The study objective was to find out whether extensively developed charophyte meadow (built by Chora tomentosa) induce the spatial heterogeneity of basie physical-chemical water properties of a large, clearwater and tachymictic Lake and, so, with a long period of water overturn (Lake Lednica, Wielkopolska Region, mid-Western Poland) . Current trophic state might be assumed as moderately eutrophic with impressive water clariry. Submersed vegetation is well developed and predominated by charophytes forming dense beds. In the period of intense development of vegetation (between 17th May and 6th July 2004) physical-chemical analyses (water temperature, pH, oxygen concentration, conductMty, total dissolved substances) were performed five times at five sampling sites. Three of them were macrophyte-free open water regions (with varied contribution of charophytes in the phytolit-toral zone) and two further were shallow littoral sites: one close to a bathing place with sparse Tolypella glomerata specimens and the other in dense Chora tomentosa bed. The results showed lower conductivity and TDS values, and higher pH and the concentration of oxygen inside charophyte meadow as compared to the rest of sampling sites. By contrast, at the other littoral site but without plant cover conductivity and TDS revealed the highest values. Results are discussed in the light of habitat forming power of charophytes contrasted with long water overturned the Lake is subject to.
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The impact of grazing has been discussed in many organisms, and it has been shown that it has considerable influence on the structural variation of vegetation, resulting in its tussocky appearance. Such spatial heterogeneity results in plants formations that facilitate other species by providing safe sites against predation and physical stress. On the Trnovski gozd plateau in western Slovenia, a secondary habitat of the rare, monotypic endemic umbelliferous species, Hladnikia pastinacifolia, was colonized by a relatively isolated population of the wide spread opilionid species, Phalangium opilio. We hypothesized that in this stony pasture, the impact of the structurally heterogenous vegetation that results from low-intensity grazing benefits both species on hot summer days. For this purpose, we classified vegetation formation types (VFT) and measured temperature and relative humidity in places settled by Ph. opilio individuals during their daily rest. According to the predominant species, we recognized six VFTs: Carlina acaulis, Ruta divaricata, Koeleria pyramidata, Juniperus communis, understory vegetation and grazed turf. Only the spiny C. acaulis and unpalatable R. divaricata facilitated H. pastinacifolia, while also acting as nurse plants. On the other hand, Ph. opilio preferred the understory, but also settled on the other VFts, except Ruta. During the highest daily temperature of about 38[degrees]C, Ph. opilio avoided the grazed turf. Temperature preferences refer to frequencies of individuals rather than to absolute temperature values. The frequencies were independent of sex and the VFTs. Females were more frequently found in tussocks showing higher temperature profiles (Juniperus, Koeleria). During the hot daytime, Ph. opilio was most sensitive to relative humidity, and less so to temperature, whilst the height of settlement varied in adjusting these two environmental factors. The distribution of males significantly differed between the subsequent morning, midday and evening series, but not between the morning and evening ones, while in females it differed between all the three time series. In habitats, such as stony dry pastures, low grazing intensity can maintain persistent populations of H. pastinacifolia and Ph. opilio even in extremely hot weather. Such grazing is the most convenient measure for protecting both species, especially the highly endangered Hladnikia. The major threat to such habitats is afforestation by Pinus nigra, which deserves additional management. We found that in stony pastures, low grazing intensity assures the structural heterogeneity of vegetation required to maintain persistent populations, of both H. pastinacifolia and Ph. opilio.
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