Projections of increased temperatures and more frequent drought events question the persistence of extant tree species in their current distributions because these long-lived but slowly regenerating species might not be able to keep pace with rapidly changing climatic conditions, neither by adapting to new conditions nor by migrating fast enough to colonise new suitable habitats. Effects of climate change make the continuation of forest uncertain in zones where trees are currently at the edge to steppe. On the other hand, the simultaneous unbroken CO2-increase is considered to mitigate water stress during the growing season to some extent, a fact that complicates predictions of vegetation change.
In this project, we focus on the regeneration stage of Pinus sylvestris, which forms extensive forest stands at low elevations of the Central Alpine valleys. To test the potential of autochthonous P. sylvestris (Leuk, Valais, Switzerland: 600 mm, 9 °C) to withstand aggravated drought under different environmental conditions (elevated CO2, light conditions), seedlings will be grown from seeds in a common garden with mobile rainshelters and free-air CO2 enrichment (FACE). During the growing season (Mar–Sep), seedlings will be exposed to ambient (390 ppm) and future (570 ppm) CO2 concentrations and two precipitation regimes that correspond to a wet Central Alpine vegetation season (150% of the long-term average in the Rhone Valley) and a dry Central Alpine vegetation season (80% of the long-term average). Over three years of seedling development, we will measure growth metrics, stomatal conductance, water use efficiency (δ13C and δ18O ratios), non-structural carbohydrates and frost hardiness of autochthonous, low elevation P. sylvestris in comparison to other P. sylvestris provenances (Grisons, Valais high, Spain, Greece), provenances of more drought-resistant P. nigra (Spain, Austria, Bulgaria, Greece) and minimally frost-resistant but most drought-resistant P. halepensis (Spain, Bulgaria, Greece). The experiment also includes acclimated provenances of North-American Pseudotsuga menziesii (Switzerland, Germany), which has been planted in Europe for more than 100 years. Results will show whether interactive effects of light conditions and increased CO2 will compensate for transiently or continuously aggravated drought during early stages of tree growth, to what degree autochthonous P. sylvestris can adapt to future conditions in comparison with other provenances and species, and whether forest continuation in general will be at risk or can be assured by adding more drought resistant species or/and provenances to autochthonous forests.