The carbon dynamics of northern natural ecosystems
contribute significantly to the global carbon balance.
Periodic disturbances to these dynamics include insect
herbivory. Larvae of autumn and winter moths (Epirrita
autumnata and Operophtera brumata) defoliate mountain
birch (Betula pubescens) forests in northern Scandinavia
cyclically every 9–10 years and occasionally (50–150 years)
extreme population densities can threaten ecosystem
stability. Here we report impacts on C balance following a
2004 outbreak where a widespread area of Lake Torneträsk
catchment was severely defoliated. We show that in the
growing season of 2004 the forest was a much smaller net
sink of C than in a reference year, most likely due to lower
gross photosynthesis. Ecosystem respiration in 2004 was
smaller and less sensitive to air temperature at nighttime
relative to 2006. The difference in growing season uptake
between an insect affected and non-affected year over the
316 km2 area is in the order of 29 × 103 tonnes C equal to a
reduction of the sink strength by 89%.