Stem hydraulic properties and xylem vulnerability to embolism in three co-occurring Mediterranean shrubs at a natural CO2 spring

Seasonal changes in hydraulic properties and vulnerability to xylem embolis m of Erica arborea L., Myrtus communis L. and Juniperus communis L. were an alysed by comparing plants at two locations in a Mediterranean environment. A distinct atmospheric CO2 concentration ([ CO2]) gradient exists between the two sites with higher [CO2] in the proximity of a natural CO2 spring (7 00 mu mol mol(-1)). Changes in native embolism in E. arborea and M. communi s indicated rather clear seasonal segregation by species and by the growth [CO2]. J. communis had constantly lower percentage embolism than the other two species (the effect of site being not consistent). Differences in summe r embolism among species and between sites were in accordance with vulnerab ility curves. Volumetric fractions also showed seasonal and site-dependent changes. Mean specific hydraulic conductivity was strongly affected by site in E. arborea and M. communis (in opposite directions). Hydraulic properti es varied as a function of shoot biomass and leaf area, and the latter incr eased with increasing sapwood area; differences between sites were somewhat significant in M. communis. Foliage biomass increased with stem biomass; E . arborea had higher values of foliage biomass at similar values of stem bi omass at the control site. Altering branch biomass allocation may influence or not (depending on the species) hydraulic adjustment. Plant responses to resource imbalances caused by increasing [CO2] tend to compensate for the imbalance by changes in hydraulic properties and biomass allocation. Howeve r, the plasticity or compensation ability of any particular species appears limited, and effective compensation for large changes in resource balance caused by environmental forcing factors may require changes in species comp osition.