This is shown by a recent paper that examines how the treeline position has changed in the Pyrenees in the last 50 years. According to the study, the treeline has advanced 40 meters on average, and the decrease in livestock pressure is one of the main causes. Contrary to the belief so far, climate change would play a much less important role.

Prat d'Aguiló (2) - Panorámica_

Prat d’Aguiló (Lleida) – Author: Aitor Ameztegui

Solsona, 18 March 2016. Forests have their own dynamics: they can gain ground, lose it or stay stable. To understand which variables may affect these dynamics, Aitor Ameztegui and his team compared the changes in treeline position in the Catalan Pyrenees between 1956 and 2006. According to them, there were two key factors potentially playing a role in the dynamics of this limit: land-use changes and climate change.

The study, carried out by researchers of the Forest Sciences Centre of Catalonia (CTFC), the CREAF, the UB and the CSIC, has been published this month in the prestigious journal Global Ecology and Biogeography, which also illustrates its cover with a picture of the Pyrenean treeline. The results show that forests have actually moved upwards almost 40 meters on average, but also show high variability on the response, since no changes were observed in more than 60% of the study area.

The areas showing a greater treeline advance were those in which there has been a stronger change in land use. Indeed, the stocking density in the Pyrenees was much higher fifty years ago, and it was mostly sheep in transhumance, a pasture system that kept the forest down. The livestock rate has fallen since, and it is now more common to see cattle and mare herds, which cause less damage than sheep on tree seedlings.

In contrast, the study found no significant evidence linking changes in treeline with increasing temperatures. This result is very important, because this limit is known to be highly influenced by climate in many areas worldwide. However, as Ameztegui specifies “it does not mean that the climate did not play a role at all, but it was much smaller than the role of land use changes”.

There is still room for further forest advance

The treeline is defined as the maximum elevation at which trees can grow. In the Catalan Pyrenees, it is located between 2000 and 2300 meters high. At this elevation we can find the transition from subalpine areas, dominated by mountain pine forests, to alpine areas with only herbaceous or shrubby vegetation. Experts point out that without disturbance caused by humans, this limit could reach elevations between 2200 and 2500 m.

The upward shift of the treeline is yet another consequence of the abandonment of traditional farming activities, which have also caused a progressive forest expansion and densification. The loss of open spaces and the typical mosaic in mountain areas – characterized by a combination of forests and grasslands – may have important effects on biodiversity, favouring species that prefer forest habitats to those needing open areas. However, Ameztegui clarifies that Pyrenean forests “still have room to move upwards, as current livestock still conditions the treeline position”.

More information:

Aitor Ameztegui, Lluís Coll, Lluís Brotons and Josep M. Ninot. (2015) Land-use legacies rather than climate change are driving the recent upward shift of the mountain tree line in the Pyrenees. Global Ecology and Biogeography. DOI: 10.1111/geb.12407