Drift of aquatic macrophyte propagules was investigated in a wetland along the River Rhône, during the first flood after the growing season (i.e. in the winter of 1995-1996). Input and output drift ...were studied at the beginning, around the top, and at the end of the river overflow in the upper reach of a cut-off channel. The soil propagule bank was sampled along the study area before and after the flood. The amount and composition of viable propagule drift and bank were determined, analysed and compared. Drift densities and richness were on average higher at the outlet of the channel than at the inlet (respectively: 23.2 vs 13.1 viable propagules/ 100 m3 of water and 8.7 vs 2.6 taxa per sample). Immigrating taxa were mostly in the form of helophyte seeds, whereas numerous resident hydrophyte species left the disturbed area rather as vegetative propagules. Temporal variability in propagule bank structure was weak, and mean bank densities did not change before and after the flood (respectively: 33 047 ± 10 510 vs 35 653 ± 15 070 viable propagules/m2 of ground, including Chara). However, the density of Elodea canadensis significantly increased after the flood while that of Eleocharis acicularis decreased. This contrast suggests that flood responses vary among species. Despite a broad overlap in the taxa (18 out of 25 were common both to drift and bank collections), no significant relationship occurred in composition or structural changes between flood drift and propagule bank. Flood acted as a means of distribution of existing propagules and also as a provider of new vegetative dispersal units.
Gap models have a rich history of being used to simulate individual tree interactions that impact species diversity and patterns of forest succession. Questions arise, however, as to whether these ...same models can be used to study the response of forest structure and composition under a changing climate.
The abundance of a given species is usually expressed in terms of either the number of individuals per unit area or volume (i.e., density), or biomass. These two abundance metrics generate different ...results at both the statistical analysis level (e.g., comparison of means) and the ecological level (e.g., diversity comparisons). We seek here to unify different abundance metrics using the formula A = N(B/N)
k/3
where A is the abundance of a given species, k represents a fractional dimension, N is the number of individuals per unit of area and B is the biomass of the sampled species. When k = 0, A is density and when k = 3, A is the biomass. A value of k = 1 would give abundance approximately proportional to the sum of the length of individuals and k = 2 would give abundance approximately proportional to the sum of their surface areas. Metrics intermediate between density, length, area and biomass are possible using non-integer values of k. Applying this methodology to ichthyological data characterized by highly variable intraspecies biomass, we examined the effect of the abundance metric on the results of a three-factor analysis of variance (depth, season and site). In some cases, differences which could not be seen with either density or biomass could be seen with intermediate metrics. We suggest that many ecological results could be usefully evaluated in terms of the effect of the fractional dimension of sampling. In some cases, such an approach could identify the optimal metric.
The degree of general applicability across Europe currently achieved with several forest succession models is assessed, data needs and steps for further model development are identified and the role ...physiology based models can play in this process is evaluated. To this end, six forest succession models (DISCFORM, ForClim, FORSKA-M, GUESS, PICUS v1.2, SIERRA) are applied to simulate stand structure and species composition at 5 European pristine forest sites in different climatic regions. The models are initialized with site-specific soil information and driven with climate data from nearby weather stations. Predicted species composition and stand structure are compared to inventory data. Similarity and dissimilarity in the model results under current climatic conditions as well as the predicted responses to six climate change scenarios are discussed. All models produce good results in the prediction of the right tree functional types. In about half the cases, the dominating species are predicted correctly under the current climate. Where deviations occur, they often represent a shift of the species spectrum towards more drought tolerant species. Results for climate change scenarios indicate temperature driven changes in the alpine elevational vegetation belts at humid sites and a high sensitivity of forest composition and biomass of boreal and temperate deciduous forests to changes in precipitation as mediated by summer drought. Restricted generality of the models is found insofar as models originally developed for alpine conditions clearly perform better at alpine sites than at boreal sites, and vice versa. We conclude that both the models and the input data need to be improved before the models can be used for a robust evaluation of forest dynamics under climate change scenarios across Europe. Recommendations for model improvements, further model testing and the use of physiology based succession models are made.
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