Question: What is the effect of a gradient in moose density on reproduction, growth and functional composition of the field layer vegetation in a boreal forest, and how is this effect modified by habitat productivity?

Location: Northwest of Umeå, Västerbotten, northern Sweden.

Methods: Field layer vegetation was surveyed in an experimental setup with simulation of three different moose densities and a control in eight study sites along a gradient of habitat productivity.

Results: We found that increased moose density led to decreased cover and reproductive effort of a browsed dwarf shrub (bilberry, Vaccinium myrtillus L.) and increased cover and reproductive effort of a non‐browsed graminoid (wavy hair‐grass, Avenella flexuosa (L.) Drejer). Increased moose density led to increased light availability and probably reduced competition from V. myrtillus. Total reproductive effort in the field layer vegetation increased, height decreased and cover of light‐demanding species and graminoids increased with increasing moose density. The effects of moose density were modified by the productivity gradient, leading to a higher relative increase in light availability and reproductive effort in highly productive areas than in low productive areas.

Conclusions: Increased light availability was an important indirect effect of moose density, leading to less competition for light and a shift towards early successional species. The effect of moose density on light availability was modified by habitat productivity, leading to stronger relative effects in highly productive areas than in low productive areas.