Long-term dynamics of shrub facilitation shape the mixing of evergreen and deciduous oaks in Mediterranean abandoned fields

Recovery of Mediterranean forests after field abandonment is a slow process, even without propagule limitations. This is mainly due to stressful conditions for seedling establishment. In this context, shrubs play a critical role in facilitating tree recruitment, but how this process unfolds after field abandonment is not entirely known. We evaluated the long-term dynamics of facilitation by the nurse shrub Retama sphaerocarpa in the recruitment of two ecologically contrasting oaks, the evergreen Quercus ilex and the deciduous Quercus faginea. Thirty years after field abandonment, we dated shrubs and oaks established in an old field to estimate the annual recruitment rates and investigate temporal recruitment patterns. For oaks, we differentiated recruitment at two microsites: open areas or under shrub. To assess how nurse shrubs modulated environmental stressors, we modelled oak recruitment as a function of climatic variables. For the evergreen oak, we assessed these effects within each microsite. Finally, we estimated the annual interaction index between shrubs and evergreen oak juveniles as a function of climatic conditions. Each species showed different recruitment pulses during colonization. Recruitment rate was the highest for the shrub, followed by the evergreen oak. Oak colonization under shrubs was appreciable 20 years after field abandonment, when shrub cover reached 2.2%, and concentrated under medium and large shrubs older than 7 years. Shrubs not only accelerated evergreen oak colonization but also attenuated the fluctuations of recruitment pulses. For the evergreen oak, the interaction index indicated facilitation dominance in years with more arid summers (precipitation < 47 mm and heat waves longer than 8 days) and competition in wetter summers. Synthesis. Oak colonization in Mediterranean abandoned fields progressed slowly, and only two decades after abandonment pioneer shrub population grew to a level in size and number that could effectively trigger facilitation and accelerate tree recruitment. The shrub nurse effect was unbalanced between oak species, only benefiting the evergreen oak, and it was more prevalent in arid years. Our study illustrates the build-up of shrub facilitation during forest recolonization and the varying nature of this process among climatically different years and ecologically distinct species. This information provides insights for assessing and managing Mediterranean forest recovery.