Insect outbreaks in northern-boreal forests are
expected to intensify owing to climate warming,
but our understanding of direct and cascading
impacts of insect outbreaks on forest ecosystem
functioning is deficient. The duration and severity
of outbreaks by geometrid moths in northern
Fennoscandian mountain birch forests have been
shown to be accentuated by a recent climatemediated
range expansion, in particular of winter
moth (Operophtera brumata). Here, we assess the
effect of moth outbreak severity, quantified from
satellite-based defoliation maps, on the state of
understory vegetation and the abundance of key
vertebrate herbivores in mountain birch forest in
northern Norway. We show that the most recent
moth outbreak caused a regional-scale state change
to the understory vegetation, mainly due to a shift
in dominance from the allelopathic and unpalatable
dwarf-shrub Empetrum nigrum to the productive
and palatable grass Avenella flexuosa. Both these
central understory plant species responded significantly
and nonlinearly to increasing outbreak
severity. We further provide evidence that the
effects of the outbreak on understory vegetation
cascaded to cause strong but opposite impacts on
the abundance of the two most common herbivore
groups. Rodents increased with defoliation, largely
mirroring the increase in A. flexuosa, whereas ungulate
abundance instead showed a decreasing trend.
Our analyses also suggest that the response of
understory vegetation to defoliation may depend
on the initial state of the forest, with poorer forest
types potentially allowing stronger responses to
defoliation
