On the Occurence in Romania of Potentillo albae-Quercetum petraeae Libbert 1933 Association

The paper presents a plant community of dry-mesic oak forests that is new to Romania, Potentillo albae-Quercetum petraeae Libbert 1933. This association belongs to Quercion petraeae alliance and its range extends over the subcontinental area of Central Europe. A multivariate analysis (cluster analysis, non-metric multidimensional scaling ordination) of a large phytosociological database was used to identify diagnostic species, interspecific associations and the gradient of species composition. Based on species composition and site parameters, a group of relevees in oak forests that correspond to the diagnosis of Potentillo-Quercetum was identified. The Romanian relevees corresponding to this association were previously assigned, pro parte, to Carici montanae-Quercetum Gergely 1962 and Quercetum robori-petraeae Borza 1959. The relations with these and other potentially related two associations, Genisto tinctoriaeQuercetum petraeae Klika 1932 and Quercetum petraeae-cerris Soó 1963, are discussed. The vegetation and site conditions of the Potentillo albae-Quercetum association in Romania are illustrated in a phytosociological table with 25 relevees and a distribution map is provided.


Introduction
In 1962 Gergely described the association Carici montanae-Quercetum petraeae from Trascau Mountains (western part of Transylvanian basin, Romania).He noticed the resemblance with the Central European association Potentillo albae-Quercetum Libbert 1933, but stated that the phytocoenoses from Trascau Mt. are different because of the presence of Carex montana and some leguminous species and the absence of spontaneous Turkey oak (considered a specific taxon for the last association, based on the works of Hungarian and Austrian authors).Yet, he asserted that Potentillo albae-Quercetum is present in the Transylvanian Plain, in the area of mixed forests with sessile oak (Quercus petraea) and Turkey oak (Quercus cerris).Since then no other references to this association have been made and no description of the syntaxon was provided for Romania.Forests of sessile oak and Carex montana, regarded as association or subassociation to Genisto tinctoriae-Quercetum petraeae Klika 1932, were reported in the Huedin basin (Csüros et al., 1969), the mountains around Brasov-south-eastern Transylvania (Danciu, 1972;Kovacs, 1979).Ularu (1972) described a Carex montana variant of Quercetum robori-petraeae Borza 1959.In the recent Romanian syntaxonomic works (Coldea and Pop, 1996;Kovacs, 2004;Sanda et al., 2001Sanda et al., , 2008) ) neither Carici montanae-Quercetum nor Potentillo-Quercetum were taken into account.
Considering the phytosociological information gathered since the Gergely's publication (Gergely, 1962) and the statistical tools available nowadays, the aim of this study is to investigate the syntaxonomical position of oak forests with Carex montana and the hypothesis regarding the presence of Potentillo albae-Quercetum in Romania.This endeavor is also important in the context of the nowadays efforts towards sustainable forest management and conservation of all representative forest ecosystems from Romania (Donita and Biris, 2003;Abrudan et al., 2009;Ioras et al., 2009;Stancioiu et al., 2010Stancioiu et al., ). ler, 1992;;Chytrý, 1997;Roleček, 2005Roleček, , 2007)), taking into account the diagnostic species, the site conditions and the similarity of the overall species composition.Other syntaxa accounted for comparison are Genisto tinctoriae-Quercetum Klika 1932 and Quercetum petraeae-cerris Soó 1963, as these associations were considered in the published literature as being linked with Carici montanae-Quercetum and Potentillo albae-Quercetum respectively.

Results and discussion
The cluster analysis of the 105 selected relevees of drymesic oak forests from Romania leads to the separation of three clusters, as it is indicated by the crispness function of classification in JUICE.The first ten diagnostic species and their fidelity for each cluster are presented in Tab. 1.The species resulted as diagnostic for clusters are also convergent in the ecological and sociological meanings (verified by interspecific associations in JUICE), therefore the solution could be ecologically accepted (i.e. it is not just a statistical evidence).Two clusters correspond to the well known associations Genisto tinctoriae-Quercetum petraeae Klika 1932 (Cluster 2) and Quercetum petraeae-cerris Soó 1963 (Cluster 3) respectively.
The Cluster 1 is intuitively similar to Potentillo albae-Quercetum Libbert 1933 and the arguments for this classification are given below.
(i) Among the 43 diagnostic species for Cluster 1 almost all of the species found as diagnostic for this association in Central Europe, by different phytosociologists are included.In each region (country) there are some specific taxa, like Fragaria moschata in Czech and Slovak relevees, and Pulmonaria angustifolia in Polish relevees.Other species, like Melampyrum nemorosum or Galium sylvaticum are replaced in Romania by their ecologically similar taxa-

Materials and methods
This study is based on the analysis of a database for over 1400 relevees stored in TURBOVEG (Hennekens and Schaminée, 2001) corresponding to oak forests from Romania.The relevees were sampled according to Braun-Blanquet methodology and originate partly from literature (801) and partly from our field work (610).In order to have an accurate image of the gradient in species composition, methods of numerical analysis were applied.Firstly, from the entire database only those relevees having the dominants Quercus petraea, Q. robur or Q. cerris were kept.Out of these, dry-mesic oak forests relevees were selected based on the results of the cluster analysis: the clusters with diagnostic species common for oak-hornbeam forests (Carpinion) or thermophilous forests (Quercion frainetto) were eliminated.Finally 105 relevees were selected.The analysis was performed using: the presence-absence data for species cover (in order to diminish the inconstancy in the assessment of species cover by different authors), the Sørensen coefficient for the similarity between relevees and the linkage method of flexible beta (β = -0.25, a value verified as giving the best results (McCune et al., 2002)).The optimal number of clusters corresponds to the maximum crispness value (Botta-Dukát et al., 2005).For measuring the species' fidelity the phi coefficient was used, with the cut off value of 0.30 (Chytrý et al., 2002;Willner et al., 2009).To visualize de variation between the clusters identified by the classification, the selected relevees were ordinated by the means of the non-metric multidimensional scaling (NMS) method.The distances between relevees in the floristic space were calculated by the mean of Sørensen dissimilarity, for presence-absence data.The arrangement of phytosociological tables and calculation of species fidelity were performed using the JUICE software (Tichý, 2002), and the classification and ordination of relevees by using PC-ORD (McCune and Mefford, 2006).
The absence of such species does not affect the association's validation.The Romanian relevees include several species with regional distribution: Iris ruthenica, Phyteuma tetramerum, Crocus banaticus, Melampyrum bihariense, Helleborus purpurascens, Lathyrus transsilvanicus.Considering the concept of association in a broader sense, such elements could eventually represent differential species for a local subassociation.
(ii) There are similarities regarding ecological groups of species.One characteristic of the association is the mixture of xerophytes, mesophytes and acidophytes.The differences compared to acidic oak forests are, firstly, the species of intermittently wet and heavy soils-some of them common in meadows of Molinion alliance (Potentilla alba, Serratula tinctoria, Carex pallescens, Succisa pratensis, Ranunculus polyanthemos, Carex montana, Potentilla erecta, Poa angustifolia, Molinia caerulea agg., etc.).Secondly, the presence of eu-mesotrophic species of oak-hornbeam forests (like Stellaria holostea, Galium schultesii, Carpinus betulus) indicates more favorable ionic and mineral properties of the soil as compared to the case of acidic forests.The occurrence of acidophilous species (Luzula luzuloides, Calamagrostis arundinacea, Pteridium aquilinum, Veronica officinalis, Hieracium murorum, Genista tinctoria, Vaccinium myrtillus etc.) is in consensus with the descriptions published until now (Matuskiewicz and Kozlowska, 1991;Chytrý, 1997;Roleček, 2005Roleček, , 2007)).For example, Luzula luzuloides is also present in the Central European relevees of Potentillo-Quercetum with frequencies of up to 47% and cover indices of 3.This species was found to be diagnostic, in the context of dry-mesic forests, for the alliance (Chytrý, 1997) or even for the association (Roleček, 2005).Moreover, the poor development of the moss layer and the higher number of species in phytocoenoses are arguments for not including these forests into Genisto tinctoriae-Quercetum. On the other hand, the differences against thermophilous associations consist of the cooler, more humid and poorer sites, located in the area of the mesic forests, thus corresponding with the significance of Quercion petraeae alliance-more humid and slightly cooler than Quercion pubescenti-petraeae or Aceri tatarici-Quercion ( Jakucs, 1960;Chytrý and Horak, 1997).The studied forests are located outside of the optimal range of thermophilous oaks, Q. frainetto, Q. pubescens or Q. pedunculiflora, therefore the species of Quercion frainetto or Aceri tatarici-Quercion are missing.
(iii) The subboreal species (Vaccinium sp., Pyrola sp.), present in Romanian relevees, are considered (Wallnöfer et al., 1993) as a major trait for Potentillo-Quercetum, against peri-pannonian Quercetum petraeae-cerris.Even Norway spruce (Picea abies) occurs, partly due to natural causes, inside or close to the sessile oak stands in south-eastern Transylvania -reflecting also the continental nuances of the climate.
(iv) The ecological conditions are equivalent with those from the Central European range of the association.The most peculiar is the imperfect drainage of the soil.The higher elevation of Romanian phytocoenoses compensates the southern latitude while the Carpathian arch reduces the submediterranean influences from the south and the continental ones from the east.A curious analogy makes reference to the distribution in the peripheral areas of watersheds (Libbert, 1933;Roleček, 2005).
The NMS ordination of dry-mesic oak forests of Romania shows a clear separation of the three clusters (Fig. 1).The first 2 axes jointly explain 70% of the variation of floristic composition in the original n-dimensional space.
In some cases Carex montana may reach high cover on xeric-oligotrophic sites, in transitional communities towards Genisto tinctoriae-Quercetum, but this syntaxon could be easily recognized by site physiography (moderately to steep slopes, shallow and skeletal soil) and the lower number of species.Another association considerably linked to Potentillo-Quercetum, but not identical, is Quercetum robori-petraeae.This was originally defined (Borza, 1959;Soó, 1951) as xeric association, but in time its meaning became too heterogeneous: nowadays it is included in Genisto germanicae-Quercion Neuhäusl et Neuhäuslová-Novotná (1967), although many relevees correspond to oak-hornbeam forests.
In order to describe the vegetation and site conditions of These forests are developed on flysh, paleogenic sediments (marls and calcareous sandstones), volcanic sediments, limestone or conglomerates, at altitudes between Fig. 2. Distribution of the Potentillo albae-Quercetum association in Romania 400-800 m, on the upper part of the slopes, on plane or gently inclined surfaces.The soils are slightly to moderately acidic (pH = 4.6-5.3),with stagnic properties due to the higher proportion of clay in the B horizon.The mean annual temperature is 7-8°C and the mean annual rainfall is 550-650 mm.The continentality index (I c = T max -T min ) is between 22 and 23, indicating a subcontinental climate.

Conclusions
Both intuitively and by statistical analysis (classification and ordination methods, fidelity measure and interspecific associations) it is demonstrated that Potentillo albae-Quercetum Libbert 1933 association is present in Romania.This syntaxon is confined to specific site conditions-the same as in its known range and indicated by the similar group of diagnostic species, it poses a high floristic diversity and could be clearly distinguished from other related associations.Its extent to the south is possible due to the climatic compensations of the higher altitudinal sites and the "shadow" provided by basins.This paper contributes to the knowledge on the distribution and variability of the Potentillo-Quercetum association, as a consequence of the valuable formalized classifications initiated by the Czech School of phytosociology.It is expected that the results will contribute to the development of proper strategies for the conservation of the representative forest habitats, that should be implemented in the future at local or regional level.