Shrub species affect distinctively the functioning of scattered Quercus ilex trees in Mediterranean open woodlands

A low tree stand density has been showed as necessary to thrive with summer drought in semiarid Mediterranean open woodlands. Shrub encroachment of these open woodlands is currently recommended to guarantee the persistence of the system, due to the nursery effect of shrubs on tree seedling. However, the increase in abundance and cover of a shrub understory in these water limited woodlands could bring consequences to tree overstory functioning. The present study analyzes the physiological status of scattered Quercus ilex L. trees in paired adjacent plots with and without the presence of a shrubby understory in CW Spain. Two contrasting shrub strategies were addressed in order to take into account possible species-specific effects: a dense-shallow rooting shrub (Cistus landanifer L.) and a sparse-deep rooting shrub (Retama sphaerocarpa (L.) Boiss). Leaf water potential (at predawn and midday), leaf gas exchange parameters (net photosynthetic rate and stomatal conductance), leaf nitrogen content and chlorophyll fluorescence transients (maximum photochemical efficiency and performance index, sensu Strasser et al., 2004) were measured during three consecutive summers. Trees growing with Cistus as understory showed significant lower leaf water potential, leaf gas exchange parameters, leaf nitrogen content and chlorophyll photochemical efficiency than trees growing without shrub competence. However, the presence of the legume Retama did not affect significantly the physiological state of Q. ilex. Thus, we conclude that the presence of a shrubby understory has the potential to modify the functioning of scattered trees, but these effects are species-specific.

Research highlights
► Effects of two shrub understory strategies on oak functioning was analyzed.
► A dense-shallow rooting shrub competed hugely affecting trees functioning widely.
► A sparse-deep rooting shrub showed a slight interaction for soil resources.
► Favoring shrub presence for open woodland management needs specific planning.