Treelines have drawn persistent research interest as they can respond markedly to climate. However, the mechanisms that determine tree seedling recruitment and the response of the forest-tundra ecotone to environmental changes remain poorly understood. We hypothesise that treeline tree seedling performance depends on the interplay between climatic and soil nutritional changes and facilitative and competitive interactions between trees and shrubs. We conducted a seedling transplantation experiment with Betula pubescens at a subarctic treeline, in northern Sweden, which followed a full factorial design with four treatment factors relating to environmental regimes of stress and resource availability: site (forest vs treeline); temperature (/ – passive warming); shrub presence (/ – Vaccinium myrtillus removal); and nutrient availability (/ – NPK addition). During three growing seasons we assessed the establishment and performance of Betula . Th e experimental manipulations caused highly signifi cant eff ects on seedling performance. Although Vaccinium enhanced seedling survival and reduced the eff ects of excessive solar radiation and insect herbivory, the seedlings growing with the shrub had a poorer performance by the end of the experimental period. Also, seedlings in the forest had a poorer performance than those at the treeline. Betula seedlings showed a very pronounced and positive response to passive warming and to nutrient addition, but such eff ects were more evident at the treeline site and often interacted with the presence of Vaccinium . Th is experiment shows that shrub – tree interactions are important drivers of subarctic treeline dynamics and that they vary with time and space. Facilitation, competition, herbivory and environmental changes at the tree seedling stage act as important fi lters in structuring the forest – tundra ecotone. We demonstrate that changes in this ecotone cannot be simply predicted from changing temperature patterns alone, and that complex interactions need to be considered, not only between shrubs and trees, but also with herbivores and between warming and soil nutrient availability. Th e eff ect of global warming on recent vegetation dynamics has been widely studied during the last few years because increases in temperature are expected to modify the dis-tribution of some major vegetation zones, especially at northernmost latitudes, where global warming is amplifi ed (IPCC 2007). Two northern vegetation zones of particular importance in terms of extent, biological singularity and land – atmosphere interactions are the tundra and the boreal/ subarctic forest (Callaghan et al. 2002). Treelines have moved substantially during the Holocene in response to changes in climate (Tinner and Th eurillat 2003). As the treeline has been associated with the mean July air isotherm of 11 ° C at subarctic latitudes (K ö rner 1998), we expect a northward (latitudinal) and upward (elevational) relocation of subarctic – alpine treelines in the future. How-ever, some changes in treeline have been shown to depend more on herbivory and change in land use than on climate change (Cairns and Moen 2004, Van Bogaert et al. 2011). As