Long-term forest-line dynamics in the French Pyrenees: an accelerating upward shift related to forest context, global warming and pastoral abandonment

Abstract. Worldwide, the upper forest line has climbed over the past decades, shaping mountain landscapes in response to global changes. In European mountains, this recent trend is a continuation of the forest transition initiated in the mid-19^th century, when forest extent was minimal. This study aimed to reconstruct the forest-line dynamics for the entire French Pyrenees from the mid-19^th century until today. To ascertain the forest-line elevational shift for the 114 municipalities studied, three digital land-use maps (dated 1851, 1993 and 2010) were employed. The forest-line shift velocity was calculated for two periods delineated by these maps. We applied linear mixed-effect models to investigate the influence of human and environmental drivers on the forest-line shift. The mean upward shift was 0.9 m.yr^-1 during the 1851–1993 period but was four-fold higher during the 1993–2010 period (3.5 m.yr^-1). During the first period, the forest-line shift coincided with the isotherm upward shift, resulting from global warming. However, during the second period, despite an acceleration, the forest line lagged behind the isotherm upward shift, deepening its climatic debt. Furthermore, during the first period, the forest line shifted upward seven times faster in the eastern Pyrenees, where the mountain pine, a pioneer species, formed the forest line and pastoral abandonment occurred earlier, than in the western Pyrenees (1.3 vs. 0.2 m.yr^-1). Conversely, in the following period, the shift occurred three times as fast in the western Pyrenees, where abandonment became widespread, as in the eastern Pyrenees (5.6 vs. 2.1 m.yr^-1). In addition, during the second period, the closed forest line climbed twice as fast as the forest line (5.6 m.yr^-1), indicating a pronounced densification of the subalpine forest. Our innovative approach integrates a large spatial scale and temporal depth and sheds new light on the interrelationships between global warming, pastoral abandonment and the forest-line upward shift.