Effects of prolonged flooding on the distribution and biomass of emergent species along a freshwater wetland coenocline

Water levels were raised 1 m for two years in 10 cells of an experimental wetland complex located in the Delta Marsh, Manitoba, Canada. The mean area covered by each of the 5 dominant emergent species in these cells declined significantly during the first year of flooding. There was no significant difference in the total acreage of emergents between flooding years. Three species were completely (Carex atherodes Spreng, Scolochloa festucacea (Willd.) Link) or almost (Scirpus lacustris L. spp. glaucus (Sm.) Hartm.) eliminated from the cells. Typha glauca Godr. and Phragmites australis (Cav.) Trin. Still covered in the second year of flooding about 40 and 25%, respectively, of the area that they had occupied in the cells in the preflooding year. The mean elevation at which the two surviving species were found both increased 0.1 and 0.2 m over preflooding elevations during the first and second year of flooding, respectively. There is no evidence that during the flooding years any species migrated upslope toward more optimal water depths.Aboveground standing crop for all five species declined significantly during the first year of flooding, but was not significantly different between the two flooding years. Total belowground biomass did not differ between spring and fall, but declined significantly (about 20%) during the first year of flooding and remained about at that level during the second year. In the spring following the two years of flooding, mean total belowground biomass was only 40 g m^-2, less than 10% of the belowground biomass during the preflooding year.Paper No. 77 of the Marsh Ecology Research Program, a joint project of Ducks Unlimited Canada and the Delta Waterfowl and Wetlands Research Station.     

Climate change induced elevational range shifts of Himalayan tree species

Global warming may force montane species to shift upward to keep pace with their shifting climate niche. How species differences in such distribution shifts depend on their elevational positions, elevation-dependent warming rates, and other environmental constraints, or plant functional traits is poorly understood. Here, we analyzed for 137 Himalayan tree species how distribution shifts vary with elevational niche positions, environmental constraints, and their functional traits. We developed ecological niche models using MaxEnt by combining species survey and botanical collections data with 19 environmental predictors. Species distributions were projected to 1985 and 2050 conditions, and elevational range parameters and distribution areas were derived. Under the worst-case RCP 8.5 scenario, species are predicted to shift, on average, 3 m/year in optimum elevation, and have 33% increase in distribution area. Highland species showed faster predicted elevational shifts than lowland species. Lowland and highland species are predicted to expand in distribution area in contrast to mid-elevation species. Tree species for which species distribution models are driven by responses to temperature, aridity, or soil clay content showed the strongest predicted upslope shifts. Tree species with conservative trait values that enable them to survive resource poor conditions (i.e., narrow conduits) showed larger predicted upslope shifts than species with wide conduits. The predicted average upslope shift in maximum elevation (8 m/year) is >2 times faster than the current observations indicating that many species will not be able to track climate change and potentially go extinct, unless they are supported by active conservation measures, such as assisted migration.     

The push and pull of climate change causes heterogeneous shifts in avian elevational ranges

Projected effects of climate change on animal distributions primarily focus on consequences of temperature and largely ignore impacts of altered precipitation. While much evidence supports temperature‐driven range shifts, there is substantial heterogeneity in species' responses that remains poorly understood. We resampled breeding ranges of birds across three elevational transects in the Sierra Nevada Mountains, USA, that were extensively surveyed in the early 20th century. Presence–absence comparisons were made at 77 sites and occupancy models were used to separate significant range shifts from artifacts of false absences. Over the past century, rising temperature pushed species upslope while increased precipitation pulled them downslope, resulting in range shifts that were heterogeneous within species and among regions. While 84% of species shifted their elevational distribution, only 51% of upper or lower range boundary shifts were upslope. By comparison, 82% of range shifts were in a direction predicted by changes in either temperature or precipitation. Species were significantly more likely to shift elevational ranges than their ecological counterparts if they had small clutch sizes, defended all‐purpose territories, and were year‐round residents, results that were in opposition to a priori predictions from dispersal‐related hypotheses. Our results illustrate the complex interplay between species‐specific and region‐specific factors that structure patterns of breeding range change over long time periods. Future projections of increasing temperature and highly variable precipitation regimes create a strong potential for heterogeneous responses by species at range margins.     

Plant community,primary productivity,and environmental conditions following wetland re-establishment in the Sacramento-San Joaquin Delta,California

Wetland restoration can mitigate aerobic decomposition of subsided organic soils, as well as re-establish conditions favorable for carbon storage. Rates of carbon storage result from the balance of inputs and losses, both of which are affected by wetland hydrology. We followed the effect of water depth (25 and 55 cm) on the plant community, primary production, and changes in two re-established wetlands in the Sacramento San-Joaquin River Delta, California for 9 years after flooding to determine how relatively small differences in water depth affect carbon storage rates over time. To estimate annual carbon inputs, plant species cover, standing above- and below-ground plant biomass, and annual biomass turnover rates were measured, and allometric biomass models for Schoenoplectus (Scirpus) acutus and Typha spp., the emergent marsh dominants, were developed. As the wetlands developed, environmental factors, including water temperature, depth, and pH were measured. Emergent marsh vegetation colonized the shallow wetland more rapidly than the deeper wetland. This is important to potential carbon storage because emergent marsh vegetation is more productive, and less labile, than submerged and floating vegetation. Primary production of emergent marsh vegetation ranged from 1.3 to 3.2 kg of carbon per square meter annually; and, mid-season standing live biomass represented about half of the annual primary production. Changes in species composition occurred in both submerged and emergent plant communities as the wetlands matured. Water depth, temperature, and pH were lower in areas with emergent marsh vegetation compared to submerged vegetation, all of which, in turn, can affect carbon cycling and storage rates.     

Postbreeding elevational movements of western songbirds in Northern California and Southern Oregon

Migratory species employ a variety of strategies to meet energetic demands of postbreeding molt. As such, at least a few species of western Neotropical migrants are known to undergo short‐distance upslope movements to locations where adults molt body and flight feathers (altitudinal molt migration). Given inherent difficulties in measuring subtle movements of birds occurring in western mountains, we believe that altitudinal molt migration may be a common yet poorly documented phenomenon. To examine prevalence of altitudinal molt migration, we used 29 years of bird capture data in a series of linear mixed‐effect models for nine commonly captured species that breed in northern California and southern Oregon. Candidate models were formulated a priori to examine whether elevation and distance from the coast can be used to predict abundance of breeding and molting birds. Our results suggest that long‐distance migrants such as Orange‐crowned Warbler (Oreothlypis celata) moved higher in elevation and Audubon's Warbler (Setophaga coronata) moved farther inland to molt after breeding. Conversely, for resident and short‐distance migrants, we found evidence that birds either remained on the breeding grounds until they finished molting, such as Song Sparrow (Melospiza melodia) or made small downslope movements, such as American Robin (Turdus migratorius). We conclude that altitudinal molt migration may be a common, variable, and complex behavior among western songbird communities and is related to other aspects of a species’ natural history, such as migratory strategy.     

Predictive Occurrence Models for Bat Species in California

Abstract: The ability to accurately predict the potential occurrence of species of management concern is useful for wildlife managers, particularly for those whose management activities involve large areas where sampling is difficult due to logistical or financial constraints. During the summers of 2002 and 2003, we used mist nets to capture bats (Myotis yumanensis, M. californicus, M. evotis, M. thysanodes, Eptesicus fuscus, Lasionycteris noctivagans, Tadarida brasiliensis, Antrozous pallidus, Lasiurus borealis, and Lasiurus cinereus) in Whiskeytown National Recreation Area in north-central California, USA. We used landscape-scale variables, logistic regression, and Akaike's Information Criterion (AIC_c) to model species distributions and produce spatially discerning predictive occurrence maps. We developed a priori models that we used to determine which landscape-scale variables best discriminated between capture sites and non-capture sites. The odds of capturing a bat were 3.3 greater when total edge increased by 10,000 m, whereas for Yuma myotis (Myotis yumanensis), the odds of predicting presence were 0.2 greater when distance to lakes and ponds decreased by 2,000 m. Elevation was important in predicting the distribution of silver-haired bats (Lasionycteris noctivagans) and big brown bats (Eptesicus fuscus). Increasing elevation by 400 m decreased the odds of capturing a silver-haired bat by 0.1 and a big brown bat by 0.4. Classification accuracy for our models ranged from 80.9% for all bat species combined to 72.3% for Yuma myotis and silver-haired bats. Predictive occurrence models can be valuable to bat conservation efforts because they provide spatial data important for evaluating the effects of management activities on species distributions.     

Tree demography suggests multiple directions and drivers for species range shifts in mountains of Northeastern United States

Climate change is expected to lead to upslope shifts in tree species distributions, but the evidence is mixed partly due to land‐use effects and individualistic species responses to climate. We examined how individual tree species demography varies along elevational climatic gradients across four states in the northeastern United States to determine whether species elevational distributions and their potential upslope (or downslope) shifts were controlled by climate, land‐use legacies (past logging), or soils. We characterized tree demography, microclimate, land‐use legacies, and soils at 83 sites stratified by elevation (~500 to ~1200 m above sea level) across 12 mountains containing the transition from northern hardwood to spruce‐fir forests. We modeled elevational distributions of tree species saplings and adults using logistic regression to test whether sapling distributions suggest ongoing species range expansion upslope (or contraction downslope) relative to adults, and we used linear mixed models to determine the extent to which climate, land use, and soil variables explain these distributions. Tree demography varied with elevation by species, suggesting a potential upslope shift only for American beech, downslope shifts for red spruce (more so in cool regions) and sugar maple, and no change with elevation for balsam fir. While soils had relatively minor effects, climate was the dominant predictor for most species and more so for saplings than adults of red spruce, sugar maple, yellow birch, cordate birch, and striped maple. On the other hand, logging legacies were positively associated with American beech, sugar maple, and yellow birch, and negatively with red spruce and balsam fir – generally more so for adults than saplings. All species exhibited individualistic rather than synchronous demographic responses to climate and land use, and the return of red spruce to lower elevations where past logging originally benefited northern hardwood species indicates that land use may mask species range shifts caused by changing climate.     

A probability-based analysis of temporal and spatial co-occurrence in grassland birds

Aim To test for non‐random co‐occurrence in 36 species of grassland birds using a new metric and the C‐score. The analysis used presence–absence data of birds distributed among 305 sites (or landscapes) over a period of 35 years. This analysis departs from traditional analyses of species co‐occurrence in its use of temporal data and of individual species’ probabilities of occurrence to derive analytically the expected co‐occurrence between paired species. Location Great Plains region, USA. Methods Presence–absence data for the bird species were obtained from the North American Breeding Bird Survey. The analysis was restricted to species pairs whose geographic ranges overlapped. Each of 541 species pairs was classified as a positive, negative, or non‐significant association depending on the mean difference between the observed and expected frequencies of co‐occurrence over the 35‐year time‐span. Results Of the 541 species pairs that were examined, 202 to 293 (37–54%) were positively associated, depending on which of two null models was used. However, only a few species pairs (<5%) were negatively associated. An additional 89 species pairs did not have overlapping ranges and hence represented de facto negative associations. The results from analyses based on C‐scores generally agreed with the analyses based on the difference between observed and expected co‐occurrence, although the latter analyses were slightly more powerful. Main conclusions Grassland birds within the Great Plains region are primarily distributed among landscapes either independently or in conjunction with one another. Only a few species pairs exhibited repulsed or segregated distributions. This indicates that the shared preference for grassland habitat may be more important in producing coexistence than are negative species interactions in preventing it. The large number of non‐significant associations may represent random associations and thereby indicate that the presence/absence of other grassland bird species may have little effect on whether a given focal species is also found within the landscape. In a broader context, the probability‐based approach used in this study may be useful in future studies of species co‐occurrence.     

Co-existence of three species of amphibious plants in relation to spatial and temporal variation: Field evidence

Changes in vegetation of Bushells Lagoon, a freshwater body fed intermittently by the Hawkesbury River in the Sydney basin, were studied over three scales of time: broad fluctuations in areas occupied by emergent plants over 28 years; fluctuations in their percentage cover over 21 months; and, over 6 months, seasonal variation in biomass of emergent plants of two native species Ludwigia peploides (Kunth) Raven ssp. montevidensis (Spreng.) Raven and Marsilea mutica Mett., and the introduced Myriophyllum aquaticum (Vellozo) Verde. Differences in distribution and abundance of plants observed between the three species were related to environmental variation in time and space. Wind and wave action varied along the shoreline, and emergent shoots of Myriophyllum were most abundant in sheltered conditions, floating leaves of Marsilea most abundant in relatively exposed conditions and emergent shoots of Ludwigia most abundant in somewhat intermediate conditions in the shallows. Chemical characteristics of the water varied in time but, unlike water temperature, had no obvious seasonal pattern. Seasonal change occurred in the biomass of Marsilea, and to a lesser extent in that of Myriophyllum, with smaller proportions of leaf material present in winter than in summer. Though water levels were apparently stable over a period of some years (1965–74), at other times they varied greatly through flooding and drought with related major change in cover of the three species. Co-existence of the three species in the lagoon may be related to differences between their plants in establishment and growth following disturbance (regeneration niche), tolerance of wind and wave action (habitat niche), or seasonal growth (phenological niche).     

Effects of an exotic invasive macrophyte (tropical signalgrass) on native plant community composition,species richness and functional diversity

1. The issue of freshwater species being threatened by invasion has become central in conservation biology because inland waters exhibit the highest species richness per unit area, but apparently have the highest extinctions rates on the planet. 2. In this article, we evaluated the effects of an exotic, invasive aquatic grass (Urochloa subquadripara– tropical signalgrass) on the diversity and assemblage composition of native macrophytes in four Neotropical water bodies (two reservoirs and two lakes). Species cover was assessed in quadrats, and plant biomass was measured in further quadrats, located in sites where tropical signalgrass dominated (D quadrats) and sites where it was not dominant or entirely absent (ND quadrats). The effects of tropical signalgrass on macrophyte species richness, Shannon diversity and number of macrophyte life forms (a surrogate of functional richness) were assessed through regressions, and composition was assessed with a DCA. The effects of tropical signalgrass biomass on the likelihood of occurrence of specific macrophyte life forms were assessed through logistic regression. 3. Tropical signalgrass had a negative effect on macrophyte richness and Shannon and functional diversity, and also influenced assemblage composition. Emergent, rooted with floating stems and rooted submersed species were negatively affected by tropical signalgrass, while the occurrence of free‐floating species was positively affected. 4. Our results suggest that competition with emergent species and reduction of underwater radiation, which reduces the number of submersed species, counteract facilitation of free‐floating species, contributing to a decrease in plant diversity. In addition, homogenisation of plant assemblages shows that tropical signalgrass reduces the beta diversity in the macrophyte community. 5. Although our results were obtained at fine spatial scales, they are cause for concern because macrophytes are an important part of freshwater diversity.     

三峡水库坝下江心洲植物群落及其性状对不同水淹强度的响应

三峡水库的运行,改变了其坝下游的水沙情势,使坝下游沙洲生境呈现出明显的水淹强度梯度变化。阐明这一梯度变化下沙洲植被组成、分布和性状结构特征,是理解植被与沙洲稳定关系的基础,更是阐明三峡工程对长江中下游地区生态环境影响的核心内容之一。选取上荆江河段第一个江心洲-太平口心滩作为研究样地,通过植被组成和分布特征的调查,对不同水淹强度下群落物种组成、多样性和功能特征进行了深入分析。结果表明：太平口心滩植被组成以草本植物为主,稀布小型灌木川三蕊柳。调查共记录物种21科33属39种,以禾本科和菊科植物为主要优势种。轻微（20-40 d）和极强水淹强度（100+d）条件下的生境物种组成同其他水淹强度生境具有显著性差异,轻微水淹强度下牛鞭草和节节草为主要优势种,极强水淹强度下虉草为主要优势种。不同水淹强度下物种多样性指数差异显著,功能多样性指数和生物多样性指数趋势基本一致。随着水淹时间的延长,植被更倾向于表现出花果期位于出露期、植株高度更加低矮、须根系、进行营养繁殖的功能性状。江心洲植被的群落结构和功能性状特征都在水淹梯度下呈现出明显的梯度变化特征。这些研究结果表明水淹强度的梯度变化是沙洲植物群落变化的重要驱动因子,为进一步研究沙洲植物群落动态变化以及沙洲植被协同演替机制,明晰大坝影响下的生态环境变化提供重要依据。     

Biotic interactions affect the elevational ranges of high‐latitude plant species

Ecological theory suggests that positive plant–plant interactions can extend species distributions into areas that would otherwise be unfavourable. However, few studies have tested this hypothesis, and none have explicitly examined the associated prediction that inter‐specific interactions between plants may broaden species altitudinal distributions. Here we test this prediction, using fine‐scale species distribution data for 156 bryophytes, lichens and vascular plants spanning a 900 m elevational gradient in north‐western Finland and Norway, analysed with a niche modelling approach. Species altitudinal ranges of all three groups of plants were more accurately predicted when including the cover of any of the 24 most wide‐spread and abundant species (‘dominants’) than when using abiotic variables alone, emphasizing the importance of including relevant biotic predictors in species distribution models. Half of the models showed that species had very low probabilities of occurrence under high cover of dominants, suggesting a strong negative impact of dominant species. Similarly, for species that are predicted to occur irrespective of dominant species cover, 62% of models showed narrower species altitudinal distributions when occurring under high dominant cover, with contractions of species’ lower and upper elevational limits being common. Nonetheless, high cover of dominant species was associated with upslope range extension in 43 species, and a net range expansion in nearly 10% of all models. Species distributional responses to dominants were only weakly related to species traits, with larger range contractions associated with arctic‐alpine dominants. Therefore, dominant species appear to exert a strong influence on the elevational distribution of other species in high latitude environments.     

The immediate and long‐term effects of water drawdown on macrophyte assemblages in a large subtropical reservoir

1. Disturbances play a central role in determining the spatial and temporal dynamics of many plant communities. In our study of macrophyte assemblages at 150 sites in five arms of a large subtropical reservoir (Itaipu Reservoir, Brazil–Paraguay border), we used co‐occurrence null models and spatiotemporal analyses to describe the patterns in the assemblages during a historically large water drawdown in 2000, in comparison with the previous year (1999) and subsequent years (2001–07). A C‐Score co‐occurrence index tested the null hypothesis of random structure during the drawdown period. A detrended correspondence analysis and multiresponse permutation procedure were used to verify whether species composition differed before, during and after the disturbance. 2. In contrast to our expectations, the null models showed that the macrophyte assemblages were spatially structured during the drawdown (2000), although species composition was significantly different from the previous year (1999) and also changed in the following years (2001–07). Significant species co‐occurrence patterns were generated by the drawdown disturbance, with species extinctions and colonisation by new species from propagules and seed bank germination. 3. The randomness we expected in 2000 actually occurred in 2001, probably because the reestablishment of normal water level enabled both submersed and free‐floating species to recolonise the shore that emergent species had inhabited since the drawdown. Biotic interactions appeared to increase during the years after the disturbance and the habitat preferences of the aquatic macrophytes were re‐established, resulting in higher similarities in aquatic macrophyte species composition in the years after the drawdown.     

Ecophysiological responses of five emergent-wetland plants to diminished water supply: an experimental microcosm study

Freshwater wetlands are fundamentally tied to hydrology as they are often found along the boundaries between terrestrial uplands and open waters. Although wetland systems are frequently prone to extended periods of flooding and exposure, the degree of water deprivation may intensify during periods of low precipitation or drought. Therefore, the purpose of this study was to evaluate plant–water relations in five emergent macrophytes (Carex alata, Juncus effusus, Peltandra virginica, Saururus cernuus, and Justicia americana) to simulated drought conditions. Weekly evaluations of tissue water content and xylem water potential (free energy of water in xylem tissues) were conducted on plants grown in experimental microcosms over a 9-week period. Plant performance was also evaluated in each species by monitoring the changes in plant biomass, leaf area, and survival. The results indicate that J. effusus and P. virginica performed better in both flooded and moderately dry conditions, and plants that maintained higher water content in water logged soils (i.e., J. americana) were less tolerant to drying conditions. This study also illustrates the importance of periodic water withdrawal on plant performance. In general, plants that were subjected to both flooded and dry conditions responded better physiologically than plants that were either continuously flooded or received extended droughts (≥4 weeks). Therefore, provided the duration of water deficit is not extensive, short periods of water withdrawal can enhance the performance and water relations in some emergent-wetland plant species.     

Predicting tree death for Fagus sylvatica and Abies alba using permanent plot data

Question: How well can mortality probabilities of deciduous trees(Fagus sylvatica) and conifers (Abies alba) be predicted using permanent plot data that describe growth patterns, tree species, tree size and site conditions? Location: Fagus forests in the montane belt of the Jura folds (Switzerland). Method: Permanent plot data were used to develop and validate logistic regression models predicting survival probabilities of individual trees. Backward model selection led to a reduced model containing the growth‐related variable ‘relative basal area increment’ (growth‐dependent mortality) and variables not directly reflecting growth such as species, size and site (growth‐independent mortality). Results: The growth‐mortality relationship was the same for both species (growth‐dependent mortality). However, species, site and tree size also influenced mortality probabilities (growth‐independent mortality). The predicted survival probabilities of the final model were well calibrated, and the model showed an excellent discriminatory power (area under the receiver operating characteristic curve = 0.896). Conclusion: Mortality probabilities of Fagus sylvatica and Abies alba can be predicted with high discriminatory power using a well calibrated logistic regression model. Extending this case study to a larger number of tree species and sites could provide species‐ and site‐specific tree mortality models that allow for more realistic projections of forest succession.     

Observed range dynamics of South African amphibians under conditions of global change

Changes in the size and location of species ranges are a recognized response to environmental changes. We assessed recent range dynamics of South African amphibians using data consolidated by the South African Frog Atlas Project, which provided both historical (1905–1995) and recent distribution data (1996–2003). Sampling differences were accounted for using a correction factor and subsampling protocols. Changes to the size of species ranges and shifts in the mean range centre for latitude, longitude and altitude were assessed. Most (70%) of the assessed species showed range contractions. Of the expected climate change predictions, only upslope shifts were supported (mean 47.6 m). There were no significant mean latitudinal shifts for the assessed group of species. Species of the Bushveld region shifted northwesterly and upslope, while species of the Winter Rainfall region also shifted northwesterly. The small, though significant, general taxon‐wide trends where minor in comparison to the wide variety of idiosyncratic, species‐specific range changes that were observed. The species‐specific observed range changes that were documented should be used to further investigate range change drivers on an individual species basis.     

Modelling ecological impacts of the acidification of Welsh streams: temporal changes in the occurrence of macroflora and macroinvertebrates

Models are developed which predict changes in macrofloral and macroinvertebrate assemblages in response to surface water acidification. Empirical relationships between assemblage type and water chemistry are used to predict the probabilities of species occurrences during acidification, as recreated by the hydrochemical model, MAGIC. The water chemistry of two streams is simulated between 1844 and 2124. From 1958, alternative scenarios involved either moorland or conifer forest. From 1984, sulphate deposition was either constant or reduced by 50%. Alternative ecological models driven by pH or total hardness are compared. The floral model showed minor differences between scenarios, probabilities of species occurrence changing gradually and reaching stable values by around 1964. For certain invertebrate species the occurrence probabilities changed rapidly over relatively short periods, for example in the late 20 h century under moorland with constant deposition. Reduced sulphate deposition prevented decrease of acid sensitive species in moorland scenarios but not under forest, which greatly accelerated faunal changes irrespective of deposition pattern. Differences between the pH and hardness models indicated that the effects of these parameters should be separated in future studies. Where an earlier model showed only step changes in invertebrate assemblage type, this new approach can model more precise taxonomic shifts occurring with acidification. Such changes could be important to conservation, or as early indicators of response to pollution. The tentative simulations here suggest that large taxonomic changes may occur over relatively short periods during acidification.     

Connecting species’ geographical distributions to environmental variables: range maps versus observed points of occurrence

Connecting the geographical occurrence of a species with underlying environmental variables is fundamental for many analyses of life history evolution and for modeling species distributions for both basic and practical ends. However, raw distributional information comes principally in two forms: points of occurrence (specific geographical coordinates where a species has been observed), and expert-prepared range maps. Each form has potential short-comings: range maps tend to overestimate the true occurrence of a species, whereas occurrence points (because of their frequent non-random spatial distribution) tend to underestimate it. Whereas previous comparisons of the two forms have focused on how they may differ when estimating species richness, less attention has been paid to the extent to which the two forms actually differ in their representation of a species’ environmental associations. We assess such differences using the globally distributed avian order Galliformes (294 species). For each species we overlaid range maps obtained from IUCN and point-of-occurrence data obtained from GBIF on global maps of four climate variables and elevation. Over all species, the median difference in distribution centroids was 234 km, and median values of all five environmental variables were highly correlated, although there were a few species outliers for each variable. We also acquired species’ elevational distribution mid-points (mid-point between minimum and maximum elevational extent) from the literature; median elevations from point occurrences and ranges were consistently lower (median −420 m) than mid-points. We concluded that in most cases occurrence points were likely to produce better estimates of underlying environmental variables than range maps, although differences were often slight. We also concluded that elevational range mid-points were biased high, and that elevation distributions based on either points or range maps provided better estimates.     

The occurrence of herbs and woody juveniles in a hardwood floodplain forest in relation to flooding and light

Abstract. A study of the distribution of herbs, seedlings and vegetative propagules of woody species in a hardwood flood-plain forest along the Upper Rhine in France revealed that the occurrence of most species is significantly correlated to elevation above river level and light transmission in summer. Species confined to higher-lying sites which are only occasionally and briefly flooded in the growing season show most damage upon flooding. Tall herb species occur on sites where more than 5% daylight reaches the herb layer and they only reach a dense cover where flooding is occasional. The occurrence of woody juveniles is negatively correlated with tall herb cover and largely confined to more shaded sites or more frequently flooded sites. The results indicate that both shading and flooding are important for regeneration of woody species and for maintaining species diversity in hardwood flood-plain forests.