Drivers of alpine-treeline-ecotone dynamics in the Pyrenees

While alpine-treeline ecotones are expected to shift upwards and densify under climate warming, observed dynamics vary across mountain ranges. Most studies focus solely on elevational shifts, yet different dimensions may respond to different drivers-limiting our ability to predict ecosystem responses. We examined multiple aspects of treeline ecotone changes (elevational shifts, spatial-pattern changes, and infilling) in the temperate mountain range of the Pyrenees, focusing on the eastern French sector, and uncovered interactions between climate, topography, land use, and lithology at different spatial scales.

We studied 626 treeline ecotones in the eastern French Pyrenees to examine elevational shifts, spatial-pattern changes, and infilling in relation to climate, land-use, and habitat drivers. Regression and factorial analyses identified key drivers and recurrent environmental combinations affecting treeline ecotone dynamics.

Three environmental clusters revealed contrasting regional dynamics across the eastern French Pyrenees. Western treeline ecotones showed the strongest upward shift, central treeline ecotones exhibited the highest local infilling, and eastern high-elevation treeline ecotones displayed downward shift and diffuse patterns. Overall, local topography influenced fine-scale infilling, while regional land-use and biogeographic contexts controlled broader treeline ecotone elevational shift, highlighting how different drivers operate at multiple spatial scales.

Understanding the effects of climate change requires considering multiple dimensions beyond elevational shifts, as each aspect of treeline ecotone dynamics responds differently to drivers operating at different spatial scales. Management strategies should adapt to multi-scale contexts rather than uniform approaches, particularly where treeline ecotone structure indicates scale-dependent processes.