Climate change will result in the shift of environmental conditions, which eventually questions the persistence of tree species in their current ecological niches. Aggravated summer drought has been projected for the central valleys of the European Alps, which may jeopardize the continuity of forests at the forest-steppe ecotone, especially if fire frequency should increase as predicted. In the dry Swiss valley Valais, the dominant tree species at the forest-steppe ecotone, Scots pine (Pinus sylvestris), has repeatedly failed to regenerate after stand-replacing forest fires. We examined whether the recruitment bottleneck is related to transient lack of ectomycorrhizal (ECM) fungi and whether seasonal drought following a fire event is the main limiting factor for seedling growth. Changes in ECM diversity after fire were tested with a soil-heating experiment, whereas the long-term effect of stand-replacing forest fires on ECM diversity was examined with bioassays of soil samples from 11 forests with different time since last fire and unburned adjacent forest communities, respectively. Effects of drought and climate warming on germination, survival and seedling growth of local and more southern pine provenances (Penyagolosa mountains, ES, located at the southern limit of the species) were tested in a common garden experiment.

Results/Conclusions

Heating soil samples to 60 and 70 °C reduced ECM diversity from 46 (control) to 32 species, whereas lower temperature (45 °C) showed no pronounced effect. Bioassays of soils with different time since last fire suggest that ECM re-colonisation is a long-term process. The similarity of ECM communities between burnt soils and adjacent forest soils increased significantly in the time frame of 2 to 20 years since last fire. Given unlimited ECM inocculum, water shortage was the primary cause of germination failure and pine seedling mortality in the common garden experiment. Autochthonous Scots pine seedlings had lower survival rates than seedlings from Penyagolosa mountains. Differences in growth rates between local and southern provenances were small in the first year but increased in older seedlings. In conclusion, the recruitment bottleneck of Scots pine after forest fires in the Valais can be partly explained by transient ECM poverty. Seasonal drought events further limit seed germination as well as survival and growth of Scots pine seedlings, especially of local provenances. The introduction of more drought-adapted Scots pine provenances may be a possibility to prevent future recruitment failure in general, and after fire events in particular.