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The cryptogamic vegetation of loess cliffs has been studied by the author together with B.O. van Zanten, Dutch bryologist (State University Groningen, Dept. of Plant Ecology, Haren, The Netherlands) for the past three yars. Since 1997 the research has been sponsored by the Hungarian Research Fund (OTKA, Project No. T 022575). The aim of the project is the floristical, phytogeographical and ecological investigation of the less known cryptogamic communities of loess cliffs in the Pannonian Basin. Until now we have visited a great part of Hungary where loess cliffs occur, as at the edges of the Great Hungarian Plain, the Tolna and Baranya Hills and on the lower slopes of the mountain ranges (see fig. 1). Within 50 field days the cryptogamic communities of 151 cliffs were studied, 1760 moss and lichen specimens were collected and 52 phytosociological relevés were made.

Although Diluvial loess covers the greater part of the Pannonian Basin, only on a small proportion of its area are cliffs formed. During the erosion of loess deposits, the formation of vertical cliffs is typical in cases, if the necessary erosion basis is at disposal. Among natural circumstances this happens along big or smaller rivers (Danube, Tisza, Zagyva, Hernád, Bodrog,) and lakes (Balaton), at the escarpment of loess plateaus and on the loess covered foothills of mountain ranges. Even among seminatural conditions huge gorges and cliffs can develop, as the result of secondary erosion of roads (hollow roads). Artificial cuts in loess valleys and plateau edges, like by the construction of highways, quarries, vineyard terraces, entrance of vine cellars, or even behind village houses can result in vertical surfaces, which are colonised within 30-50 years from the neighbouring natural loess habitats with their cryptogamic communities. But not all kind of primary or secondary loess cliffs are equally suitable for the settlement of bryophytes and lichens. The derasion loess (secodrarily redeposited on slopes) and infusion loes (secondarily sedimented in water) have a physical structure, which is no good for the cryptogamic communities. They optimally develop only on the aeolic loess (primarily deposited during the glacial periods from the air) with a grain size of 20-50 mm, which can form cliffs in Hungary up to 60 m height, but never as a uniform layer. It is sandwiched by sand and interglacial soil bends, which again do not favour the settlement of cryptogams. Finally, only the loess of pale yellow colour, preserved among semiarid conditions, with a CaCo₃ content above 8% (sometimes up to 40% in Hungary) and basic reaction, is good for these communities. The CaCo₃ content of aeolic loess studied by SEM appears in the form of special microtubules, neddle cristals and incrustations on the quarz and other mineral particles, as a result of palaebiotic and palaeoclimatic processes and cements the particles together (Pécsi 1993). The typical loess cryptogams are calciphilous and bound to this special structure and adapted to them by their growth habits and life strategies. The so called „brown loess” in western Hungary or in western Europe has been transformed by leaching and weathering and do not bear the above properties. Their CaCo₃ content is usually well below 5% and their reaction is neutral or acidic.

But the significance of loess cliffs, as habitat for living communities, far exceeds their area. On the typical loess cliffs in the Pannonian Basin (see fig. 2) from the steppe forest mosaic through arid bushes and semidesert dwarf scrub to cryptogamic desert all kind of communities occur according to the microrelief conditions. The same is true for the animal (bird, wasp, spider, etc.) communities. The higher plant communities of loess cliffs were well studied in Hungary and their rôle in the plant succession is summarised by Zólyomi and Fekete (1994). At the same time, the micro-communities of vertical cliffs are very little known. Karczmarz (1960) described bryophyte communities of the loess cliffs in Poland and Gallé (1964, 1974, 1975) described lichen communities from Hungary and from northern Yugoslavia. These communities are easily overlooked even by an average passer by botanist, as they appear from a distance on the dry yellowish cliff as dark dirty greyish patches. During the investigation of our cliff communities we have found relationship with the cryptogamic communities of flat surfaces of Judean and Middle East deserts (Frey, Herrnstadt and Kürschner 1990, 1991), the semideserts of southeastern Spain (Ros & Guerra, 1987), the very dry habitats in Tibet and in Inner Mongolia (Tan & Zhao 1997) and of British Columbia (McIntosh 1989, 1997). This relationship is manifested by the predominant growth habit (solitary plants in herds) and life strategy („innovative settler” type of Frey & Kürschner 1991a, 1991b) and by the floristic composition. During our three years studies we have found 9 moss species new to Hungary (see also van Zanten 1999a, 1999b). 5 of them have special importance in this respect. We collected on almost all natural dry loess cliffs Pterygoneurum compactum, a species recently described from the arid parts of SE Spain by Cano, Guerra & Ros (1994). We also discovered at a few localities, mostly in southern Hungary an other rare species, Pterygoneurum crossidioides, described from the Dead Sea deserts by Frey, Herrnsadt & Kürschner (1990), which is, according to them, especially common on the aeolian loess deposits north of Qumran (see fig. 6). Pterigoneurum squamosum was also found, which became known from SE Spain (Segarra & Kürschner 1998) and seems to be widespread on the loess cliffs in southern Hungary, slowly replaced northwards by the common Pterygoneurum ovatum (see fig. 7). By these new findings the known number of Hungarian Pterygoneurum species is doubled (see Hungarian key on page xx). In addition, Hilpertia velenovskyi (Schiffn.) Zander and Crossidium crassinerve (De Not.) Jur., both considered as very rare protected plants in Hungary, were discovered at numerous new localities on loess cliffs (see the figs 8-10). A future task will be to revise the poorly known Hungarian representatives of Tortula Sect. Crassicostatae, which are typical for the arid habitats and seem to be widespread also on our loess cliffs.

How can we interpret such a high rate of extrazonality, the occurrence of desert plant communities in the forest steppe zone of the Carpathian Basin? The explanation lies in the special orographic conditions of loess cliffs. It is easy to understand, how far modifies the vertical or near vertical surface the amount of precipitation, which is proportional to the cosinus of the degree of inclination (fig. 3b). This amounts in the case of 70, 75, 80, 85 and 90^o inclination only 0.342, 0.259, 0.174 and 0.087x the total precipitaton, that is 193, 146, 98 and 49 mm/year accordingly and none on the vertical surface, with at least 7 dry months per year, compared to an average Hungarian lowland station, Szeged, with its 565 mm/year precipitation and 2-3 semiarid months. Even if we take in account, that the irradiation and air moisture conditions are different, these

are real desert conditions! If we compare the Gaussen-Walter climatic diagrams of some average stations in Hungary (see the upper row of fig. 5 based on Borhidi in Walter & Lieth, 1960-1966), with the conditions on the near vertical cliffs of the same stations calculated this way by us (second row), and compare with the diagrams of desert and semidesert stations (lower two rows based on Walter & Lieth l.c.), the conditions on cliffs are strikingly similar to the latter two. The irradiation conditions on near vertical and vertical surfaces (see figs 3a and 4, calculated by our colleagues G. Patkó & T. Stumpfhauser) just increase this situation in early spring and in late autumn. Cryptogams, as it was pointed already by Boros (1964) are satisfied by much smaller microclimatic spaces to fulfil their special demands, than the vascular plants, therefore their occurrence out of their continuous vegetational belt is more common than that of the higher plants.

In the future we continue the systematic survey of Hungarian loess cliffs, the collection of distributional data on their bryophytes and lichens, the study of their communities, growth forms, life strategies and the taxonomic revision of critical plant groups. We wish to extend our investigations, in collaboration with the concerned fellow bryologists in the neighbouring countries, where loess cliffs occur, like in Yugoslavia, Romania, Bulgaria, Ukraine, Slovakia and Poland.