Tree rings have been used to both reconstruct past climate, and to estimate and project carbon uptake of forest ecosystems. Here we show that large groups of trees of the dominant tree species within widely-distributed circumpolar forest sites show opposing growth trends during recent warming. These opposing growth trends are present at a sub-chronology level and, if averaged into chronologies, may have contributed to the widely reported overall decreased temperature sensitivity of high-latitude chronologies over recent decades. Unlike previous studies, we find that temperature sensitivity has actually increased for most individual trees at these sites. This recent, widespread divergence in growth response seems unique over the past three centuries, and may relate to different microsite responses of individual trees to temperature-induced drought stress or other factors. This divergence needs to be taken into account in dendroclimatic reconstructions, estimations of global warming impacts, and carbon uptake projections.
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