The Swiss stone pine (Pinus cembra) is the 'queen' of the upper forest line. Gnarled trees up to 500 years old, with fragrant wood, they once hemmed the forest line in many parts of the Alps. Alpine farming, damage from game and diseases as well as the long-term decimation of the spotted nutcracker (once mistakenly fought as a pest of the Swiss stone pine) have caused their populations to dwindle. The only substantial contiguous forests remaining in Switzerland are in the Engadine and Valais. The species now faces another problem: climate change. If the climate continues to get warmer and drier, the Swiss stone pine could be displaced by fast-growing competitors from lower altitudes, namely spruce, fir, pine, and deciduous species.
Its cold tolerance would give the species a competitive advantage at higher elevations, but could it make the move quickly enough? Researchers from WSL, ETH Zurich and the University of Zurich examined whether juvenile trees have the genetic make-up they need for the future. The Swiss stone pine is a leisurely species, taking its time to reproduce and not forming fertile cones until the age of 40 to 60 years. The fear is that the seeds from adult trees that are germinating today are adapted to the cooler and wetter climate of the past which, according to climate models, is set to disappear.
The research team analysed over 3,000 genes from several hundred seedlings and adult trees at high and low elevations in the tree's Swiss distribution range. It identified which gene variants are advantageous for which environmental conditions, and the populations and elevations where these variants occur. Juvenile trees at high elevations were found to have the genetic make-up to cope with both the current and future climate, as lead author Benjamin Dauphin and his colleagues report in the journal Global Change Biology.
Left behind by climate change
By contrast, they found that young trees at low elevations mostly had the 'wrong' gene variants, which would be disadvantageous in the future, warmer and drier climate. "The offspring of the trees living today will be less well adapted to a warmer future in these places," says Felix Gugerli of the WSL Ecological Genetics research group, who led the study. When species are overwhelmed by climate change due to their long generation time, experts refer to this as an 'adaptation debt’.
For the Swiss stone pine to move to higher elevations, the right genes are not enough: it also needs the spotted nutcracker, which hides the tree's seeds as a food reserve. Many of these seeds are not eaten and can then germinate. In addition, Swiss stone pines can only grow where there is enough raw humus. In many places, this does not yet exist at high elevations because soil development is an extremely lengthy process.
These factors combined with other challenges – damage from game or skiers, pathogenic fungi that benefit from the warmer climate – could spell trouble for the Swiss stone pine in some places. "We won't lose the species altogether, but its occurrence will be further reduced and increasingly fragmented," says Gugerli. This will hinder the exchange of individuals and could lead to inbreeding. In some Alpine valleys, the Swiss stone pine could even become extinct.
Since this species, together with the larch, is integral to the typical forest ecosystem at the upper treeline, its decline would put an entire biological community at risk – not only the spotted nutcracker but also a multitude of fungi, lichens and insects that are native to these forests.