Inertia or reprise? Reconstructing recent alpine treeline dynamics in relation to climate change.

Alpine treelines (the altitudinal limit of the habitat at which trees are capable of growing) are considered monitors of the effects of climate change on forest growth and dynamics. Treelines are expected to react to current climate warming by showing upslope migrations. However, treeline dynamics are often characterized by lagged responses to rising temperatures, that is, treeline inertia. We investigated how the treeline responded to climate in a Pyrenean site with an intense Mountain pine (Pinus uncinata) regeneration but also abundant dead trees.

Treeline in Ordesa, Spanish Pyrenees

Forest growth around the treeline in Ordesa National Park (Spanish Pyrenees)

Dendrochronology allowed us to reconstruct the treeline dynamics (growth, tree recruitment, and death) and to build an age structure of Pyrenean Mountain pine forests, and relate them to temperature reconstructions of the study area. This analysis showed profuse pine recruitment since the 1980s. Most current Pyrenean Mountain pines were recruited in the first half of the 18th century, a warm period when growth was stable, while old treeline trees recruited not only in those decades but also in previous warm periods. Pine deaths concentrated in the cool transition between the mid 17th and the early 18th centuries and mainly from 1820 to 1860, when growth declined as a consequence of temperatures rapidly dropping at the end of the Little Ice Age.

Dendrochronological reconstruction and population dynamics at the study site.

Dendrochronological reconstruction and population dynamics at the study site.

Only the amount of dead pines at the treeline was negatively related to temperatures, indicating that cool periods cause high adult mortality rates and trigger long-term treeline decline. But this decline was interrupted by intense regeneration and treeline encroachment, two features that characterize recent treeline dynamics in some mountains. This concurs with the view of a rapid response of alpine treelines to climate during the late 20th century.

More information: J. Julio Camarero, José María García-Ruiz, Gabriel Sangüesa-Barreda, Juan Diego Galván, Arben Q. Alla, Yasmina Sanjuán, Santiago Beguería, and Emilia Gutiérrez (2015) Recent and intense dynamics in a formerly static Pyrenean treeline. Arctic, Antarctic, and Alpine Research, In-Press.

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