Interaction between weeding, outplanting date and site in open oak woodland plantations

Abstract

In this paper we explore ways of improving the field performance of oak plantations for the regeneration of open woodlands in Mediterranean environments. Using the evergreen, sclerophyllous holm oak species (Quercus ilex L. subsp. ballota (Desf.) Samp.), we analyzed the effects of the outplanting date (October vs. February), vegetation competition (weeding vs. untreated), the environmental conditions imposed by the outplanting site, and the interactions between all these factors. For test purposes, 3 blocks of 200 one-year-old holm oak nursery-grown seedlings were set up in four sites in SE Spain (Becerra, Mágina, Jarosa and Hinojosa). Half of each block (plot) was weeded to prevent competition from other vegetation. In each plot (weeded or not) half of the seedlings were outplanted in October 2019 and the other half in February 2020. In total, 2400 holm oak seedlings were planted to be monitored over the course of two growing seasons. Survival and growth-related parameters such as height, root collar diameter, number of leaves and total and fractional aerial biomass were recorded. Weeding was shown to be the most important factor affecting both survival and seedling growth, especially in sites with a milder autumn and winter, and a long, intense summer, such as Jarosa and Hinojosa. In colder locations, however, October outplanting was more advantageous, especially if accompanied by weeding (Becerra and Mágina). In cases where competition from native species is not controlled, it would be advisable to plant in October, in terms of both survival and development of the seedlings, especially in colder sites. In general terms, the best option was outplanting in October plus weeding, while the worst option was outplanting in February with no weeding. This study provides new tested recommendations on how to improve the success of oak plantations in different Mediterranean environments.