Spatial dynamics of alpine tree lines under global warming: What explains the mismatch between tree densification and elevational upward shifts at the tree line ecotone?

Aim Most studies focusing on the alpine tree line 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 relationship 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: (a) to quantify the mismatch between the two processes at a regional scale, and (b) to identify its site-specific determinants. Taxon Pinus uncinata (Ramond ex DC.) Location French eastern Pyrenees. Methods An object-oriented supervised classification procedure was performed on historical (1953) and current (2015) aerial photographs. Based on the resulting rasters, densification of the tree line ecotone and upward shift of the tree line were estimated at the two dates in 191 sites, then standardized, before finally being compared. Three site clusters were derived (no mismatch, densification prevalence and 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 relationship 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 tree line spatial dynamics, since these two ecological processes are not controlled by the same drivers.