Ana M. Sánchez

Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands

The worldwide phenomenon of shrub encroachment in grass-dominated dryland ecosystems is commonly associated with desertification. Studies of the purported desertification effects associated with shrub encroachment are often restricted to relatively few study areas, and document a narrow range of possible impacts upon biota and ecosystem processes. We conducted a study in degraded Mediterranean grasslands dominated by Stipa tenacissima to simultaneously evaluate the effects of shrub encroachment on the structure and composition of multiple biotic community components, and on various indicators of ecosystem function. Shrub encroachment enhanced vascular plant richness, biomass of fungi, actinomycetes and other bacteria, and was linked with greater soil fertility and N mineralization rates. While shrub encroachment may be a widespread phenomenon in drylands, an interpretation that this is an expression of desertification is not universal. Our results suggest that shrub establishment may be an important step in the reversal of desertification processes in the Mediterranean region.

Seed size, shape and persistence in dry Mediterranean grass and scrublands

Seed size and shape, measured as the variance of the three main dimensions, have been proposed as good indicators for predicting seed persistence. We tested whether these variables were robust predictors of seed persistence in the soil for 58 abundant herbaceous species, primarily annuals, in grass and scrubland of central Spain. Seed persistence was estimated from data on germinable seed banks, while seed weight and shape were measured using fresh seeds collected in the study area. There was a significant tendency for species with persistent seeds to have smaller seeds than species with transient seeds. Seed shape was not, however, related to persistence and we did not find any clear seed weight/shape threshold for predicting persistence. The binary logistic model of seed bank type as a function of seed weight was significant and explained 67% of total variability. Supplementary information about dormancy, environmental conditions of habitat, predation and attack by pathogens has to be used to elaborate more accurate general predictive models of seed persistence.

Abundance and habitat segregation in Mediterranean grassland species: the importance of seed weight

Abstract We analysed the relationship between seed traits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same area. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Multiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small-seeded species without specialised dispersal structures are over-represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and small-seeded species were relatively more abundant in the upper than the lower slope zones. This pattern is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability. Abbreviations: CSA = Cross species analysis; PIA = Phylogenetically independent analysis; PIC = Phylogenetically independent contrast. Nomenclature: Castroviejo (1986–1999), except taxa yet to be covered which follow Tutin et al. (1964–1980).

Dispersal mechanisms in Lavandula stoechas subsp. pedunculata: autochory and endozoochory by sheep

It is normally assumed that Lavandula stoechas subsp. pedunculata (Miller) Samp. ex Rozeira (Labiatae) is dispersed by autochory in spite of the clear pioneer nature of the species. This paper examines the efficiency of autochorous dispersal (seed rain) and the possibility that the species is also dispersed endozoochorally by sheep. Seed rain was measured using pitfall traps and adhesive strips in summer 1999. The viable seed content in sheep dung was measured by greenhouse germination of dung collected monthly in the summers of 1998 and 1999. Two experimental tests were also conducted to ascertain whether dung-borne seeds could be established under field conditions and to evaluate the effect of added dung in the establishment of seeds taken from plants. The recorded seed rain was 2544 seeds m � 2 inside the Lavandula patches, with an aggregated distribution. Autochory around the mother plant fitted a negative exponential distribution, with 90.5% concentrated in a 0‐30 cm radius and a maximum distance of 1 m. Viable Lavandula seeds were found in 73% of the examined dung samples, with an average of 5.5 seeds per sample (6 g) and a high interand intra-annual variation. This high seed content, together with the daily sheep movements over several kilometres, make the species highly dispersible, possibly explaining its clear pioneer nature. Moreover, available data suggest that seeds in sheep dung can germinate and establish under natural conditions, and that dung addition has a positive effect on species establishment.

Environmental control of germination in semi-arid Mediterranean systems: the case of annuals on gypsum soils

Whether or not it is time for germination is a dilemma for annuals since they have only a single opportunity to reproduce successfully. This is critical for species living in stressful and unpredictable environments such as Mediterranean habitats. In order to clarify the environmental cues of germination, four annuals of different families, all of them occurring very frequently on gypsum soils, were selected and their germination observed under 14 climate scenarios, mimicking temperature and photoperiod conditions from autumn to spring, at two levels of water availability (continuous irrigation versus an initial single irrigation event followed by a progressive soil desiccation). In spring scenarios, two seed-storage conditions were compared: dry cold and room conditions. In the absence of water limitation, germination decreased from early autumn to late spring. Water scarcity always reduced germination, especially in early spring. Our results suggest a facultative winter germination behaviour and highlight the crucial role of dry cold storage in reducing spring germination. In conclusion, Mediterranean ephemerals showed a very plastic germination response that allows them to take advantage of favourable environmental conditions from autumn to spring. This environmental cueing is combined with the ability to dilute the risk through a variable rate of seed dormancy that, according to bet-hedging strategies, increases from secure autumn to riskier spring.

Female reproductive output in a Mediterranean shrub: effects from inflorescence to population.

Inflorescence effects have been poorly studied, in spite of the functional relevance of the inflorescence in fruit and seed ecology. The present study focused on the effects of inflorescence size and phenology, and flower position within the inflorescence, in relation to fruit and seed production of the Mediterranean shrub Ononis fruticosa. Variability in fruit and seed production, seed weight and germination were estimated and modelled. Results confirmed that the most important predictors in seed production were inflorescence flowering time and flower position within the inflorescence. Thus, the number of mature seeds per fruit was higher in earlier inflorescences and in basal positions. On the other hand, predation was higher in fruits in basal positions. In fact, seed predation seemed to be the most important factor controlling final seed production. Models at the plant level suggested a negative incidence of geitonogamous pollination and resource limitation, which were also observed at the fruit level. This study confirmed the relevance of inflorescence effects on the reproductive output of O. fruticosa. Although the underlying processes could not be identified, our results provide several hypotheses for future experimental studies.

Endothelium‐dependent relaxation of isolated splanchnic arteries from cirrhotic patients: Role of reactive oxygen species

Aim:  To examine the endothelium‐dependent relaxation of splanchnic arteries during cirrhosis as well as the role of reactive oxygen species in this relaxation using hepatic arteries from cirrhotic patients undergoing liver transplantation and mesenteric arteries from liver donors.

How might edaphic specialists in gypsum islands respond to climate change? Reciprocal sowing experiment to infer local adaptation and phenotypic plasticity

Background and Aims Local adaptation and phenotypic plasticity are considered key mechanisms for coping with climate warming, especially for plant species that inhabit island-like habitats. In Spain a complete guild of edaphic specialists, most of them threatened, occurs in gypsum outcrops, but how these species will respond to climate change has received little attention. Methods A reciprocal sowing experiment was performed to determine the extent of local adaptation and phenotypic plasticity in five gypsophytes with contrasting distributions along a climate gradient. Germination, seedling growth and survival were recorded during a 4-year period. Key Results Plants responded plastically according to their positions along the regional climate gradient, as well as locally between matched locations. All species exhibited highly plastic responses and stress-tolerant behaviours, especially in terms of seedling survival during summer drought. However, no evidence of local adaptation was detected in any of the locations, where local individuals never performed better than those from other sites. In some sites, both germination and seedling recruitment were higher irrespective of parent plant origin. Conclusions The lack of local adaptation to drought displayed at the regeneration stage indicates limited capacity for in situ genetic response to new climate scenarios. Nevertheless, a plastic response along the climatic gradient does suggest a wider species-level capacity to enable these edaphic specialists to cope with increasing aridity over coming decades.

Winter is coming: plant freezing resistance as a key functional trait for the assembly of annual Mediterranean communities

Background and Aims In Mediterranean annual plants, germination mainly occurs during the autumn and only those seedlings that survive winter freezing can flower and produce seedlings in spring. Surprisingly, the effect of freezing events as an abiotic determinant of these communities remains unexplored. The present study aimed to investigate how freezing events affect annual Mediterranean communities and whether their functional structure as related to freezing resistance is linked to the main biotic and abiotic determinants of these communities. Methods In 120 plots located on a semi-arid Mediterranean steppe (Spain), the community functional structure related to the lethal temperature causing 50 % frost damage (LT50 trait) in seedlings was estimated and summarized as the community-weighted mean (CWM-LT50) and its functional diversity (FD-LT50). Plots were stratified according to distance to rabbit shelters and latrines as a proxy for rabbit density, proximity to Stipa tenacissima and spring water availability, where annual species abundance was recorded in all plots over three consecutive years. Key Results Annual species were able to resist a threshold temperature of -4 °C and most had LT50 values around the absolute minimum temperature (-9.5 °C) in the three years. Higher rabbit densities led to lower CWM-LT50 and higher FD-LT50 values. Plots close to Stipa tussocks had higher CWM-LT50 values whereas water availability had no effects. Conclusions High freezing resistance was extended among winter annual species, suggesting the presence of an association between historical environmental filtering and low winter temperatures. However, the community functional structure related to freezing resistance remained variable among scenarios with differences in herbivory pressure and distance to perennial vegetation. The trends observed indicate that traits that allow plants to deal with herbivory may also promote freezing resistance, and that tussocks can act as nurses via microclimatic amelioration of harsher winter conditions.

Assessing seed and microsite limitation on population dynamics of a gypsophyte through experimental soil crust disturbance and seed addition

Understanding the factors limiting population growth is crucial for species management and conservation. We assessed the effects of seed and microsite limitation, along with climate variables, on Helianthemum squamatum, a gypsum soil specialist, in two sites in central Spain. We evaluated the effects of experimental seed addition and soil crust disturbance on H. squamatum vital rates (survival, growth and reproduction) across four years. We used this information to build integral projection models (IPMs) for each combination of management (seed addition or soil disturbance), site and year. We examined differences in population growth rate (λ) due to management using life table response experiments. Soil crust disturbance increased survival of mid to large size individuals and germination. Contributions to λ of positive individual growth (progression) and negative individual growth (retrogression) due to managements varied among years and sites. Soil crust disturbance increased λ in the site with the highest plant density, and seed addition had a moderate positive effect on λ in the site with lowest plant density. Population growth rate (λ) decreased by half in the driest year. Differences in management effects between sites may represent a shift from seed to microsite limitation at increasing densities. This shift underscores the importance of considering what factors limit population growth when selecting a management strategy.