Canopy vs. soil effects of shrubs facilitating tree seedlings in Mediterranean montane ecosystems

8 pages, 1 figure, 3 tables, 53 references. We are especially grateful to José Antonio Hódar, Jorge Castro and Sergio de Haro for their contributions in the design and revisions of the experiment. We thank the Consejería de Medio Ambiente (Junta de Andalucía) and the Direction of the Sierra Nevada National Park for permission for field work. Constructive suggestions from Thomas Kitzberger and two anonymous referees improved the final version of the manuscript.

Question: Is the facilitative effect of nurse shrubs on early recruitment of trees mediated by a ‘canopy effect’ (microclimate amelioration and protection from herbivores), a ‘soil effect’ (modification of soil properties), or both? Location: Two successional montane shrublands at the Sierra Nevada Protected Area, SE Spain. Method: Seedlings of Quercus and Pinus species were planted in four experimental treatments: (1) under shrubs; (2) in open interspaces without vegetation; (3) under shrubs where the canopies were removed; (4) in open interspaces but covering seedlings with branches, mimicking a shrub canopy. Results: Both effects benefited seedling performance. However, microclimatic amelioration due to canopy shading had the strongest effect, which was particularly pronounced in the drier site. Below-ground, shrubs did not modify soil physical characteristics, organic matter, total N and P, or water content, but significantly increased available K, which has been shown to improve seedling water-use efficiency under drought conditions. Conclusions: We propose that in Mediterranean montane ecosystems, characterised by a severe summer drought, pioneer shrubs represent a major safe site for tree early recruitment during secondary succession, improving seedling survival during summer by the modification of both the aboveand below-ground environment.

This study was supported by a PFPU-MECD grant to L. Gómez-Aparicio, and projects FEDER 1FD97-0743-CO3-02 and REN 2000-3176-E from MCYT.

Peer reviewed