Vegetation diversity and vertical structure as indicators of forest disturbance

Abstract

Plant species composition and vertical structure of oak-mixed Atlantic woodlands differing on disturbance regime were studied in the Basque Country, northern Spain. Four different disturbance groups were considered depending on the time since last thinning and the presence/absence of grazers. Plant species cover varied among disturbance groups for 17 out of the total 47 species present in the study area. In general, disturbance by clear-cutting favoured the cover of Betula alba, Castanea sativa and Quercus robur. The shrub species most affected by disturbance were heathers. Among these, the most drastic effect was found for Calluna vulgaris, Daboecia cantabrica and Erica vagans which disappeared in those woodlands subjected to grazing. Several nemoral species, such as Vaccinium myrtillus, Blechnum spicant and Lamiastrum galeobdolon, were absent in these same grazed forests. By contrast, generalist species, such as Ruscus aculeatus, Geranium robertianum and Veronica officinalis, were mostly present in the grazed woodlands. Diversity was measured as Species Richness as well as with the Shannon’s and Simpson’s indices. Plant species richness did not vary with disturbance treatment, however, it was highest for trees and shrubs at those woodlands in an advanced stage of regeneration, and for herbs in the lowest and highest disturbance groups. Besides, only the third vertical layer (2.5–10 m) was affected by disturbance treatment. Simpson’s was the best index to explain the observed differences in the structure of woodlands subjected to unalike levels of disturbance. Species diversity was highest in woodlands with lowest disturbance or in an advanced state of regeneration. Woodlands with lowest disturbance had the highest diversity of trees and those woodlands in an advanced state of regeneration showed the highest diversity of shrub species. Regarding vertical structure, diversity was mostly affected in the intermediate layer (2.5–10 m), with highest values found in woodlands at the lowest disturbance group, i.e. at least 100 years of no disturbance. Two indices of evenness, i.e. Pielou’s and Simpson’s were also calculated in this study. Results here presented suggest that plant species composition, percent cover of individual plant species, and the Simpson’s diversity index (applied considering the different growth-forms and vertical layers) are suitable and complementary indices to evaluate disturbance at the studied woodlands.