EFFECT OF ENVIRONMENTAL FACTORS ON THE FLORISTIC VARIABILITY OF FLOODED FORESTS AND DIFFERENTIAL ADAPTABILITY OF SPECIES ON VARIATION OF ENVIRONMENTAL GRADIENTS (RAVNI SREM, SERBIA)

摘要

The multiple regression CCA model shows how the pronounced gradients of three hydrological factors (length, frequency and level of flooding above the surface of forest soil), in synergetic activity with five other environmental factors (ECe, pH, distance from river bed, altitude and canopy of stand) influence the pattern of species variability and differentiation of flood forests. According to the inter-set correlation, the environmental factors representing the gradients of distance from river bed, altitude and alternation of length of flooding and surface waterlogging of forest soils were shown to be the most important for structure of floristic variability. The environmental variables used in this ordination explain 72.18% of total variability of species appearance. The cumulative percentage of variance for species explained by influence of first four canonical axes is 45.5%. The percentage of variance of species-environmental factors relationship, as a fraction of total explainable dispersion (inertia) of species score is one third for the first two axes, while for the first four axes 63.1% of variation may be explained with these eight environmental factors. According to SIMPER procedure, first 50% of cumulative contribution to floristic differentiation among the three hydrologically different forest types is composed of 19 (out of the total of 111) taxa, listed according to the importance of their individual contribution: Carpinus betu-lus, invazivna wrsta Amorpha fruticosa, Fraxinus angustifo-lia, Salix cinerea, Quercus robur, Myosotis palustris, Polygonum hydropiper, Populus alba, Mentha arvensis, Carex vulpina, Galium palustre, Rumex sanguineus, Ly-copus europaeus, Frangula alnus, Oenanthe fistulosa, Salix fragilis, Euphorbia helioscopia, Hordeum murinum and Veronica chamaedrys. The Gaussian response model to species abundance (Gradient species packing), applied to dominant and certain adventive ligneous species, suggests a strong adaptability pattern of species toward the environmental gradients. Regarding the length of flooding, the reached ecological optimums are clearly defined. The lowest values of optimums at the given gradient among the species included in the model were shown by field maple and common hornbeam, while the greatest values for this hydrological variable were shown by grey willow and black alder, as expected. The most tolerant species were pedunculate oak, field ash and the invasive shrub species false indigo bush as the most euryvalent species from the aspect of length of flooding, while the Euro-American poplar and black alder were the most stenovalent. Very similar relationships among the species were also present regarding the gradient of reached levels of surface water, clearly indicating a certain co-linearity between these two factors, as supported by the multiple regression CCA model.