dynamics
When we explore the Pannonian region on foot, we see different climatic steppes. How should we understand this variability? In which character- istics are the stands different from each other? Is it possible to distinguish “types”? Are all of these “types” of the same importance? What relation- ships are there among these types? How did they develop? What factors explain their existence or their changes?
These questions can be answered by the theory of Bálint Zólyomi and Gábor Fekete, which de- scribes and explains the origin, development, dif- ferentiation and dynamical relationships of the different steppes developed in the forest steppe
zone of the Pannonian region. This comprehen- sive theory is a reconstruction, describing thou- sand-years long vegetation history of a huge area, and the original appearance and distribution of a particular, formerly extended vegetation type in it, namely the Pannonian loess steppes, already shrunk to small fragments by now. Recognition of phytosociological relationships is mostly based on phytogeographical analogies. Space for time approach was used for the description of finer scale and short-term differentiation as well. The most important recognition of the theory is that it turns special attention to the loess steppe mead- ow from the actual range of variation. The reason for this is that it is the “best-organised” grassland type being in equilibrium with the climate of the zone. The composition of steppe meadow is the richest and many other grassland types can be de- rived from it, so it isa totipotent form. For testing the theory, detailed studies in community ecol- ogy, population biology and ecophysiology have been performed during the last twenty years by the research-team of Gábor Fekete. The results of these studies have supported the theory from various aspects.
What is the mechanism of differentiation? If the balance of factors ensuring the coexistence of many species breaks down, particular species combinations will be selected and their abun- dance increases. If the balance is disturbed only slightly and for a short period of time, regulat- ing mechanisms are able to rebalance the system. Differentiation occurs, when the intensity of the disturbing factor crosses a threshold and the giv- en process starts to strengthen itself by positive feedbacks. An example for positive feedback is when the steppe opens up due to drought or over- grazing and reaches a certain openness, soil ero- sion starts, which leads to further opening. The feedback is positive, thus the open grassland state increases the probability that there will be open vegetation in the future. From the permanently open grassland broad-leaved xero-mesophilous species are selected out, while the ones tolerat- ing the changed environment are maintained. As a consequence, the species composition changes , a new type evolves, and a particular step of the
0 20 40 60 80 100 Borítás (%) S B C F E BR szárazabb --- nedvesebb Stipa Bothriochloa Chrysopogon Festuca Brachypodium 0 10 20 30 40 S B C F E BR Fajszám (2×2m) szárazabb --- nedvesebb 0 20 40 60 80 100 S B C F E BR 0 40 80 120 160 200 S B C F E BR Összes borítás (%) szárazabb --- nedvesebb 0 20 40 60 80 100 S B C F E BR %- os hasonlóság szárazabb --- nedvesebb Fig. 11
The alteration of species composition, species groups and commu- nity characteristics along a phytiosociological gradient (coenoclin) in the loess grasslands of Isaszeg. The coenoclin was developed by ranking those vegetation types which were recognizable in the field from the driest habitat towards the more humid ones. Patch types: BR – Brachypodium pinnatum type, E – forest steppe type, F – loess steppe type dominated by Festuca spp., C – Chrysopogon gryllus type, B – Bothriochloa ischaemum type, S – Stipa capillata type. Each vegetation patch (except for Bothriochloa ischaemum type) is dominated by a single grass species, so the dominance ty- pes can be delineated unambiguously. The cover of xero-mesophi- lous species decrease sharply along the gradient from more humid types towards drier ones. The abundance of xerophilous species (ignoring the extremes of the gradient) remains rather constant. The number and total cover of sub-ordinate species decrease slightly towards the drier extreme of the gradient. When compa- ring the species composition of the different vegetation types, considerable overlap becomes obvious between them. On the ave- rage, 60 % of the species occur in all of the patches. Consequently, we can declare that there are only slight differences among the vegetation patches when dominant species are ignored.
B F H E D R C L S 1983-2002 1983-1988 &1988-2002 B F H E D R C L S 1983-2002 &1988-2002 B E H F D R C L S
Fig. 12 Changes in the steppe vegetation following the cessation of grazing ( about 1980), nearTard (based on Imelda Somodi’s work).
Patch types: F– Festuca rupicola type, D – Danthonia alpina type and S – Sieglingia decumbens type represent vegetation states close to natural, while the H – Chrysopogon gryllus type and R – vegetation dominated by shrubs have transitional character. B –Bothriochloa ischaemum type, C – Calamagrostis epigeios type, E –Elymus repens type and L – Leontodon hispidus type (the latter have disappeared by 2002) developed as a result of degradation. The transition probabilities between patch types can be calculated on the bases of the maps on the left side. Figures on the right show the graphs visualizing the most frequenttransitions. Arrows of different colours represent different transition probabilities. Discernibly, transition probabilities are not constant and the most characteristic transitions of different periods are also differ. The graph is closed concerning the whole duration of the survey (ignor- ing the shrub dominated type), what means that each patch types can transform into any other else. An admirable experience of the 20-year-long survey is the fact, that the effect caused by the cessationof grazing can only be recognized after a longer period. Almost no difference can be found between the first two maps. Accordingly, no sign of grazing can be detected in the first 5 years, yet later it becomes striking.
In general, the breakdown of balance or equi- librium can take place in different ways. Although there is only one real equilibrium, there are many possible quasi-equilibrium states. Moreover, if the equilibrium of the system is a result of the interactionsof different forces, like in the steppe meadow, changes even in the relative strengthof the forces can lead to a high variety of “break- down” states. Thus, it is not surprising that the classification of the variety that developed due to different disturbances is a very hard task. Smaller disturbances are easy to be recognised on the field, and botanists often match these changes with “in- dicator species”. We observe for example, that on the mounds of wild bore clonal species with run- ners spread vegetatively swiftly, such as Fragaria or Teucrium chamaedrys. In case of slight over- grazing, Euphorbia pannonica, Eryngium camp- estre, or Ononis spinosa will reach higher cover, while in case of the cessation of the management causing litter accumulation the abundance of Galium verum increases. Filipendula vulgaris or Stipa species can flower in multitudes following burning. These are well-known field anecdotes, with which many field botanist will agree, but there will be others, who will insist on something totally different. In the lack of systematic, com- prehensive and statistically correct experimental studies we should not analyse these examples. We should rather conclude that steppe vegetation
reacts to smaller disturbances in various ways, however, these changes will not be conserved in the long run, and thus the steppe regenerates. For real differentiation the effect must be long-term and must act at larger scale.
In the present landscape, under the present climate, geomorphologic differences lead to real differentiation. Grazing can increase further the differences between the slopes of northern and southern exposition. In case of moderate graz- ing, forest steppe meadows rich in broad-leaved graminoids and forbs can be preserved on the northern slope, while on the southern slope broad-leaved species disappear and a Festuca rupicola dominated steppe rich in narrow-leaved, grazing- and drought-tolerant species differenti- ates. The degree of the differentiation may vary stand by stand. However, there are characteristics of the spatial organization that differ consistently
between the southern and northern slopes as it was proved by the studies of András Horváth.
According to the analyses of soil samples there is no major gradient in the soil parameters along the slope in steppe meadows, even if the slope is steep. This means that soil dynamics are balanced, there is no considerable erosion. On the other hand, on rocky slopes with shallow soils and scat- tered trees, the conditions are not adequate for the development of closed grasslands. The shad- ing and root-competition of trees as well as the dry leaves and the shallow soil with rock outcrops prevent the development of closed grasslands. In these places the soil dynamics are not balanced, erosion is continuous. Even if a kind of grass- land similar in appearance to steppe meadows develops under these conditions, its organisation and dynamics are different and can not reach the equilibrium which is characteristic for the steppe meadow. This more dynamic, more patchy type with more interaction with the neighbouring vegetation types (forest, forest fringe, shrubland, open rocky grassland) is called slope steppe.
A special pathway of the differentiation is the secondary succession towards steppe after defor- estation. This process was studied by Gábor Fekete and Klára Virágh in a foothill landscape on loess, close to the village Isaszeg. In the studied area a dynamic mosaic of different types of grasslands has developed following the logging. The patches of Brachypodium pinnatum living originally in the understorey layer of the forest, grew sideways and patches of Bromus erectus filled in the gaps among them mostly. Festuca rupicola appeared early in the shifting to the steppe, its patches be- came larger and larger with time. Some patches of Brachypodium pinnatum could preserve the broad-leaved forest species for a long time, even for centuries, while its newly established patch- es are colonized by steppe species. The result of competition between dominant species is influ- enced by the weather. In wet years the patches of Brachypodium pinnatum grow larger, while dry years favour Festuca rupicola. Patch dynamics can be characterised by the transition probabili- ties among the particular patch types.
Spontaneous fires and grazing by large herbiv- ores were part of the natural disturbance regime of ancient steppes. Plant and animal species (e.g.
small mammals, insects) have been adapted to re- current wildfire effects, which does not influence the survival of populations significantly. These natural disturbances in general do not lead to the differentiation of new vegetation types. Although it may happen that an area is not burnt for years, then a very strong fire appears due to the accu- mulated litter, which causes severe damages and changes in the vegetation. Large herbivores mov- ing in herds can also cause larger disturbance , since they usually graze the grassland with dif- ferent intensities. There are overgrazed and tram- pled parts, we do not know, however, whether these effects are long-lasting enough to give rise to a new vegetation type.
Traditional pasturing, as a management was probably more frequent and more regular than natural disturbances. Therefore grazing by do- mestic animals more often leads to overgraz- ing. In an overgrazed loess steppe broad-leaved grasses and forbs disappear; they are replaced by rosette-forming or small sized creeping species and narrow-leaved grasses. In an extreme situa- tion even the forest steppe meadow of a northern slope can differentiate to a stage similar to dry steppes on the southern slopes. When grazing intensity decreases, the species rich forest steppe meadow can regenerate again, although it is only possible if the soil has not been degraded severely and if there are propagule sources in the sur- roundings.
Permanent plot studies showed that the rank of species in the dominance hierarchy is surpris- ingly stable over time despite the variance in the abundances caused by weather fluctuations. Dominance ranks can change, however, due to grazing pressure. Dominant species can be sup- pressed locally and a former subordinate can get the first rank. This leads to the development of the so called dominance-types which further enrich the diversity of vegetation in loess valleys. Without grazing, topographical differences are the leading factors of differentiation. In the dis- cussed case differentiation is limited to the Brach- ypodium pinnatum type on the northern slopes, and Festuca rupicola and Stipa species (mainly Stipa capillata) dominated types on the southern slopes. Patches of Bromus erectus, Calamagrostis epigeios, Chrysopogon gryllus or Bothriochloa is-
chaemum can further complement the dominant types in the case of recent logging or permanent, higher intensity grazing. Grazing-tolerant forbs dominated (Leontodon hispidus, Potentilla are- naria, Hieracium and Thymus species) patches may develop due to severe trampling and graz- ing of grasses. Grazing affects erosion processes as well, so on the upper and bottom verges of the loess valley further types can differentiate locally, for example patches dominated by Bromus iner- mis or Elymus repens or even by Astragalus cicer.
Quantitative ecological studies reveal that there could be abrupt shifts in the dominant grasses from type to type while the overall species composition does not change much. By investi- gation along a mesic-xeric coenoclin (a gradient of changing species composition) it is shown that species richness decreases towards the xeric end of the gradient (BOX1). On this end, the physi- ognomic differentiation and floristical poorness of Stipa capillata dominated stands is apparent. Xerophilous and generalist forbs are present along
the full gradient, while broad-leaved xero-mes- ophilous species are absent from xeric sites. For- est species are only present on the other (mesic) end, in the Brachypodium pinnatum dominated type. However, these types can easily transform into one another, the differentiation is partial. The dynamical relationships among the types, the so-called patch-dynamics can be studied by repeated vegetation mapping and monitoring in permanent quadrates. (BOX2). Understanding probabilities of patch-transitions and revealing their driving factors provide bases for conserva- tion management.
What is the level of the naturalness of these vegetation mosaics developed as a consequence of human land use and disturbance? There are many kinds of disturbance resulting in a variety of vegetation types. Still there is a major direc- tion of differentiation along the moisture gradi- ent (mesic-xeric) in every case. So, according to this and to the structure and history of the forest steppe zone, there are three major ecological spe- cies groups which can be distinguished in general: forest-patch - forest fringe species, forest steppe meadow species and steppe species. If the natu- ral disturbance is replaced by human disturbance we can still call the vegetation natural or better
semi-natural as these two kinds of disturbance do not differ significantly from each other either in intensity or in extent. The grassland types devel- oped under these conditions can transform into one another. Slight degradation is followed by quick regeneration. However, the observed quick and spectacular dynamics of loess steppes can be misleading. These swift transformations are only possible if propagules are available. Therefore one of the most important features is the capacity of the landscape to hold and preserve these species for centuries and millennia. Consequently, plan- tation of alien trees or meliorisation with fertilis- ers or by sowing seeds to natural grasslands might cause serious problems, such as an irreversible loss of diversity and misfunctioning.