Age-related change in canopy traits shifts conspecific facilitation to interference in a semi-arid shrubland

Shifts between facilitation and interference and their importance in shaping plant population and community dynamics have received wide recognition. Nevertheless, the causes and spatio-temporal scales of these shifts are poorly understood, yet strongly debated. This study tested the hypothesis that age-related changes in canopy structure shift the effect of a nurse shrub on their protégé from facilitation to interference, using as a model system the interaction between the dwarf shrub Sarcopoterium spinosum and conspecific new recruits, in the shrubland of the transition area between the Mediterranean and the semi-arid climatic zones of Israel. Foliation level (i.e. the percentage of canopy surface area covered with leaves), a measure of shrub canopy structure, increased with age. Shading level was significantly and positively related to foliation level. Densities of new recruits in the shrubland showed a unimodal response to canopy structure and cover: the highest densities were associated with canopies presenting low and medium foliation levels (providing 71 and 82% shade, respectively), while high foliation levels (93% shade) and open spaces among canopies were characterized by very low densities. A related field experiment using shading nets revealed that seedling survival rates followed a similar unimodal pattern, with the highest survival (ca 60%) detected in moderate shade (70%), twice as much as in full sun, and the lowest survival (ca 10%) observed in extreme shade (90%). These results support the study hypothesis on age-dependent interactions. Thus, in a semi-arid shrubland ecosystem, the transition of the "nurse shrub" from "young" to "old" stage can shift facilitation to interference. Hence, the age structure of established shrub populations determines a) the availability of suitable sites for seedling recruitment and b) the balance between facilitation versus interference effects on seedling establishment.