Positive and negative interactions at different life stages of a colonizing species (Quercus humilis)

1 The downy oak Quercus humilis has recently recolonized the Causse du Larzac plateau in southern France. We studied the influence of the shrubs Buxus sempervirens and Juniperus communis on Q. humilis establishment, and of Buxus on the growth of established Q. humilis individuals. 2 Percentage germination of experimentally planted Q. humilis was higher under shrubs than in nearby open areas and higher on the north than the south side of the canopy. Germination where part of the canopy has been removed was similar to that away from the shrubs, suggesting that the facilitation mechanism is related to changes in microclimate rather than to a soil effect. 3 When exposed to sheep for 1 month, 100% of 326 unprotected oak seedlings were grazed, causing 44% mortality. The presence of Buxus and Juniperus improved seedling survival by protecting them against sheep grazing and summer drought. Predation by rodents was however greater under shrub cover. 4 The highest leaf dry mass of oak seedlings was recorded under Juniperus where light conditions seem more favourable for growth than under Buxus (direct effect) or in grassland (indirect effect). The growth of naturally established individuals of Q. humilis (in terms of total leaf mass per annual branch and width of rings) was lower under Buxus than in grassland but the values became similar once the canopy was overtopped. 5 The balance between positive and negative interactions varied in relation to the life stage of Q. humilis and the two shrub species. Regeneration of Q. humilis in open grassland was prevented by grazing. The protection offered by shrubs continues to offset the negative interference on growth, particularly under Buxus, so that plants could survive to overtop the shrub canopy and reach maturity. The succession pathway therefore depends closely on the distribution of shrubs in the grassland.