Local and regional climatic constraints of shrub and tree growth near the treeline

Many treeline and shrublines are not responding to climate warming as fast as expected. This lack of responsiveness could be explained by other non-thermal, climate drivers operating at the cold edge of distribution of trees and shrubs. To determine which are the main climate drivers of tree and shrub radial growth near the treeline we measured ring width and related it to climate variables (temperature, precipitation, snow depth) and vegetation greenness (NDVI, Normalized Different Vegetation Index). We compared two dwarf shrub (Vaccinium uliginosum, Dryas octopetala) and three tree species (Larix sibirica, Picea obovata, Pinus uncinata) sampled in three treeline sites: Polar or Northern Urals, Southern Urals, and Spanish Pyrenees. Dwarf shrubs presented lower first-order autocorrelation (AR1) than trees, excepting in the N. Urals site. In this site, V. uliginosum showed a negative growth trend, whereas this tendency was observed in P. obovata and P. uncinata trees from the S. Urals and Pyrenees sites, respectively. Shrub and tree growth indices correlated with NDVI at different months. Trees showed stronger and positive growth responses to warmer summer conditions and also negative responses to precipitation in the N. and S. Urals. The growth of D. octopetala in the Pyrenees was enhanced by prior-winter and current-spring precipitation showing a strong correlation with May snow depth (r = 0.66, p = 0.0006, period 1998−2020). Dwarf shrubs and trees coexisting near the treeline differently responded to regional climate variability. Our findings indicate a strong dependence of shrubs and trees on local (e.g., snow depth) and regional (e.g., growing-season air temperature) climate conditions, respectively.