Do shrubs facilitate oak seedling establishment in Mediterranean pine forest understory?

- Understory vegetation cover influenced oak seedlings in two clear pine forests.
- Understory shrubs had positive effects on oak survival at the more stressful site.
- Shrubs improved growth of the less drought-tolerant oak at the less stressful site.
- Grass cover decreased seedling survival and growth.
- Managers should preserve understory shrubs in harsh conditions.

Shrubs have been shown to facilitate tree seedling establishment in open Mediterranean ecosystem habitats, but their effects in forests have been much less explored. We investigated the role played by shrubs in seedling emergence, survival and growth for two co-occurring oak species - Quercusilex and Quercuspubescens - in the understory of clear Aleppo pine stands (10 m2/ha). Acorns of both species were sown in two sites in South-East France that contrasted in terms of former land-use (pastoral vs agricultural), soil thickness (shallow vs deep) and type of understory (shrubs + grass vs only shrubs). Oak seedlings had a higher survival and growth on the former agricultural site with deeper soil. In general, the more stress-tolerant Q. ilex performed better than Q. pubescens. In the harsher site, seedling survival decreased with grass cover but increased with shrub cover. Shrub and grass cover decreased seedling diameter and had no effect on seedling height. In the more fertile site, shrub cover had no influence on seedling survival but had a species-specific effect on seedling growth: shrubs mostly competed with Q. ilex but ameliorated Q. pubescens growth, leading to changes in the two species performance ranking at high shrub cover. We conclude that shrubs can act as nurses for oak seedling establishment in pine forest understory, particularly in harsh conditions and for stress-intolerant species. In harsh conditions, shrub cover should be factored in as a way to promote pine forest diversification toward mixed pine-oak stands.