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Vegetational conditions in the Danube area
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Vegetational
Conditions in the Danube Area
Vegetationsverhaltnisse
im Donauraum
Les types de vegetation dans la region danubienne
PBI JAKUCS, Debrecen*
Abstract: The vegetation of the Danube Basin is extremely diverse. Sub-Atlantic influences are experienced in the western parts, sub-continental influences in the southeastern parts, whereas Balkan influences dominate in the south. Uplands, damp depressions, sandy soils and saline soils promote the development of special ecosystems. As a result of anthropo zoogenous agencies, the original vegetation has undergone substantial alteration, and has in places been destroyed. The individual elements of the vegetation pattern merge to form an interlocking whole. Zusammenfassung: Die Vegetation des Donaueinzugsgebiets ist hochst abwechslungsreich. Die westlichen Teile liegen im subatlantischen, die stidostlichen im subkontinentalen, die siidlichen dagegen im balkanischen Einflul3bereich. Hochgebirge, feuchte Niederungen, Sand- und Salzboden begiinstigen die Entfaltung spezieller 6kosysteme. Die urspriinghche Vegetation wurde durch anthropozoogene Einwirkungen stark veriindert, an vielen Stellen vernichtet. Die Einzelelemente des Vegetationsbildes gehen kontinuierlich ineinander iiber. R&urn& La vegetation du territoire du Danube et de son bassin versant est t&s variee. Elle subit au Nord l’influence subatlantique, au Sud-Est l’influence subcontinentale, au Sud l’influence balkanique. Les hautes montagnes, les regions humides, de sables et de sols salins favorisent le developpement des ecosystdmes spdciaux. La vegetation primaire a 6th considerablement supprimee par les influences anthropozoogdnes ou completement andantie a maints endroits. Les eldments du mosaique v&g&l passent de l’un a l’autre d’une manihe continue.
It was in 1863 that A. KERNER’s work Das ffflanzenleben der Dorm.dind~ was published. The book, based on accurate observations and giving descriptions of the vegetation of the Hungarian Great Plain, the Carpathians, the Czecho-Moravian Mountains and the Alps, is regarded as a classic in its field. For the above mentioned territories, KERNER’s work is a basic reference book even today. Not more than a centruy ago, account was given of wide range, splendid woodlands, immense flowering grasses, flood-plain groves, swamps, etc.; today, however, one can but find the remainders of all this. Thus, when a survey is given of the natural vegetation of the drainage area of the great Danube River of South East Central Europe, brief reference should be made first to the changed vegetational conditions due to great changes in the landscape, The vegetation of the region was classified by KERNER into formations primarily according to physiognomical appearance. After 1900, on the basis of the laws of association, researchers focused attention on the * Prof. Dr. Pal JAKUCS, Department of Botany, Kossuth University, Debrecen 10, Hungary
investigation of coenoses, and today studies of ecosystems are predominant. The ecosystems are inner, energetically associated subunits of the biosphere, which have been developed in any part of the earth in complex correlation of the potent actual environmental factors (relief, climate, soil, etc.). The green-plant components of the varied ecosystems, by their oxygen production as well as their primary phytomass production due to assimilation, are the base and assurance of any higher animal and human life. In a given natural region, the ecosystems develop to attain the highest-order bioproduction, corresponding to environmental conditions. Their qualitative and quantitative marks thus indicate the potential productivity value of the region. With the development of human civilization, however, transformation of natural regions assumed an accelerating rate which, in the majority of cases, involved the substitution of natural ecosystems, according to demand, by artificial ones (agriculture), or even their complete elimination (urbanization, industry, transport, etc.). The continually accelerating rate of regional transformation is inflicting on the ever increasing natural eco-
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systems harmful side effects which, in the last analysis, may bring about irreversible damages to the ever growing masses of mankind. For this reason, the great task of the future is to eliminate, through multilateral and through study of surviving naturaI e~systems and through utilization of the information thus obtained, the damages already caused, and to secure a sound basis for the work of developing the highest attainable bioproduction of a region. The most si~ic~t part of this task is the territorial reconstruction of the original natural vegetation. This is why the subsequent survey of the vegetation of the Danube area is solely concerned with the potential vegetation. 1. General Remarks on Vegetation in the Danube Area The 817 thousand square km watershed area joined up by the 2 860 km long Danube comprises the territories of essentially seven countries. The river itself, while flowing from W to SE, strings up three major basins: the Upper or Bavarian, the Middle or Carpathian, and the Lower Danubian basin. Passing on from one into the other, it breaks through, at picturesque gorges, a number
of ranges of the Alps, C~pa~~s, and Balkans. To its watershed area belong the East Alps, Carpathians, Dinaric Alps, Serbian Central Mountains, and the northern part of the Balkan Mountains. From its spring to its delta, it passes through regions of the most different structure, relief, and climate. For this reason, the Danube is referred to as Europe’s most varied, most heterogeneous, and most complex river. The vast variety of en~onmental factors is reflected in the vegetation as well. From alpine grasslands through rustling pinewoods to the unwooded steppe, the mosaics of zonal, intra- and extrazonal patches of vegetation form the landscape into a multicolored carpet. In general, the whole area of the Danubian system of basins is climatically favorable for the evolution of forests, even the lowland parts of the Middle and Lower basins are cuftivated steppes, brought about by the activity of man. At these places, however, the distribution of precipitation may very often reach the limit critical for trees. Thus, in view of vegetation, these areas join the forest steppe belt. Spacious, ancient, unwooded steppes, on the other hand, do not occur in the Danube area, apart from the eastern parts of the Rumanian plain, which is proved by the fact that even the barest heaths can be afforested. Nevertheless. Fig. 1
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P~ticipation of Pontic (a) and Central European (b) floristic elements in the xerotherm scrub forests of the Danube area Anteii der pontischen (a) und mitteleuropaischen (b) Florenelemente in den xerothermen Buschwaldern des Donauraums
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Participation des elements de flore pontiens (a) et centreurop~ens (b) dans les taillis xirothermes du Bassin Danubien
1. Swabian Alb (RiiHL 1954); 2-3. Frank Alb (GAUCKLER 1938); 4. Bohemian Basin; 5. Viennese Basin; 6. Moravian Karst; 7. Pannonian Hills; 8. Western Slovakia; 9. South Hungary: Mecsek Mountains; 10-I 1. Hungarian Northern Mountains; 12. Kazane-Defile, Lower Danube; 13. West Bulgaria: Golo-Bodo Mountains; 14. East Bulgaria: Balkan Mountains; 15. Dobrogea. (4-15 see JAKUCS 1961, Fig. 61)
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even within the woodland zone, at larger or smaller patches there are ecologic factors hindering the growth of trees and the evolution of forests. Such factors are special soil or base rock, especially their aridity, extremities of geomorphology, great heights above sea level, etc. Vegetation at such place is either completely specific (edaphic, intrazonal vegetation), or is in correlation with that of other vegetational zones (extrazonal vegetation). The Danube area is one of the transitional climatic territories of Europe, where climate is characterized by the intermingling of oceanic, continental, and Mediterranean influences. Continental influences display a gradually increasing tendency toward the east, and especially in the basins. At the same time in the SE, there appears a strong sub-Mediterranean influence. As to vegetation, this influence results in the fact that while in the western part of the Danube area deciduous forests are dominated by the beech tree, which prefers a more balanced sub-Atlantic climate, toward the east the increase in the extremities of temperature (hard, cold winters and dry summer heat spells lasting sometimes for weeks) drives the beech back to the microclimatically cool damp, or higher mountainous regions, and among the leafy trees the oak, maple, and lime gain predominance. On passing from W to E, the parallel weakening of central European character and strengthening of continental influence in the vegetational aspect can be demonstrated by areal-type analyses performed in scrub forests. Scrub forests are developed as transition between open grass-
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lands and closed tree forests, and can be regarded as extrazonal representatives of forest steppe vegetation. Characteristic of them is a mosaic like appearance together with edaphic xerotherm grasses. They have evolved in the whole Danube area generally on the southern slopes of lower mountains. Dominant in their foliage around the upper reaches of the Danube is the pedunculate oak (Quercus robur), the piliferous oak (Quercus pubescens), and the eastern hornbeam (Carpinus orientalis). Their rich shrub and grass level is characterized by the multitudinous appearance of Eurasian and sub-Mediterranean plants. In Fig. 1, the percentile distribution of central European and Pontic floristic elements of scrub forest cenoses from 14 different regions of the Danube area are presented to support the statement that in the aspect of vegetation, the shift toward the continental is a phenomenon parallel to the regression of central European character. It should be noted that the tendency of areal-type changes from central European (sub-Atlantic) toward Pontic (subcontinental), calculated for scrub forests, appears quite similarly within each vegetational unit in the Danube area with a stronger or weaker character. Another general feature in the vegetation of the areas SE of the Pannonian basin is, in addition to the strengthening of eastern traits, the more and more multitudinous occurrence of the floristic elements of the Balkans (Fig. 2). The vertical evolution of high-mountain vegetation is also very interesting to demonstrate climatic differences
Fig. 2 l
Ideal vegetational profiles from various regions of the Danube area, supposing identical altitudes above sea level, and identical base rocks (JAKUCS 1971)
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Idealisierte Vegetationsprofile von verschiedenen Gebieten des Donauraums unter Voraussetzung einer gleichen Meereshohe und eines gleichen Gesteins (JAKUCS 1971)
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Profils de vegetation id&l&s de diverses regions du bassin danubien supposant la m6me altitude et la m8me roche originaire (JAKUCS 1971)
1. Atlantic; 2. Subatlantic - Central European; 3. Central European; 4. Submediterranean - Subcontinental; 5. Subcontinental - Continental. F = Fagetalia; C = Carpino-Fagetea; Q = Quercetea pubescenti-petraeae; FB = Festuco-Brometea
S-N
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between forest steppe, occupying an intermediate position between temperate-belt deciduous forests and the steppe, and the extrazonal high-mountain pinewoods of the zone of coniferous forests. Within the area of macrozonal beech forests, oak-woods, forest steppes, and extrazonal coniferous forests, one fourth part is occupied by edaphic or intrazonal vegetational units: aquatic and subsoil water influenced vegetation, sand, alkali, rock, alpine, etc., vegetations. 1. Croatian Karst (Risnjak); 2. Dinaric Alps (Bjelasnica); 3. Balkan mountains (Rila); 4. Southern Carpathians (Pareng); 5. Northern Carpathians (High Tatra); a = subnival region; c = Swiss pine forests; ch = alpine grasses; d = forests (after I. HORVAT and T. P6CS - in P&S 1968) Fig. 3
Altitude belts of several high-mountain regions of the Danube area Hohenstufen der Vegetation in einigen Hochgebirgen des Donauraums Zones de vegetation dans certaines regions de hautes montagnes du Bassin Danubien
between the north-western and southeastern parts of the Danube area. The beech forests at the upper reaches of the Danube are predominant forests in the zone at 400 to 900 m above sea level. In the Carpathian basin, their presence is characteristic at altitudes between 600 to 1200 m, on the slopes of the Southern Carpathians and the Balkan Mountains the ecological range of their conditions of life can be found between 1000 to 1 700 m (Fig. 3). Before giving a short survey of the most important units of vegetation in the Danube area, it should be mentioned that vegetation, in contradiction to the more homogeneous western regions, is more variegated, multicolored, richer in species in the southern and southeastern regions. This is not only due to the greater variety of environmental factors, but is in correlation with the history of vegetation. The flora, driven back from Central Europe by the Pleistocene glaciation, has weathered out the extreme fluctuations in climate among the Balkan Mountains, as in a refuge area. This is why in the more central regions of the Danube area woodland ecosystems are 30-50 percent poorer in species than their equivalents in the Balkans.
2.1.1. Beech forests The most important forest-forming deciduous tree of the climatically better-balanced hilly and mountainous territories of the Danube area is the European beech (Fagus silvatica), distributed in Central Europe. In soils not too damp, not too easily drying, and relatively rich in nutrients, where winter colds are not too strong and the annual mean temperature is between 4-12 “C, where extremities are comparatively not wide, and where precipitation is relatively evenly distributed over the year and not exceeding 2 000 mm, beech forests may appear. The foregoing ecological conditions are dominant and very characteristic of the territories of the Danube area from the river’s source to the Pannonian basin; in the Black Forest, in the Swabian and Frank Albs, in the Bavarian woods, and in the part of the Bavarian basin ranging up to the East Alps between altitudes of 400 to 900 m above sea level. Thus, the most extensive, continuous zonal beech forests are to be found here. Within the ring of the Carpathians and among the Dinaric Alps, they have been repelled to higher regions. In various areas in the southern part of the Danube valley, they even reach the height of 1 800 m, forming at places the absolute timberline. In these northern territories of the Balkans, Fagus sylvatica is frequently replaced by the species Fagus moesiaca, distributed in the Balkans.
2.1. Zonal Vegetation
Beech forests are generally of high growth (25-35 m), their canopy is very often mixed with scattered specimens of ash (Fraxinus excelsior), mountain maple (Acer pseudop&anus), and in the mixed zone by spruce and fii. The foliage is always highly dense, so the floor is shady. Usually only 50 to 80 percent of the sunlight can penetrate the forest. Thus the undergrowth of beech forests consists mainly of shade tolerant and umbrageous species; the shrub layer is scantily developed. Very characteristic, however, are the vernal geophytes, which store the nutrients necessary for flowering in their underground parts, bring flowers during the more favorable light conditions before foliation, and with the ripening of their crops after foliation complete their annual cycle of life (Fig. 4).
Nearly half the territory of the Danube area is occupied by oak and beech forests belonging to the zone of temperate-belt deciduous forests. The rest is divided by halves
A number of associations, subassociations, variants, and other subunits of beech forests are known in the Danubian basin. (For a comprehensive survey and bibliography see
2. Most Important
Types of Vegetation in the
Danube Area
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Primarily for these two reasons, the original sites of beech forests have remained covered, at a high percentage, up to our times. In the westernmost parts of the Danube area, especially on acidic soils, beech forests have been transformed by forestry into monocultures of fir and spruce. 2.1.2. oakwoods
Oakwoods of pure stand are forestry areas of lowlands and of the generally more arid soils of hilly regions. They are much more varied in comparison with beech forests; the canopy is constituted by several species of oak. In the Danube area, the most frequent are the pure and mixed cultures of the sessile oak (Quercus petrae), the pedunculate oak (Quercus robur), the Austrian oak (Quercus cemk), the piliferous oak (Quercus pubescens), and the Hungarian oak (Quercus farnetto). Besides oaks, there is a frequent occurrence of other broad leafed trees (birch, field maple, lime, elm, etc.). In the intermingling zone with beech, transition is very often indicated by oakwoods mixed with hornbeam and forming an independent belt (Fig. 5). Fig.4 Submountainous mountains
beech-tree forests in the North Hungarian
Submontaner Buchen-Hochwald Futaie de h&raie submontagneuse Hongrie septentrionale
hn nordungarischen
Gebirgsland
darts les montagnes de la
SO6,1963.) In general, two great groups can be discriminated depending on whether the soil is calcareous or poor in calcium. Beech forests on soils of high basicity (generally developed on limestone base rocks) are more typical, richer in species, and with numerous species characteristic of beech forests only. On the other hand, on eluviated soils of low basicity (formed generally on silica rocks, e.g., granite, gneiss, sandstone, clay, and slate), beech forests are poorer in species and their acidity indicating dominant species are common with those of oakwoods of similar soil structure. Characteristic of these is the evolution of rich moss strata. Within the two main groups, the basis for further dismembering may arise partly from the geographical position (associations under the influence of regional units, e.g., Illyric, Dacian, Carpathian, Alpine), partly from finer ecological differences of more local value (mountainous, submountainous position, rendzina, brown forest soil, podzolic soils, etc.). The habitat of beech forests is generally not advantageous for agricultural cultivation. Beech is favorably regenerating without any help even in case of intensive use of timber.
Fig. 5 Oakwood, the climatic-zonal association in the central parts of the Danube area at the altttude of 300-400 m Eichenwald, die klimabedingte Assoziation im zentralen Donauraum in 300-400 m HGhe Foret de chgnes, association v&&ale des regions centrales danubiennes sit&e entre 300-400 m d’altitude
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Unlike beech forests, oakwoods are more tolerant to extremities of temperature. They can survive, without any major damage, both spells of dry summer heat and hard winter colds. Consequently, their main territories of distribution in the Danube area are the Carpathian basin, the lower, Rumanian and Bulgarian sections of the Danube valley, and the northern middle-height mountainous and hilly territories of the Dinaric Alps. Since the woody regions of the forest steppe are generally represented by oakwoods, their dimensions are further extended to these areas. Their habitats on hilly and lowland regions, however, are optimal areas for a number of agricultural plants, too, thus, in the present landscape, they have been greatly diminished in their stand. If, however, we make a comparison within the Danube area of the potential sites for oakwoods with those of beech forests, the ratio is in favor of oakwoods. The canopy of closed oak forests under natural conditions is 12-15 m high, in mixed stand the other species of trees are usually evolved in the second, lower gallery. Even inside oakwoods with completely closed canopy, conditions of temperature and light are very favorable as compared to beech forests. This is why the shrub layer is usually highly developed (species of Comus, Euonymus, Ligustrum, Crataegus, Prunus, etc.). The softstemmed vegetation is also varied and, particularly in the spring and early summer months, there is an abundance of flowers.
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the forest steppe belt. The Viennese Basin, the Hungarian Little and Great Plains, and the Transsylvanian basin are the westernmost part of the forest steppe belt that is continuously stretching through the Rumanian Plain east of Bucharest, in an eastern direction from the Danube area, through Voronezh and Saratov, to somewhat beyond the Urals. The forest steppe belt is characterized by the mosaical variation of patches of woodland and steppe, or open parklike forests, particularly oakwoods rich in steppe elements. Annual precipitation is round 500 mm, the increasing extremities of temperature (hard, cold winter-hot dry summer) set up for the trees zonal limits. Continuous, closed woodland areas rich in hydrophilous species are evolved rather edaphically along the great rivers. The macroclimatic forest steppe appears extrazonally, lococlimatically on the souther slopes of the lower mountains surrounding the plains, bringing about the so-called slopeforest steppes in the aspect of vegetation. The mosaical patches of forest and steppe in the forest steppe belt are in a constant dynamic change, in which a very important role is played by the fringes between wooded and woodless ecosystems, since the fringes do not only protect and enclose the forest ecosystems, but at the same time ecologically prepare their extension. In forest steppes, the essence of this process consists in the following (see JAKUCS, 1971):
2.1.3. Forest steppes, steppes
In case of zonal and extrazonal (slope) forest steppes, very similar in their structural patterns but different in their dimensions, the absolute boundary of forest ecosystems is brought about by edaphic aridity effects. The theoretically uniform, continuous physiological boundary belt of trees was, owing to constantly working anthropozoogenic and other natural mechanical effects, in most cases very probably rather early disrupted, or completely failed to evolve. The patches of woodland, broken up into mosaics, or capable only of mosaical development are, thus, in contact with disadvantageous conditions at longer peripheral surfaces, which in this way can exert increased inhibitory effect on afforestation. On the other hand, at fringe areas of extended length there begins an increased development of ecotones, which can thus exert their protective and preparatory effect to a greater extent. The struggle, both spatially and temporally, leads to the adaptation of the fringe belt to the physiological-theoretical boundary, i.e., a closing-straightening regeneration of the fringe. Since the artificial and natural mechanical influences, or in secular dimensions, the continuous change of inhibitory physiological factors mostly subsist and work, the physiological aridity boundary of the forest steppe can only be judged and evaluated in practice by its dynamically fluctuating mosaicity.
The regions in the SE parts of the Danube area, lower than 300-400 m, fall into the mixed transitional vegetational belts between deciduous forests and the steppe, i.e., into
Forests in the forest steppe belt of the Danube area are mostly oakwoods with pedunculate oak as the dominant species, but mixed with Ulmus campestnh, Acer tataricum,
The fundamental discrimination according to the basicity of soil is valid for oakwoods, too. Oakwoods with eluviated, acidic soils are more frequent in the more balanced subAtlantic climate of the western basins. The mass species in them (Luzula, Deschampsia, t/‘accinium, etc.) are common with beech forests of acidic soil. Dominant in the canopy is Quercus robur, with frequent occurrence of Qu. petraea, Betuh per&la, and Populus tremula. Intense mossiness of the surface and large-scale appearance of fungi are also characteristic. As opposed to oakwoods of acidic soil, the majority of oakwoods in the Danube area belong to the so-called xerotherm oakwoods of relatively high basicity ratio (Fig. 6). These forests can be further dissected on the basis of whether dominance is achieved in them by plant species favoring sub-hlediterranean, W or E Balkanic, subcontinental, or central European climatic influences. This differentiation is of regional nature; however, further cenological differences of territorial value are brought about within them by the varied local ecological conditions (e.g., separation of closed tree forests and scrub forests).
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Fig. 6 Occurrence of Quetcetea pubescenti-pelrueue Verbreitung xerothermer Eichenwtider
forests in Europe (JAKUCS 197 1)
in Europa (JAKUCS 1971)
Distribution des forits de Querceleu pubescenti-petrueue en Europe (JAKUCS 1971)
Acer campestre, etc. The shrub layer is well developed, comprising shrubs of eastern distribution (Amygdalus nanu, Prunus fruticosa, Spiraea media, etc.). In the grass level, in addition to species characteristic of xerotherm oakwoods, there are always present common elements with steppes. The soil is generally chernozemic with transitions thereof of brown forest soils. The unwooded steppes are rich in species of grasses, together with geophytes, papilionaceae, and labiates. In summer these areas become completely dry, but they have a characteristic feature, i.e., a period of weaker second flowering in autumn. The cenoses of the forest steppe belt have been evolved most typically on more bound loessy soils. As these have been for long the most
important corn growing regions, owing to their favorable properties, very few remains of the original vegetation have been preserved up to our time. 2.1.4. Pinewoods
In the sub-Alpine parts of the mountainous regions of the Danube area, et a height above the deciduous beech forest belt, a considerable area is occupied, up to the timberline, by coniferous forests. They are essentially representatives of the sub-Arctic evergreen forest belt, stretching northward from the Danube area. The evergreen coniferous trees, well resistant to frost, can endure long spells of cool and cold weather, require much precipitation, but avoid climatic extremities. Their chief habitats in the Danubian
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basins are the East Alps, the lower mountainous regions north of the Alps, the Carpathians, Dinaric Alps, and the high mountains of the Balkans. The most typical stands have been evolved between 600 to 1500 m in the Alps, 1 100 to 1600 m in the Northern Carpathians, 1200 to 1800 m in the Southern Carpathians, and 1400 to 2 300 m in the Balkans. The main forest forming trees are the fir (Abies alba) and the Norway spruce (Picea abies). Mixed with these in the highest zones are various species of pines, the yellow pine (Pinus cembra) and the larch (Lank decidua). In the Balkans, they may be replaced by other species of pines, e. g., pinus peuce, Pinus heldreichii, Plea omorica, etc. (Fig. 7). Coniferous forests are generally homogeneous; the separate layers are represented by one or two predominant species. The grass layer is poor with no seasonal aspect. The damp climate inside the stand of closed forests is very often favorable for a rich mossy level. The soils eluviated by much precipitation facilitate the growth of fungi in the warm summer months. The coniferous forests of the Danubian basins are distinguished from one another primarily by their domin-
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ant trees (spruce, fir, pine, larch), but since each of these species has its own ecological requirements, the habitational-ecological relations of the dissociating units are also different. Regional differences are brought about (e.g., in the South and North Carpathians and Dinaric Alps) by the occurrence of several different species of herbaceous plants. As to their function, more than 90 percent of coniferous forests are lumbering areas. The economical value of pines has greatly contributed to the artificial increase of their habitats (mainly at the expense of adjoining beech forests). 2.2. Edaphic or Intrazonal
Ecosystems
ln the Danube valley within the area of the beech and oak belts, primarily dependent on the macroclimate, and within the forest steppe belt, about one quarter is occupied by such edaphic or intrazonal vegetational units, where the subsistence of the ecosystem is determined by one or more ecological factors of special local importance. Owing to the great number of influencing factors (relief, base rock, altitudinal conditions, water supply, radiational conditions, etc.) and their great number of combinations, there is a possibility for a very highly varied evolution of vegetation. In the aspect of the landscape, these factors bring about a multicolored carpet of vegetation, whose patches, being local in nature, are specific for smaller areas or portions of the landscape in the Danubian basins, too. This variety and the great numbers prevent us from dealing with them in merit within the scope of the present discussion. Thus, in the following a brief survey will be given of their most important groups. 2.2.1. Aquatic and water influenced
Fig. 7 l
Distribution of fir (Abies alba) in Europe and the high-mountain region of the Danube area (after RUBNER)
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Verbreitung der WeiDtanne (Abies alba) in Europe und in den Hochgebirgen des Donauraums (nach RUBNER)
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Distribution du sapin (Abies alba) en Europe et dans les hautes montagnes du bassin danubien (d’aprks RUBNER)
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vegetation
The vegetation of the standing waters and rivers of the Danube area has developed according to the depth and the streaming properties of the water. Depths larger than 2 m only facilitate the evolution of hair-weed vegetation, which can be rooted, floating, upheld on the surface or immersed. The plants usually utilize for their metabolism the oxygen and carbon dioxide dissolved in water. On the other hand, water depths shallower than 2 m give rise to the evolution of swamp vegetation, where plants can root under the water, but the assimilating surfaces rise above the water level. Most frequent representatives of the latter are reeds and sedges. Vegetation in nondirectly open water areas (e.g. watersides), but where subsoil water level is high, displays two main types. If destructed parts of plants can very slowly oxidize in the waterlogged soil, the developing turf is agglomerated under the consecutive plant generations. This process is advantageous for marshlands, marsh meadows rich in mosses, and for marsh forests with the dominance of alder (Alnus). If, on the other hand, the
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2.2.3. Rockland
oxidation process of the soil is secured by continuous or frequent supply of fresh ground water, then swamp meadows poor in moss, and gallery forests rich in willow, aspen, ash, and ehn become dominant. It should be further noted that in the various altitude belts of mountainous regions, cenoses developing in dependence of water (springs, mountain creeks, etc.) extend usually to smaller areas, but are more sharply discernible from the vegetation of the surroundings.
vegetation
The character of vegetation on the emerging coarse base rock of mountainous areas is primarily determined by the thickness of the soil evolved, and the rate of desiccation. These properties, on the other hand, are dependent on the features of the rock, relief conditions, and microclimatic relations. Depending on at what time and by which one or ones of these ecological factors predominance is gained, the usually more stenecious cenoses of the extraordinarily varied units of rockland vegetation appear. In the Danube area, the plant species of rock vegetation are also geographically well isolated from one another. The Dacian elements in the Balkan Mountains
In the Danube area, aquatic, marsh and swamp vegetation had been widespread mainly along the big rivers, in their flood plains, and around lowland lakes and depressions without outlet until the great river-control works began intensively in the middle of the last century. Their area has decreased to one fifth of the original, owing to the regulational draining works.
2.2.2. Vegetation
and high-mountain
on sandy soils
In the Danubian basins, major areas are covered by continuous sandy surfaces of fluvial origin. These soils always display specific vegetation differing from the macra zonal vegetation of the environment. These deviations are primarily due to the structure of sand soils. The water retaining and conducting properties of coarse grain sand (and similarly of gravel soils) is slight, this is why they cannot develop continuous capillary systems. Thus when the climate is semiarid (e.g., in the forest steppe zone), it is only the surface layers of the soil that get dry, even during great droughts, while the deeper strata, having better water retaining properties, possess enough humidity to promote the evolution of forest ecosystems. A very good example of this phenomenon is the better afforestation, with respect to environment, of the largest sandy areas of the Danube basins and Central Europe (Marchfeld on the Morava), in Hungary the Little Plain, the DanubeTisza midregion, the NyirsBg, in Yugoslavia the Deliblat, and in Rumania the territory round the Lower Danube. Under semihumid climatic conditions, the properties of Fig. 8 sand are manifested in a reverse way, the coarse structure of the soil cannot retain the great amount of precipitations d a Dolomite vegetation in the Hungarian middle-height mountains. water, becomes waterlogged, thus the area is turned into Forest areas are xerotherm oakwoods and scrub forests of Quercus pubescens, the unwooded parts are barren grounds marshland before long. (This is the case primarily in the with rock grasses western territories of the Danube area.) I-
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Vegetation on sand undergoes a great number of phases from faring the wind blown sand to developing closed forests (annual grasses, sand heath meadows, closed sand heath forests, and closed sand forests). Less calcareous sand soils are characterized by sub-Atlantic, the more calcareous ones by sub-Mediterranean, Balkanic, and both types by various endemic plant species.
Dolomitvegetation in den Gebirgen Ungams. Die Wider bestehen aus xerothennen Eichenbestinden und aus Buschwtidem der Flaumeiche (Quercus pubescens), die waldfreien Teile sind natiirliche KahlflSchen mit Felsgras
0 V@tation de dolomite dans les montagnes hongroises. Les for&s sent cornposSes de chenes xkrophiles et de taillis de ch8nes pubescents, sur les endroits ddcouverts poussent des herbes de roches
56
Geoforum
and Southern Carpathians, the Illyric species in the Dinaric Alps, and the species of Alpine-Carpathian distribution are all differentiated. Relicts of innate character and of previous climatic periods are very frequent. Rockland vegetation has developed successively from pioneer mosslike cenoses through rock grasses, slope steppes, and mosaical scrub forests and rock forests, parallel with the deepening of the soil and improving water supply, to closed tree forests (Fig. 8).
617 1
salty soils of the inner continent develop with special salt enduring and halophilous alkali soil vegetation. Even a few centimeters of difference in the soil surface levels may bring about, as to alkali and salt accumulation, enormous differences. This is why the vegetation of alkali soils consists of micromosaics of cenoses usually small in area, but different in character. The water conditions of alkaline habitats are also very different. They are capable of extreme changes even in the course of one single year. Plants of such regions are very often xeromorphous in appearance. They are very often succulent plants, with great quantities of salt stored in their stems, which enables them to exert great suction force. According to their distribution, they are usually eastern continental species. The vegetation has much in common with that of the salt steppes of the Soviet Union, or even of Central Asia. The alkali steppe vegetation of the Danube area is a remainder of the post-glacial hazel phase, which has secondarily spread over to great areas since water controlling and deforestation began in the Middle Ages.
High mountain, woodless vegetation is, in view of ecological variety, similar to rock vegetation, but is more dependent on light and heat conditions induced by the increase of altitude above sea level. In the Danube area from altitudes above the timberline, referred to above in connection with pines, the succeeding belt is formed by sub-Alpine shrubs. These are replaced by Alpine grass, then by the flowering and cryptogamic plants of the subnivean belt. Their habitat is relatively small, but all the more varied are their individual traits. It is just this specific character that made them favored investigational objects to researchers of older times, too. 2.2.4. Vegetation of alkali soils
References
In some lowland regions of the Danube area (Carpathian basin, Rumanian plain) where the amount of precipitation is exceeded by evaporation, and where it is facilitated by the properties of the soil, the easily soluble salts of subsoil water (alkaline salts, sodium, potassium) come up to the soil surface with the upward trend of water, and are precipitated on the surface (Fig. 9). In this way do the
ADAMOVIC, L. (1909): Vegetationsverhaltnisse In: Vegetation der Erde, Band 11. DIHORU, G. and N. DONITA (1970): Eabadag (Die Flora und Vegetation Bucurgti.
der Balkanlander.
Flora si vegetatia Podi@ui der Norddobrudscha).
DOSTAL, .I., ed. (1958): Exkursionsftihrer fur die XII, Internationale Phytogeographische Exkursion durch die Tschechoslowakei. Praha. ELLENBERG, H. (1963): Alpen. Stuttgart.
Vegetation
Mitteleuropas
mit den
GAUCKLER, K. (1938): Steppenheide und Steppenheidewald der Frankischen Alb in pflanzensoziologischer und geographischer Betrachtung; Ber. bayer. bot. Ges., 23, pp. 5 - 134. GRADMANN, R. (1950): Alb, 4. Aufl. Stuttgart.
Das Pflanzenleben
der Schwabischen
HORVAT, I. (1954): Pflanzengeographische europas; Vegetatio. S/6, pp. 4344447
Gliederung
Siidost-
JAKUCS, P. (1961): Die phytozonologischen Verhaltnisse der Flaumeichen-Buschwalder Sudostmitteleuropas. Budapest. KERNER,
\
\
I
Fig. 9 Distribution of alkali vegetation after WENDELBERGER 1950) Verbreitung der Alkali-Vegetation nach WENDELBERGER 1950)
in Central
Europe
in Mitteleuropa
(completed
(erganzt
Distribution de la vegetation cotyledon&e en Europe (complete d’apres WENDELBERGER 1950)
Pflanzenleben
der Donaulander.
Wald und Umwelt;
MEUSEL,
Vergleichende
H. (1943):
RUBNER, K. and K. REINHOLD Europas. Berlin.
Schweiz.
Arealkunde. (1953):
Innsbruck.
Z. Forstwes.. Berlin.
Das natiirliche
Waldbild
RUHL, A. (1954): Ein Beitrag zur Kenntnis der Trockenwllder und warmeliebenden Waldgesellschaften Siiddeutschlands; Angew. Pfl. Soziol., Festschrift Aichinger, pp. 423-436. P&S, T. (1968): Nbvenyfoldrajz es okol6gia. Novenytan, vol. 2, pp. 489-649. SO6,
Centrale
A. (1963):
KLOTZLI, F. (1968): 119, pp. 264-334.
R. (1945):
Nov&zyfoldrajz
In: Hortobhgyi,
(Pflanzengeographie).
Z6LYOM1, B. (1957): The zonal plant Acta biol. hung., 1, pp. 7-8.
associations
Budapest. of Hungary
47
6/7 1
Vegetational
Conditions in the Danube Area
Vegetationsverhaltnisse
im Donauraum
Les types de vegetation dans la region danubienne
PBI JAKUCS, Debrecen*
Abstract: The vegetation of the Danube Basin is extremely diverse. Sub-Atlantic influences are experienced in the western parts, sub-continental influences in the southeastern parts, whereas Balkan influences dominate in the south. Uplands, damp depressions, sandy soils and saline soils promote the development of special ecosystems. As a result of anthropo zoogenous agencies, the original vegetation has undergone substantial alteration, and has in places been destroyed. The individual elements of the vegetation pattern merge to form an interlocking whole. Zusammenfassung: Die Vegetation des Donaueinzugsgebiets ist hochst abwechslungsreich. Die westlichen Teile liegen im subatlantischen, die stidostlichen im subkontinentalen, die siidlichen dagegen im balkanischen Einflul3bereich. Hochgebirge, feuchte Niederungen, Sand- und Salzboden begiinstigen die Entfaltung spezieller 6kosysteme. Die urspriinghche Vegetation wurde durch anthropozoogene Einwirkungen stark veriindert, an vielen Stellen vernichtet. Die Einzelelemente des Vegetationsbildes gehen kontinuierlich ineinander iiber. R&urn& La vegetation du territoire du Danube et de son bassin versant est t&s variee. Elle subit au Nord l’influence subatlantique, au Sud-Est l’influence subcontinentale, au Sud l’influence balkanique. Les hautes montagnes, les regions humides, de sables et de sols salins favorisent le developpement des ecosystdmes spdciaux. La vegetation primaire a 6th considerablement supprimee par les influences anthropozoogdnes ou completement andantie a maints endroits. Les eldments du mosaique v&g&l passent de l’un a l’autre d’une manihe continue.
It was in 1863 that A. KERNER’s work Das ffflanzenleben der Dorm.dind~ was published. The book, based on accurate observations and giving descriptions of the vegetation of the Hungarian Great Plain, the Carpathians, the Czecho-Moravian Mountains and the Alps, is regarded as a classic in its field. For the above mentioned territories, KERNER’s work is a basic reference book even today. Not more than a centruy ago, account was given of wide range, splendid woodlands, immense flowering grasses, flood-plain groves, swamps, etc.; today, however, one can but find the remainders of all this. Thus, when a survey is given of the natural vegetation of the drainage area of the great Danube River of South East Central Europe, brief reference should be made first to the changed vegetational conditions due to great changes in the landscape, The vegetation of the region was classified by KERNER into formations primarily according to physiognomical appearance. After 1900, on the basis of the laws of association, researchers focused attention on the * Prof. Dr. Pal JAKUCS, Department of Botany, Kossuth University, Debrecen 10, Hungary
investigation of coenoses, and today studies of ecosystems are predominant. The ecosystems are inner, energetically associated subunits of the biosphere, which have been developed in any part of the earth in complex correlation of the potent actual environmental factors (relief, climate, soil, etc.). The green-plant components of the varied ecosystems, by their oxygen production as well as their primary phytomass production due to assimilation, are the base and assurance of any higher animal and human life. In a given natural region, the ecosystems develop to attain the highest-order bioproduction, corresponding to environmental conditions. Their qualitative and quantitative marks thus indicate the potential productivity value of the region. With the development of human civilization, however, transformation of natural regions assumed an accelerating rate which, in the majority of cases, involved the substitution of natural ecosystems, according to demand, by artificial ones (agriculture), or even their complete elimination (urbanization, industry, transport, etc.). The continually accelerating rate of regional transformation is inflicting on the ever increasing natural eco-
48
Geoforum
systems harmful side effects which, in the last analysis, may bring about irreversible damages to the ever growing masses of mankind. For this reason, the great task of the future is to eliminate, through multilateral and through study of surviving naturaI e~systems and through utilization of the information thus obtained, the damages already caused, and to secure a sound basis for the work of developing the highest attainable bioproduction of a region. The most si~ic~t part of this task is the territorial reconstruction of the original natural vegetation. This is why the subsequent survey of the vegetation of the Danube area is solely concerned with the potential vegetation. 1. General Remarks on Vegetation in the Danube Area The 817 thousand square km watershed area joined up by the 2 860 km long Danube comprises the territories of essentially seven countries. The river itself, while flowing from W to SE, strings up three major basins: the Upper or Bavarian, the Middle or Carpathian, and the Lower Danubian basin. Passing on from one into the other, it breaks through, at picturesque gorges, a number
of ranges of the Alps, C~pa~~s, and Balkans. To its watershed area belong the East Alps, Carpathians, Dinaric Alps, Serbian Central Mountains, and the northern part of the Balkan Mountains. From its spring to its delta, it passes through regions of the most different structure, relief, and climate. For this reason, the Danube is referred to as Europe’s most varied, most heterogeneous, and most complex river. The vast variety of en~onmental factors is reflected in the vegetation as well. From alpine grasslands through rustling pinewoods to the unwooded steppe, the mosaics of zonal, intra- and extrazonal patches of vegetation form the landscape into a multicolored carpet. In general, the whole area of the Danubian system of basins is climatically favorable for the evolution of forests, even the lowland parts of the Middle and Lower basins are cuftivated steppes, brought about by the activity of man. At these places, however, the distribution of precipitation may very often reach the limit critical for trees. Thus, in view of vegetation, these areas join the forest steppe belt. Spacious, ancient, unwooded steppes, on the other hand, do not occur in the Danube area, apart from the eastern parts of the Rumanian plain, which is proved by the fact that even the barest heaths can be afforested. Nevertheless. Fig. 1
. ..
;
S/7 1
P~ticipation of Pontic (a) and Central European (b) floristic elements in the xerotherm scrub forests of the Danube area Anteii der pontischen (a) und mitteleuropaischen (b) Florenelemente in den xerothermen Buschwaldern des Donauraums
‘
Participation des elements de flore pontiens (a) et centreurop~ens (b) dans les taillis xirothermes du Bassin Danubien
1. Swabian Alb (RiiHL 1954); 2-3. Frank Alb (GAUCKLER 1938); 4. Bohemian Basin; 5. Viennese Basin; 6. Moravian Karst; 7. Pannonian Hills; 8. Western Slovakia; 9. South Hungary: Mecsek Mountains; 10-I 1. Hungarian Northern Mountains; 12. Kazane-Defile, Lower Danube; 13. West Bulgaria: Golo-Bodo Mountains; 14. East Bulgaria: Balkan Mountains; 15. Dobrogea. (4-15 see JAKUCS 1961, Fig. 61)
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even within the woodland zone, at larger or smaller patches there are ecologic factors hindering the growth of trees and the evolution of forests. Such factors are special soil or base rock, especially their aridity, extremities of geomorphology, great heights above sea level, etc. Vegetation at such place is either completely specific (edaphic, intrazonal vegetation), or is in correlation with that of other vegetational zones (extrazonal vegetation). The Danube area is one of the transitional climatic territories of Europe, where climate is characterized by the intermingling of oceanic, continental, and Mediterranean influences. Continental influences display a gradually increasing tendency toward the east, and especially in the basins. At the same time in the SE, there appears a strong sub-Mediterranean influence. As to vegetation, this influence results in the fact that while in the western part of the Danube area deciduous forests are dominated by the beech tree, which prefers a more balanced sub-Atlantic climate, toward the east the increase in the extremities of temperature (hard, cold winters and dry summer heat spells lasting sometimes for weeks) drives the beech back to the microclimatically cool damp, or higher mountainous regions, and among the leafy trees the oak, maple, and lime gain predominance. On passing from W to E, the parallel weakening of central European character and strengthening of continental influence in the vegetational aspect can be demonstrated by areal-type analyses performed in scrub forests. Scrub forests are developed as transition between open grass-
49
lands and closed tree forests, and can be regarded as extrazonal representatives of forest steppe vegetation. Characteristic of them is a mosaic like appearance together with edaphic xerotherm grasses. They have evolved in the whole Danube area generally on the southern slopes of lower mountains. Dominant in their foliage around the upper reaches of the Danube is the pedunculate oak (Quercus robur), the piliferous oak (Quercus pubescens), and the eastern hornbeam (Carpinus orientalis). Their rich shrub and grass level is characterized by the multitudinous appearance of Eurasian and sub-Mediterranean plants. In Fig. 1, the percentile distribution of central European and Pontic floristic elements of scrub forest cenoses from 14 different regions of the Danube area are presented to support the statement that in the aspect of vegetation, the shift toward the continental is a phenomenon parallel to the regression of central European character. It should be noted that the tendency of areal-type changes from central European (sub-Atlantic) toward Pontic (subcontinental), calculated for scrub forests, appears quite similarly within each vegetational unit in the Danube area with a stronger or weaker character. Another general feature in the vegetation of the areas SE of the Pannonian basin is, in addition to the strengthening of eastern traits, the more and more multitudinous occurrence of the floristic elements of the Balkans (Fig. 2). The vertical evolution of high-mountain vegetation is also very interesting to demonstrate climatic differences
Fig. 2 l
Ideal vegetational profiles from various regions of the Danube area, supposing identical altitudes above sea level, and identical base rocks (JAKUCS 1971)
0
Idealisierte Vegetationsprofile von verschiedenen Gebieten des Donauraums unter Voraussetzung einer gleichen Meereshohe und eines gleichen Gesteins (JAKUCS 1971)
l
Profils de vegetation id&l&s de diverses regions du bassin danubien supposant la m6me altitude et la m8me roche originaire (JAKUCS 1971)
1. Atlantic; 2. Subatlantic - Central European; 3. Central European; 4. Submediterranean - Subcontinental; 5. Subcontinental - Continental. F = Fagetalia; C = Carpino-Fagetea; Q = Quercetea pubescenti-petraeae; FB = Festuco-Brometea
S-N
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between forest steppe, occupying an intermediate position between temperate-belt deciduous forests and the steppe, and the extrazonal high-mountain pinewoods of the zone of coniferous forests. Within the area of macrozonal beech forests, oak-woods, forest steppes, and extrazonal coniferous forests, one fourth part is occupied by edaphic or intrazonal vegetational units: aquatic and subsoil water influenced vegetation, sand, alkali, rock, alpine, etc., vegetations. 1. Croatian Karst (Risnjak); 2. Dinaric Alps (Bjelasnica); 3. Balkan mountains (Rila); 4. Southern Carpathians (Pareng); 5. Northern Carpathians (High Tatra); a = subnival region; c = Swiss pine forests; ch = alpine grasses; d = forests (after I. HORVAT and T. P6CS - in P&S 1968) Fig. 3
Altitude belts of several high-mountain regions of the Danube area Hohenstufen der Vegetation in einigen Hochgebirgen des Donauraums Zones de vegetation dans certaines regions de hautes montagnes du Bassin Danubien
between the north-western and southeastern parts of the Danube area. The beech forests at the upper reaches of the Danube are predominant forests in the zone at 400 to 900 m above sea level. In the Carpathian basin, their presence is characteristic at altitudes between 600 to 1200 m, on the slopes of the Southern Carpathians and the Balkan Mountains the ecological range of their conditions of life can be found between 1000 to 1 700 m (Fig. 3). Before giving a short survey of the most important units of vegetation in the Danube area, it should be mentioned that vegetation, in contradiction to the more homogeneous western regions, is more variegated, multicolored, richer in species in the southern and southeastern regions. This is not only due to the greater variety of environmental factors, but is in correlation with the history of vegetation. The flora, driven back from Central Europe by the Pleistocene glaciation, has weathered out the extreme fluctuations in climate among the Balkan Mountains, as in a refuge area. This is why in the more central regions of the Danube area woodland ecosystems are 30-50 percent poorer in species than their equivalents in the Balkans.
2.1.1. Beech forests The most important forest-forming deciduous tree of the climatically better-balanced hilly and mountainous territories of the Danube area is the European beech (Fagus silvatica), distributed in Central Europe. In soils not too damp, not too easily drying, and relatively rich in nutrients, where winter colds are not too strong and the annual mean temperature is between 4-12 “C, where extremities are comparatively not wide, and where precipitation is relatively evenly distributed over the year and not exceeding 2 000 mm, beech forests may appear. The foregoing ecological conditions are dominant and very characteristic of the territories of the Danube area from the river’s source to the Pannonian basin; in the Black Forest, in the Swabian and Frank Albs, in the Bavarian woods, and in the part of the Bavarian basin ranging up to the East Alps between altitudes of 400 to 900 m above sea level. Thus, the most extensive, continuous zonal beech forests are to be found here. Within the ring of the Carpathians and among the Dinaric Alps, they have been repelled to higher regions. In various areas in the southern part of the Danube valley, they even reach the height of 1 800 m, forming at places the absolute timberline. In these northern territories of the Balkans, Fagus sylvatica is frequently replaced by the species Fagus moesiaca, distributed in the Balkans.
2.1. Zonal Vegetation
Beech forests are generally of high growth (25-35 m), their canopy is very often mixed with scattered specimens of ash (Fraxinus excelsior), mountain maple (Acer pseudop&anus), and in the mixed zone by spruce and fii. The foliage is always highly dense, so the floor is shady. Usually only 50 to 80 percent of the sunlight can penetrate the forest. Thus the undergrowth of beech forests consists mainly of shade tolerant and umbrageous species; the shrub layer is scantily developed. Very characteristic, however, are the vernal geophytes, which store the nutrients necessary for flowering in their underground parts, bring flowers during the more favorable light conditions before foliation, and with the ripening of their crops after foliation complete their annual cycle of life (Fig. 4).
Nearly half the territory of the Danube area is occupied by oak and beech forests belonging to the zone of temperate-belt deciduous forests. The rest is divided by halves
A number of associations, subassociations, variants, and other subunits of beech forests are known in the Danubian basin. (For a comprehensive survey and bibliography see
2. Most Important
Types of Vegetation in the
Danube Area
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Primarily for these two reasons, the original sites of beech forests have remained covered, at a high percentage, up to our times. In the westernmost parts of the Danube area, especially on acidic soils, beech forests have been transformed by forestry into monocultures of fir and spruce. 2.1.2. oakwoods
Oakwoods of pure stand are forestry areas of lowlands and of the generally more arid soils of hilly regions. They are much more varied in comparison with beech forests; the canopy is constituted by several species of oak. In the Danube area, the most frequent are the pure and mixed cultures of the sessile oak (Quercus petrae), the pedunculate oak (Quercus robur), the Austrian oak (Quercus cemk), the piliferous oak (Quercus pubescens), and the Hungarian oak (Quercus farnetto). Besides oaks, there is a frequent occurrence of other broad leafed trees (birch, field maple, lime, elm, etc.). In the intermingling zone with beech, transition is very often indicated by oakwoods mixed with hornbeam and forming an independent belt (Fig. 5). Fig.4 Submountainous mountains
beech-tree forests in the North Hungarian
Submontaner Buchen-Hochwald Futaie de h&raie submontagneuse Hongrie septentrionale
hn nordungarischen
Gebirgsland
darts les montagnes de la
SO6,1963.) In general, two great groups can be discriminated depending on whether the soil is calcareous or poor in calcium. Beech forests on soils of high basicity (generally developed on limestone base rocks) are more typical, richer in species, and with numerous species characteristic of beech forests only. On the other hand, on eluviated soils of low basicity (formed generally on silica rocks, e.g., granite, gneiss, sandstone, clay, and slate), beech forests are poorer in species and their acidity indicating dominant species are common with those of oakwoods of similar soil structure. Characteristic of these is the evolution of rich moss strata. Within the two main groups, the basis for further dismembering may arise partly from the geographical position (associations under the influence of regional units, e.g., Illyric, Dacian, Carpathian, Alpine), partly from finer ecological differences of more local value (mountainous, submountainous position, rendzina, brown forest soil, podzolic soils, etc.). The habitat of beech forests is generally not advantageous for agricultural cultivation. Beech is favorably regenerating without any help even in case of intensive use of timber.
Fig. 5 Oakwood, the climatic-zonal association in the central parts of the Danube area at the altttude of 300-400 m Eichenwald, die klimabedingte Assoziation im zentralen Donauraum in 300-400 m HGhe Foret de chgnes, association v&&ale des regions centrales danubiennes sit&e entre 300-400 m d’altitude
52
Unlike beech forests, oakwoods are more tolerant to extremities of temperature. They can survive, without any major damage, both spells of dry summer heat and hard winter colds. Consequently, their main territories of distribution in the Danube area are the Carpathian basin, the lower, Rumanian and Bulgarian sections of the Danube valley, and the northern middle-height mountainous and hilly territories of the Dinaric Alps. Since the woody regions of the forest steppe are generally represented by oakwoods, their dimensions are further extended to these areas. Their habitats on hilly and lowland regions, however, are optimal areas for a number of agricultural plants, too, thus, in the present landscape, they have been greatly diminished in their stand. If, however, we make a comparison within the Danube area of the potential sites for oakwoods with those of beech forests, the ratio is in favor of oakwoods. The canopy of closed oak forests under natural conditions is 12-15 m high, in mixed stand the other species of trees are usually evolved in the second, lower gallery. Even inside oakwoods with completely closed canopy, conditions of temperature and light are very favorable as compared to beech forests. This is why the shrub layer is usually highly developed (species of Comus, Euonymus, Ligustrum, Crataegus, Prunus, etc.). The softstemmed vegetation is also varied and, particularly in the spring and early summer months, there is an abundance of flowers.
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617 1
the forest steppe belt. The Viennese Basin, the Hungarian Little and Great Plains, and the Transsylvanian basin are the westernmost part of the forest steppe belt that is continuously stretching through the Rumanian Plain east of Bucharest, in an eastern direction from the Danube area, through Voronezh and Saratov, to somewhat beyond the Urals. The forest steppe belt is characterized by the mosaical variation of patches of woodland and steppe, or open parklike forests, particularly oakwoods rich in steppe elements. Annual precipitation is round 500 mm, the increasing extremities of temperature (hard, cold winter-hot dry summer) set up for the trees zonal limits. Continuous, closed woodland areas rich in hydrophilous species are evolved rather edaphically along the great rivers. The macroclimatic forest steppe appears extrazonally, lococlimatically on the souther slopes of the lower mountains surrounding the plains, bringing about the so-called slopeforest steppes in the aspect of vegetation. The mosaical patches of forest and steppe in the forest steppe belt are in a constant dynamic change, in which a very important role is played by the fringes between wooded and woodless ecosystems, since the fringes do not only protect and enclose the forest ecosystems, but at the same time ecologically prepare their extension. In forest steppes, the essence of this process consists in the following (see JAKUCS, 1971):
2.1.3. Forest steppes, steppes
In case of zonal and extrazonal (slope) forest steppes, very similar in their structural patterns but different in their dimensions, the absolute boundary of forest ecosystems is brought about by edaphic aridity effects. The theoretically uniform, continuous physiological boundary belt of trees was, owing to constantly working anthropozoogenic and other natural mechanical effects, in most cases very probably rather early disrupted, or completely failed to evolve. The patches of woodland, broken up into mosaics, or capable only of mosaical development are, thus, in contact with disadvantageous conditions at longer peripheral surfaces, which in this way can exert increased inhibitory effect on afforestation. On the other hand, at fringe areas of extended length there begins an increased development of ecotones, which can thus exert their protective and preparatory effect to a greater extent. The struggle, both spatially and temporally, leads to the adaptation of the fringe belt to the physiological-theoretical boundary, i.e., a closing-straightening regeneration of the fringe. Since the artificial and natural mechanical influences, or in secular dimensions, the continuous change of inhibitory physiological factors mostly subsist and work, the physiological aridity boundary of the forest steppe can only be judged and evaluated in practice by its dynamically fluctuating mosaicity.
The regions in the SE parts of the Danube area, lower than 300-400 m, fall into the mixed transitional vegetational belts between deciduous forests and the steppe, i.e., into
Forests in the forest steppe belt of the Danube area are mostly oakwoods with pedunculate oak as the dominant species, but mixed with Ulmus campestnh, Acer tataricum,
The fundamental discrimination according to the basicity of soil is valid for oakwoods, too. Oakwoods with eluviated, acidic soils are more frequent in the more balanced subAtlantic climate of the western basins. The mass species in them (Luzula, Deschampsia, t/‘accinium, etc.) are common with beech forests of acidic soil. Dominant in the canopy is Quercus robur, with frequent occurrence of Qu. petraea, Betuh per&la, and Populus tremula. Intense mossiness of the surface and large-scale appearance of fungi are also characteristic. As opposed to oakwoods of acidic soil, the majority of oakwoods in the Danube area belong to the so-called xerotherm oakwoods of relatively high basicity ratio (Fig. 6). These forests can be further dissected on the basis of whether dominance is achieved in them by plant species favoring sub-hlediterranean, W or E Balkanic, subcontinental, or central European climatic influences. This differentiation is of regional nature; however, further cenological differences of territorial value are brought about within them by the varied local ecological conditions (e.g., separation of closed tree forests and scrub forests).
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Fig. 6 Occurrence of Quetcetea pubescenti-pelrueue Verbreitung xerothermer Eichenwtider
forests in Europe (JAKUCS 197 1)
in Europa (JAKUCS 1971)
Distribution des forits de Querceleu pubescenti-petrueue en Europe (JAKUCS 1971)
Acer campestre, etc. The shrub layer is well developed, comprising shrubs of eastern distribution (Amygdalus nanu, Prunus fruticosa, Spiraea media, etc.). In the grass level, in addition to species characteristic of xerotherm oakwoods, there are always present common elements with steppes. The soil is generally chernozemic with transitions thereof of brown forest soils. The unwooded steppes are rich in species of grasses, together with geophytes, papilionaceae, and labiates. In summer these areas become completely dry, but they have a characteristic feature, i.e., a period of weaker second flowering in autumn. The cenoses of the forest steppe belt have been evolved most typically on more bound loessy soils. As these have been for long the most
important corn growing regions, owing to their favorable properties, very few remains of the original vegetation have been preserved up to our time. 2.1.4. Pinewoods
In the sub-Alpine parts of the mountainous regions of the Danube area, et a height above the deciduous beech forest belt, a considerable area is occupied, up to the timberline, by coniferous forests. They are essentially representatives of the sub-Arctic evergreen forest belt, stretching northward from the Danube area. The evergreen coniferous trees, well resistant to frost, can endure long spells of cool and cold weather, require much precipitation, but avoid climatic extremities. Their chief habitats in the Danubian
64
basins are the East Alps, the lower mountainous regions north of the Alps, the Carpathians, Dinaric Alps, and the high mountains of the Balkans. The most typical stands have been evolved between 600 to 1500 m in the Alps, 1 100 to 1600 m in the Northern Carpathians, 1200 to 1800 m in the Southern Carpathians, and 1400 to 2 300 m in the Balkans. The main forest forming trees are the fir (Abies alba) and the Norway spruce (Picea abies). Mixed with these in the highest zones are various species of pines, the yellow pine (Pinus cembra) and the larch (Lank decidua). In the Balkans, they may be replaced by other species of pines, e. g., pinus peuce, Pinus heldreichii, Plea omorica, etc. (Fig. 7). Coniferous forests are generally homogeneous; the separate layers are represented by one or two predominant species. The grass layer is poor with no seasonal aspect. The damp climate inside the stand of closed forests is very often favorable for a rich mossy level. The soils eluviated by much precipitation facilitate the growth of fungi in the warm summer months. The coniferous forests of the Danubian basins are distinguished from one another primarily by their domin-
Geoforum
ant trees (spruce, fir, pine, larch), but since each of these species has its own ecological requirements, the habitational-ecological relations of the dissociating units are also different. Regional differences are brought about (e.g., in the South and North Carpathians and Dinaric Alps) by the occurrence of several different species of herbaceous plants. As to their function, more than 90 percent of coniferous forests are lumbering areas. The economical value of pines has greatly contributed to the artificial increase of their habitats (mainly at the expense of adjoining beech forests). 2.2. Edaphic or Intrazonal
Ecosystems
ln the Danube valley within the area of the beech and oak belts, primarily dependent on the macroclimate, and within the forest steppe belt, about one quarter is occupied by such edaphic or intrazonal vegetational units, where the subsistence of the ecosystem is determined by one or more ecological factors of special local importance. Owing to the great number of influencing factors (relief, base rock, altitudinal conditions, water supply, radiational conditions, etc.) and their great number of combinations, there is a possibility for a very highly varied evolution of vegetation. In the aspect of the landscape, these factors bring about a multicolored carpet of vegetation, whose patches, being local in nature, are specific for smaller areas or portions of the landscape in the Danubian basins, too. This variety and the great numbers prevent us from dealing with them in merit within the scope of the present discussion. Thus, in the following a brief survey will be given of their most important groups. 2.2.1. Aquatic and water influenced
Fig. 7 l
Distribution of fir (Abies alba) in Europe and the high-mountain region of the Danube area (after RUBNER)
l
Verbreitung der WeiDtanne (Abies alba) in Europe und in den Hochgebirgen des Donauraums (nach RUBNER)
l
Distribution du sapin (Abies alba) en Europe et dans les hautes montagnes du bassin danubien (d’aprks RUBNER)
6/71
vegetation
The vegetation of the standing waters and rivers of the Danube area has developed according to the depth and the streaming properties of the water. Depths larger than 2 m only facilitate the evolution of hair-weed vegetation, which can be rooted, floating, upheld on the surface or immersed. The plants usually utilize for their metabolism the oxygen and carbon dioxide dissolved in water. On the other hand, water depths shallower than 2 m give rise to the evolution of swamp vegetation, where plants can root under the water, but the assimilating surfaces rise above the water level. Most frequent representatives of the latter are reeds and sedges. Vegetation in nondirectly open water areas (e.g. watersides), but where subsoil water level is high, displays two main types. If destructed parts of plants can very slowly oxidize in the waterlogged soil, the developing turf is agglomerated under the consecutive plant generations. This process is advantageous for marshlands, marsh meadows rich in mosses, and for marsh forests with the dominance of alder (Alnus). If, on the other hand, the
Geoforum
55
6i7l
2.2.3. Rockland
oxidation process of the soil is secured by continuous or frequent supply of fresh ground water, then swamp meadows poor in moss, and gallery forests rich in willow, aspen, ash, and ehn become dominant. It should be further noted that in the various altitude belts of mountainous regions, cenoses developing in dependence of water (springs, mountain creeks, etc.) extend usually to smaller areas, but are more sharply discernible from the vegetation of the surroundings.
vegetation
The character of vegetation on the emerging coarse base rock of mountainous areas is primarily determined by the thickness of the soil evolved, and the rate of desiccation. These properties, on the other hand, are dependent on the features of the rock, relief conditions, and microclimatic relations. Depending on at what time and by which one or ones of these ecological factors predominance is gained, the usually more stenecious cenoses of the extraordinarily varied units of rockland vegetation appear. In the Danube area, the plant species of rock vegetation are also geographically well isolated from one another. The Dacian elements in the Balkan Mountains
In the Danube area, aquatic, marsh and swamp vegetation had been widespread mainly along the big rivers, in their flood plains, and around lowland lakes and depressions without outlet until the great river-control works began intensively in the middle of the last century. Their area has decreased to one fifth of the original, owing to the regulational draining works.
2.2.2. Vegetation
and high-mountain
on sandy soils
In the Danubian basins, major areas are covered by continuous sandy surfaces of fluvial origin. These soils always display specific vegetation differing from the macra zonal vegetation of the environment. These deviations are primarily due to the structure of sand soils. The water retaining and conducting properties of coarse grain sand (and similarly of gravel soils) is slight, this is why they cannot develop continuous capillary systems. Thus when the climate is semiarid (e.g., in the forest steppe zone), it is only the surface layers of the soil that get dry, even during great droughts, while the deeper strata, having better water retaining properties, possess enough humidity to promote the evolution of forest ecosystems. A very good example of this phenomenon is the better afforestation, with respect to environment, of the largest sandy areas of the Danube basins and Central Europe (Marchfeld on the Morava), in Hungary the Little Plain, the DanubeTisza midregion, the NyirsBg, in Yugoslavia the Deliblat, and in Rumania the territory round the Lower Danube. Under semihumid climatic conditions, the properties of Fig. 8 sand are manifested in a reverse way, the coarse structure of the soil cannot retain the great amount of precipitations d a Dolomite vegetation in the Hungarian middle-height mountains. water, becomes waterlogged, thus the area is turned into Forest areas are xerotherm oakwoods and scrub forests of Quercus pubescens, the unwooded parts are barren grounds marshland before long. (This is the case primarily in the with rock grasses western territories of the Danube area.) I-
l
Vegetation on sand undergoes a great number of phases from faring the wind blown sand to developing closed forests (annual grasses, sand heath meadows, closed sand heath forests, and closed sand forests). Less calcareous sand soils are characterized by sub-Atlantic, the more calcareous ones by sub-Mediterranean, Balkanic, and both types by various endemic plant species.
Dolomitvegetation in den Gebirgen Ungams. Die Wider bestehen aus xerothennen Eichenbestinden und aus Buschwtidem der Flaumeiche (Quercus pubescens), die waldfreien Teile sind natiirliche KahlflSchen mit Felsgras
0 V@tation de dolomite dans les montagnes hongroises. Les for&s sent cornposSes de chenes xkrophiles et de taillis de ch8nes pubescents, sur les endroits ddcouverts poussent des herbes de roches
56
Geoforum
and Southern Carpathians, the Illyric species in the Dinaric Alps, and the species of Alpine-Carpathian distribution are all differentiated. Relicts of innate character and of previous climatic periods are very frequent. Rockland vegetation has developed successively from pioneer mosslike cenoses through rock grasses, slope steppes, and mosaical scrub forests and rock forests, parallel with the deepening of the soil and improving water supply, to closed tree forests (Fig. 8).
617 1
salty soils of the inner continent develop with special salt enduring and halophilous alkali soil vegetation. Even a few centimeters of difference in the soil surface levels may bring about, as to alkali and salt accumulation, enormous differences. This is why the vegetation of alkali soils consists of micromosaics of cenoses usually small in area, but different in character. The water conditions of alkaline habitats are also very different. They are capable of extreme changes even in the course of one single year. Plants of such regions are very often xeromorphous in appearance. They are very often succulent plants, with great quantities of salt stored in their stems, which enables them to exert great suction force. According to their distribution, they are usually eastern continental species. The vegetation has much in common with that of the salt steppes of the Soviet Union, or even of Central Asia. The alkali steppe vegetation of the Danube area is a remainder of the post-glacial hazel phase, which has secondarily spread over to great areas since water controlling and deforestation began in the Middle Ages.
High mountain, woodless vegetation is, in view of ecological variety, similar to rock vegetation, but is more dependent on light and heat conditions induced by the increase of altitude above sea level. In the Danube area from altitudes above the timberline, referred to above in connection with pines, the succeeding belt is formed by sub-Alpine shrubs. These are replaced by Alpine grass, then by the flowering and cryptogamic plants of the subnivean belt. Their habitat is relatively small, but all the more varied are their individual traits. It is just this specific character that made them favored investigational objects to researchers of older times, too. 2.2.4. Vegetation of alkali soils
References
In some lowland regions of the Danube area (Carpathian basin, Rumanian plain) where the amount of precipitation is exceeded by evaporation, and where it is facilitated by the properties of the soil, the easily soluble salts of subsoil water (alkaline salts, sodium, potassium) come up to the soil surface with the upward trend of water, and are precipitated on the surface (Fig. 9). In this way do the
ADAMOVIC, L. (1909): Vegetationsverhaltnisse In: Vegetation der Erde, Band 11. DIHORU, G. and N. DONITA (1970): Eabadag (Die Flora und Vegetation Bucurgti.
der Balkanlander.
Flora si vegetatia Podi@ui der Norddobrudscha).
DOSTAL, .I., ed. (1958): Exkursionsftihrer fur die XII, Internationale Phytogeographische Exkursion durch die Tschechoslowakei. Praha. ELLENBERG, H. (1963): Alpen. Stuttgart.
Vegetation
Mitteleuropas
mit den
GAUCKLER, K. (1938): Steppenheide und Steppenheidewald der Frankischen Alb in pflanzensoziologischer und geographischer Betrachtung; Ber. bayer. bot. Ges., 23, pp. 5 - 134. GRADMANN, R. (1950): Alb, 4. Aufl. Stuttgart.
Das Pflanzenleben
der Schwabischen
HORVAT, I. (1954): Pflanzengeographische europas; Vegetatio. S/6, pp. 4344447
Gliederung
Siidost-
JAKUCS, P. (1961): Die phytozonologischen Verhaltnisse der Flaumeichen-Buschwalder Sudostmitteleuropas. Budapest. KERNER,
\
\
I
Fig. 9 Distribution of alkali vegetation after WENDELBERGER 1950) Verbreitung der Alkali-Vegetation nach WENDELBERGER 1950)
in Central
Europe
in Mitteleuropa
(completed
(erganzt
Distribution de la vegetation cotyledon&e en Europe (complete d’apres WENDELBERGER 1950)
Pflanzenleben
der Donaulander.
Wald und Umwelt;
MEUSEL,
Vergleichende
H. (1943):
RUBNER, K. and K. REINHOLD Europas. Berlin.
Schweiz.
Arealkunde. (1953):
Innsbruck.
Z. Forstwes.. Berlin.
Das natiirliche
Waldbild
RUHL, A. (1954): Ein Beitrag zur Kenntnis der Trockenwllder und warmeliebenden Waldgesellschaften Siiddeutschlands; Angew. Pfl. Soziol., Festschrift Aichinger, pp. 423-436. P&S, T. (1968): Nbvenyfoldrajz es okol6gia. Novenytan, vol. 2, pp. 489-649. SO6,
Centrale
A. (1963):
KLOTZLI, F. (1968): 119, pp. 264-334.
R. (1945):
Nov&zyfoldrajz
In: Hortobhgyi,
(Pflanzengeographie).
Z6LYOM1, B. (1957): The zonal plant Acta biol. hung., 1, pp. 7-8.
associations
Budapest. of Hungary