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

中文翻译：

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与夏季变暖和干燥有关的亚高山树木死亡率上升

1. 气温升高和水分亏缺增加，预计将增加背景树木死亡率–树木死亡率低（约0.5％–2％- ^1年），这是在没有突然，粗暴干扰事件的情况下森林人口统计过程的特征。 （例如火）。当在几十年和大面积地区复合时，即使背景树木的死亡率略有增加（例如<0.5％year ^-1），也可能导致森林群落和碳储藏潜力的变化，甚至可大于或大于扰动所引起的变化。
2. 我们使用1982年至2019年美国科罗拉多州Niwot Ridge的树木死亡率观测资料，研究了四个亚高山针叶树的背景树木死亡率的时间变化如何反映气候和气候遥相关的变化。在37年中，对13个永久性地块（分别位于不同的工地条件，林分年龄和物种组成中）的单独标记的树木（初始种群5,043）进行了9次新的死亡率普查。
3. 背景树木的死亡率主要归因于不利的气候和竞争压力（71.2％）和树皮甲虫活动（23.3％），而很少有树木因风（5.3％）和野生动植物影响（0.2％）而死亡。由于树的压力和蠹虫的平均年化树死亡率超过所有三倍代表初步普查之间（0.26％，比去年^-1，1982- 1993年/ 1994年）和最近的人口普查（0.82％，比去年^-1，2008–2019）。树木死亡率较高与最高夏季最高气温，夏季水分不足和ENSO（拉尼娜）负异常有关，某些亚种群（树木大小，年龄和树种）受到干旱的影响更大。例如，在较老的林分（> 250年）中，较大和较老的树木比较小和较年轻的树木更容易死亡。因林分类型而异的亚种群之间的树木死亡率和对气候的敏感性差异可能会导致林分组成和结构发生意想不到的变化。
4. 综合。树木死亡率较高与夏季气候更干燥干燥之间的密切关系表明，气候变暖将继续增加亚高山森林的本底死亡率。再加上扰动的增加和适于幼苗生长的湿冷年频率的下降，树木死亡率的上升有可能推动亚高山树木种群数量的下降。