Mari Cruz Díaz-Barradas

Effects of Pinus pinea litter on seed germination and seedling performance of three Mediterranean shrub species

Pinus pinea L. is a coniferous tree inhabiting the Mediterranean Basin, which appears widely distributed in the Iberian Peninsula, frequently associated to Mediterranean shrub species, such as Cistus salvifolius L., C. libanotis L. and Halimium halimifolium (L.) Willk. (Cistaceae). This study investigated the effects (chemical and physical) of P. pinea needles on the seed germination and early growth of these three shrub species. Laboratory experiments were carried out in order to assess an expected negative effect by using different treatments to seeds (physical layer and two aqueous extracts concentrations). Seed germination of all species was inhibited when treated with aqueous extracts and the degree of inhibition was higher when using the extract with the highest concentration, while no effect was observed with the physical layer treatment. Seedling performance was also sensitive to the different treatments, showing the most stressed values with the high concentration extract. The results from this study highlight the importance of allelopathy of P. pinea needles as a mechanism for controlling seed germination and seedling performance of some Mediterranean shrub species.

Season-dependent and independent responses of Mediterranean scrub to light conditions

Semi-arid plant species cope with excess of solar radiation with morphological and physiological adaptations that assure their survival when other abiotic stressors interact. At the leaf level, sun and shade plants may differ in the set of traits that regulate environmental stressors. Here, we evaluated if leaf-level physiological seasonal response of Mediterranean scrub species (Myrtus communis, Halimium halimifolium, Rosmarinus officinalis, and Cistus salvifolius) depended on light availability conditions. We aimed to determine which of these responses prevailed independently of the marked seasonality of Mediterranean climate, to define a leaf-level strategy in the scrub community. Thirty six leaf response variables - involving gas exchange, water status, photosystem II photochemical efficiency, photosynthetic pigments and leaf structure - were seasonally measured in sun exposed and shaded plants under field conditions. Physiological responses showed a common pattern throughout the year, in spite of the marked seasonality of the Mediterranean climate and of species-specific differences in the response to light intensity. Variables related to light use, CO2 assimilation, leaf pigment content, and LMA (leaf mass area) presented differences that were consistent throughout the year, although autumn was the season with greater contrast between sun and shade plants. Our data suggest that in Mediterranean scrub shade plants the lutein pool could have an important role in the photoprotection of the photosynthetic tissues. There was a negative linear correlation between the ratio lutein/total chlorophylls and the majority of leaf level variables. The combined effect of abiotic stress factors (light and drought or light and cold) was variable-specific, in some cases enhancing differences between sun and shade plants, while in others leading to unified strategies in all scrub species.

How do Mediterranean shrub species cope with shade? Ecophysiological response to different light intensities

Under natural conditions, light exposure for Mediterranean shrubs can be highly variable, especially during cloudy days or under a canopy, and can interfere with other environmental factors such as temperature and water availability. With the aim of decoupling the effect of radiation and temperature from water availability, we conducted an experiment where two perennial and three summer semi-deciduous shrub species were subjected to different levels of irradiation. In order to follow plant responses to light exposure, we measured gas exchange, photosystem II photochemical efficiency, photosynthetic pigments and leaf mass area in spring and summer. Results showed that all study species presented a plastic response to different light conditions, and that light-related traits varied in a coordinated manner. Summer semi-deciduous species exhibited a more opportunistic response, with higher photosynthesis rates in full sun, but under shade conditions, the two strategies presented similar assimilation rates. Stomatal conductance did not show such a drastic response as photosynthetsis, being related to changes in WUE. Daily cycles of Fv /Fm revealed a slight photoinhibitory response during summer, mainly in perennial species. In all cases photosynthetic pigments adjusted to the radiation level; leaves had lower chlorophyll content, higher pool of xanthophylls and higher proportion of the de-epoxydaded state of xanthophylls under sun conditions. Lutein content increased in relation to the xanthophyll pool under shade conditions. Our results evidenced that radiation is an important driving factor controlling morphological and physiological status of Mediterranean shrub species, independently of water availability. Summer semi-deciduous species exhibit a set of traits with higher response variability, maximising their photosynthetic assimilation under different sun conditions.

Species-specific effects of the invasive Hieracium pilosella in Magellanic steppe grasslands are driven by nitrogen cycle changes

AimsThe principal objective was to evaluate the interference by the invasive species, H. pilosella, on native grassland species at the physiological performance level. We hypothesised that the invasive species is able to alter the nitrogen uptake of native plant species, and can modify community functioning.MethodsThis study was performed under field conditions in the Magellanic Steppe (Argentina). We compared stable isotope signatures, nutrient content and several functional physiological traits in four grassland species with and without H. pilosella interference.ResultsWe found significant interference effects from the invasive species on native species, mostly through changes in nitrogen uptake. The variation in the natural abundance of foliar δ15N suggests that the native plants switched nitrogen sources due to interference with the exotic species. A linear relationship between chlorophyll and proline content that disappears when species are under H. pilosella interference, suggests major changes in the N allocation of native species. Grassland species under interference with exotic species exhibit lower photochemical efficiency and higher water use efficiency. Canonical discriminant analysis evidenced the existence of a different set of functional traits between invasive and native plants, and also among native species with and without H. pilosella interference.ConclusionsOur results support the hypothesis that H. pilosella exerts intense interference with native species through shifting the N sources available for native species, a lower leaf N content, and increasing water stress.

Water source partitioning among plant functional types in a semi-arid dune ecosystem

Co-ordinating Editor: José Paruelo Abstract Question(s): The aim of this work was to characterize the main water sources used by the woody plant community of a semiarid coastal dune ecosystem. We ask: Do the seasonal water sources used by plants differ between functional types? Does the seasonal drought promote shifts towards deeper soil layers in all plants? Does greater use of deep soil water enables the maintenance of a more favorable plant water status? Do watersourcesuse strategies mirror overall drought strategies? Location: Semiarid coastal dune system in Doñana Biological Reserve, southwest Spain. Methods: We analyzed the oxygen isotope composition of xylem water of fourteen coexisting woody species and compared it to water sources (soil water at different depths, rain and groundwater), both in spring and dry summer. Bayesian isotope mixing models were used to estimate the proportion of each water source used by plants. We tested the influence of different (categorical) traits on plants’ watersourceuse. We evaluated the relationships between the use of deep soil water and ecophysiological parameters related to water economy (leaf δ13C and reflectance waterindex). Results: A greater similarity between species was found in periods of higher water availability (spring). Contrastingly, during the dry period the traits considered showed a significant effect on the water sources used and a high interspecific differentiation was observed. Accordingly, species clustered in five wateruse functional groups, exploring water from shallower to deeper soil layers. Greater use of deep soil layers in the dry season was linked to a better seasonal maintenance of plant water status. Conclusions: Coexisting plant functional types segregated along a spectrum of watersourceuse under extremely dry conditions, evidencing great soil water partitioning. Relevant seasonal wateruse shifts towards deeper soil layers were observed, but not in all species. Furthermore, we confirmed that the watersourceuse behavior accompanied broader strategies of drought resistance.