Many studies in northern Europe, North and South America, describe regional trends of population densification at altitudinal and polar tree lines during the 20th century. The purpose of this study was (1) to ascertain if this regeneration enhancement is present across the alpine ecotones of the Pyrenees, (2) if synchronous recruitment trends are common among the studied populations and (3) to determine the tree limit stability during recent decades. Twelve Pinus uncinata tree line populations were studied on the Iberian eastern range of the Pyrenees. Rectangular plots ranging from 940 to 7600 m2 were set along the forest-alpine grassland transition; more than 3600 P. uncinata individuals were mapped. Tree size and age were used to establish the demographic structure at each stand, and to characterize abrupt or smooth transition patterns along the tree line ecotone. A new procedure for estimating missing rings in off-centre cores was developed to ensure a correct interval for the age-classes distribution analysis. Past and recent synchronous recruitment trends (mid 19th century, second half of the 20th century) were apparent at the tree line over the studied area of the Pyrenean range. The ecotone densification since the 1950s occurred in the context of climatic warming and substantial land use abandonment. Both gradual and step-like transition patterns in tree age and size along the ecotone were observed. Regeneration enhancement in the last approximately 30 years appears as an abrupt change in population age structures, which could indicate the importance of feedback mechanisms for tree line recruitment dynamics. In 50% of the surveyed tree lines ecotone densification has been coupled to tree limit shifts in the recent past. This indicates both great tree limit sensitivity to short-term climatic changes and the presence of differential tree line dynamics at a regional scale. Synthesis. The observed past and recent synchronous recruitment trends suggest the presence of regional climatic factors modulating tree line structure and dynamics. However, tree line dynamics in the Pyrenees have been widely affected by local anthropogenic activities. We suggest that the presence of step-like tree line transitions in tree age can be considered an evidence of recent human-induced disturbances when no other major natural disturbances affect the tree line dynamics.
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