Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 79. (Budapest 1987)
Debreczy, Zs.: Fluctuating-dynamic equilibrium of photophil, xerophil rupicolous plant communities and scrub woods at the lower arid woodland limit
land patch changes and rupicolous elements enter in the place of the earlier woodland vegetation, and finally the rupicolous grassland takes over the place of the woodland patch (see also interpretation at Fig. 2). As a result, the proportions of woodland and grass coverage remain constant over a longer period at the average level of the volume of water that can be utilized and evaporated. JAKUCS reported his cyclical succession models with stages marked by a short black line (that is, without soil) and this model has been adopted in Hungarian textbooks (HORTOBÁGYI & SIMON 1981, p. 197-198). However, the model does not answer some important questions: what happens to the rendzina that may have developed over several centuries (HARGITAI, ex verb.) below the scrub forest (Fig. 2, a 2 , d 2 )l What happens to the favourable endowments of the microrelief and water flow that may appear to be by chance but that have been shown to act regularly over the long term, and that from the outset determine the most favourable places in the scrub wood? When a pubescent oak grows old and falls, what happens to the young Fraxinus ornus saplings, to the oak seedlings hidden in the interior of the scrub wood to the thick undergrowth of Ligustrum and Cotinus, to the Crataegus monogyna growing strong in the shade of the scrub wood, to the Euonyrnus verrucosus! The oak can still be renewed more effectively through its acorns that roll there in the protective shade than under the Cotinus coggygria clinging to the surrounding poor, thin soil and creeping out to the skeletal soil with a high drain off and poor water retention (Figs 19-20). The author began his investigation (Debreczy 1965-85, see later) on the same area where JAKUCS (1972, p. 89, 90, 91), carried out his investigations, and at his advice 4) and guidance. After the initial cenological survey and comparative tests, the key question was raised once again : what maintains the equilibrium of woodland and grassy areas on dry south-facing slopes? There can be no doubt that the area was untouched ; vines had been cultivated 200 m distance lower down on Pannonian sand and loess and after the phylloxera plague (1860) they were abandoned and remained practically uninhabited. The area studied is separated from the surrounding area by a steep slope that is difficult to climb. On the basis of the author's study of the local and microclimate, the properties of the soils, including their utilisation of water, and from observations in other directions, the most acceptable working hypothesis seemed to be to demonstrate the "self-defensive" properties of the fluctuating-dynamic succession model for scrub woods and the inhibitional succession model for rupicolous associations. According to this, the total foliage surface and its size (density) of scrub woods is determined by the quantity of precipitation effectively available ( !) in the area, that is, the quantity of water available after deduction of flow-off and higher temperature causing rapid evaporation due to the exposition, and its distribution, is determined by the unequal possibility for utilisation of the water, that is, its division between scrub wood patches that have developed on areas gaining and absorbing water and the edaphic-microclimatic semi-arid patches with a water loss. The fluctuating movement of the woodland edge in the scrub woods at the lower woodland limit with a semi-arid microclimate can be compared to the movement that also determines the size of desert cushion shrubs, which in the final analysis determines the size of the plant (here, the dominant trees) as the average over a period of many years of the expansion in humid years and the contraction in dry years. According to the investigations reported in the author's study on succession and denudation — in contrast with Jakucs' view — the expansion and contraction does not occur in the interior of the scrub wood but at its edge, outside the drip-line. In observing the processes in the area over a period of many years (since 1965), the author also observed mainly expansion at the edge of the scrub woods, even if the combined influence of the drought and other factors (e. g. frost damage) caused a certain amount of shedding and destruction of polycormon. The hypothesised destruction on a large scale did not occur until the dry summer of 1983 and the exceptionally cold winter of 1984/85 followed by another drought in 1985 and 1986. It was then that the author gained the proof needed concerning the fluctuation dynamics of scrub forests that is summed up in the present paper. Sample area studied, methods Area studied — The broad band around Lake Balaton 100 km in length and 40 km width, ranging from Permian red sandstone to Triassic limestone and dolomite strata, from Pannonian sand to Pliocene basalt hills, Pleistocene loess and Holocene sediment in composition and with climates ranging from Praeillyricum to Eupannonicum, has been studied in detail and it proved to be a very suitable sample area. The present investigations were conducted in one of the most thoroughly studied sampie areas of the Balaton Upland, in the same area where Pál Jakucs drew his vegetation map of the fine distribution of scrub woods and rupicolous associations on the basis of aerial photoThe author takes this opportunity to thank Pál Jakucs lor his heuristic choice of theme and for the thorough expounding of his theory that encouraged the author in his investigations; he also expresses his gratitude to Gábor Fekete for his professional consultation.
