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Wesuls D.,University of Hamburg | Pellowski M.,University of Hamburg | Suchrow S.,University of Hamburg | Oldeland J.,University of Hamburg | And 2 more authors.
Ecological Indicators | Year: 2013

Persistence or disappearance of plants under grazing pressure has led to their categorisation as grazing increasers or decreasers. We aimed to extend this classical indicator concept in rangeland ecology by interpreting the shape of species responses and trait patterns modelled along continuous grazing gradients at different spatial scales. Taking transects of two different lengths, we recorded the cover of vascular plant species along grazing gradients in central Namibian rangelands. We used a hierarchical set of ecologically meaningful models with increasing complexity-the HOF (Huisman-Olff-Fresco) approach-to investigate species' grazing responses, diversity parameters and pooled cover values for two traits: growth form and life cycle. Based on our modelling results, we classified species responses into eight types: no response, monotonic increasers/decreasers, threshold increasers/decreasers, symmetric unimodal responses, left skewed and right skewed unimodal responses. The most common category was that of no response (42% of the short and 79% of the long transect responses). At both scales, decreaser responses with higher grazing pressure were more frequent than increaser responses. Monotonic and threshold responses were more frequent along the short transects. Diversity parameters showed a slight but continuous decline towards higher grazing intensities. Responses of growth form and life cycle categories were mostly consistent at both scales. Trees, shrubs, dwarf shrubs, and perennials declined continuously. Woody forbs tended to show a symmetric unimodal distribution along the gradients, while herbaceous forbs and annuals showed skewed unimodal responses towards lower grazing intensities. The different grazing response types proposed in this study allow for a differentiated picture of niche patterns along grazing gradients and provide a basis to use species as indicators for a continuum of vegetation states altered by livestock impact. The general decline of plant diversity with increasing grazing intensities highlights the importance of reserves that are less impacted by grazing to support the resilience of the studied system. © 2012 Elsevier Ltd. Source


Margalef O.,CSIC - Institute of Earth Sciences Jaume Almera | Canellas-Bolta N.,Botanic Institute of Barcelona IBB CSIC ICUB | Canellas-Bolta N.,University of Barcelona | Pla-Rabes S.,Ecological Research Center and Forestry Applications | And 10 more authors.
Global and Planetary Change | Year: 2013

The Rano Aroi mire on Easter Island (also known as Rapa Nui; 27°09'S, 109°27'W, 430m above sea level) provides a unique non-marine record in the central South Pacific Ocean for reconstructing Late Pleistocene environmental changes. The results of a multiproxy study on two cores from the center and margin of the Rano Aroi mire, including peat stratigraphy, facies analysis, elemental and isotope geochemistry on bulk organic matter, X-ray fluorescence (XRF) core scanning and macrofossil analysis, were used to infer past water levels and vegetation changes. The chronology was based on 18 14C AMS dates for the upper 8.7m. The extrapolated age for the base of the sequence is 70kyr, which implies that this record is the oldest paleolimnological record on Easter Island. The recovered Rano Aroi sequence consists of a radicel peat formed primarily from the remains of sedges, grasses and Polygonaceae that have accumulated since Marine Isotopic Stage (MIS) 4 (70kyr BP) to the present. From 60 to 40kyr BP (MIS 3), high precipitation/runoff events were recorded as organic mud facies with lighter δ13C, low C/N values and high Ti content, indicating higher detritic input to the mire. A gradual shift in δ13C bulk organic matter from -14% to -26%, recorded between 50 and 45calkyr BP, suggests a progressive change in local peat-forming vegetation from C4 to C3 plant types. Post-depositional Ca and Fe enrichment during sub-aerial peat exposure and very low sedimentation rates indicate lower water tables during Late MIS 3 (39-31calkyr BP). During MIS 2 (27.8-19calkyr BP), peat production rates were very low, most likely due to cold temperatures, as reconstructed from other Easter Island records during the Last Glacial Maximum (LGM). Geochemical and macrofossil evidence shows that peat accumulation reactivates at approximately 17.5calkyr BP, reaching the highest accumulation rates at 14calkyr BP. Peat accretion decreased from 5.0 to 2.5calkyr BP, coinciding with a regional Holocene aridity phase. The main hydrological and environmental changes in Rano Aroi reflect variations in the South Pacific Convergence Zone (SPCZ), Southern Westerlies (SW) storm track, and South Pacific Anticyclone (SPA) locations. © 2013 Elsevier B.V. Source


Schulz K.,University of Greifswald | Timmermann T.,University of Greifswald | Steffenhagen P.,LUP Luftbild Umwelt Planung GmbH | Zerbe S.,Free University of Bozen Bolzano | Succow M.,Institute of Botany and Landscape Ecology
Hydrobiologia | Year: 2011

Rewetting can strongly affect the matter balance of peatlands. Owing to evidence of increasing CH4 emissions and P mobilisation after rewetting, the effects of peatland restoration on climate, eutrophication risks and related controversies are discussed. Our study focuses on the role of helophytes in the carbon and nutrient balance of rewetted fen grasslands of NE Germany. We hypothesise that the helophytes Carex riparia, Glyceria maxima, Phalaris arundinacea, Phragmites australis and Typha latifolia differ in biomass production and nutrient standing stock according to site conditions and harvest time. We analysed the helophyte biomass three times a year and continuously measured water levels and quality. Biomass production, nutrient standing stock and litter accumulation were highly species specific and depended on nutrient availability, mean water levels and harvesting time. We conclude that helophytes store considerable amounts of carbon and temporarily improve the water quality by withdrawing high amounts of nutrients from the top soil during the growing season, and by reducing nutrient discharges. Restoring peatlands as effective nutrient and carbon sinks in the landscape should favour highly productive potentially peat-forming helophytes as Phragmites australis by establishing adequate water levels. If nutrients are to be removed from the degraded peatland, then management can combine the restoration of helophyte stands by rewetting with harvesting measures. © 2011 Springer Science+Business Media B.V. Source


Brunbjerg A.K.,University of Aarhus | Cavender-Bares J.,University of Minnesota | Eiserhardt W.L.,University of Aarhus | Ejrnaes R.,University of Aarhus | And 13 more authors.
Journal of Plant Ecology | Year: 2014

Aims Studies integrating phylogenetic history and large-scale community assembly are few, and many questions remain unanswered. Here, we use a global coastal dune plant data set to uncover the important factors in community assembly across scales from the local filtering processes to the global long-term diversification and dispersal dynamics. Coastal dune plant communities occur worldwide under a wide range of climatic and geologic conditions as well as in all biogeographic regions. However, global patterns in the phylogenetic composition of coastal dune plant communities have not previously been studied. Methods The data set comprised vegetation data from 18463 plots in New Zealand, South Africa, South America, North America and Europe. The phylogenetic tree comprised 2241 plant species from 149 families. We calculated phylogenetic clustering (Net Relatedness Index, NRI, and Nearest Taxon Index, NTI) of regional dune floras to estimate the amount of in situ diversification relative to the global dune species pool and evaluated the relative importance of land and climate barriers for these diversification patterns by geographic analyses of phylogenetic similarity. We then tested whether dune plant communities exhibit similar patterns of phylogenetic structure within regions. Finally, we calculated NRI for local communities relative to the regional species pool and tested for an association with functional traits (plant height and seed mass) thought to vary along sea-inland gradients. Important Findings Regional species pools were phylogenetically clustered relative to the global pool, indicating regional diversification. NTI showed stronger clustering than NRI pointing to the importance of especially recent diversifications within regions. The species pools grouped phylogenetically into two clusters on either side of the tropics suggesting greater dispersal rates within hemispheres than between hemispheres. Local NRI plot values confirmed that most communities were also phylogenetically clustered within regions. NRI values decreased with increasing plant height and seed mass, indicating greater phylogenetic clustering in communities with short maximum height and good dispersers prone to wind and tidal disturbance as well as salt spray, consistent with environmental filtering along sea-inland gradients. Height and seed mass both showed significant phylogenetic signal, and NRI tended to correlate negatively with both at the plot level. Low NRI plots tended to represent coastal scrub and forest, whereas high NRI plots tended to represent herb-dominated vegetation. We conclude that regional diversification processes play a role in dune plant community assembly, with convergence in local phylogenetic community structure and local variation in community structure probably reflecting consistent coastal-inland gradients. Our study contributes to a better understanding of the globally distributed dynamic coastal ecosystems and the structuring factors working on dune plant communities across spatial scales and regions. © 2014 The Author. Source


Bennert H.W.,Ruhr University Bochum | Horn K.,Bro fur Angewandte Geobotanik und Landschaftskologie BaGL | Kauth M.,Bergmannsheil Universittsklinikum | Fuchs J.,Leibniz Institute of Plant Genetics and Crop Plant Research | And 5 more authors.
Annals of Botany | Year: 2011

Background and AimsInterspecific Diphasiastrum hybrids have been assumed to be homoploid and to produce well-formed spores serving sexual reproduction. If this were the case, forms intermediate between hybrids and parents or hybrid swarms should be expected. The purpose of this study was: (1) to check whether homoploidy consistently applies to the three hybrids throughout their Central European range; (2) to examine whether their genome sizes confirm their parentage as assumed by morphology; and (3) to perform a screening for detection of ploidy levels other than diploid and variation in DNA content due to backcrossing.MethodsFlow cytometry was used first to measure the relative DNA values [with 4′,6-diamidino-2-phenylindole (DAPI) staining] and ploidy level as a general screening, and secondly to determine the absolute DNA 2C values [with propidium iodide (PI) staining] in a number of selected samples with the main focus on the hybrids.Key ResultsA considerable variation of DNA 2C values (5·267·52 pg) was detected between the three European Diphasiastrum species. The values of the diploid hybrids are highly constant without significant variation between regions. They are also intermediate between their assumed parents and agree closely with those calculated from their putative parents. This confirms their hybrid origin, assumed parentage and homoploid status. Considerably higher DNA amounts (9·4810·30 pg) were obtained for three populations, suggesting that these represent triploid hybrids, an interpretation that is strongly supported by their morphology.ConclusionsDiploid hybrids have retained their genetic and morphological identites throughout their Central European range, and thus no indications for diploid backcrossing were found. The triploid hybrids have probably originated from backcrossing between a diploid gametophyte of a hybrid (derived from a diplospore) and a haploid gametophyte of a diploid parental species. By repeated crossing events, reticulate evolution patterns arise that are similar to those known for a number of ferns. © The Author 2011. Source

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