The Arctic and alpine biome is rapidly warming, which might be causing an encroachment of relatively tall woody shrub vegetation into tundra ecosystems, which will probably result in an overall positive feedback to climate warming. This encroachment is, however, believed to remain limited to the relatively warm parts of the biome, where taller shrubs may displace shorter species. Still, climate sensitivity of shrub growth strongly differs between species and sites and High Arctic dwarf shrub species may respond rapidly to increasing temperatures in absence of taller species. In addition, it remains largely unknown whether shrubs from different functional groups from the same sites respond similarly to climate drivers. In the present study we examined the climate-growth relationships of six different site-species combinations: one evergreen and one deciduous shrub species at two alpine sites, and one evergreen dwarf shrub species at two High Arctic sites. We compared linear mixed models for each combination, explaining existing shrub growth data with site-specific interpolated monthly and seasonal climate data from the gridded meteorological dataset CRU TS4.00. Shrub growth rates were found to be sensitive to summer climate for all species at all sites. Continued and projected warming is thus likely to stimulate a further encroachment of shrubs in these systems, at least through a densification of existing stands. Dwarf shrub growth strongly responded to the recent warming at both High Arctic sites, contrasting with previous work suggesting that shrub expansion might remain limited to warmer tundra regions. At the alpine sites, growth of evergreen shrubs was found to be more dependent on summer climate than growth of deciduous shrubs, perhaps because these evergreen species are less prone to herbivory. However, biome-wide generalizations at the functional group level may be difficult to interpolate to the species level. Micro-site conditions, such as the determination of growing season length and winter soil temperatures, and influence on growing season soil moisture by snow depth, may determine the strength of the climate-growth relationships found.

中文翻译：

---

一个更温暖、更绿的寒冷世界：夏季变暖增加了高山和高北极苔原的灌木生长

北极和高山生物群落正在迅速变暖，这可能导致相对高大的木本灌木植被侵入苔原生态系统，这可能会导致对气候变暖的整体积极反馈。然而，这种侵占被认为仅限于生物群落中相对温暖的部分，在那里较高的灌木可能会取代较短的物种。尽管如此，灌木生长的气候敏感性在物种和地点之间存在很大差异，并且在没有较高物种的情况下，高北极矮灌木物种可能对温度升高做出快速反应。此外，来自同一地点的不同功能群的灌木是否对气候驱动因素有类似的反应，在很大程度上仍然未知。在本研究中，我们研究了六种不同地点-物种组合的气候-增长关系：两个高山地点的一种常绿灌木和一种落叶灌木，以及两个高北极地区的一种常绿矮灌木。我们比较了每种组合的线性混合模型，用来自网格气象数据集 CRU TS4.00 的特定地点插值的月度和季节性气候数据来解释现有的灌木生长数据。发现灌木生长率对所有地点的所有物种的夏季气候都很敏感。因此，持续和预计的变暖可能会刺激这些系统中灌木的进一步侵占，至少是通过现有林分的致密化。矮灌木生长对两个高北极地区最近的变暖反应强烈，与之前的工作形成对比，表明灌木扩张可能仍然仅限于温暖的苔原地区。在高山地区，发现常绿灌木的生长比落叶灌木的生长更依赖夏季气候，这可能是因为这些常绿物种不太容易食草。然而，功能组级别的生物群落范围的概括可能难以插入到物种级别。微场地条件，例如生长季长度和冬季土壤温度的确定，以及积雪深度对生长季土壤水分的影响，可以决定所发现的气候-生长关系的强度。