Red foxes increase reproductive output of white spruce in a non-mast year.

• Over many decades, red foxes increase nutrient concentrations and affect vegetation diversity on their dens in the boreal woodlands, a nutrient-poor environment. • Denning activity also affects the growth and reproduction of white spruce, a mast-seeding conifer. • White spruce on fox dens are taller and produce more cones in non-mast years than trees off dens. • The effects of denning are subject to the mast-seeding cycle, as cone production in mast years was not elevated on dens. Predators may act as ecosystem engineers by modifying their physical environment through non-trophic interactions. Red foxes (Vulpes vulpes) are ecosystem engineers in the boreal woodlands, a nutrient-poor environment, where their denning activity increases soil nutrient concentrations, affecting the diversity of vegetation around dens. Since dens can persist for many decades and tree growth is enhanced on dens, dens can be used to study the long term impacts of nutrient additions on community and ecosystem processes. We examined the reproductive output of a mast-seeding conifer, white spruce (Picea glauca), on 10 red fox dens and paired control sites at the boreal treeline near Churchill, Manitoba, in July 2019. We estimated cone production in both the current non-mast year (2019) and the previous mast year. The number of cones produced per tree and per hectare was significantly higher on dens than control sites in the non-mast year, but did not differ in the mast year. Higher cone production on dens was partly driven by trees being larger on dens. These results suggest that red fox ecosystem engineering activity affects white spruce reproduction, since increased soil nutrients on dens allow for higher cone production, but the effect of red fox denning is limited by interactions with mast-seeding mechanisms. For example, weather cues may override the effects of denning in mast years, while trees on dens may be able to allocate more resources to reproduction in non-mast years, but store the same amount of resources as trees off dens to produce similarly large cone crops in mast years. Altered resource availability in this nutrient-limited landscape could have additional ecological implications, by affecting the foraging patterns of seed predators and spruce range expansion. [ABSTRACT FROM AUTHOR]