DOI: 10.5061/dryad.1nm650h
title: Data from: Can novel pest outbreaks drive ecosystem transitions in northern-boreal birch forest?

description: 1. 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 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 adds to the list of
 mechanisms that could help explain the bistability of tree-cover across
 the boreal biome.
description: forest_data_2010_pubData on mountain birch forest damage and recovery in August 2010.forest_data_2016_pubData on mountain birch forest damage and recovery in August 2016.Rscript_all_analyzes_pubR script to reproduce all analyzes and plots reported in Vindstad et al. "Can novel pest outbreaks drive ecosystem transitions in northern-boreal birch forest?"README. Explanation of variablesExplanation of the variables found in the datafiles forest_data_2010_pub.txt and forest_data_2016_pub.txtdefolitation_data_pubData on yearly percentage fall in NDVI for sampling transects for 2001-2010defoliation_data_pub.txt