Subalpine ecosystems are natural laboratories to study the evolution of global warming, since their dynamics are particularly sensitive to temperature changes. However, the alpine and subalpine environments also count on a long history of human use. In a recent study, we analysed the contribution of climate-human interactions to historical forest recruitment dynamics.
In this new article in Global Change Biology, we analysed three high-elevation pine forests (Pinus heldreichii and Pinus uncinata) in southern Europe. We selected these forests because they harbour some of the current oldest known trees in Europe, exceeding 1000 years of life (e.g. Italus and Adonis). These forests are located in the Pyrenees (north-eastern Spain), Pollino Massif (southern Italy) and Mt. Smolikas (northern Greece).
Using dendrochronology, we reconstructed the germination date of 1,146 trees and analysed the long-term recruitment dynamics in the three study forests. Curiously, our results showed coupled peaks of tree establishment in the late 15th and early 16th centuries. So, the next step was to investigate the potential drivers and interactions of such widespread tree recruitment. The similitude in mountain forests located thousands of kilometres away pointed towards a regional-wide driver; therefore, we analysed the influence of temperature changes, the effects of historical human practices and the role of potential synoptic climatic drivers (e.g. North Atlantic Oscillation).
The peak of tree recruitment followed the late-medieval demographic crisis of the 14th and 15th centuries, which was documented by recurrent famines, the Black Death pandemic and political turmoil. During Medieval times, human land use deeply affected the mountain systems of Europe. At that time, mountain pastures represented a fundamental resource for the local population. The late-medieval crisis led to a large-scale depopulation of rural areas and, therefore, the agro-pastoral activities in the following decades and centuries terminated or decreased considerably. Abandoned pastures were progressively dominated by pines after a period when stands with mature reproductive trees were restored and soils capable of harbouring pine saplings were formed. In other words, forests reclaimed spaces that humans had occupied in the past.
In addition to the societal/sanitary dimension, the beginning of the Little Ice Age around 1450 CE turned climate towards cooler and more unstable conditions, promoting a rapid cessation of grazing activities in the high-elevation belt. All in all, the current abundance of old trees and mature forests in southern European mountains may be a consequence of this historical process. These long-living forest ecosystems have been storing carbon for centuries, protecting at the same time unique biodiversity. Their strict conservation is therefore essential to keep these last stands of ancient trees.
Currently, a new wave of forest expansion is taking place in subalpine environments. In this case, the cessation of traditional land use started in the second half of the 20th century, and a gradual conversion of subalpine grasslands to forests is being observed. The exodus of rural population towards cities is the cause behind this recent land use abandonment, but the effect on rewilding mountain landscapes is being similar to that of the early phase of the late-medieval crisis. However, warmer temperatures due to climate change and increasing air concentrations of N and CO2 may also be favouring the current rapid forest expansion.