Comparing forest expansion (1930s–present) by invasive and native trees in relation to climatic, socio-economic, and landscape variables

Over the past century, large areas of agricultural land across Europe have been abandoned, often transitioning to forests. This study examines the relationships between climatic, socio-economic, and landscape variables and forest expansion by the invasive non-native black locust (Robinia pseudoacacia L.) and native tree species in the Italian municipalities. Forest expansion was assessed by comparing a digitised 1936 forest type map with more recent forest maps (1999–2017), reclassified into black locust, native forests, and non-forested land. We modelled the probability and extent of forest expansion, accounting for non-linear relationships and non-Gaussian distributions. Expansion by native trees and black locust shows both similarities and differences. For black locust, spatial patterns and landscape diversity are strongly linked to both the probability of establishment and subsequent expansion. In contrast, expansion by native trees is primarily linked to landscape diversity, temperature, and rainfall, while spatial patterns are more relevant after expansion begins. Farm size is also associated with these dynamics: native tree expansion decreases with farm size but increases beyond 20 ha, whereas black locust expansion consistently declines as farm size grows. Mean individual income does not appear to influence either process. This study shows that native and invasive trees respond differently to landscape structure, spatial context, and farm size, while economic variables such as income play a minor role. These contrasting patterns support the use of the municipal scale to interpret forest change and to inform spatial planning aimed at distinguishing areas of natural recovery from those susceptible to invasion.