Increasing rates of subalpine tree mortality linked to warmer and drier summers

Warming temperatures and rising moisture deficits are expected to increase the rates of background tree mortality–low amounts mortality (~0.5%–2% year?1), characterizing forest demographic processes in absence abrupt, coarse-scale disturbance events (e.g. fire). When compounded over multiple decades large areas, even minor increases <0.5% year?1) can cause changes communities carbon storage potential that comparable or greater than those caused by disturbances. We examine how temporal variability for four subalpine conifers reflects climate teleconnections using observations from 1982 2019 at Niwot Ridge, Colorado, USA. Individually marked trees (initial population 5,043) 13 permanent plots—located across a range site conditions, stand ages species compositions—were censused new nine times 37 years. Background was primarily attributed stress unfavourable competition (71.2%) bark beetle activity (23.3%), whereas few died wind (5.3%) wildlife impacts (0.2%). Mean annualized beetles more tripled all stands between initial censuses (0.26% year?1, 1982–1993/1994) recent (0.82% 2008–2019). Higher were related warmer maximum summer temperatures, deficits, negative anomalies ENSO (La Niña), with effects drought some subpopulations (tree size, age species). For example, older (>250 years), larger likely die smaller younger trees. Differences sensitivity among varied type may lead unexpected shifts composition structure. Synthesis. A strong relationship higher warmer, drier conditions implies warming will continue forests. Combined disturbances declining frequency moist-cool years suitable seedling establishment, increasing have drive declines populations.