Is the interaction between Retama sphaerocarpa and its understorey herbaceous vegetation always reciprocally positive? Competition-facilitation shift during Retama establishment

摘要

Retama sphaerocarpa is a Mediterranean shrub that when adult, facilitates the establishment of herbaceous plants under its canopy. We test the hypothesis that during the establishment of R. sphaerocarpa seedlings, the interaction with the herbaceous plants is negative. We carried out a greenhouse experiment in which seedlings of R. sphaerocarpa were grown under different conditions of competition with herbs, watering and date of emergence. Measurements of seedling mortality, biomass and growth were taken during the first growing season. We found a significant relationship between R. sphaerocarpa seedling mortality and competition in early spring, presumably due to higher water demand of herbaceous plants. Generally, presence of herbaceous species, lower availability of water and late emergence had negative effects on biomass and growth of Retanza seedlings. Additional water compensated for the negative effects of competition, except on leaves and cladodes of Retama seedlings, suggesting that other resources, such as light, could be the subject of competition. In contrast, Retama seedlings exerted a positive influence on the herbaceous plants by increasing their survival and biomass, probably as a consequence of the high availability of nutrients provided by the Rhizobia nodules in the roots of Retama seedlings. We concluded that, at the regeneration stage of the shrub, the interaction between the herbaceous vegetation and the shrub is negative for the shrub and positive for the herbs. This suggests a shift from competition to facilitation with age of Retama, as reciprocal positive interactions have been described between herbaceous plants and adult individuals of the shrub. (C) 2004 Elsevier SAS. All rights reserved.