Partitioning of water and nitrogen in co-occurring Mediterranean woody shrub species of different evolutionary history.

We studied the interspecific and intraspecific variation in the development of water stress and in the use of different water and nitrogen sources during the spring (wet season) and summer (dry season) in a shrub community in NE Spain. We measured shoot water potentials, stable deuterium isotopic composition (delta D) of xylem sap, leaf mass per area, leaf N and C concentrations, gas exchange, leaf delta(13)C, and leaf delta(15)N of the dominant species (Quercus coccifera, Arbutus unedo, Pistacia lentiscus, Erica multiflora, Globularia alypum). The delta D, the delta(13)C and the shoot water potential values showed diurnal, seasonal, intraspecific and interspecific variation in the source and use of water. There was also seasonal, intraspecific and interspecific variation in the foliar delta(15)N and N concentrations. In summer, some species (A. unedo, P. lentiscus and E. multiflora) presented significantly different delta D values in morning and afternoon measurements likely indicating that they used different sources of water during the day, and a dual root system in these species. We conjecture that dew may be one of these water sources. Species predawn water potential was negatively correlated with species xylem water delta D. There was also a positive correlation between delta(13)C and delta D in P. lentiscus, species for which we took additional samples from nearby sites. These results suggest that the access to water from greater depths allowed the maintenance of more favourable plant water supply. Multivariate principal component analysis based on the studied hydrological and isotope variables clearly separated the seasons (wet spring and dry summer) and the species. The species resulted separated according to their evolutionary history (Pre-Mediterranean and Mediterranean) and the associated root and functional traits. These results show water (and nitrogen) partitioning among coexisting species of the same functional type (Mediterranean woody shrubs). They also show the great intraspecific plasticity of responses to resource availability.