Description of the herbaceous-subshrub layer in Quercus robur, Q. petraea and Q. pyrenaica stands from northwestern Iberia

A botanical inventory was conducted following the Braun-Blanquet methodology, a widely used phytosociological approach for the characterization of plant communities. In this method, species abundance is jointly estimated with vegetation cover using a six-point ordinal scale, whereas sociability reflects the degree and spatial pattern of aggregation among individuals of the same species within the floristic community. This approach allows for a detailed assessment of species composition, structure, and relative dominance in the sampled stands. The aim of this study is to analyse and characterize the herbaceous–subshrub layer in Atlantic oak forest stands located in the northwestern Iberian Peninsula, dominated by Quercus robur L., Quercus petraea (Matt.) Liebl., and Quercus pyrenaica Will., with particular attention to the botanical composition and structural organization of the understory vegetation.

For the classification of species within each vegetation stratum, four strata were considered: arboreal (height > 5 m), shrub (height between 1.5 and 5 m), subshrub (height < 5 m, restricted to woody species), and herbaceous (non-woody individuals of any height). The study was conducted in areas of homogeneous vegetation covering relatively large surfaces, always exceeding 100 m², while also accounting for different degrees of tree-canopy openness. The results showed that, irrespective of canopy openness, the most abundant species were Pteridium aquilinum (L.) Kuhn and Hedera helix L. However, analysis of the influence of canopy openness revealed marked differences: (i) in closed-canopy areas, Trifolium repens L., Lonicera periclymenum L., and Ranunculus repens L., were dominant; (ii) in areas with partially open canopy, Agrostis curtisii Kerguelen, Anemone nemorosa L., and Hyacinthoides non-scripta (L.) Chouard ex Rothm., were very abundant; and (iii) in canopy clearings, Taraxacum officinale L. and Agrostis stolonifera L. predominated.