What are the environmental factors determining the secondary forest diversity in mountain ecosystems? A case study in the Gran Paradiso National Park, Italy

Details

Supervisors

Ponette, Quentin ; Garbarino, Matteo

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en gestion des forêts et des espaces naturels, à finalité spécialisée

Abstract

enLand abandonment since the 19th century has driven a widespread expansion of secondary forests across the European Alps, generating heterogeneous mosaics whose composition and structure are still poorly understood. Trajectories of these stands do not converge towards a single natural state but remain shaped by the climatic, topographic and anthropogenic conditions. The factors driving this diversity, however, have received comparatively little attention, limiting our ability to predict which forest type will develop at a given site. This master's thesis characterises the diversity of secondary forests in the Gran Paradiso National Park (Western Italian Alps). The analysis is based on 283 sampling plots. Forest structure, composition and regeneration were measured in the field and combined with a set of climatic, topographic, landscape and anthropogenic descriptors. Eight forest types were identified and grouped into two macro-groups: a broadleaved and mixed group, and a larch-dominated group. A principal component analysis revealed two main ecological gradients: a species-composition gradient separating the two macro-groups and a maturity-and-legacy gradient opposing mature, well-established stands to pioneer, newly-established ones. A Random Forest analysis identified elevation as the single dominant driver, accounting for 64.5% of the total variable importance, far ahead of southness and mean diurnal temperature range; anthropogenic and 1954 landscape variables together contributed less than 10%. These results converge towards a hierarchical interpretation of secondary forest diversity. A primary thermal-accumulation filter, acting through elevation and secondarily through slope aspect and temperatures, determines which successional trajectory is ecologically possible at a given site. A secondary filter, linked to land-use legacies inherited from the 1954 landscape, controls how far a stand has progressed along its trajectory. The type of secondary forest developing at a given location in the Park is therefore primarily determined by where it lies on the thermal-accumulation gradient, and only secondly by its land-use history. This framework provides a synthetic and transferable answer to the question of what drives secondary forest diversity in mountain landscapes.