Shrub expansion has occurred across much of the arctic tundra over the past century. Increasing dominance of woody vegetation is expected to have global influences on climate patterns and lead to local changes in hydrological function and nutrient cycling. Changing abiotic conditions associated with shrubs will likely alter the relative fitness of neighbouring plants resulting in distinct community composition. Here, we use an extensive set of paired abiotic and biotic data to investigate the capacity for Alnus alnobetula (green alder) patches to modify the habitat of the local plant community at the taiga–tundra ecotone of the Northwest Territories, Canada. Plots were established across topographic positions in ten alder patches and adjacent, alder-free tundra. Habitat corresponded to the strongest gradient of among-site variation in abiotic measures and plant community composition, indicating that alder patch growing conditions were distinct from those of alder-free tundra. Slope position was generally unimportant in determining environmental conditions. Alder patches changed the vertical structure of the understory by increasing the maximum height of birch. Tall shrubs also decreased the richness of tundra specialists, suggesting that these species face competitive pressures from shrub expansion at the southern edge of their ranges. Our findings demonstrate that tall shrub patches can substantially modify their local environment in taiga–tundra ecotone systems, altering available habitat and acting as niche constructors for the local plant community. These habitats will therefore be important to consider in regional predictions of hydrology, nutrient cycling, and biodiversity as shrubs continue to expand across the arctic.

在过去的一个世纪中，许多北极苔原都发生了灌木丛扩张。预计木质植被的主导地位将对气候格局产生全球影响，并导致水文功能和养分循环的局部变化。与灌木有关的非生物条件的变化可能会改变邻近植物的相对适应性，从而导致不同的群落组成。在这里，我们使用了广泛的非生物和生物配对数据来研究Alnus alnobetula的能力（绿色al木）斑块，以改变加拿大西北地区的taiga-tundra生态交错带的当地植物群落的栖息地。在十个al木斑块和相邻的无al木苔原中跨地形位置建立了图。生境对应于非生物措施和植物群落组成的部位间变化的最大梯度，表明al的生长条件不同于无al的苔原。在确定环境条件时，斜坡位置通常并不重要。der木斑块通过增加桦树的最大高度来改变林下层的垂直结构。高大的灌木丛也降低了苔原专家的丰富度，表明这些物种在其山脉南部边缘面临灌木丛扩张带来的竞争压力。我们的发现表明，高灌木灌木丛可以在针叶林苔原过渡带系统中极大地改变其当地环境，改变可用的栖息地，并成为当地植物群落的生态位建设者。因此，随着灌木在北极地区的不断扩展，这些生境将在水文学，养分循环和生物多样性的区域性预测中予以考虑。