Dynamics of understorey herbaceous plant diversity following shrub clearing of cork oak forests: A five-year study

Cork oak forest management has been traditionally oriented towards optimization of cork production. Shrub clearing is a traditional silvicultural practice aimed to: (1) facilitate cork extraction (conducted every nine years); (2) increase cork yield by reducing competition from neighbouring shrubs; and (3) reduce fire risk by decreasing fuel load. These silvicultural practices, however, may interfere with current objectives of conservation and sustainable use of biodiversity. The effects of shrub clearing on different diversity components were assessed in three structurally contrasting forests sites, located in Southern Spain. A fenced experimental plot (1 ha) was established at each forest site. Half of each plot was shrub-cleared and the other half was left unmanaged (control stand). Afterwards, the presence of herbaceous species was monitored during five consecutive years in a total of 120 permanent quadrats (1 m2). Species richness (α- and γ-diversity) increased with the clearing, mainly by expansion of open grassland species, while β-diversity declined due to the homogenizing process associated with this expansion. Thus, the herbaceous species composition was modified by these silvicultural practices, especially the second year following treatment application. Effects differed across forest stands, being more marked in the Open Woodland, while there was no significant effect observed in the closed Forest. Understorey herbaceous communities were resilient to shrub clearing and initial diversity values were approximately restored after five years. The high resprouting potential of shrubs contributed to this resilience. Based on the results of this study, we propose several low costs strategies to be incorporated in forest management plans to reconcile cork oak extraction with the maintenance of biodiversity.