Microsite manipulation in lowland oak forest restoration results in indirect effects on acorn predation

Taking advantage of facilitation mechanisms to counteract harsh environmental conditions may prove to be a successful strategy in the restoration of degraded man-made ecosystems. Seed sowing is often a viable and less expensive technique to restore forest cover in environments subjected to heavy anthropic disturbance. In this context, nurse plants and shield objects can improve microsite conditions, enhancing the emergence and growth of seedlings. However, their presence can also create preferred microhabitats for seed predators, and as a result, may alter their movement decisions and foraging behavior. In this study, we investigated whether nurse shrubs and artificial shade structures, with and without previous soil amelioration, could facilitate acorn emergence. We also tested their effects on the predation of acorns by small mammals, within the framework of a larger project aimed at restoring a Quercus robur L. forest. The research was conducted in a former roadbuilder’s yard located along the Mediterranean Corridor, in Northern Italy. The yard had been in use during the widening of a highway that runs parallel to a stretch of the Corridor. The area was restored in 2014. Mechanical preparation of the soil included two treatments, with and without the addition of compost and zeolitite. Acorns were then sown in plots in different microsites, i.e., 1) close to a shrub (Cytisus scoparius L., Scotch broom), 2) to the north and 3) south of a shield object (a 30 cm x 30 cm wood particle board), and 4) without any protection element (control). We recorded acorn emergence and predation during the first growing season. At the end of the season, overall acorn emergence was observed to be low, and, surprisingly, was higher in the non-amended soil treatment. The amended soil had a higher temperature and a lower water volumetric content in the summer (JJA). No evidence of direct facilitation on emergence by shield objects or Scotch broom was found, but indirect effects were detected. Acorn predation was generally high (67.45%), but was negatively affected by the presence of C. scoparius. The distance of the plot from the rainwater drainage ditch surrounding the study site also influenced winter predation; a larger number of acorns were removed by rodents at longer distances from the drainage ditch. In order to restore degraded sites to lowland oak forests through direct seeding, it is necessary to deal with complex interactions between direct and indirect facilitation mechanisms, competition and predation. Predation was particularly important because of the high predation pressure on acorns.