Externally held water - a key factor for hair lichens in boreal forest canopies

- Maximum water storage in Bryoria was among the highest found for fruticose lichens.
- External water in hydrated thalli was four to five times higher than internal water.
- Water holding capacity (mg H2O cm−2) increased with 69% from lower to upper canopy.
- High storage of external water contributes to the success of Bryoria in canopies.
- Bryoria retains substantial amounts of water in canopies of old boreal forests.

Lichens hold water inside (internal pool) and outside their body (external pool). Yet, external pool size is not known in hair lichens dominating boreal forest canopies. Here we quantify morphological traits and internal/external water in two widespread Bryoria species along Picea abies canopy-height gradients: Bryoria fuscescens at 5-20 m and Bryoria capillaris at 15-20 m. Dry mass and specific thallus mass (STM) of intact B. fuscescens increased with height, while STM of individual branches did not. Maximum water holding capacity (mg H2O cm−2) increased with height, but did not differ between the species. Bryoria had much larger external (79-84% of total) than internal water pools, trapping water by dense clusters of thin, overlapping branches. They thus increase water storage in boreal forest canopies and influence hydrology. High external water storage extends hydration periods and improves lichen performance in upper canopies, and thereby contributes to the success of these hair lichens.