Aim: Global change effects on forest ecosystems are increasingly claimed to be context dependent, indicated by interactions between global and local environmental drivers. Most examples of such context dependencies originate from temperate systems, while limited research comes from the boreal biome. Here we set out to test if interactions between climate warming, nitrogen deposition, land-use change resulting in increasing forest density and soil pH drive long-term changes of understory vegetation in boreal forests. Location: Sweden Time period: 1953-2012 Major taxa studied: Vascular plants Methods: We used long-term (50 years) National Forest Inventory data on forest understory vegetation in Sweden to model the combined effects of climate warming, nitrogen deposition, increase in forest density (tree basal area) and soil pH. Results: Our results identify increasing temperature, nitrogen deposition and denser, shadier forest conditions as main drivers of understory vegetation changes during this time period. More importantly, we found that these effects varied with local conditions, i.e. that the change towards a more nitrophilic understory vegetation was more pronounced at low than high soil pH. Forest density was an important modulator of nitrogen deposition and temperature increase, with effects generally decreasing with density. Decreased cover of ericaceous dwarf shrubs was driven by both forest density and nitrogen deposition, with a stronger effect at low than at high pH. Main conclusions: Our results highlight that to understand forest ecosystems´ response to global change, and to make adequate management decisions to mitigate the effects of global change, we need to understand how changes in local environmental factors (forest density and soil pH) interact with global-scale drivers (nitrogen deposition and climate warming). Neglecting such interactions will lead to incorrect estimations of effects. In our case, we would e.g. have underestimated the eutrophication effects on acid soils, which constitute a considerable part of the boreal biome. --

The variables Rich_Forb_increase, Low_Cover_increase, Tall_Forb_increase and Low_Forb_increase indicate if a vegetation type has increase in a spatial unit (1=increase).

Dwarf_Shrub_Cover_change indicates the change over time in the aggregated cover of dwarf shrubs (0.01=1% cover).

The basal areas are in m² per ha.

Nitrogen deposition is in mg N per m².

Temperature change is in Celsius degrees.

Spatial coordinates are in SWEREF 99 TM.