Accounting for environmental stress in restoration of intertidal foundation species

1. Restoration of foundation species in historical habitat may be difficult if adult facilitation is obligatory for survival of early life stages. 2. On intertidal Mediterranean coasts, large-scale loss of the dominant forest-forming macroalga Ericaria amentacea, have prompted restoration efforts using recruits. Yet, early life stages may be more susceptible to the abiotic stress that characterizes their habitat. 3. We tested strategies to enhance resilience of lab-cultured juveniles of E. amentacea to environmental stress in historical habitat lacking conspecifics. Juveniles were exposed in culture to fluctuations of the dominant physical stressors, irradiance and temperature, and then outplanted in upper and lower zones of their native intertidal range. 4. Without adult canopy, juvenile outplant survival was limited to the lower tidal range, with nearly complete mortality in the upper zone. Survival was also strongly determined by spatial clumping of recruits. Longer-term growth in the lower zone was enhanced by fluctuating mild stress in culture, with variable irradiance and concurrent heat pulses increasing post-outplant cover by 40-60% after 4 months. Clumping also promoted growth across experimental treatments. 5. Synthesis and applications: Reliance on self-faciliation feedbacks is a common barrier to foundation species restoration in high-stress habitats. Our results suggest that without adult habitat amelioration, environmental stress limits recruit survival in intertidal algal forest. Yet, exposure to transient, low environmental stress in culture and outplanting of clumped individuals may confer resilience and allow successful establishment of early life stages in zones of reduced abiotic stress, providing a source for gradual colonization of more high stress areas.Funding provided by: European CommissionCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000780Award Number: LIFE16 NAT/IT/000816Percent cover and the degree of spatial aggregation of Ericaria amentacea individuals (i.e. clumping) on each tile were determined post-culture (week 0) by photo analysis (ImageJ v1.52, National Institute of Health, USA). Percent cover was measured by manual thresholding to select pixels in the correct color spectrum that corresponded to the area covered by E. amentacea juveniles. Clumping was assessed by the number and size distribution of polygons (larger than 2 mm2) resulting from aggregated individuals identified in the thresholding process. Length was determined visually. After outplant, tiles were monitored at 2–5 week intervals over 4 months (3, 6, 8, 12, 15 wks) as allowed by weather conditions. Survival (presence/absence) was determined by visual inspection in the field and change in percent cover was assessed by photographic analysis