Effect of revegetation method (seedling outplanting versus direct seeding) on holm oak root architecture: Implications for restoration success under a global change scenario

Oaks develop robust taproots that enable them to access water from deeper soil layers and thrive in drought-prone environments. Understanding how different revegetation methods influence oak root development is therefore crucial for improving restoration strategies. In a common garden experiment, we compared the root systems of holm oaks (Quercus ilex L.) established through two revegetation methods: seedling outplanting after nursery cultivation and direct acorn seeding. After five growing seasons, we excavated holm oak root systems to a depth of 50–60 cm using a bulldozer and scanned them with terrestrial laser scanning (TLS). Two TLS-derived metrics described the apical dominance gradient of the taproot (index of principal axis dominance and path fraction), while another assessed root ramification (total number of forks per meter). Manual measurements were also taken for taproot diameter, branching root diameters, root branching density, and root:shoot ratio. Moreover, we assessed the resprouting capacity of seeded and planted oaks harvested two years earlier in the same common garden. Multivariate analyses and generalized linear models revealed significant differences between outplanting and direct seeding in root characteristics. Seeded individuals showed greater apical dominance and a higher root:shoot ratio, whereas outplanted ones developed more root ramifications and root branching density, with thicker taproot and branching roots, and a higher ratio of the mean branching root diameter to the taproot diameter. Furthermore, plants from the seeding treatment exhibited a slight but significantly higher resprouting capacity than those from the outplanting treatment. These results indicate that direct seeding promotes a more natural root structure, with stronger taproots and better resprouting capacity than outplanting. Our findings suggest that direct seeding may provide a more nature-based solution than outplanting for forest restoration by emulating natural regeneration. In the mid-term, the root development shown by direct acorn seeding may enhance the ability of holm oaks to withstand disturbances such as droughts in the Mediterranean Region under global change.