Rock climbing is popular and the number of climbers rises worldwide. Numerous studies on the impact of climbing on rock-dwelling plants have reported negative effects, which were mainly attributed to mechanical disturbances such as trampling and removal of soil and vegetation. However, climbers also use climbing chalk (magnesium carbonate hydroxide) whose potential chemical effects on rock-dwelling species have not been assessed so far. Climbing chalk is expected to alter the pH and nutrient conditions on rocks, which may affect rock-dwelling organisms. We elucidated two fundamental aspects of climbing chalk:

1. Its distribution along non-overhanging climbing routes was measured on regularly spaced raster points on gneiss boulders used for bouldering (ropeless climbing at low height). These measurements revealed elevated climbing chalk levels even on 65% of sampling points without any visual traces of climbing chalk.
2. The impact of climbing chalk on rock-dwelling plants was assessed with four fern and four moss species in an experimental set-up in a climate chamber.

The experiment showed significant negative, though varied effects of elevated climbing chalk concentrations on the germination and survival of both ferns and mosses. The study thus suggests that along climbing routes elevated climbing chalk concentration can occur even were no chalk traces are visible and that climbing chalk can have negative impacts on rock-dwelling organisms.