Pinus nigra anthropogenic treelines in the central Apennines show common pattern of tree recruitment

Treeline position, structure, and composition are shaped by natural and anthropogenic factors. At human-disturbed treelines, it is particularly difficult to disentangle the specific role of natural and anthropogenic drivers controlling tree recruitment dynamics following land abandonment. We tested for a possible common regeneration pattern of Pinus nigra Arn. at four upper treeline ecotones in the central Apennines (Italy). The sites were selected based on (1) the occurrence of natural encroachment of P. nigra above 1600 m a.s.l., and (2) the mountain top elevation higher than 2000 m a.s.l. We assessed structure and spatiotemporal patterns of P. nigra advancing regeneration using point and surface pattern analyses. We mapped, measured, and dated 845 trees sampled on a total surface area of 336 ha. P. nigra is the only tree species expanding at high altitude and features a scattered process that started 35–40 years ago, with a maximum recruitment frequency between 1995 and 2003. Pinus regeneration appeared over-dispersed along the slope at a scale range of 12–18 m. We found spatial segregation between saplings and young trees at intermediate distances (8–17 m) and small patches of young trees distributed along the treeline ecotone. The spatial pattern of P. nigra encroachment in the central Apennines revealed a replicable model independent of treeline topography and local disturbance histories.