Aim

Most studies focusing on the alpine treeline responses to climate warming have used either the tree densification within the ecotone or its elevational upshift as indicators. However, it is acknowledged that the relation between densification and upshift is spatially heterogeneous, making inferences and comparability among studies tricky. The lack of consistent empirical evidence on this potential mismatch and its drivers leads us to focus on this issue in this study. The aim was twofold: (i) to quantify the mismatch between the two processes at a regional scale, and (ii) to identify its site-specific determinants.

Location

French eastern Pyrenees

Time period

1953-2015

Major taxa studied

Pinus uncinata (Ramond ex DC)

Methods

An object-oriented supervised classification procedure was performed on historical (1953) and current (2015) air-photos. Based on the resulting rasters, densification of the treeline ecotone and upward shift of the treeline were estimated at the two dates in 191 sites, then standardized, before finally being compared. Three site clusters were derived (no mismatch, densification prevalence, upshift prevalence). After having characterized their spatial patterns through join count statistics, a multinomial logistic regression model was computed to identify the correlates of these clusters among a list of site variables.

Results

No spatial pattern among the categories of responses emerges at a local scale, but buffers with no mismatch tend to aggregate at a larger scale. Changes in minimum air temperatures, site elevation, mean slope, slope morphometry and lithology appear as significant drivers of the observed mismatch, implying that the relation between densification and elevational upshift is context-specific.

Main conclusions

Our findings suggest that both densification and upshift should be considered in quantitative analyses of treeline spatial dynamics, since these two ecological processes are not controlled by the same drivers.