Various studies have focused attention in Europe, and in Spain in particular, on determining how forest masses “move” and what the influence of warming is, which creates the conditions for them to occupy spaces at higher altitudes. “High mountain plants have also been especially sensitive to warming,” recalls Francisco Lloret, Creaf researcher and professor of Ecology at the UAB.
Scientists from the European Gloria network for monitoring high mountain flora have observed an increase in the number of alpine species that grow at higher levels in the Pyrenees, as a consequence of the increase in temperatures.
Normally, high mountain plants are limited by cold conditions while “an increase in temperatures allows them to live better,” says Lloret. This same pattern is observed in the high mountain vegetation of central and northern Europe. The exception is the high Mediterranean mountains (Sierra Nevada), where this plant variety decreases, or which is attributed to a decrease in available water. The scientists (who published their work in Science in 2012) calculated a displacement towards the top of 2.7 meters on average in the species studied.
Furthermore, the forest limit is rising in altitude in the Pyrenees. Trees, especially conifers, are colonizing higher elevations. This was indicated by a study led by Aitor Ameztegui and published in Global Ecology and Biogeography in 2015. However, this increase in the altitudinal limit of the forest is also due to the fact that the pressure of livestock, especially sheep, has decreased (due to abandonment of agricultural and livestock practices), which favors the growth of young trees.
These analyzes showed that the Pyrenean forest line has moved upwards on average almost 40 meters between 1956 and 2006. However, in most sites it has remained unchanged (61.8%) or has advanced moderately, between 25 and 100 meters (23.7%), and only 9.2% of the locations have advanced more than 100 meters. “We see clear evidence that the increase in temperatures is modifying the vegetation in small plants, at high mountain levels. In the case of pine trees, it is masked because livestock pressure has decreased and that causes the trees to rise to higher levels,” says Lloret.
The first studies on this “movement of forests” towards higher altitudes were developed more than 20 years ago by Josep Peñuelas and Martí Boada. Comparing old photographs, they confirmed that the beech forests were rising in elevation in the high areas of Montseny. However, both suggested that lower livestock pressure may also play a role: and, therefore, the fact that beech trees can grow better. Other complementary studies detected that the growth of beech rings at low altitudes was decreasing, indicating that they were growing less due to the drought.
Another strong impact on Catalan forests is caused by drought. “At lower levels, drought, which to a certain extent we can associate with climate change, is causing significant mortality in forest masses, sometimes in combination with forest pests,” adds Lloret. In summary, although climatic conditions are improving for forests in high mountains, they are becoming more stressful for them at lower altitudes.