Running headline: Assembly rules along a long stress gradient Summary 1. A central issue of community ecology is finding rules that explain the composition and abundance of co-existing species. Nowadays two main processes, environmental filtering and limiting similarity are thought to play the main roles in structuring communities. Their relative importance under different environmental conditions, however, is still not properly clarified. 2. We studied the strength and the effect of environmental filtering (causing convergence) and limiting similarity (causing divergence) in 137 sample plots along an extremely long environmental gradient ranging from open sand grasslands to highly productive marshes, using a trait based approach. The main environmental gradient (i.e. productivity) was characterised by the Normalized Difference Vegetation Index, an indicator of aboveground live biomass. Cover of the plant species was estimated visually. Values of 11 plant traits were collected from field measurements and databases. Mean and dispersion of the trait values of the plots were quantified by community-weighted means and Rao's quadratic entropy. Trait convergence and divergence were tested by randomization tests, followed by the study of changes in effect size along the productivity gradient by fitting generalized additive mixed models (GAMM). 3. For vegetative traits we found mainly convergence, indicating the filtering effect of environmental constraints, while traits related to regeneration showed divergence. 4. The strength of convergence in vegetative traits generally decreased as productivity grew, indicating that while under harsh conditions environmental constraints strongly limit the possible trait values; under more benign conditions various water and nutrient-use strategies are adaptable. At high productivity, the strength of divergence in regenerative traits decreased. Since the larger diversity of vegetative traits found here reduces competition, the importance of diverse reproductive strategy is probably lower. 5. Synthesis: Our results partly support the stress-dominance hypothesis, but reveal that assembly rules are more complex. The relative importance of environmental filtering and limiting similarity depends on the trait and on the environmental conditions of the habitat. Traits related to resource use are generally limited by environmental filtering, and this restriction is weakening as conditions become more favourable, while traits related to regeneration are constrained by limiting similarity and are more diverse under harsh conditions.

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Question: Based on the spontaneous vegetation development of old-fields in the Ny ırs eg and Kiskuns ag sand regions (Hungary), we aimed to answer the following questions using the chronosequence method: (1) how do the proportions of different functional groups change during succession;(2) which target species establish successfully in the old-fields during the course of succession; and (3) how successful is spontaneous succession in the recovery of target grasslands?Location: Two sand regions of the Great Hungarian Plain: (1) the Ny ırs eg sand region (East Hungary, acidic sand, moderately continental climate) and the Kiskuns ag (Central Hungary, calcareous sand, continental climate). Methods:Altogether 24 old-fields were classified into young (<10-yr-old), middle-aged (10-20-yr-old) and late-succession (20-40-yr-old) old-fields; four fields in each age category. For baseline vegetation reference, three open and three closed sand grassland stands in both regions were sampled in the vicinity of the old-fields. The percentage cover of vascular plants was recorded in five 2 9 2-m plots in each field, in early May and late June 2012. We used life forms, clonal spreading traits and Ellenberg indicator values for nutrients in the analysis. Species of Festuco-Brometea class were considered as target species.Results: The cover of hemicryptophytes and geophytes increased, the cover of short-lived species decreased with time. Cover of species without clonal spreading ability decreased, while cover of species with clonal spreading ability increased with increasing field age. The cover of invasive species decreased with increasing field age. The majority of target species had established already in the young and middle-aged old-fields, although their cover was significantly higher in the two older age groups.Conclusion: Spontaneous succession can be a vital option in recovery of sand grassland vegetation in Central Europe; the majority of the species pool of sandy grasslands can be recovered in the first 10-20 yrs. However, the success of grassland recovery can be strongly influenced by the surrounding species pool and can be slow if seed dispersal is limited. Spontaneous succession is most promising when the target species of grasslands immigrate at the very beginning of the succession, within the first few years.

Invasion of alien plant species is one of the main reasons for biodiversity loss in terrestrial ecosystems. However, alien plant species are not evenly distributed in the landscape. We studied which factors determine the actual level of neophyte invasion in a landscape with heterogeneous land-use and which habitats are the most infected. Since neophyte species with different life-forms can respond differently to the factors determining the invasion, species groups of annual, herbaceous perennial and woody neophytes were also analyzed separately. The study was conducted within the field site network of the Kiskun-LTER program (Hungary), in 16 sites of 5 km  5 km. Fifteen habitat types were distinguished belonging to five major land-use/land cover types (agricultural land, abandoned agricultural land, tree plantation, semi-natural grassland and semi-natural forest). Present and past land-use, landscape composition and environmental variables were included as factors with a potential impact on the level of invasion. The most important factor determining invasion level was present habitat type, followed by the past habitat type of the location and landscape context. Tree plantations, agricultural habitats and recently abandoned agricultural habitats had the highest level of invasion. As expected, annual neophytes were most abundant in agricultural habitats, while perennial herbaceous neophytes were most abundant in old-fields and plantations, and woody neophytes in tree plantations. Past agricultural land-use was reflected in the higher levels of invasion of annuals and perennials, and past forestry practice resulted in higher levels of invasion of woody neophytes. In a landscape with a higher proportion of tree plantations, not only the tree plantations, but primary woodland [ 2 4 0 _ T D $ D I F F ] patches also showed higher levels of invasion by woody neophyte species. Our results indicate the importance of present and past land-use in plant invasion and suggest that tree plantations are hot-spots of plant invasion and threaten the remnants of semi-natural vegetation.

QuestionsWhat are the main floristic patterns in the Pannonian and western Pontic steppe grasslands? What are the diagnostic species of the major subdivisions of the class Festuco‐Brometea (temperate Euro‐Siberian dry and semi‐dry grasslands)?LocationCarpathian Basin (E Austria, SE Czech Republic, Slovakia, Hungary, Romania, Slovenia, N Croatia and N Serbia), Ukraine, S Poland and the Bryansk region of W Russia.MethodsWe applied a geographically stratified resampling to a large set of relevés containing at least one indicator species of steppe grasslands. The resulting data set of 17 993 relevés was classified using the TWINSPAN algorithm. We identified groups of clusters that corresponded to the class Festuco‐Brometea. After excluding relevés not belonging to our target class, we applied a consensus of three fidelity measures, also taking into account external knowledge, to establish the diagnostic species of the orders of the class. The original TWINSPAN divisions were revised on the basis of these diagnostic species.ResultsThe TWINSPAN classification revealed soil moisture as the most important environmental factor. Eight out of 16 TWINSPAN groups corresponded to Festuco‐Brometea. A total of 80, 32 and 58 species were accepted as diagnostic for the orders Brometalia erecti, Festucetalia valesiacae and Stipo‐Festucetalia pallentis, respectively. In the further subdivision of the orders, soil conditions, geographic distribution and altitude could be identified as factors driving the major floristic patterns.ConclusionsWe propose the following classification of the Festuco‐Brometea in our study area: (1) Brometalia erecti (semi‐dry grasslands) with Scabioso ochroleucae‐Poion angustifoliae (steppe meadows of the forest zone of E Europe) and Cirsio‐Brachypodion pinnati (meadow steppes on deep soils in the forest‐steppe zone of E Central and E Europe); (2) Festucetalia valesiacae (grass steppes) with Festucion valesiacae (grass steppes on less developed soils in the forest‐steppe zone of E Central and E Europe) and Stipion lessingianae (grass steppes in the steppe zone); (3) Stipo‐Festucetalia pallentis (rocky steppes) with Asplenio septentrionalis‐Festucion pallentis (rocky steppes on siliceous and intermediate soils), Bromo‐Festucion pallentis (thermophilous rocky steppes on calcareous soils), Diantho‐Seslerion (dealpine Sesleria caerulea grasslands of the Western Carpathians) and Seslerion rigidae (dealpine Sesleria rigida grasslands of the Romanian Carpathians).

Development of semi-natural vegetation has recently been a primary concern of restoration efforts. A primary management question is whether active intervention is required or spontaneous secondary succession could suffice. We studied 54 old-fields in central Hungary, which differed in time since abandonment but which had similar environmental conditions and management histories. The sites were grouped into four age groups according to the time elapsed since cultivation abandonment: 1-5, 6-10, 11-23 and 24-33 yr. In each old-field we recorded the species and estimated their abundances. We grouped species in two ways: according to life form (annuals, biennials, perennials, woody plants) and according to coenological behaviour (weeds, sand and steppe generalists, specialists). We analysed the changes in species number and abundance in these categories as a function of site age.Contrary to other successional studies, the total number of species did not change significantly among the four age groups. A significant change was detected between the first two age groups as to life-form composition. Species number and abundance of annuals decreased, while the perennials and woody plants increased. As to coenological behaviour, species number changed only in the first two age groups, while abundance changed in the first three. Weeds quickly disappeared and specialists established and spread, while the species number and abundance of generalists did not change significantly. We concluded that the basic shifts in species composition are almost completed within 10 yr. Most of the late successional species colonized and weeds disappeared. We conclude that there was no need for active intervention in this system: the spontaneous secondary succession leads to semi-natural vegetation.

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