Resumen de La recent expansió forestal a europa (1985-2015). Patrons, impulsors i implicacions en el creixement del bosc
In Europe, forest area has been increasing in many regions since the early 20th century, mainly due to the abandonment of farmland that has induced widespread forest expansion across the continent. Despite the relevance of this phenomenon, many questions remain poorly understood mostly due to the lack of studies addressing the issue at a global scale. In this thesis, we aimed at exploring the spatial patterns of the forest expansion in Europe for the last decades (1985-2015) and its main socioenvironmental drivers in order to investigate its consequences on secondary forests establishment and growth and on landscape composition and configuration. To achieve this goal, we used different land-cover maps and other remote sensing datasets. In Chapter 2, we addressed the association between forests cover increase and its spatial pattern change in Europe (1990-2012) through a set of landscape metrics regarding land cover diversity and habitat fragmentation and connectivity. Results showed that landscapes experiencing forest area increase exhibit a significant decrease in their land cover diversity, and an increase in both forest defragmentation and connectivity. However, these changes in land cover diversity were not directly attributable to forest increase but to the land cover initial composition. In addition, we determined that forest expansion patterns also depended on elevation and geographical position, with forest defragmentation being more frequent in forest-dominated landscapes concentrated in northern and Eastern Europe and new patch proliferation in the less forested landscapes in southern and western regions. In Chapter 3, we examined the magnitude and distribution of recently established forests (1992-2015) across socioecological gradients in Europe. We classified the established forests into regenerating after disturbances or secondary forests expanded into agricultural areas after abandonment, and we determine main socioenvironmental factors associated while we evaluated the effect of these patterns and the land use legacy (i.e. regeneration vs. expansion) on forest productivity by using the Enhanced Vegetation Index. Results determined that forest area increased in Europe (0.06% year-1) caused mainly by forest expansion in Mediterranean and in Eastern temperate regions, while regeneration was particularly relevant in the boreal region. Both forest expansion and regeneration had a greater magnitude in highly forested and/or highly diverse landscapes, suggesting that landscape composition largely determined the local forest increase across Europe. Conversely, the rest of socioenvironmental factors showed contrasting association with forest expansion and regeneration depending on the climatic domain. Moreover, the analysis of EVI temporal trends revealed that expanding forests had higher EVI values than regenerating forests except in the warmer and drought-prone areas where, probably, they cannot benefit from the biological and physicochemical legacies of former agricultural soils for tree growth. In Chapter 4, we put the focus on the Iberian Peninsula to explore changes in the pattern of secondary forest establishment (1985-2014) and its implications for forest growth (aboveground biomass), for the main forest leaf-habit types. Results showed expansion rates in the Iberian Peninsula (0.31% year-1) above average Europe and that secondary forests were increasingly established in places with better environmental conditions (i.e. higher water availability, at lower elevations and on less steep slopes). In addition, results highlighted a key role of summer precipitation, temperature, slope and forest cover, and the lesser role of drought events, on secondary forest growth. Particularly, we observed that warm temperatures compromise the growth of needleleaf forests while broadleaf secondary forest benefits more from summer precipitation. Our results may improve the knowledge of forest expansion and can be useful in addressing some European afforestation strategies (e.g. Green Deal) because they provide valuable information about which sites may be prioritized while visualize the potential of passive expansion of forests as a cost-effective strategy of habitat restoration.
