Same nurse but different time: temporal divergence in the facilitation of plant lineages with contrasted functional syndromes

Plant facilitation assembles functionally contrasted communities in dry lands. Nurse plants are often early colonizers with xerophytic traits that were mostly selected during the dry Quaternary (Quaternary syndrome), while beneficiary plants tend to be late-successional species with mesophytic traits that evolved mainly during the more humid Tertiary (Tertiary syndrome). Integrating plant facilitation within the community assembly theory requires a better understanding of the ontogenetic development of the nursing abilities that benefit functionally contrasted species. We assessed whether the same nurse plant facilitates species with Quaternary and Tertiary syndromes in an ecosystem under severe abiotic stress conditions imposed by aridity and gypsum soil toxicity. We hypothesized that both functional types find suitable microsites for seedling establishment underneath the same nurse but their optimal regeneration niches are temporally segregated along the ontogenetic development of the nurse. We carried out a sowing experiment along a 40-year ontogenetic gradient of the nurse shrub Ononis tridentata. Seeds from five Tertiary and five Quaternary species were sown and seedling emergence monitored. While the nurse age did not affect the seedling emergence of Quaternary species, it significantly increased that of Tertiary species. These results were corroborated for elder ontogenetic stages in non-manipulated plants in the field. Juveniles of Quaternary species were able to grow beneath nurse plants along their whole ontogenetic gradient excepting beneath Ononis seedlings, while Tertiary species were only facilitated by mature nurses.Synthesis. Our results show that plant nursing abilities evolve ontogenetically in a different way for beneficiary Quaternary and Tertiary plant lineages. The finding of a plant species that plays a role as key assembler of early- and late-successional species in plant dynamics broadens the scope of facilitation in the community assembly theory.