Different Tree Recruitment Patterns at Treelines Are Associated With the Atlantic Multi-Decadal Oscillation Across the Northern Hemisphere.

Tree recruitment is one of the most critical processes in forest dynamics. However, prediction of forest dynamics is largely limited by a lack of understanding on how climate variability drives tree recruitment. Modes of large-scale atmospheric variability, such as the Atlantic Multi-decadal Oscillation (AMO), exert a strong influence on climate and forest ecosystems worldwide. Yet, the linkages between forest dynamics and AMO at large spatial scales, particularly in climatically marginal tree populations such as treelines, remain unknown. Using field survey data from 68,011 trees at 120 treeline sites across the Northern Hemisphere, we investigated tree recruitment dynamics and their relationships with climate and the AMO during 1951-2010. We identified increasing tree recruitment on the Tibetan Plateau and Central China at mid-latitudes, whereas a decreasing trend was observed in Scandinavia-Kola and Ural-Siberia at high latitudes. An inverse recruitment pattern was documented between mid-latitude Western North America and high-latitude Alaska-Northern Canada. Regional warming and changes in precipitation were linked to spring and summer AMO variability. In general, tree recruitment increased with warming and suitable moisture conditions and snow cover. Conversely, low precipitation in the non-growing season and warming-induced drought stress reduced recruitment. We provide evidence on how large-scale atmospheric circulation patterns influence treeline dynamics by modulating regional climatic limitations on tree recruitment at hemispheric scales. Our results have notable implications to forecast future forest shifts in climatically harsh environments.