Facilitation of Quercus ilex in Mediterranean shrubland is explained by both direct and indirect interactions mediated by herbs

摘要

1. Competitive and facilitative interactions shape plant communities. Whereas a number of studies have addressed competition and direct facilitation among plants in dry ecosystems, indirect facilitation has received little attention.ud2. We investigated the relative importance of direct and indirect facilitation by the nurse plant Retama sphaerocarpa on late-successional Quercus ilex seedlings mediated by herb suppression in a Mediterranean shrubland in 2006 and 2007. We also studied whether facilitation outcome depended on the size of the facilitated seedlings. 3. A field experiment was carried out to test the effect of (i) position of Q. ilex seedling with respectudto shrub canopy (under shrubs or in gaps), (ii) herb competition (presence or absence), and (iii) seedlingudsize. 2006 was an average rainfall year while 2007 had a much more humid spring and a dryerudsummer than 2006.ud4. In both years, nurse shrubs reduced seedling mortality whereas herbs increased it. In the averageudrainfall year, seedling mortality under shrubs was unaffected by herbs whereas in gaps it was signifi-udcantly higher in presence of herbs. This showed that the nurse shrub indirectly facilitated the seedlingsudby reducing the competitive capacity of herbs. Conversely, facilitation was predominatelyuddirect during the humid spring and dry summer year since herbs hindered seedling survival similarlyudunder the nurse shrub and in gaps. The nurse shrub directly facilitated the seedlings by reducingudseedling photoinhibition and water stress.ud5. Improvement of environmental conditions by Retama benefited smaller seedlings but not largerudseedlings since the nurse shrub reduced mortality of smaller seedlings relative to that in gaps, butudthis effect was not observed for larger seedlings. This indicates that individuals within a seedlingudpopulation may not have the same response to facilitation.ud6. Synthesis. Both indirect and direct facilitation are important mechanisms for Q. ilex regenerationudin Retama shrubland and their importance seems to vary with climatic conditions. Indirect facilitationudby release of herb competition under nurse shrubs is important in years of dry springs whenudcompetition between nurse shrubs and herbs is high, whereas direct facilitation mediated by microclimateudamelioration increases with summer aridity