Mediterranean shrub assemblage of holm oak forests (Quercus ilex L.) is driven by aridity and soil texture rather than forest biomass

Understory vegetation represents a large part of forest biodiversity and plays an important role in forest structure and functioning. However, little is known about how it is shaped by environmental gradients and overstory biomass. In this study, we analyse the changes in the taxonomic and functional composition of the understory communities in Quercus ilex L. forests underlined by changes in forest biomass, soil and climate. To do so, we used ordination techniques and a hierarchical model of species composition (HMSC) across a large aridity gradient in the Iberian Peninsula. Aridity and soil clay content were the main factors responsible for the differences in taxonomic composition, with a lowest importance but significant role of soil fertility. Increasing aridity led to higher leaf mass per area at species level for the present shrubs. However, increased leaf mass per area had no effect at community level because highly sensitive species to aridity were relatively rare. We evidence a turnover in Quercus ilex forests along the studied aridity gradient, both taxonomic and functional, reflecting shrub species selection and adaptation to environmental conditions which agree with the potential compositional changes required to deal with a warmer and drier climate. • Shrub species composition is driven by aridity and soil characteristics. • Forest biomass is not relevant to determine species composition or diversity. • When the effect of aridity on species abundance increases, LMA increases as well. • Species functional traits are affected by aridity without impact at community level.