Influences of 20th-century warming at the upper tree line contingent on local-scale interactions: evidence from a latitudinal gradient in the Rocky Mountains, USA

Aims Twentieth-century climate, the spatial pattern of tree establishment and positive feedback influence upper tree line ecotones. Here, I investigate how these factors interact to gain a more holistic understanding of how broad-scale abiotic and local-scale site conditions regulate tree establishment within upper tree line ecotones. Location A latitudinal gradient (c. 35-45 degrees N) in the US Rocky Mountains. Study sites (n = 22) were located in the Bighorn (BH), Medicine Bow (MB), Front Range (FR) and Sangre de Cristo (SDC) mountain ranges. Methods Dendroecological techniques were used to reconstruct tree establishment dates that were compared with 20th-century climate data using correlation and regime shift analyses. Spatial patterns of tree establishment were analysed by Ripley's K and used to determine local-scale interactions capable of ameliorating broad-scale climate inputs through positive feedback. Results Significant correlations (P < 0.01) between tree establishment and climate were confined to the FR, where a positive correlation was found with summer (June-August) and cool season (November-April) temperature range (T-max - T-min). These trees were almost exclusively situated in a random spatial pattern. Similar patterns exist in the BH, yet their establishment was contingent on the availability of local shelter in the lee of boulders. Trees in the MB and SDC were primarily clustered in space and had no significant correlations with climate. Considerable lag times exist between regime shift changes in climate towards more favourable growing conditions and corresponding shifts in tree establishment in all mountain ranges except the FR, where synchronous shifts occurred in the early 1950s. Main conclusions These results suggest that the influence of broad-scale climate on upper tree line dynamics is contingent on the local-scale spatial patterns of tree establishment and related influences of positive feedback. This research has important implications for understanding how vegetation communities will respond to global climate change.