Trends and fluctuations in species abundance and richness in Mediterranean annual pastures

Abstract. Floristic data collected from permanent plots during 14 consecutive years are used to model the frequency of the 62 most abundant species in relation to post-ploughing succession, topography and rainfall in annual Mediterranean grasslands located in a Quercus rotundifolia dehesa. The interannual dynamics of species richness are also analysed. From 1980 to 1993, presence/absence data of grassland species were noted in five 20 cm X 20 cm permanent quadrats placed at random in 1980 in 14 permanent plots on a south-facing slope along the topographic gradient. Weekly autumn rainfall data over the 14 years were analysed using a profile attributes index and Hybrid Multidimensional Scaling to arrange the years according to their autumn rainfall pattern. Generalized Linear Models were used to fit the species richness and species frequency according to topographic position, age since the last ploughing episode, total rainfall in the growing season and autumn rainfall pattern using a forward stepwise procedure. The richness model includes all of these variables, and reveals a relatively high goodness-of-fit (71 %). The fact that the meteorological factors play a key role in modelling richness forces us to include them if we wish to use richness as an indicator of the degree of disturbance in these highly fluctuating annual pastures. Models of species dynamics show that although roughly 33 % of the species have a successional behaviour, the majority are more dependent on temporal heterogeneity associated with rainfall or spatial heterogeneity linked to the topographic gradient.     

Numerical syntaxonomy of the Asplenietalia petrarchae in the Iberian Peninsula

Abstract. A numerical syntaxonomic study of rupicolous vegetation of the Asplenietalia petrarchae in the Iberian Peninsula is described. The study is based on 37 local tables (OSUs) of different rupicolous communities from limestone cliffs found in the eastern half of the Iberian Peninsula. Three classification approaches were followed: (1) ‘hard partition’ techniques, (2) ‘fuzzy partition’ and (3) ordination partition. In order to find a direct solution, we tried an evaluation system based on the inertia associated with a CCA, Canonical Correspondence Analysis, and its relationships with the inertia of a similar CA, Correspondence Analysis. The most effective solutions were found through spatial partitions, and the best partition was obtained with CA considering five segments per axis. Among the fuzzy partitions the best solution was obtained with two clusters and a fuzziness value of 1.20. The best hierarchical partition was Complete Linkage with Chord Distance as a resemblance function. With the GRID procedure applied to the CA diagram, three different groups were distinguished. The first group is related to the Saxifragion boisseriano-reuterani, including five OSUs from Baetic ranges; the second group is very close to the Cosentinio-Lafuenteion rotundifoliae and includes 11 OSUs from thermo-mediterranean areas; the third group, including 21 OSUs basically related to meso-mediterranean areas, is strongly related to the Teucrion buxifolii.     

Spatial variation in a grassland on soil rich in heavy metals

Abstract. The vegetation in a grassland area in the prefecture of Kilkis (N. Greece), known for its surface Cu-mineralization, was studied. 43 quadrats were established along a transect through an area where the vegetation formed patches of different size. Cover-abundance and frequency estimates for all species were made. Normal Association Analysis revealed five quadrat groups characterized by Trifolium scabrum, Linaria pelisseriana, Anthoxanthum ovatum, Gypsophila muralis and Minuartia hirsuta ssp. falcata. 43 soil samples were analysed for pH, organic C, CaCO₃, total content of Fe, Zn, Pb, Cu, Mn, Ca, Mg, K, Na and soil texture. From a discriminant analysis performed on soil data five soil groups resulted, which are highly related to the five vegetation groups. Thymus sibthorpii, Minuartia hirsuta ssp. falcata and Rumex acetosella are the most important taxa with respect to physiognomy and patchi-ness of the vegetation. The number of species in each group of quadrats is affected by the relative favourableness or severity of the soil conditions. Metal contents, mainly Zn, Cu, Mg and Na, and soil texture are considered to be among the main factors controlling the structure and physiognomy of the vegetation.     

Effect of local competition on resprouting of Arbutus unedo after clipping

Abstract. To evaluate the effects of local competition on the growth and size variability of sprouts following disturbance in a natural population of Arbutus unedo in Catalonia, plants were cut at the base and a neighbor removal experiment was performed. Removal of neighbors resulted in an increase in the number and biomass of sprouts at 2 and 7.5 months after clipping. Number and biomass of sprouts was also correlated with initial plant size (stump area). Inter-genet competition appeared to be symmetric, and acted to delay the onset of interference among sprouts within a genet. Size variability of sprouts on an individual was positively correlated with their density on the stump, supporting the hypothesis that competition among ramets (sprouts) within a genet is asymmetric. Reduced inter-genet competition from neighbor removal resulted in an increase in the number and biomass of sprouts growing from a stump. This resulted in an increase in the asymmetry of competition among sprouts, and therefore an increase in the size variability of these sprouts.     

Germination and regeneration mechanisms in Mediterranean degenerate forests

Abstract. Germination and developmental behaviour during juvenile stages are of importance in relation to succession mechanisms acting on plant communities. This is particularly so in the Mediterranean region, where the climatic summer drought causes stress. We investigated and compared germination and development of four tree species in two parts of the French Mediterranean region where the original dominant tree species (Quercus pubescens) has been largely replaced by Quercus ilex in the southern part, and Castanea sativa in the northern part. Observed differences in germination rates and developmental behaviour, especially shootroot ratio and the role of water supply, suggest that other species may eventually replace the current dominant species.     

Evidence for lottery recruitment in Mediterranean old fields

Abstract. The hypothesis of lottery establishment ( Sale 1977 ) explains coexistence of species with similar niches through processes of stochastic recruitment. This initial idea forms the basis for a variety of mathematical models, but has not been tested empirically. This study is a field investigation of lottery establishment for plants with a seed bank, using Canonical Correspondence Analysis to compare the compositions of the vegetation and the seed bank according to different hypotheses on the mechanisms of establishment. This method was used for a data set from old fields from southern France. The weighted lottery (i.e. a random draw from the seed pool, weighted by the frequencies of each species) appeared as the best suited hypothesis to explain the high degree of similarity between the vegetation and the seed bank and the relative spatial distributions of the species. Several mechanisms are probably interacting, depending on the life histories of the species. Modelling and experimental approaches are needed to further test the hypothesis of lottery recruitment.     

Effects of cutting and nitrogen deposition on biodiversity in Cantabrian heathlands

Questions: Does the diversity of heathland vegetation change when subjected to experimental disturbances such as cutting and nitrogen fertilization? Do changes in the vegetation structure negatively affect the regeneration of the dominant species Calluna vulgaris? Is cutting an alternative method of conserving the diversity and maintaining the structure of heathlands in the Cantabrian Mountains? Location: Calluna vulgaris heathlands on the southern slopes of the Cantabrian Mountain range, NW Spain. Methods: A total of 60 plots were treated with different combinations of cutting and twice the estimated atmospheric deposition of nitrogen (56 kg‐N.ha^?1.yr^?1). The changes in the cover values of the species present were monitored over a five year study period. The cover values were used to calculate abundance and species richness. Results: Fertilizing with nitrogen allows biodiversity to increase over time. However, the greatest biodiversity is associated with the cutting plus fertilization treatment, since cutting allows gaps to be opened that are easily colonized by pioneer annual species, while fertilization mainly favours an increase in the mean number of perennial herbs (graminoids and forbs). Increased perennial herb richness also corresponds to a rise in their cover values. The recovery of the dominant woody species in these communities, Calluna vulgaris, is not impeded by the increase in perennial herbs species' cover values. Conclusions: In the Calluna vulgaris heathlands studied, cutting plus fertilization allowed an increase in biodiversity over time. No displacement of the dominant woody species, Calluna vulgaris, is observed due to the presence of the perennial herbs. Cutting patches of heathland is recommended as a mechanism for maintaining high vegetation diversity, when grazing is not possible.     

Effects of herbivory on the growth and reproduction of Picris hieracioides in the Mediterranean region

Abstract. The impact of herbivorous insects on the growth and reproduction of Picris hieracioides (Compositae) was followed in three consecutive years by comparing plants treated with an insecticide and untreated plants in an old-field population abandoned for 15 yr (P15). In the third year the interactions between herbivory and competition were analysed as well. Pre-dispersal seed predation of seed heads was investigated in three adjacent old-fields abandoned for one (P1), 15 (P15) and 40 yr (P40) in the region of Montpellier, France. Fields differed in vegetation cover and floristic composition and also in density and above-ground biomass of Picris hieracioides plants (from 246 g per plant in P1, 13.3 g in P15, down to 3.3 g in P40, for the first year of study). The cumulative rainfall between January and July, the period of Picris growth, decreased from 1992 to 1994. Moreover, in 1992, three times as much rainfall occurred in May and June – the period of stem elongation – as in 1993 and 1994. As a consequence, environmental stress increased during the study period. Herbivorous invertebrates had a negative effect on the growth and reproduction of individual plants of Picris hieracioides in P15. The effect was not constant with time, being apparent in only two years out of three. These two years (1992 and 1994) had contrasting climatic characteristics, whereas the year with no apparent effect (1993) was climatically intermediate. When herbivory had effects on growth, the insecticide-treated plants had a significantly higher above-ground biomass and produced twice as many seed heads as the control plants. There was no interaction between herbivory and competition. The percentage of damaged seed heads was significantly lower in the P1-field than in the P40-field. The P15-field had intermediate levels of predation. Predation on seed heads increased whereas biomass and reproductive output decreased between 1992 and 1994 in the three old-fields. Therefore, the results on seed predation (but not for above-ground biomass and reproduction) tend to confirm‘the environmental stress hypothesis’ of Mattson & Haack (1987) that states that plants under stress are more liable to be attacked by grazers.     

Post-fire bryophyte dynamics in Mediterranean vegetation

Abstract. Bryophyte dynamics after fire in the Mediterranean macchia of Southern Italy was studied both by diachronic and synchronic approaches. Changes of bryophyte cover and species composition were found in relation to both age and fire intensity. During the first 2 yr after fire, bryophytes dominated the plots which had experienced the highest fire intensity while herbs were dominant in plots affected by lighter fires. Pioneer species, such as Funaria hygrometrica, Barbula convoluta and Bryum dunense, characterized recent intense fires, whereas Bryum torquescens, B. radiculosum and B. ruderale were dominant after less intense burning. Pleurochaete squarrosa, Tortula ruraliformis and Tortella flavovirens dominated intermediate successional stages. Pleurocarpous mosses were dominant only in the older closed stands. Different patterns of regeneration strategies were described: spores dominated early stages of intense fire, while vegetative propagules characterized later successional stages and less severely burned areas. Although bryophytes usually have a low abundance in Mediterranean vegetation, their role in post-fire vegetation dynamics may be locally enhanced according to burning conditions.     

Seed banks of Erica australis and Calluna vulgaris in a heathland subjected to experimental fire

Abstract. Soil samples were collected before and after an experimental fire on a heathland in the province of León (Spain). The seed banks were assessed by counting the numbers of seedlings appearing of Erica australis and Calluna vulgaris. A total of 2285 germinable seeds/m² before burning and 1177 germinable seeds/m² after the fire were estimated for Erica australis, and 90 and 690 germinable seeds/m², respectively, for Calluna vulgaris. After the fire the number of germinable seeds/m² of Erica australis had decreased, whilst there was a considerable increase in the number of germinable seeds/m² of Calluna vulgaris. Neither seedlings nor sprouts of Calluna vulgaris had appeared in the field plots 10 months after the fire. Erica australis did recover mainly by sprouting after fire.     

Prediction of occurrence of vascular plants in deciduous forests of South Sweden by means of Ellenberg indicator values

Abstract. In this study we present a new method for predicting the occurrences of species using data from deciduous forests in South Sweden. Complete species lists of vascular plants were compiled from 101 stands and from representative sample plots inside the stands. Soil samples from each stand were collected for determination of pH and nitrogen mineralization. Presence-absence data for species were fitted to the values of four environmental variables - soil moisture, soil reaction (pH), soil nitrogen and light - by means of Linear (Multiple) Logistic Regression (LLR), and Gaussian (Multiple) Logistic Regression (GLR). First, these values were estimated by calculating the weighted averages of Ellenberg indicator values. Second, the estimates for reaction and nitrogen were substituted by the real measurements of pH and mineralized NH₄⁺, keeping the Ellenberg estimates for light and moisture. The models were validated by an independent test data set. In general, the models had high predictive abilities. GLR fitted the species occurrences better to the environmental variables than LLR, but had a lower accuracy of prediction of species occurrence in the stands. The use of soil measurements instead of Ellenberg indicator values did not improve the predictive abilities of the models. The environmental conditions in the stand test set were successfully estimated by using species data from the plots. When using the species lists of the stands instead of plot data, a slightly better predictive ability was obtained. The collection of plot data, however, is easier and less time-consuming. The accuracy of prediction differed considerably between species.     

The vertical distribution of below-ground biomass in grassland communities in relation to grazing regime and habitat characteristics

Abstract. 40 sites, representing different pasture types in Northwest Spain, were sampled in respect of their floristic composition, distribution of above and below-ground biomass and environmental and physical variables. Five plant community types were identified by classification techniques of plant species composition. These communities were then characterized in terms of the percentage of ground covered by herbaceous and shrub vegetation, stones, rocks and gaps as well as their topographic location and characteristics of the shallow soil (pH, organic matter, nitrogen and calcium content). Bio-mass was assessed in terms of above-ground structures, surface crowns and three below-ground layers to a depth of 10 cm. Three types of grazing regime were distinguished: Concentrated Intense Grazing in early spring (CIG), Extended Intense Grazing throughout the spring (EIG), and Non-Intense Grazing (NIG). Grazing regime showed the highest association with plant community type and three broad categories were identified: xeric stressed pastures, which nevertheless received CIG, mesic pastures with EIG, and three kinds of NIG mesic pastures. The xeric communities had the highest proportion of aboveground biomass, as a consequence of their greater proportion of woody perennials. These xeric communities displayed a more gradual reduction in below-ground biomass with depth than mesic pastures, a likely consequence of the low water content in the upper soil layers. The mesic communities had a high concentration of below-ground biomass in the upper layers when they were intensely grazed. However, when grazing was low (i.e. NIG situations), these communities had greater variability in biomass profiles than any of the other pasture types. Possible causes of the patterns in biomass distribution of the intensely grazed pastures are discussed.     

Changes in chalk-grassland structure and species richness resulting from selective nutrient additions

Abstract. A series of fertilization experiments was carried out over a 5-yr period in a chalk grassland in Limburg (The Netherlands) as part of a study of the maintenance of species richness in species-rich grasslands. Phosphorus and nitrogen were shown to be the most limiting nutrients. Addition of both elements doubled above-ground production, and species richness dropped ca. 50 % in 0.01-m² subplots, relative to controls. However, neither the above-ground production nor plant growth-forms were sufficient to explain the observed changes in species richness. Small-scale structural heterogeneity of the vegetation is probably critical for maintaining high levels of richness. Historically, high nitrogen, low phosphorus conditions were rarely encountered in the Dutch landscape and few species appear adapted to these conditions. Among the chalk grassland species, Brachypodiumpinnatum seems well adapted to these conditions, where it dominates and excludes most other species. A detailed understanding of the small-scale processes responsible for maintenance of species richness is critically important in efforts to maintain the biodiversity of natural ecosystems.     

Time and space in the community structure of a species-rich limestone grassland

Abstract. The community structure of a species-rich grassland was investigated at a small spatial scale (0.001 m²), to determine whether evidence suggesting assembly rules could be found in temporal or spatial variation in either species richness or guild proportions. The community was alvar limestone grassland on the island of Öland, Sweden. Three sites were sampled: two lightly grazed, the other recently ungrazed. Plots with and without fertilizer were compared. Evidence was sought for restriction on the ability of species to co-occur within a limited spatial area. Restriction due to a limited number of niches available,‘niche limitation’, could be manifest as lower variance in quadrat richness than expected under a null model (i.e. RV_r, the ratio of observed: expected variance in richness, would be < 1.0). In several cases, RV_r values were significantly < 1.0, even using a patch model to allow for possible spatial variation in the environment. Low RV_r values were found only at the smallest square quadrat size, 10 cm². On Fertilized plots in the years immediately after application of fertilizer, low RV_r could not be demonstrated. Explanations of low RV_r other than niche limitation are considered, such as environmental heterogeneity (present and/or historical) and limitations to the co-occurrence of individual plant modules. Assembly rules based on guild membership were sought by looking at the variance across quadrats in the proportions of species from morphological guilds. An assembly rule would be seen as relatively constant proportion, estimated via RV_gp, the ratio of observed: expected variances in guild proportions. Significant guild proportionality was found in some cases. There was no evidence of guild proportionality in the years after the application of fertilizer. The significant effects in RV_r were more numerous than expected on a random basis, though not observed in every site in every year. Similar trends were seen in RV_gp. At the space/time scales examined, the species in a plant community may be constrained by assembly rules only intermittently, e.g. when resources are more limiting (Wiens 1977). Under this concept, when competition is relaxed, such as following fertilizer application, there is a temporary microhabitat ‘waterhole’ in which more species can coexist, and the assembly rules break down, at least temporarily whilst the species composition adjusts. There was some indication of a return to more deterministic community structure four years after fertilization commenced. Variants of van der Maarel's Carousel model were tested. A Niche-limited Carousel Model (i.e. a model in which there is some limitation in the number of species that can occupy a microsite) would imply restricted variation in richness through time for a single quadrat (temporal RV_r). Overall differences between years in species richness were demonstrated, and their effect removed; after this adjustment there was support for the Niche-limited Carousel Model. The extent of this limitation varied between sites. There were also consistent differences between quadrats in species richness. There was little evidence for constancy of guild proportions through time. The site that showed the strongest community structure in time and space, least year-to-year variation in mean species richness, and least response to fertilizer perturbation, is that on the shallowest soil. Possibly the thin soil results in greater resource limitation, supporting suggestions that assembly rules are stronger when resources are more limited.     

Climatic discrimination of Mediterranean broad-leaved sclerophyllous and deciduous forests in central Spain

Abstract. Climatic differences between three types of deciduous (Quercus pyrenaica) and three types of sclerophyllous (Quercus rotundifolia) Mediterranean forests in the Spanish Sistema Central were analyzed by means of Canonical Discriminant Analysis and Jancey's Discriminant Analysis, applied in successive steps to data from 252 meteorological stations. Climatic data included temperature and precipitation records as well as bioclimatic indices. Discriminant analysis was applied to broad-leaved sclerophyllous and deciduous forest communities sampled at each meteorological station using phytosociological methods. Annual and seasonal (summer, spring) water availability are the most important factor controlling the distribution of the two physiognomic forest types; southwestern associations of Quercus pyrenaica and Q. rotundifolia differ from their colder homologues by annual and monthly temperatures; western associations were separated from eastern ones in terms of annual and seasonal precipitation gradients. Discriminant analysis was a good technique to explore climatic gradients not shown by other general ordination or classification methods.     

How do species dominating in succession differ from others?

Abstract. Data on succession were collected from 15 seres starting on bare ground in man-made habitats (i.e. sites disturbed by various mining activities, bulldozed sites, ruderal urban sites, the exposed bottom of a destroyed water reservoir, and abandoned fields), all in the western part of the Czech Republic, Central Europe. The period for which the data on succession were available ranged from 12 to 60 yr. 56 species reaching dominance in some period of succession were selected (the criterion being: at least 25 % cover in at least one year in any sere); they were compared for biological and ecological traits with other species participating in the seres (167 species with at least 1 % cover in at least one year in any sere), and with the Central European flora as a whole. Significant differences between the species dominating in succession and others were found for the following traits: life form, life strategy, pollination mode, and ability of lateral spread. Dominant species differed from the regional flora in distribution of life strategies, pollination mode and immigration status. The results suggest that an ‘ideal successional dominant’ is a tall, wind-pollinated plant, often a geophyte capable of intensive lateral spread, requiring high nutrient supply and sufficient site moisture. The set of traits contributing to achieving dominance by a species in human-made habitats includes both features occurring independently of phylogenies (life strategy, pollination mode, plant height, moisture demands) and those which dominant species probably share due to their common ancestors (nutrient demands, capability of extensive lateral spread).     

Small-scale spatial dynamics of plant species in a grassland community over six years

Abstract. In a species-rich mountain grassland in the Krkonosse Mts., Czechoslovakia, data from four permanent plots of 50 cm x 50 cm were recorded annually from 1985 to 1990 to study the spatial dynamics of the species. Plots were divided into 15 x 15 subplots and the number of vegetative units of all plants within each subplot was determined. There was not much net change at the plot level, but the subplots were very dynamic. Two aspects of the spatial dynamics of the species were followed: (1) persistence, i.e. the tendency of the species to remain in the same subplot, and (2) long-distance spreading, i.e. movement to subplots beyond the immediate neighbourhood. Species differed widely in their persistence and longdistance spreading and were classified into mobility types: long-range guerrilla, short-range guerrilla, phalanx and 'sitting’. The mobility types were, to a certain extent, correlated with the growth form of plants, but some species of one growth form showed different types of small-scale dynamics and some species with different growth forms had the same spatial dynamics.     

Effect of allogenic processes on successional rates in former river channels

Abstract. Former braided channels of a floodplain subjected to the same river regime and regulation are supposed to experience different rates of ecological succession. These differences are assumed to be related to the position of the channel within the floodplain and the subsequent scouring effect of floods. These hypotheses were tested on six former braided channels of the upper Rhône River that have the same age of isolation from the main channel. Channels were discriminated by their substrate grain size, the coarser grain size being related to the stronger effect of floods. Aquatic vegetation indicates the successional stage of the channels through their trophic degree, with on one hand typically eutrophic species, such as Lemna minor and Ceratophyllum demersum, and, and on the other hand, mesotrophic species, such as Berula erecta or Callitriche platycarpa. According to a new analysis, the matching between the two data sets delineated the correlation between the grain size and the trophic degree that corresponds to the successional stages. Two of the six channels are in the most advanced stages of succession as marked by eutrophic species occurring on fine substrate. Two others are in an earlier successional stage with coarse substrate and mesotrophic species, although they have the same age as the latter ones, demonstrating a lower successional rate. The difference in such rates results from the scouring effect of floods which slows down successions by removing fine sediment and vegetation. Succession is also slowed down by ground water supplies in two channels not subjected to a noticeable scouring effect.     

Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation

Abstract. Four herbaceous plant species of a sand dune area and several herb species of an open early‐successional patch were investigated for the occurrence of a simple relationship between aboveground biomass and plant cover. Without exception linear regressions of aboveground biomass on plant cover were found with slope factors depending on the growth form of the species. These results suggest that (early) growth of herbaceous plants in low and (temporarily) open vegetation is not affected by possible constraints caused by a decreasing ratio of plant cover to aboveground biomass. The obtained linear relationships could be used for rapid non‐destructive determination of aboveground biomass by image‐analysis of cover data.