Key Factors in Direct Acorn Seeding for the Successful Restoration of Open Oak Woodlands

Little is known about the effectiveness of direct acorn seeding when subjected to different environmental conditions, but without the interference of acorn predation. In order to assess and improve its success, a total of 2400 holm oak acorns from 20 mother trees were sown in three blocks across four sites from southern Spain. Each block was divided into two subplots, in one of which weed competition was mechanically controlled over the study period. Acorns were weighed, protected by a seed shelter, and sown at four different dates between December 2019 and March 2020. Emergence, survival and growth were registered until November 2021. In this last sampling, the aboveground biomass was harvested. Emergence varied from 53.7% of acorns sown in December to 29.4% of those sown in March while survival success after two growing seasons was 27.1% for acorns sown in December as compared to 9.2% of acorns sown in March. Survival success clearly decreased at a higher rate over time in non-treated subplots (12.7%) as opposed to those where the weeds were controlled (22.5%). Emergence, growth, and survival success were highly dependent on the site and the mother tree from which the acorn came. The mass of the emerged acorns was higher than the mass of those that did not emerge. The mean acorn mass of each mother tree was positively correlated with the germination rate and emergence of the seedlings, and with the survival of emerged seedlings and final survival success. In conclusion, we recommend selecting large acorns, sowing early (December), controlling weed competition over time, and selecting mother trees with high germination potential. Seeding success will be lower in drier sites and drier years.