Climate change – Bad news for montane forest herb layer species?

Global warming presents a threat to plant species distributed at montane or alpine altitudes if the topography does not allow upward shifts in distribution ranges. Nevertheless, the species might also benefit from increasing temperatures and secondary effects on dominant species (e.g. bark beetle outbreaks or summer drought affecting the canopy species). As a consequence, disturbance frequency in montane forests might increase and light availability for herb layer species will increase. We addressed these interactions in a common garden experiment in Central Germany at different altitudes, representing cold and moist vs. warm and dry conditions. We investigated three montane species with different life forms, including a herb (Trientalis europaea), a grass (Calamagrostis villosa) and a dwarf shrub (Vaccinium myrtillus) under three shading treatments (3%, 28% and 86% of full sunlight). We hypothesized that montane species are at a disadvantage in the lowland, with the dwarf shrub suffering more than the grass. Furthermore, we hypothesized an antagonistic interaction of increased temperature and increased light conditions. While T. europaea and V. myrtillus showed only slightly responses to low altitude conditions, C. villosa displayed a nearly fifteen fold increase in biomass production, despite higher observed herbivory levels in the lowland. We failed to show an antagonistic effect of increased temperature and increased light availability, as all study species suffered from deep shade conditions and grew best under full light conditions at both sites. In conclusion, both improved temperature and light conditions might be principally beneficial for the investigated boreal species, in particular for the grass species C. villosa.