The boreal biome exhibits distinct alternative ecosystem states with high and low
levels of tree cover. Insect outbreaks facilitated by climate warming could potentially
drive transitions from high to low tree cover states. We investigated whether two key
premises for such outbreak-induced transitions – critical thresholds (tipping points)
and positive feedbacks that could maintain alternative states – are present in the
northern-boreal mountain birch forest of Fennoscandia. Here, climate warming has
promoted range expansions of defoliating geometrid moths, resulting in novel, severe
multispecies outbreaks, most recently during 2002-2010.
2. We conducted regional-scale field surveys of forest damage and recovery in 280
mountain birch stands in a northeast Norway immediately after the outbreak (2010)
and six years later (2016). Satellite-derived time series of the normalized difference
vegetation index (NDVI) provided an index of stand defoliation during the outbreak
period.
3. The proportion of dead stems per stand displayed a bimodal distribution, with stands
generally being either lightly or severely damaged. This was due to a critical threshold
in the relationship between defoliation and stem mortality, with mortality rates
increasing abruptly in stands experiencing a mean drop in NDVI of more than 4 %
during the outbreak. The two key forest regenerative pathways – basal sprouting and
sapling production – both displayed positive feedbacks with surviving stems and
trees, so that regeneration success declined with increasing damage to the mature tree This article is protected by copyright. All rights reserved.
layer. These feedbacks imply that stands which have been forced across critical
defoliation thresholds and suffered collapses of living tree cover may struggle to
recover, especially if the loss of positive regenerative feedbacks is compounded by
ungulate browsing on birch recruits.
4. Synthesis. The north Fennoscandian mountain birch forest displays critical thresholds
and positive feedbacks that conform to theoretical expectations for a system that could
be vulnerable to abrupt and persistent changes of state in the face of novel,
climatically facilitated insect outbreaks. These findings deepen the understanding of
the persistent losses of tree cover that have occasionally been observed after outbreaks
in this system in the past, and add to the list of mechanisms that could help explain the
bistability of tree cover across the boreal biome.
Critical threshold, Epirrita autumnata, facilitation, forest regeneration, Global
change ecology, Operophtera brumata, positive feedback, range expansion
