(Table 1) Cover of shrub and tree species, and number of stems (>=3.5 cm DBH) during 1976-1977 and 2009-2010, respectively@en

Shrubs and trees are expected to expand in the sub-Arctic due to global warming. Our study was conducted in Abisko, sub-arctic Sweden. We recorded the change in coverage of shrub and tree species over a 32- to 34-year period, in three 50 x 50 m plots; in the alpine-tree-line ecotone. The cover of shrubs and trees (<3.5 cm diameter at breast height) were estimated during 2009-2010 and compared with historical documentation from 1976 to 1977. Similarly, all tree stems (>=3.5 cm) were noted and positions determined. There has been a substantial increase of cover of shrubs and trees, particularly dwarf birch (Betula nana), and mountain birch (Betula pubescens ssp. czerepanovii), and an establishment of aspen (Populus tremula). The other species willows (Salix spp.), juniper (Juniperus communis), and rowan (Sorbus aucuparia) revealed inconsistent changes among the plots. Although this study was unable to identify the causes for the change in shrubs and small trees, they are consistent with anticipated changes due to climate change and reduced herbivory.受全球变暖影响，亚北极地区的灌木与乔木预计将发生扩张。本研究于瑞典亚北极区域的阿比斯库（Abisko）展开。我们在高山林线交错带（alpine-tree-line ecotone）的3块50 m×50 m样地内，记录了32至34年间灌木与乔木物种的盖度（coverage）变化：于2009-2010年估算了胸径（diameter at breast height）小于3.5 cm的灌木与乔木的盖度，并与1976-1977年的历史调查记录进行对比；同时对所有胸径≥3.5 cm的树干进行了记录与定位。研究发现，灌木与乔木的盖度已出现显著提升，其中尤以矮桦（Betula nana）和山桦（Betula pubescens subsp. czerepanovii）最为突出，同时欧洲山杨（Populus tremula）也实现了种群定植。其余物种，包括柳属（Salix spp.）、欧洲刺柏（Juniperus communis）以及花楸（Sorbus aucuparia），在不同样地间的盖度变化并不一致。尽管本研究未能明确灌木与幼树盖度变化的具体成因，但该结果与气候变化以及植食性压力降低所预期的变化趋势相符。