Spatial patterns of plant richness across treeline ecotones in the Pyrenees reveal different locations for richness and tree cover boundaries

Aim To forecast the responses of alpine flora to the expected upward shift of treeline ecotones due to climatic warming, we investigated species richness patterns of vascular plants at small spatial scales across elevational transects. Location Richness patterns were assessed at local scales along the elevational gradient in two undisturbed treeline ecotones and one disturbed treeline ecotone in the Spanish Pyrenees. Methods We placed a rectangular plot (0.3–0.4 ha) in each treeline ecotone. We estimated and described the spatial patterns of plant richness using the point method and Moran’s I correlograms. We delineated boundaries based on plant richness and tree cover using moving split windows and wavelet analysis. Then, to determine if floristic and tree cover boundaries were spatially related, overlap statistics were used. Results Plant richness increased above the forest limit and was negatively related to tree cover in the undisturbed sites. The mean size of richness patches in one of these sites was 10–15 m. Moving split windows and wavelets detected the sharpest changes in plant richness above the forest limit at both undisturbed sites. Most tree cover and plant richness boundaries were not spatially related. Main conclusions The upslope decrease of tree cover may explain the increase of plant richness across alpine treeline ecotones. However, the detection of abrupt richness boundaries well above the forest limit indicates the importance of local environmental heterogeneity to explain the patterns of plant richness at smaller scales. We found highly diverse microsites dominated by alpine species above the forest limit, which should be monitored to describe their response to the predicted upward shift of forests.