The Bugac Rabbit Project. Part I. Description of the ...

Abstracta

Botanica 17(1-2):187-196,1993 10 Department of Plant Taxonomy and Ecology, ELTE, Budapest

The Bugac Rabbit Project. Part I. Description of the study site and vegetation map M. Kertész1, J. Szabó1 & V. Altbäcker2 1 Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences, Herman Ottó út 15, H-1022 Budapest, Hungary. 2 Department of Ethology, Eötvös University, Jávorka u. 14, H-2131 Göd, Hungary. Aerial photography, Impact of animals, Juniper Forest, Succession, Vegetation types. Abstract: The Bugac Juniper Forest consists of a mosaic of woods, shrubs and xeric grasslands lying on a sand dune system, and a part of the forest is a core area of the Man and Biosphere Program. Compared to the other sand dune areas of the Kiskunság region, the Bugac Juniper Forest is composed of more differentiated patches interspersed with l ocally poorer vegetation. The succession of vegetation on drought-prone, sandy regions of the Carpathian Basin and especially on the calcareous sands between the Danube and Tisza rivers is less predictable than that on other soil types, but it is hypothesized that this difference from sites of the same climatic and edaphic conditions is mainly due to the high abundance of herbivores, particularly wild rabbits, relative to the low productivity of the grassland. The present state of the forest is considered to be the result of secondary succession consequent on burning, heavy grazing and forestry intervention. However, lack of an adequate species pool for this edaphic semi-desert might be also involved. Nevertheless, the Bugac Juniper Forest is rich in xerofrequent plant species demonstrating a wide variety of adaptations to drought conditions. In order to document the present status of this unique biotope and provide a basis for future studies on the development of the forest, a systematic map of the vegetation within a 35 hectare area was prepared. The aim of this study is to estimate the relative extent of the main vegetation types and to start to describe the nature of current successional processes. Mapping was carried out based on aerial photographs. A 1:1000 scale was chosen and 18 vegetation types were distinguished and the resulting map was digitized in order to incorporate it into a GIS. Keywords:

Introduction The Bugac Juniper Forest is a unique, protected landscape in the Kiskunság National Park, central Hungary. It consists of a mosaic of woods, shrubs and xeric grasslands lying on a sand dune system, and part of the forest is a core area of the Man and Biosphere Program. Due to its peculiarities as well as for methodological reasons the forest has become the focus of extensive research activity. Thus, the Bugac Juniper Forest has been a "classical" site for botanical researches (Szodfridt 1968, Simon 1984, Konecsni 1984, Simon & Rajkai 1985) and investigations have become even more intensive since the establishment of the Kiskunság National Park in 1975 (Tóth 1985). The dominant vegetation and land use types of the Bugac-Bócsa

Protected Area, including the Bugac Juniper Forest, have been mapped at a scale of 1:25000 by the Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences (RISSAC) and a team from the Department of Plant Taxonomy and Ecology of L. Eötvös University (ELTE), Budapest, has been working in Bugac since the National Park was founded. Their main field of interest is the eco-physiological adaptation of grassland species and the grassland community to water and nutrient shortage (Kovács-Láng & Mészáros-Draskovits 1985, Kovács-Láng et al. 1986, Kalapos 1989, 1991a, 1991b). The water regime of grasslands is studied partly in cooperation with RISSAC (Szabó 1985, Szabó & Keszei 1985, Kertész 1991).

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Zoological surveys have also been carried out in order to compare different habitat types. The influence of the rabbit on the vegetation has been under study by a joint team of the Ethology Dept, Dept. Plant Taxonomy & Ecology of ELTE and RISSAC since 1989 (Altbäcker et al. 1991a, Altbäcker et al. 1991b, Szabó et al. 1991). The Bugac Juniper Forest is of particular interest as compared to the other sand dune areas of Kiskunság, it is composed of differentiated patches interspersed with locally poorer vegetation. It is hypothesized that this difference from sites of the same climatic and edaphic conditions is mainly due to the abundance of wild rabbits in this area relative to the low productivity of the grassland. According to our recent findings, rabbits at Bugac prefer juniper shrubs as burrowing sites and for hiding, and mainly forage in the grasslands surrounding the juniper clumps (Altbäcker et al. 1991a, Altbäcker et al. 1991b). The results of fencing off experimental plots also indicate that the extreme openness of the grassland areas is almost certainly due to the activity of the rabbits (Szabó et al. 1991). Therefore, in order to document the present status of this unique biotope and provide a basis for future studies on the development of the forest, it was decided to systematically map the vegetation within a 35 hectare area. In doing so it was the aim of this study to estimate the relative extent of the main vegetation types and to describe the nature of current successional processes. Description of the study site Before considering the vegetation in detail, it is necessary to provide a more general description of the study area. Physical geography The Bugac sand dune region is situated between the Danube and Tisza rivers at about 46°N and 19°E and covers an area of about 50 km 2 . The elevation is approximately 115 m above sea level but dunes 15-20 m higher also occur. Although the longitudinal dunes typically run in a NW-SE direction, their shape is rather irregular indicating that the dominant wind direction and the plant cover changed several times during the development of the present surface. Climate The climate of the Carpathian Basin is moderate continental with predominantly Mediterranian

Kertész, Szabó & Atbäcker: Bugac Rabbit Project 1.

and oceanic characteristics. Within the Carpathian Basin the climate in the central part of the region between the Danube and Tisza rivers is relatively warm and dry. The average temperature is 11 °C and the annual precipitation is 500 m. On the basis of general climate statistics the summer months are considered semi-arid. The microclimate of the grasslands has been studied by Almádi et al (1986) more specially. They found it extreme with above 70 ° C midday surface temperature in summer. Since 1983 the Kiskunság area has been suffering from a serious drought. Although several seasons were relatively moist the second half of the last decade was warmer and drier than the long term average. Soil The soil cover is generally coarse calcareous sand containing a very low 0-2% of organic matter (Simon & Rajkai 1985). Close to the soil surface the calcium carbonate content is usually much less than in the parent material due to leaching, and under juniper or pine litter the lime can be completely leached out from the top 1-5 cm and the pH drops below 6. Moving sand surfaces also occur. The humus content of the root zone varies depending on the vegetation and relief. Under forest and in the valleys it can exceed 1%. The depth of the humus horizon is 5-10 cm in open grassland and 10-25 cm in closed grassland and forest. Buried humus layers 20-30 cm thick also can be found and they are expected to play a significant role in the redistribution of subsurface moisture. Coarse sand has an extreme moisture . regime. Whereas the field capacity is very low (6-12 volumetric per cent), the saturated conductivity is very high (500-1500 cm/day) and the conductivity drops close to zero if the moisture content is less than the field capacity. This means that moisture does not disappear from horizons below the root zone even during serious drought (Kertész 1991). The vegetation and its history Centuries ago the sand dune region was covered by an open oak forest. During the late medieval era the forest was burnt several times and heavily grazed by cattle. Juniper was spread during the secondary succession under grazing pressure. In this century and especially 20-30 years ago, additional tree species, some non-indigenous, were

Abstracta Botanica 17 (1993) planted. The present state of the forest is therefore considered to be the result of secondary succession consequent on burning, heavy grazing and forestry intervention. The succession of vegetation on drought-sensitive, sandy regions of the Carpathian Basin and especially on the calcareous sands between the Danube and Tisza rivers, is less predictable and less "regular" than on other soil types such as the grassland succession on loess. It is possible that the reason for the adequate species pool for this edaphic semi-desert being so poor because of the relatively great distance from the potential source of species (Fekete 1991) represented on the surrounding mountains and margins of the Balkans. Nevertheless, the Bugac Juniper Forest is rich in xerofrequent plant species demonstrating a wide variety of adaptations to drought conditions (Kalapos 1989, 1991a, 1991b). A high proportion of the more common plant species are considered to be poisonous or of medicinal value. The animals of the forest

The Bugac Juniper Forest, due to its extreme microclimate and mosaic-like vegetation structure, has an animal community of relatively limited species diversity composed by species well adapted to the semiarid environment. Contrary to the open sandy meadow (puszta) surrounding the dune system, where the insects are the most abundant plant consumers, inside the juniper forest mammalian herbivores are of highest importance. Nevertheless, a few insect species especially adapted to live among the sandhills are good examples of close links between species like the ant Cataglyphis and the antlion Myrmeleon formicoidea. Green and sand lizards hunt for snouted grasshoppers while rollers (Coracias coracias) feed on lizards. There are a relatively few decomposer species because of the lack of available biomass; that is why rabbit dunghills last for years. The bird community of the juniper-poplar forest is relatively diverse compared to the cultivated forests nearby (Sasváry & Csörgó 1989). The characteristic nesting species are the longtailed tit, nightingale, black woodpecker, huupoo, blackcap, roller, nightjar, chaffinch and goldfinch, goshawk, common buzzard and short-eared owl. The fieldfare and bullfinch are abundant winter guests.

189 As most mammals of this forest are nocturnal a systematic study is still needed for estimating their significance. Among mammals rabbits were found to be the most important grazers. The movement of rabbits and smaller herbivores (mostly woodmice) is restricted to the vicinity of shelters. Roe deers and hares, the most abundant browsers of the forest, rest in the bush during daytime but leave the forest at dusk to feed in the meadow. Fox, wild ferret, feral cats and dogs, badger and goshawk are the most important predators of Bugac, hunting mainly for rabbits. Evidence is still needed if they control rabbit population, periodically infected by myxomatosis, but now increasingly immune. Method of mapping The vegetation within a 35 hectare area of the forest was mapped. This area was considered large enough to represent the major vegetation types charasteristic of the dune region and was located so as to include most of the sample plots of previous and currently running field studies. The resolution of the map was determined by the size of the smallest units to be mapped, single juniper shrubs growing on openings. As their diameters were 2-5 m, a 1:1000 scale adequate to draw patches of 2-5 mm was chosen. The mapping was carried out using aerial photographs. The photos were taken on July 3, 1989 from 400 m flight height using a 80 mm focus camera and 6 x 6 cm KODAK AEROCOLOR film. The scale of projection on the film was 1:5000. A paper montage of 1:1000 scale was used for the field verification. The photographs and the montage are produced by ÖKOPLAN GM Hungary. The field verification was carried out in July, 1990, one year after the photograph had been taken but during the same season. Thirteen mapped vegetation types were determined and delineated on the basis of the montage before the start of the field work. Bare sand was distinguished from the other grass types, the grass types from the bushes and woodlands, and within the bushes and woodlands the juniper ([J]), poplar ([P], [JP]) and pine ([S], [N]) dominated types were delineated from the others, although the two poplar- and two pinedominated types were not distinguished from each other within their pairs. All of the remaining borders were determined during field verification. Five new vegetation types ([m],[+],[B],[RJ],[PR])

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were classified in a field survey raising the total number of types to 18.

quently on the study site, it forms only a few dense (1-10 shoots per square meter) patches.

The resultant map was digitized and stored in a personal computer, using the DigiCAD program. The map of the 18 types was drawn at a scale of 1:1000, and the final version was compiled in the summer of 1991. Because a black and white map containing several hundred patches and 18 different vegetation types cannot be easily represented, we have chosen to give a general overview using a three phase map showing grasslands (light phase), forests with juniper (dark phase) and forests without juniper (medium phase).

Poplar stands [P]

Definition of the mapped vegetation types Bare surface [b]

Bare surfaces (less than 5% plant cover) can be found in two forms: roads or routes of animals and gaps between juniper shrubs (type [J]). The thin routes and narrow belts around junipers have not been drawn. Very open grassland [ol

Total plant coverage is less than 25%. It is usually classified as Festucetum vaginatae danubiale or as the grassland phase of Junipero populetum caricetosum. Dominant vascular plant species include Koeleria glanca, Alkanna tinctoria, Euphorbia cyparissias, Cynodon dactylon, Poa bulbosa,

which dominate the annual aspect of the forest in late spring and early autumn. The coverage of lichens (Cladonia spp. Parmelia pokorni) and mosses (Tortela ruralis, Tortella inclinata) can greatly exceed that of the vascular plants. Less open grassland [c]

Total plant cover is over 25 %. It is usually classified as the above type but often consisting of a mosaic of monodominant patches of Carex liparicarpos, C. stenophylla, Koeleria Blanca, Festuca vaginata or Teucrium chamaedrys. More open patches of the closed sandy grassland, Festucetum wagneri holoschoenetosum, are also included in

this category. Grassland with poplar shoots [p]

A variant of the two above mentioned types. The structure of the perennial sandy grassland is still intact but poplars have begun to invade. Milkweed-dominated grassland [m]

Variety of type [c] or less often of type [o]. Although the milkweed, Asclepias syriaca, occurs fre-

The architecture of the poplar stands, Juniperopopuletum ligustretosum, is similar to sandy oak forest. The canopy layer consists of 15-25 m tall Populus alba and P. canescens trees. The shrub

layer is often dense, sometimes two-tiered, and mainly formed by Ligustrum vulgare and Crataegus monogyna. If juniper is dominant or co-dominant in the shrub level the stand is categorized as juniper-poplar [JP]. At the grass level, in addition to the rather common forest species such as Bromus inermis, Cynoglossum viride or Solanum dulcamara, some species can be found which are characteristic of sandy oak forests e.g. Polygonatum latifolium. In the less dense patches many species typical of the open perennial grassland can be found. Young poplar stands [Y]

This is a transitional type between the open perennial grassland with poplar shoots [p] and the completely established poplar stands. The trunk diameter of the trees is from 3 to 10 cm, and they are between 2 and 10 m in height. There is no distinct shrub layer below the canopy. The structure of the preceding grassland can be still clearly recognized in the grass level but at some places weeds have started to invade. Shrub patches [S]

This is a phase of the poplar forest consisting of shrub-sized poplar trees and Crataegus monogyna, Ligustrum vulgare, Berberis vulgaris, Prunus spinosa and Rubus fruticosus. Some stands of Salix rosmarinifolia dominated by the above mentioned

shrubs are also included in this category. Juniper stands [J]

The canopy level is generally absent or contains only rarely isolated, short (5-10 m) poplar trees. The shrub level is dominated by Juniperus communis, although other shrubs may also be found occasionally. The grass level is very poor and often absent but the moss level is usually welldeveloped. Tortella inclinata, Hypnum cypressiforme and their parasite lichen, Diplostiches parasiticus dominate the moss level. Between the juniper shrubs there are small open patches or belts of perennial grassland and more rarely, bare surface. If the width of these exceeds 5 m they are

Abstracta Botanica 17 (1993) included in the corresponding category for grasslands. Juniper poplar stands /JP]

In this. category all of the characteristic components of the Bugac forest are represented. Poplar and juniper trees form the canopy which is sometimes two-layered, with the juniper often shaded by the poplar. The shrub layer consists also of juniper and the various shrub species including Ligustrum vulgare. The grass level is similar to that of the poplar phase but usually richer because this phase is generally more open. The grass layer is often denser than the perennial open grassland. Characteristic stands of the typical Festucetum vaginatae danubiale can be found in the partially shaded gaps of these stands.

Black pine plantations [N]

Where the plantation of Pinus nigra has been successful from the forestry point of view, there is no shrub and grass layer. Where the plantation is less successful the stands resemble the juniper-poplar stands with co-dominant Pinus nigra in the canopy level, although even in these cases the grass layer is poorer than in the juniper-poplar stands. Planted Scotch pine forest [S]

The younger (about 40 years old) and close stands of Pinus silvestris are similar to Pinus nigra, but the shrub and grass layers resemble those of the poplar stands. Main current successional processes

This is a rare variant of the juniper-poplar stands containing birch, Betula pendula, in the canopy level.

According to our observations since 1983, the main successional processes presently occurring in the Bugac Juniper Forest are the occupation of bare surfaces, the spreading of white and grey poplars and the opening of closed grasslands.

Dead juniper [+]

Colonization of bare surfaces

This is a variant of type [J]. At the site of dead juniper the moss layer of the former juniper bush can still be found for years or even one or two decades.

The bare surfaces are generally occupied by perennial species such as Linum hirsutum ssp. glabres-

Juniper-birch patches [B]

Planted poplar stands [P1]

These are 20-30 year old stands without a completely closed canopy. The shrub layer is very poor and the grass layer is similar to that under poplar stands. Planted black locust stands [R]

These are open stands of 8-15 m high black locust trees, Robinia pseudoacacia, in which the shrub layer is often absent. The grass level usually consists of very poor degradated patches of perennial sandy grassland dominated by Calamagrostis epigeios or Melica ciliata. Black locust with juniper [RJ]

This category includes black locust plantations containing juniper in the shrub layer. The grass level is less degradated than in the pure robinia stands (the type [R]). Poplar with black locust [PR]

Similar to the poplar stands but containing black locust in the canopy level.

cens, Koeleria glauca, Populus spp., Alkanna tinctoria, Carex liparicarpos, Euphorbia cyparissias,

and in some stands also annual grass species, mostly Secale silvestre. The most frequent species characterizing the spring ephemer aspect is Arenaria serpyllifolia and in autumn Tragus racemosus. However, except for the invasion of poplar in some places (see below) this occupation is restricted, according to the findings from our fenced plots, primarily by the grazing of the wild rabbits (Szabó et al. 1991). Physical disturbances caused by animals (from horses and humans to antlions) and wind may also help explain the limited success of this colonization. Invasion ofpoplar

The most conspicuous change in the last decade is the rapid spread of the white and gray poplars, Populus alba and P. canescens. Their roots, from which colonizing shoots can sprout, run beneath the root-zone of the herb layer. Shoots can appear at every kind of opening including bare sand, open perennial sandy grassland, Festucetum vaginatae danubiale, closed grassland in the depressions between dunes, Festucetum wagneri holoschoenetosum, and in Salir rosmarinifolia stands. Two other rare poplar species, Populus tremula and P. italica also heavily colonize their surroundings.

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The vegetation map of the study site. The scale is 1 to 2000, i.e. 1 cm is 20 m. The right edge of the first sheet fits to the left edge of the second one and the second sheet similarly fits to the third. The first sheet is the southwestern part of the map, the second one is the medium and the third one is the northeastern part. Light phase denotes grasslands, dark phase denotes forests with juniper and medium phase denotes forests without juniper.

Obviously, this process must be relatively recent or the open areas would have disappeared by now. Two hypotheses have been recently put forward to explain the fast invasion of the poplars. First, the colonization could be a consequence of the

serious drought since 1983, resulting in a water shortage and reduced resistance of the grassland to the poplar. The main problem with this explanation is that we lack information on the nature of the adaptive mechanisms affected by the drought.

Abstracta Botanica 17 (1993)

The vegetation map. Middle part. According to the second hypothesis the invasion is an outcome of a marked change in grazing practices. Presumably, large herbivores such as horses and cattle grazed the area more intensively before 1975 when the National Park was established. However, it now seems probable that the primary factor is the altered grazing regime of the wild rabbit. Since 1978 the winter snow cover has been shorter than the century average, allowing the rabbits as well as hares and roe deer to feed on plants

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other than trees or shrubs. Periodic decline in the population density of rabbits, due mainly to myxomatosis, may also result in decreased grazing pressure. Opening of the closed grassland

The opening of the closed grassland, Festucetum wagneri holoschoenetosum, is a degradative succession as it results in less organized, more variable and less stable stands. Although this process is

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The vegetation map. Third part. li mited to small, isolated patches, it results in the disppearance of a very charasteristic vegetation type from the forest. During succession, the 2-5 m broad and 10-30 m long patches disintegrate into fragments of some square meters. Among the fragments the abundance of Holoschoenus romanus and grasses is reduced and species typical of the open perennial sandy grasslands, such as Carex liparicarpos, can colonize. This succession results

in associations relatively rich in species, although their composition is rather fortuitous. The poplar shoots that are usually absent from the intact closed grassland can appear even in the first stage of this succession. The opening process is almost certainly due to the recent shortage of available water. Originally, the root zone of this vegetation type had access to an

Abstracta Botanica

17 (1993)

extra water supply from the shallow soil water table both through capillary rise and the deeper root systems of some grass species. As a consequence of the intensive canalization of the surrounding region and the serious drought, the depth of soil water table decreased from 1-1.5 m to 3 or more m in the last ten years, thus depriving the extra water supply. The hygrophilous Holoschoenus could not compete under conditions of such water deficiency and gradually lost its dominance while the mesophilous species which lived among its dense stands also disappeared within a few years to be replaced by xerotolerant species like Carex liparicarpos. This process is often paralleled by opening and change in Salix rosmarinifolia stands. When the coverage of Salix reduces other shrubs or even grasses occupy the gaps. Other important recent successional processes The black locust trees which were planted in several places, spread by clonal growth, forming mixed forests with poplar and in some places with poplar and juniper.

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nificant influence on the present vegetation pattern. Although the scale of 1:1,000 used here should provide sufficient resolution for a broad analysis of the impact of animals on the forest, finer scale maps of particular focus sites are necessary for the finer, short-term identification of the mechanisms underlying such effects. Thus, the vegetation maps provide the basis for studying both the spatial pattern of mammalian activity and the resultant changes in the vegetation. Mapping the vegetation, relief, and correlates of animal activity by means such as biotelemetry, and relating these to each other in such a way as to provide an integrated view of the forest life, is computationally very demanding. However, the possibilities provided by the recent development of GIS (Geographical Information System) software, specifically designed for performing just this kind of spatial analysis, should now facilitate a more integrated, systems-oriented approach to the study of the Bugac Juniper Forest and its future development. This study was supported by a grant from the Hungarian Academy of Sciences. We are especially thankful to Robyn Hudson (Ludwig-Maximilian University, Munich) for discussing and correcting the manuscript. Acknowledgements. No. 2316/1991

The recently introduced, adventitious milkweed, Asclepias syriaca has spread fast, occupying patches of closed or open grassland. This represents a serious problem from a conservationist point of view as milkweed may completely change the structure of the grassland. Perspectives The primary aim in compiling a vegetation map of the Bugac Juniper Forest was to describe the present status of vegetation in order to provide a basis for analyzing long-term changes in its pattern. Despite the inevitable difficulties in precisely defining categories, and differences in the levels of description used in doing so, the present map should allow more accurate monitoring of major successional processes such as the invasion of grassland by young poplars. The usefulness of the vegetation map could be increased by combining it with a map of the underlying topography, thus allowing the relation of vegetation types to relief to be studied. Obviously, analyzing the vegetation in relation to relief would provide a much fuller description of the biotope than the two sets of features separately. According to our previous observations and the results of exclusion experiments, the large mammals and particularly wild rabbits, have had a sig-

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