Hydraulic lift (HL), a process by which plants take up water from deep, wet soil layers and passively lift and release it into upper, dry soil layers has been shown to improve performance of individuals occurring next to a lifting plant, but whether this process plays a role in seedling establishment and growth remains unknown. Here, we tested the influence of HL on the interaction between Retama sphaerocarpa, a nurse lifter shrub from semi-arid Spain, and one of its associated understory species, the evergreen shrub Marrubium vulgare. Seedlings of Marrubium were planted under the canopy of Retama in three tube types that reduced or fully prevented competition and that allowed or prevented the flux of water from entering the tube. Additional seedlings in gaps between shrubs served to evaluate the overall facilitative effect of the nurse shrub.

Results/Conclusions

Hydraulic lift patterns were observed under the canopy of Retama only in tubes that allowed the flux of hydraulically lifted water. Seedling survival was greatest in treatments where competition was reduced and HL occurred or where interactions were fully prevented. Seedling biomass was greatest in the treatment where seedlings grew isolated, with no competition or HL, followed by the treatment where competition was reduced and HL allowed, and lowest in the treatment where full interactions were allowed. These results show intense competition for water between Marrubium seedlings and neighboring roots, but also a significant contribution of HL to seedling survival. The extra water supplied via HL did not fully counterbalanced competition by neighboring roots. Nevertheless, Marrubium seedlings performed better under the canopy of Retama, where microclimate conditions were milder and soil water content was higher than in gaps suggesting that the net positive effect of nurses prevail over the effects of competition in this system.