Can stress turn trees hair white? Hair covering of stems improves resilience of corticular photosynthesis against heat-stress

The hypothesis was tested that hair covering of stems improves resilience of corticular photosynthesis against heat stress. Hairy and non-hairy outer bark of Quercus ilex L. and Quercus robur L. trees was removed and optical properties measured. Additionally, structural bark traits and chlorophyll fluorescence parameters during heat stress treatment were studied. Optical analysis revealed a protective role of hairy outer bark (OB) against overheating of the underlying cortex of the stems. Hairiness decreased OB transmittance and increased thermal insulation of stems by an increased absorptance and reflectance of OB in the visible (380–720 nm) and an increased reflectance in the infrared part of the spectrum (720–900 nm). Simple linear regression analysis revealed no significant effect of stem structural traits (OB thickness (OBT), cortex density (Dcortex), cortex water content (Wcortex)) on corticular photochemistry (PScort), while optical traits of outer bark were significantly (P   45 °C) PScort of the hairy species showed a higher resilience and a better post-stress-recovery as compared to the non-hairy one. It is concluded that stem hairs play a physiologically significant role in modulating the stem energy balance due to a close interaction between optical characteristics of hairy OB and stem photochemical processes.