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Kivonat:

Loess grasslands have wide distribution in the forest-steppe region of Hungary. They can be found regularly on formerly forested sites, mostly in the final stage of secondary succession. In these grasslands Festuca rupicola is a common species. The intermediate successional stages are rare and differ from the final stage by the importance of Brachypodium pinnatum. Clones of Brachypodium pinnatum survive forest cut and they develop into species rich xeromesophilous Brachypodium communities. These grasslands can persist for a long time and slowly change into a more xeric type of Festuca rupicola dominated grasslands, typical of loess vegetation of the area. During these changes many mesic, forest and forest-edge species disappear, parallelly with the colonization of several xeric loess-steppe species. Therefore Brachypodium grasslands provide an excellent opportunity to study the transformation of plant communities. The fine-scale pattern of species coexistence during community transitions is unknown.

We studied which species pattern transformations were characteristic during the shifting process of a xeromesophilous Brachypodium pinnatum community into xerophilous communities and how the replacement of mesic and xeric species and coexistence structure changes between species happened along a successional gradient, along a varying fine-scale coenological and environmental conditions. Here the results of fine-scale within stand multispecies spatial coexistence relations are presented in stands (Forest-type, Transitional- and Steppe-type) representing the early, middle and late successional phases of xeromesophilous Brachypodium community. These successional stages also present a light gradient associated with the declining dominance of Brachypodium pinnatum.

Micro-maps of rooting individuals (or ramets) of vascular plant species were recorded in 20x110 grids of contiguous 5x5 cm micro-quadrats. Pairwise interspecific associations and multispecies spatial patterns were analysed by information theory methods of Juhász-Nagy. The significance of the associations was assessed by Monte-Carlo randomization tests based on the Site Model of Wilson.

We found a decreasing trend of frequency (standardized by the number of possible species pairs) of significant associations during the secondary succession. There were some particular coalition structure characteristic for the successional stages. Grassland types were spatially well-structured with highly comp-lex multispecies coalitions. Coalition structures were the most developed in the Transitional-type. There were great differences how the individual species were spatially assembled in the 3 stages. Early stage was characterized by a coalition of mesic species. Xeric and mesic grasses formed a coalition in the middle stage but they were segregated in the late successional stage. The pattern of species replacement was diffuse and gradual instead of  a complete replacement of species during the transformation process at stand scale.

Considering the relative role of dominant and subordinated species in the spatial organization of these communities, we found contrasting patterns of the multiple associations according to the abundance and successional status of species. Subordinated grass species were generally strongly associated to the community, that indicated indirectly they could not appear everywhere within the community. The new colonizing subordinated species prefered certain patch types (or gaps).

We concluded that micro-structure changes and the reorganization of the assemblages of subordinated species in the successional communities were slow and gradual, parallelly with the decreasing viability of Brachypodium clones. At the final successional stage of xeromesophilous Brachypodium pinnatum community the coalition structure was very similar to that of the xerophilous Festuca grasslands, except for the presence of Brachypodium pinnatum. However, complete replacement of Brachypodium pinnatum by Festuca rupicola and by other steppe grass species could take place in the long run. We believe that as the Brachypodium pinnatum community does not represent a stabile community in the Pannonian forest-steppe region, it will be converted by natural processes into a Festuca rupicola grassland wich can be considered as a steady-state formation. The shifting process on the forest to steppe gradient can be very similar to that we observed in our study site. Such a change eventually leads to the demise of many forest species.