Shrub-clearing is a frequent sylvicultural practice in cork oak forests. Main objectives are: 1) to facilitate the extraction of bark to obtain cork, every 7-9 years, 2) to increase the cork production reducing competition by neighbour shrubs, and 3) to reduce the risk of fires, decreasing the amount of fuel. However, this disturbance can have ecological impacts on the regeneration of woody species and the diversity of herbaceous understorey. In this paper we show sorne results obtained afier monitoring during five consecutive years the effect of an experimental treatment of shrub-clearing in three forest sites, in Los Alcornocales Natural Park (Cádiz-Málaga, South Spain). In each site, 40 permanent quadrats (of 1 m2) have been examined periodically; recording the presence of herbaceous species there. Half of the quadrats (20) were on the shrub-cleared forest and the other half in the adjacent control fores!. We have found that shrub-clearing seems to induce a double effect on the understorey diversity: on one hand increase the number of herbaceous species (a diversity), but on the other hand reduce the community heterogeneity (P diversity). As expected, shrub-clearing reduce significantly the overstorey leaf area index (LAI), thus increasing light availability at ground level. This increased radiation favours regeneration ofmany herbaceous species, in particular ofthose light-demanding, that readily colonise the forest understorey and in consequence generate higher values of a diversity. On the other hand, this reduction of woody cover can have detrimental effects, by excess of light (photoinhibition) and summer drought stress on many shade-tolerant herbaceous species, which are restricted to forested habitats under Mediterranean climate. The overall effects of shrub-clearing on forest biodiversity include a reduction of P diversity, through the homogenizing process associated to the colonization and expansion of common herbaceous species (mostly typical of disturbed and open sites), despite their high a diversity, and the reduction of the shade-tolerant species, with more restricted distribution. Finally, irs important to point up the high capacity ofrecuperation that these forests present in response to these sylvicultural practices. Afier only 5 years, they were reached a very similar diversity values (a y P) compared with the initials, probably due to the high expansion ofthe shrubland, which returns at level ground a luminosity conditions similar to presents before the perturbation.

32 páginas.-- Memoria presentada para el XLII Curso Internacional de Edafología y Biología Vegetal

Directores: Zavala, Miguel A. , Marañón, Teodoro y Díaz-Villa, Mª D.

Peer reviewed