Contrasting impacts of short- and long-term large herbivore exclusion on understory net CO2 exchange in a boreal forest

Across boreal forests, trees are the main living biomass carbon (C) stock, but the understory vegetation can contribute significantly to the C cycling and net forest carbon dioxide (CO2) balance. The patchy understory vegetation which consists of sunlit (i.e., lichen-like) and shaded habitats (i.e., dwarf shrub-like) is often altered by ungulate grazers. Grazers may influence understory CO2 exchange, and consequently, the forest CO2 balance. Grazing affects differently the biomass of slow growing lichens compared to the faster growing mosses and dwarf shrubs, and therefore the effects of grazing on CO2 exchange in the patchy understory vegetation could vary temporally. We studied how excluding grazing for short- and long-term affects the CO2 exchange and vegetation biomass in the understory of an oligotrophic Scots pine forest. We measured growing season (2019, 2020) CO2 exchange across sunlit and shaded habitats inside fences that had excluded large grazers for 0–1 and 25–26 years and ...