Canopy buffering effects against climatic extremes of deciduous broad‐leaved forests are higher on calcareous than siliceous bedrocks

Forest ecosystems can buffer understorey microhabitats and provide refuges for understorey species in a warming world. These effects are dependent on forest complexity and management. However, the roles of soils and bedrock types are less known although they also may affect forest complexity. We assessed differences in buffering effects of deciduous broad‐leaved forests from calcareous and siliceous soils in the temperate climate of south‐west France predicted to become dryer with climate change. We transplanted during dry and wet years, without and with neighbours (herbaceous and shrubs from forest understories and gaps), six species in gaps and dense forests of the two soil types. Target species originated from contrasting light availabilities and the two soil types. We measured transplant survival, light transmission, air temperature and vapour pressure deficit (VPD) in all treatments. Forest structure and soil parameters were also quantified on both bedrocks. Forest structure complexity was higher and light transmission lower on calcareous than siliceous soils. Relative air humidity was higher and maximum air temperature and VPD lower in the understorey of calcareous than siliceous forests, while soil nutrients and moisture were lower in siliceous soils. We found strong direct facilitation of transplants from shaded habitats by calcareous forest canopies during dry years. Facilitation of forest canopies was much weaker on siliceous bedrocks, during wet years and for species from sunny habitats. Facilitative responses were less obvious but still observed in the presence of understorey neighbours. Calcareous forests showed a stronger buffering ability than siliceous ones due to both higher facilitative effects of calcareous woody dominants and lower survival in calcareous gaps during dry years. Considering that the two forest types were old forests with similar management histories, differences in buffering ability between them were likely due to differences in forest complexity and composition induced by differences in soil types.