was used as a basis for discriminating between groupings of patches based on vegetation. A total of 39 patches were selected that represented five recognized grassland habitat types (Mueggler and Stewart 1980), as well as two disturbed types (replanting within a known habitat type). Repeated sampling in 1988 of both the insect and plant communities yielded a total of 40 grasshopper (19 664 individuals) and 97 plant species. Detrended Correspondence Analysis (DCA) indicated that patch classifications based on presence and percent cover of plants were appropriate and showed good between-group (habitat type) separation for patches along gradients of precipitation/elevation and plant community complexity. Results from undisturbed habitats showed that plant and grasshopper species composition changed over observed environmental gradients and suggested that habitat type influenced not only species presence, but also relative abundance. Discussion is presented that relates results with patch-use and core and satellite species paradigms.     

Markov models and initial floristic composition in heathland vegetation dynamics

A simple Markov model is used to test the hypothesis that the floristic composition of vegetation colonizing bare ground immediately after burning is the major factor influencing post-fire development in heathland vegetation. Data are taken from stands of different ages at time of burning in a species-rich Calluna-Arctostaphylos heath in NE Scotland. It is shown that variation in the initial floristic composition of the stands is not, in itself, sufficient to produce model simulations which match observed trends, although altering the probabilities of transition from bare ground to other states allows more successful simulations. The model supports the hypothesis that stand age before burning influences the post-fire development through the process of colonization of bare ground. After a very severe fire post-fire development may initially depend on the formation of a moss cover, although this requires further study. It is concluded that simple Markov models can provide the basis for examining successional processes when used in a comparative way.Nomenclature follows Tutin et al. (1964–80) for vascular plants and Smith (1978) for mosses.We thank Prof. C. H. Gimingham for comments on the draft, and J. M. M. Humphrey, P. Marren and the Nature Conservancy Council for permission to work on the Muir of Dinnet.     

滇池大型无脊椎动物的群落演变与成因分析

该文综合滇池大型无脊椎动物的历史资料,并结合2009—2010年的现场调查数据,对其群落演替进行了研究,并对群落衰退的成因进行了探讨。物种数下降明显,由20世纪80年代的57种降为现在的32种,群落的物种损失率高达44%。其中软体动物损失率高达75%:其次是水生昆虫(39%)。滇池全湖底栖动物密度为1776ind/m2(其中寡毛类1706ind/m2;摇蚊科68ind/m2)。近20年滇池全湖底栖动物的丰度比较发现,寡毛类的密度和生物量呈一种先急剧上升,而后明显下降的过程,而摇蚊科的密度和生物量呈现出减少的趋势。寡毛类中的耐污种相对丰度增加,如霍甫水丝蚓(Limnodrilus hoffmeisteri)成为绝对优势种,其平均相对丰度达到了74.1%。摇蚊科相对丰度减少,以前广泛分布的异腹鳃摇蚊(Einfeldiasp.)基本消失,取而代之的是羽摇蚊(Chironomus plumosus)、细长摇蚊(Ch.attenuatus)、中国长足摇蚊(Tanypus chinensis)等耐污种;软体动物种类变得单一,许多高原湖泊特有的软体动物均已消失,螺蛳(Margarya melanioides)、牟氏螺蛳(M.mondi)、光肋螺蛳(M.mansugi)在2009年被世界自然保护联盟列入了极危物种,滇池圆田螺(Cipangopaludina dianchiensis)也被列入了濒危物种。滇池的Shannon-Wiener多样性指数显著降低,尤其是草海物种多样性从20世纪50年代的2.70降到现在的0.30。半个多世纪以来总氮、总磷与物种数和多样性呈现显著负相关。底栖动物群落衰退的成因主要是生境破坏、水质恶化、蓝藻爆发、沉水植物消失、种质库匮乏等。     

Species composition and dominance structure of Gastrotrich (Gastroiricha) assemblages in water bodies of different trophic status

Species composition, density, diversity, and dominance structure in gastrotrich assemblages were studied in three lakes (dystrophic, -mesotrophic, and polytrophic) and in three small ponds of different trophic status, situated in eastern Poland. The -mesotrophic lake (the only lake with a complete lacustrine zonation) was inhabited by the species-richest assemblage (31 species), the number of species being by 26–45% higher than that found in the remaining water bodies studied. Chaetonotus oculifer, C. similis, and Lepidodermella squamata were found to occur in the lakes only, while C. ophiogaster, C. rafalskii and C. rectaculeatus were recorded only in the ponds. C. disjunctus and C. macrochaetus were common eurytopic species. Two Sphagnum bogdwellers: C. acanthodes and Ichthydium forficula, were found to occur in the lakes and ponds sampled and in the -mesotrophic lake littoral, respectively. Dasydytid species were present in one of the ponds only, no dasydytids being found in the three lakes studied.     

Diversity and dominance in planktonic rotifers

The diversity of planktonic species in a body of water can be estimated in two ways: momentary diversity and long term diversity. The former represents the number capable of coexisting at any one time, and the latter includes a seasonal or other temporal component. The relationship between the two varies mainly with salinity. Dominance is generally inversely related to diversity. In considering the total possible range of dominance for a given number of species the mean values are surprisingly restricted. An abnormally high dominance for a number of species can be an indication of pollution or some other form of environmental stress.     

Microbial community composition in sediments resists perturbation by nutrient enrichment

Functional redundancy in bacterial communities is expected to allow microbial assemblages to survive perturbation by allowing continuity in function despite compositional changes in communities. Recent evidence suggests, however, that microbial communities change both composition and function as a result of disturbance. We present evidence for a third response: resistance. We examined microbial community response to perturbation caused by nutrient enrichment in salt marsh sediments using deep pyrosequencing of 16S rRNA and functional gene microarrays targeting the nirS gene. Composition of the microbial community, as demonstrated by both genes, was unaffected by significant variations in external nutrient supply in our sampling locations, despite demonstrable and diverse nutrient-induced changes in many aspects of marsh ecology. The lack of response to external forcing demonstrates a remarkable uncoupling between microbial composition and ecosystem-level biogeochemical processes and suggests that sediment microbial communities are able to resist some forms of perturbation.     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

Acceleration of vegetation succession on eroded land by reforestation in a subtropical zone

Based on field investigations in the East River basin in Guangdong Province, south China, the processes of vegetation development and vegetation succession on bare slopes with high erosion rates are studied. Different reforestation measures have been applied to the slopes, which have resulted in very different processes of vegetation development and succession. On an experimental plot with burned forest but surviving roots, the vegetation restored naturally and quickly because there was little soil erosion. However, in the plots suffering from long-term severe soil erosion, natural vegetation recovery on these barren slopes is very slow. After 26 years, a barren slope has only been partly colonized by poorly developed vegetation composed of heliophilous herbages and scattered shrubs. On the other hand, plantation of some selected wood species on the slope land has dramatically accelerated the vegetation recovery and succession. The plots were reforested with plantation of Acacia auriculiformis in the early 1980s. Twelve years later, the vegetation cover of the artificial forests reached 90% and an understory vegetation community consisting of local species had naturally developed. Reforestation with suitable strategies may control erosion and greatly accelerate vegetation succession in the eroded slope land in the subtropical zones.     

Afromontane old-field vegetation: secondary succession and the return of indigenous species

South‐west Uganda primarily holds afromontane forests within three protected areas: Mgahinga Gorilla National Park, Echuya Forest Reserve and Bwindi Impenetrable National Park. All forests contain portions of old‐field vegetation. The central question of this study, then, is whether and how natural regeneration of afromontane vegetation would take place. A successional pathway consisting of nine plant communities was found for the first 2 years after cessation of agricultural use. A return of afromontane species despite of an initial dominance of neophytes, a diversification of life form spectra and a growing importance of endozoochory with time belonged to the conspicuous characteristics of this secondary succession. To obtain an insight into the role of birds and buffaloes as possible vectors for seed input, I examined their faeces with regard to germinable seeds. Both, bird faeces, as well as buffalo faeces contained germinable seeds originating from the forest flora. Thus, animal dispersal from the forest into the regenerating zones could be documented. The regeneration potential of the soil‐seed bank seems to be limited in its time scale, and only an initial regeneration capacity could be found. A further progressive succession into a secondary afromontane forest depends on an input of diaspores from undisturbed forest sites. Zoochory appears to be one of the most important dispersal mechanisms.     

Seed bank and vegetation composition of forest stands of varying age in central Belgium: consequences for regeneration of ancient forest vegetation

Vegetation composition differs significantly between ancientand recent forest, due to slow colonization capacity of typical forest speciesand the higher abundance of early successional species in recent forest.However, little is known about differences in persistent seed bank compositionbetween ancient and recent forest and about the interaction between seed bankand vegetation in relation with forest age. We surveyed the seed bank and theunderstorey vegetationcomposition in transects from ancient to recent forest. Seed bank and fieldlayer vegetation characteristics and similarity between seed bank andvegetationwere analysed in relation to recent forest age and distance to the ancientforest. A total of 39 species and 14,911 seedlings germinated, whichcorresponds with a seed density of 12,426 seeds/m².Total seed density is significantly higher in the youngest recent forest parcel(55 years). Also the seed bank composition in the youngest forest parceldifferssignificantly from the other parcels. After a longer period of reforestation,the seed bank approaches that of the ancient forest, suggesting seed bankdepletion, although the seed bank is permanently replenished to some extent byseed bank forming species from local disturbances. Seed bank composition doesnot change significantly with distance to the ancient forest. Similaritybetween seed bank and vegetation composition, nomatter the forest age, is very low, but decreases with increasing forest age.The most frequent species in the vegetation are absent in the seed bank andvice versa. The contribution of forest species is highin the vegetation and they almost not occur in the seed bank, while species offorest edges and clearings, and species of disturbed environments are morefrequent in the seed bank. The seed bank is mainly composed of earlysuccessional species of former forest stages or species which temporary occurinsmall-scale disturbances. The seed bank may enhance the negative effects ofearlysuccessional, mainly competitive species to the forest species richness in therecent forest. In this respect, forest management should minimise forestdisturbances, to prevent germination of competitive species form buriedseeds.     

Long-term effects of clear-felling on vegetation dynamics and species diversity in a boreal pine forest

The understorey vegetation in a lichen–Scots pine forest was monitored during 20 years before and after clear-felling. Plots with and without logging residues were compared concerning the general pattern of the vegetation dynamics and changes in species composition, dominance, richness, evenness and diversity. The succession of both treatments had a clear principal component analysis (PCA) pattern of a 'stepwise arch-shaped diverging' trend mainly driven by 'pioneer' lichens, 'reindeer' lichens and Calluna vulgaris. The difference between the residue treatments was significant regarding succession of vascular plants, bryophytes and 'reindeer' lichens. The nitrogen indicators Epilobium angustifolium and Deschampsia flexuosa were favoured on plots with logging residues.