Spatial and seasonal patterns of water use in Mediterranean coastal dune vegetation

Aims: This paper examines the water dynamics of a coastal dune plant community, addressing spatial and seasonal variations. We aimed to detect the patterns of water use by plants at the community level according to their distribution across a coastal dune gradient from beach to inland. Methods: Five sites were established: upper beach, embryo-dune, slack, foredune, and inland. Eight perennial species were collected seasonally to analyse the isotopic composition and water potential. Soil water samples at 3 depths, groundwater, and atmosphere, were obtained to determine plant water sources. Results: The species from Inland and foredune plant communities, Retama, Juniperus, and Helichrysum, showed the most stable isotopic signal throughout the year. On the contrary, the species most abundant on the upper beach, embryo-dune, and slack (Ammophila, Achillea, and Polygonum) showed the highest variability. Water deficit decreased the dependence on shallow and mid-soil layers along the beach-inland gradient. Beach and embryo-dune sites showed less negative leaf water potential values than the other positions in the dune gradient. Conclusions: Three factors mark the proportion of water sources used for vegetation in coastal vegetation: community composition, distance to the sea, and seasonality. Coastal dune vegetation exhibited a species-specific response in water uptake that was modified by its location on the gradient. From upper beach to inland, the plant communities showed a slight progressive increase in the use of water from deeper layers. This pattern was similar and overlapped with the wet to dry seasonal pattern.Ministerio de Economía y Competitividad CGL2009‐1057

The hemiparasitic shrub Osyris lanceolata (Santalaceae) does not disturb the ecophysiology of its hosts

Osyris lanceolata is a hemiparasitic dioecious shrub species growing on stabilised dunes. This study aimed to determine the spatial structure of O. lanceolata and its hosts, to understand the ecophysiological mechanisms involved in water and nutrient transfer, and quantify the physiological stress on its hosts. In the Asperillo stabilised dunes (Doñana, Spain) we selected one mixed plot (50 m × 50 m) with Pinus pinea and Juniperus phoenicea in which the coordinates of every individual, the hosts and the hemiparasitic species, were recorded. Additionally, we selected two study areas in which O. lanceolata was well represented, one dominated by P. pinea and the other by J. phoenicea. We marked 60 plants: 20 O. lanceolata with their respective hosts (10 Pinus and 10 Juniperus) and 20 free host plants. In all the marked plants we measured: leaf water potential, gas exchange, photochemical efficiency, morpho-anatomical leaf traits, pigments, and proline content. Our results evidenced that O. lanceolata is spatially associated with its hosts. Midday leaf water potential values were always between 1.5 to 3 times more negative and transpiration rates were 6-fold higher for the hemiparasite than for the hosts. Additionally, O. lanceolata plants showed a high accumulation of proline in leaves and haustorium. Although the hemiparasitic species maintains an active photosynthetic canopy, its rates of CO2 assimilation were 35–48% lower than in the hosts, which caused a 10-fold lower instantaneous water use efficiency. Through these mechanisms, the hemiparasitic plant can absorb water and nutrients from the host species. Despite this parasitic relationship, there were no significant differences in the physiological performance of the hosts in comparison to the parasite-free plants. We conclude that O. lanceolata presents a specific strategy to absorb water in this dry ecosystem, without exerting negative effects on the plant community, which might indicate that there is an equilibrium in the tradeoffs between parasitism and mutualism in the interaction between 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

Gender dimorphism in Corema album across its biogeographical area and implications under a scenario of extreme drought events

In dioecious species, traits may have evolved because of significantly different resource demands associated with male versus female sexual reproduction. It is generally assumed that females have higher reproductive costs, thereafter in long-lived species, males often exceed females in vigour and suffer lower physiological limitations under stress. However, large-scale studies of gender dimorphism and the link between survival and physiological responses and compensation mechanisms are still lacking. In this study, we compared canopy performance, photochemical efficiency, leaf water potential, Δ13C, leaf nitrogen content and δ15N in both genders of the dioecious species Corema album (Ericaceae) across all the species’ biogeographical distribution. This area corresponds to a wide climatic gradient, from temperate and humid to Mediterranean, across the Atlantic coast of the Iberian Peninsula. Our results evidenced that the species responds to the bioclimatic gradient, and most variables were significantly correlated with annual rainfall and bioclimatic indices, although gender dimorphism was only manifested in Δ13C and in leaf water potential at the peak of the drought season. Female plants have significantly higher values of Δ13C, which could be attributed to lower water use efficiency; however, male plants presented more negative leaf water potentials than females, especially in the populations with the warmest and most arid climate. We concluded that C. album populations adjust to the climatic gradient at different hierarchical scales, from physiological instantaneous to time-integrated traits, with gender dimorphism only occurring in sub-optimal environments The absence of morphological differences between genders and the maintenance of a less negative water potential in females in the dry period in southern more arid populations, suggests the existence of compensatory mechanisms at underground level.Fundação para a Ciência e a Tecnologia (FCT) UID/BIA/00329/201

Reliance on deep soil water in the tree species Argania spinosa

In South-western Morocco, water scarcity and high temperature are the main factors determining species survival. Argania spinosa (L.) Skeels is a tree species, endemic to Morocco, which is suffering from ongoing habitat shrinkage. Argan trees play essential local ecological and economic roles: protecting soils from erosion, shading different types of crops, helping maintain soil fertility and, even more importantly, its seeds are used by the local population for oil production, with valuable nutritional, medicinal and cosmetic purposes. The main objective of this study was to identify the sources of water used by this species and to assess the effect of water availability on the photosynthetic rate and stem water potential in two populations: one growing on the coast and a second one 10 km inland. Stem water potential, photosynthetic rate and xylem water isotopic composition (δ18O) were seasonally monitored during 2 years. Trees from both populations showed a similar strategy in the use of the available water sources, which was strongly dependent on deep soil water throughout the year. Nevertheless, during the wet season or under low precipitation a more complex water uptake pattern was found with a mixture of water sources, including precipitation and soil at different depths. No evidence was found of the use of either groundwater or atmospheric water in this species. Despite the similar water-use strategy, the results indicate that Argania trees from the inland population explored deeper layers than coastal ones as suggested by more depleted δ18O values recorded in the inland trees and better photosynthetic performance, hence suggesting that the coastal population of A. spinosa could be subjected to higher stress

Efecto de retama monosperma sobre el endemismo amenazado Thymus carnosus: luz, agua y patrón de distribución espacial en un sistema costero dunar

Las interacciones planta-planta suponen una parte fundamental de la ecología de las especies vegetales. Conocer los mecanismos subyacentes a las interacciones y la respuesta de las distintas especies a la presencia de otras plantas supone completar una información muy valiosa de su biología, ya que raramente ocurren de manera aislada en la Naturaleza. En esta tesis doctoral se aborda el estudio de la interacción entre Retama monosperma y Thymus carnosus. Ambas son especies nativas mediterráneas características de sistemas costeros dunares. Sin embargo R. monosperma ha mostrado en las últimas décadas un marcado carácter invasivo en las costas de Andalucía occidental; en cambio T. carnosus está considerada como especie en regresión y riesgo de extinción. Este estudio se realizó en un área compartida por ambas, la Flecha de El Rompido (Huelva, suroeste español). La mayor parte del trabajo se centró en estudiar la respuesta de ambas especies al uso de la luz y del agua en condiciones naturales. Los resultados mostraron que el efecto de la sombra de R. monosperma junto con los períodos de sequía ambiental crean unas condiciones negativas para T. carnosus, cuya respuesta fisiológica es peor que en ausencia de la influencia de R. monosperma. Profundizando en los mecanismos de uso del agua, se ha puesto de manifiesto que la presencia de R. monosperma modifica las fuentes de agua utilizadas por T. carnosus, que cuando está aislada puede utilizar agua subterránea y sin embargo en interacción con R. monosperma utiliza otras fuentes de agua no permanentes. Para determinar si la competencia entre las raíces de ambas especies era directa se desarrolló un experimento en condiciones controladas. Los resultados muestran el mismo patrón de respuesta ante la sequía que en condiciones naturales, y confirman la interacción directa de las raíces de ambas especies. Así mismo se observaron claras diferencias tanto en el patrón como en la velocidad de crecimiento y ramificación de T. carnosus y R. monosperma. Por último este estudio aborda la interacción indirecta del efecto del ganado sobre la competencia entre ambas especies, mediante el análisis de los patrones de distribución espacial. Los resultados muestran que tanto una presión herbívora alta como la ausencia total de herbivoría producen un patrón espacial de repulsión en la interacción T. carnosus - R. monosperma, en cada caso por causas muy diferentes. Al mismo tiempo, una presión baja del pastoreo permite una distribución agregada y por tanto una asociación entre ambas especies

Germination success and seedling development of Argania spinosa under different climatic conditions and browsing intensity

The present study assesses whether the germination and establishment success of Argania spinosa seeds are affected by the environmental conditions under which the mother plant has grown. Seeds from three populations with different climatic conditions and herbivory intensity were collected and sown in the laboratory after different treatments. Our study suggests that the seed germination process and initial stages of seedling growth are adaptive. Seeds from the population of Agadir with the highest herbivory pressure and high air relative humidity in summer (due to the proximity to the sea) were stimulated by acid treatment, and showed a lower root/stem ratio, which allows them to take advantage of the atmospheric water resources. Seeds from the Mountain population, where the most arid environmental conditions were found, produced early-germinating seeds with the highest root/stem ratio that would facilitate seedling establishment when the harshest environmental conditions appear in summer

Some secrets of Argania spinosa water economy in a semiarid climate

Argania spinosa is an example of an avoider tree growing under semi-arid conditions in Morocco. To assess what are the physiological strategies of this species, different variables were measured through an annual cycle in two populations located in the species’ main distribution area. Results show the expected decrease of leaf water potential (Ψ) with an increase of water-use efficiency (A/gs) with the onset of the dry season. In summer, leaf conductance (gs) was sensitive to vapour pressure deficit (VPD), and stomatal closure occurred over 30 mbar of VPD. Surprisingly, carbon isotope discrimination (δ13C) maintained very low values over the year, with almost no relationship with any physiological or morphological variable. Hence Argania spinosa presents a complex set of mechanisms to avoid water deficit, but δ13C cannot be used as an ecological tracer of long term WUE