1.  Epiphytic lichens are important biodiversity components of forest canopies worldwide, significantly contributing to ecosystem function. The relative growth rate (RGR), a measure of fitness, drives population dynamics and shapes lichens’ large-scale distributions. In a climate change scenario, we need to know how external (macro- and microclimate, and nitrogen deposition), and internal factors (cortical pigments, chlorophyll and specimen size) affect RGR in these ecologically important canopy organisms.

2.  We used dominant pendulous (hair) lichens widely distributed across the boreal biome to test the hypothesis that precipitation drives RGR of pale (Alectoria sarmentosa, Usnea dasopoga) and dark species (Bryoria fuscescens) to a different extent across a large-scale gradient from continental to oceanic climates (precipitation: 450-2600 mm) in Scandinavia (60-64° N, 5-19° E). After transplanting lichens to lower branches of Picea abies in nine boreal forest sites for one year, we used linear mixed effect models to analyze how total precipitation, rainfall, number of days with rain, temperature sum, nitrogen deposition, light, chlorophyll a (an indicator of photosynthetic capacity), and size influenced their RGR.

3.  RGR was highest in the pale species (Alectoria and Usnea) and increased with amount and frequency of precipitation, with >3 times higher RGR in the wettest compared to the driest site. The number of days with rain was a better predictor of RGR than total precipitation or rain. By contrast, RGR of the dark Bryoria weakly increased with precipitation. RGR in all species increased with light and decreased with size. Chlorophyll a concentration, boosted by moderate nitrogen deposition, increased RGR of all species.

4.  In conclusion, rainfall likely drives the distribution of the pale species due to their higher RGR and abundance in wet climates but cannot explain why Bryoria dominate drier inland forests. Our results highlight that the functional links between rainfall and RGR depends on both color of the lichens (pale versus dark pigments) and hydration traits.

5.  Synthesis. Our findings may explain the global, regional and local distribution patterns of hair lichens and help us to predict how environmental hazards such as climate change and forestry influence these important boreal canopy components.