Understanding the drivers of extensive plant damage in boreal and Arctic ecosystems: Insights from field surveys in the aftermath of damage

Abstract

The exact cause of population dieback in nature is often challenging to identify retrospectively. Plant research in northern regions has in recent decades been largely focussed on the opposite trend, namely increasing populations and higher productivity. However, a recent unexpected decline in remotely-sensed estimates of terrestrial Arctic primary productivity suggests that warmer northern lands do not necessarily result in higher productivity. As large-scale plant dieback may become more frequent at high northern latitudes with increasing frequency of extreme events, understanding the drivers of plant dieback is especially urgent. Here, we report on recent extensive damage to dominant, short, perennial heath and tundra plant populations in boreal and Arctic Norway, and assess the potential drivers of this damage. In the High-Arctic archipelago of Svalbard, we recorded that 8-50% of Cassiope tetragona and Dryas octopetala shoots were dead, and that the ratios of dead shoots increased from 2014 to 2015. In boreal Norway, 38-63% of Calluna vulgaris shoots were dead, while Vaccinium myrtillus had damage to 91% of shoots in forested sites, but was healthy in non-forested sites. Analyses of numerous sources of environmental information clearly point towards a winter climate-related reason for damage to three of these four species. In Svalbard, the winters of 2011/12 and 2014/15 were documented to be unusually severe, i.e. insulation from ambient temperature fluctuation by snow was largely absent, and ground-ice enforced additional stress. In boreal Norway, the 2013/14 winter had a long period with very little snow combined with extremely low precipitation rates, something which resulted in frost drought of uncovered Calluna plants. However, extensive outbreaks of a leaf-defoliating geometrid moth were identified as the driver of Vaccinium mortality. These results suggest that weather and biotic extreme events potentially have strong impacts on the vegetation state of northern lands.

Intraspecific Differences in Spectral Reflectance Curves as Indicators of Reduced Vitality in High-Arctic Plants

“…Overall, dwarf shrubs such as the species C. tetragona and D. octopetala [6] have varying spectral shapes in the 700 nm to~1000 nm spectral range compared with the alpine bistort B. vivipara and polar willow S. polaris. Damage ratios for C. tetragona and D. octopetala appeared not to be good indicators for vitality, but only the green leaves were measured.…”

“…This step was achieved using stepwise linear discriminant analysis (LDA) for band selection [38] and nonparametric multivariate analysis of variance technique (NPMANOVA) to validate the ability of selected bands to successfully separate all species (measured group differences are statistically significant) [39]. Secondly, plant vitality by means of optical vegetation indices was assessed [6]. On each plot for the dwarf shrub species (Cassiope tetragona and Dryas octopetala) we measured the cover of each species following a percent scale, the health status using the color of the apex leaves of each species and finally the damage ratio of each species following the methods in Bjerke et al [6].…”

“…On each plot for the dwarf shrub species (Cassiope tetragona and Dryas octopetala) we measured the cover of each species following a percent scale, the health status using the color of the apex leaves of each species and finally the damage ratio of each species following the methods in Bjerke et al [6]. The ratio of dead plants was confirmed by ecophysiological measurements such as chlorophyll fluorescence [6]. Heavy metal concentrations were used as an indicator of vegetation condition.…”

“…The relative cover of healthy vs. damaged vegetation was estimated on an area basis, viz. plants with green shoots were considered healthy, whereas shoots with brown or grey leaves were considered damaged [6]. This damage was shown to be caused by stressful conditions during winter [6].…”

“…plants with green shoots were considered healthy, whereas shoots with brown or grey leaves were considered damaged [6]. This damage was shown to be caused by stressful conditions during winter [6]. Usually, lower NDVI values (<0.6) are measured for species and sites operating within more adverse environmental conditions, while species and sites in good health had a NDVI of 0.6-0.7 with species and sites in very good condition having NDVI values > 0.7 [6,52].…”