Dynamics in human-shaped upper forest margins in southern European mountains

High-elevation areas in mountains ranges of the Mediterranean basin have been shaped by a millenary history of pastoral activities that have significantly depressed the upper forestlines. During the 20th century the same ecotones experienced the progressive decline of such practices and in some cases the following natural establishment of new cohorts of trees facilitated by the absence of relevant geo-morphological constrains and the influence of climate warming. These recent dynamics featured local differences since forestline expansion is strongly related to dispersal characteristics of the current forestline species, production of viable seeds, seed dispersal kernels and seedlings competition with ground vegetation. Our studies on upper forest margins of the Apennines (Italy) conducted over the last 20 years showed that Fagus sylvatica is the most widespread species (94%) but largely stable, whereas Pinus nigra, favored by extended plantations at lower altitude, is expanding upwards in treeless grasslands. We investigated this process in some upper ecotones sites in the Central Apennines largely controlled by microsite topography and the new tree proximity to the seed source. Similar processes occurred to other pine species naturally occurring in the Dinaric Alps of Albania and Montenegro and in the Iberian mountain ranges (Pyrenees and Iberian System) either as a high-altitude tree densification process or as a renewed treeline upshift. The increase of these young tree cohorts within the treeline ecotones can induce changes in major biogeochemical cycles, soil properties and mountain eco-hydrological processes, leading to a significant long-term plant diversity decline in species-rich grasslands. Forest densification could improve slope protection against landslides and snow avalanches but also produce more fuel and, together with climate warming, increase the risk of mountain wildfires. Further analysis at different spatio-temporal scales are necessary including better estimation of demographic and dispersal parameters in similar low-density expanding tree populations.