Interactions among plant layers in shrub-encroached Iberian dehesas and consequences for their persistance

PosterWe analyze the effect of shrub understory on the functioning and persistence of Iberian dehesas (grazed open woodlands). We study the effect of two contrasting, in terms of morphological traits and reproductive strategy, shrub species: Retama sphaerocarpa (a deep rooted shrub) and Citus ladanifer (a shallow rooted shrub), in order to account for specie-specific effects, on neighboring vegetation (tree and pasture). Pasture and tree rooting profile, soil resources availability (water and nutrients), tree and pasture production, tree physiological status and tree regeneration were assessed with and without the presence of shrubs. The results showed that the presence of shrubs deeply modify tree and pasture root systems. Trees growing either with Cistus or Retama exhibited a significantly deeper rooting profile than growing without competition, whereas herbaceous species showed the opposite trend, shifting most of their roots upwards to shallower soil horizons. Cistus appeared to be a great competitor for soil resources, what led to significantly lower tree leaf water potential, leaf gas exchange parameters, acorn production and growth. Cistus encroachment also reduced pasture yield by 68 to 98 %. Retama ameliorated top soil fertility and showed positive effect on soil moisture. Retama increased significantly pasture yield by 11 to 157 % and tree growth by 6 to 13 %, but had a neutral to negative effect on acorn yield. Both shrubs showed a similar positive effect on tree seedling emergence in spring and survivorship after summer. However, as seedling aged, this positive association strengthened with Retama and weakened over time with Cistus. To sum up, shrubs have the potential to modify the functioning and persistence of Mediterranean open oak woodlands. Thus, future management practices must bear in mind this specificity of effects and try to optimize the positive effect of shrubs without jeopardizing the productivity or functioning of the system