Understanding the drivers of extensive plant damage in boreal and Arctic ecosystems: Insights from field surveys in the aftermath of damage
Graphical abstract
Introduction
Remotely-sensed estimates of vegetation productivity have shown an increasing productivity of northern lands from 1982 to 2011, concurrently with warmer and longer growing seasons (Xu et al., 2013). This process is known as ‘greening’. However, from 2011 to 2014, Arctic regions experienced an overall decline in productivity (Bhatt et al., 2013, Park et al., 2016), in a process known as ‘browning’ (de Jong et al., 2012).
While greening occurs gradually, driven by longer and warmer growing seasons, browning is brought about by different drivers, often as stochastic biotic or weather events, that occur at distinct spatial and temporal patterns in the landscape (Phoenix and Bjerke, 2016). Increasing awareness of such events has recently led to more knowledge of their impacts on boreal and Arctic vegetation. The effects of drought and wildfires during summer are among the best-documented events (e.g. Angert et al., 2005, Beck and Goetz, 2011, Bret-Harte et al., 2013). However, browning events may take place at any time of the year, and with winters warming more rapidly than summers (Walsh et al., 2011), increased disturbance of northern lands is expected to occur during the cold season: threats to plants associated with reduced snow cover include encasement in ground-ice, which generally develops after rain-on-snow events, freezing damage following extreme winter warming, and desiccation resulting from exposure to light, wind and warmth of plants unable to access frozen soil water, known as frost drought or winter desiccation (Gudleifsson, 2009, Bjerke, 2011, Bokhorst et al., 2011, Bokhorst et al., 2016, Callaghan et al., 2011, Preece and Phoenix, 2013, Williams et al., 2015, Blume-Werry et al., 2016).
Since 2006, several incidents of frost drought following winter warming events have been reported from the sub-Arctic regions of Scandinavia (Bjerke and Tømmervik, 2008, Bokhorst et al., 2009, Bokhorst et al., 2012, Bjerke et al., 2014). These reports show that some dominant evergreen and deciduous dwarf shrubs (especially Empetrum nigrum and Vaccinium myrtillus) are vulnerable to such events. However, also conifers, e.g. Juniperus communis, Picea abies and Pinus sylvestris, which are adapted to being exposed during the winter period, occasionally suffer from frost drought after midwinter thaw episodes (Printz, 1933, Tranquillini, 1982, Kullman, 2014, Bjerke et al., 2014).
Leaf defoliation caused by outbreaks of geometrid moths is another driver of widespread damage to northern boreal vegetation (Jepsen et al., 2009, Jepsen et al., 2011, Callaghan et al., 2013, Karlsen et al., 2013, Bjerke et al., 2014, Bokhorst et al., 2015). These outbreaks have strong effects on both the canopy and the field layer.
Bjerke et al. (2014) reported on 12 additional anomalous events that drove browning during one year in the Nordic Arctic Region, while Callaghan et al. (2013) summarized various weather and biotic events affecting primary production in northern boreal and alpine parts of northernmost Sweden over multiple decades. Hence, although the impacts of some types of events are better documented than others, browning drivers rarely occur in isolation and it is generally not straightforward to identify the causes of plant damage (Sinclair and Lyon, 2005, Gunthardt-Goerg and Vollenweider, 2007).
While trend changes, such as greening, take place over several years or decades and, hence, can be studied during their progress, the sporadic nature of events in time and space means they cannot be predicted beforehand; it may only be the aftermath of damage that allow us to detect the events and evaluate possible causes (Phoenix and Bjerke, 2016). That is the case in the study reported here. During the summers of 2014 and 2015, widespread dieback of shoots of dwarf shrubs and some other cryptophytes was recorded in the Norwegian boreal and Arctic regions (64°N to 79°N). Similar types of damage were also recorded in neighbouring regions, as far east as Finland (Salo, 2014). Such widespread dieback has major impacts on ecosystem structure and functioning (Bokhorst et al., 2011, Bokhorst et al., 2012, Treharne et al., 2016). Understanding the drivers of this extensive dieback is crucial for assessing the potential recurrence frequency and ecosystem impact, and to evaluate the impacts in a broader, circumboreal and pan-Arctic context. We therefore attempted to quantify the damage ratio at selected sites within the affected regions and to identify the drivers, relying on timing of damage, phytopathological traits, meteorological data, and other environmental data.
As we studied these events retrospectively, we did not test any a priori hypotheses. However, based on self-experienced knowledge on the types of extreme events that took place during the study period, and the types of damage occurring, we hypothesized a posteriori that hazardous winter weather conditions and insect outbreaks played significant roles. We therefore studied this damage over large areas to assess whether these were the main drivers of the observed plant damage.
Section snippets
Study area
Our field locations included regions in the boreal and Arctic biomes of Norway. Spitsbergen is the main island of the High-Arctic archipelago of Svalbard (Fig. 1a). It is characterized by large inter-annual variability in air temperatures and steadily increasing precipitation rates and winter temperatures (Førland et al., 2009, van Pelt et al., 2016, Vikhamar-Schuler et al., 2016). We conducted field work at 52 monitoring plots and several additional plots around the settlements of Longyearbyen
Results from permanent sites
Physiological shoot measurements clearly show that non-green plants were either stressed or dead. Grey leaves of C. tetragona (Fig. S1) and D. octopetala (Fig. S2) had no chlorophyll fluorescence. In July 2015, Fv/Fm of pale brown and strongly anthocyanin-containing shoots of C. tetragona (Fig. S1) were 39.6% (P < 0.001) and 11.7% (P = 0.002) lower than in pure green shoots. Brown leaves of D. octopetala (Fig. S2) with green patches had 28.6% (P < 0.001) lower Fv/Fm than pure green leaves. Some brown
Discussion
Our environmental and phytopathological data point towards a winter climatic reason for most of the observed damage reported here. We did not find any evidence of outbreaks of plant pathogens, nor did we find any reports indicating unusual dispersal of air pollution. We also evaluated whether any climate-related aspects of the growing seasons could have caused damage, but we did not find any such indications. However, we did observe outbreaks of geometrid moths in boreal birch forests, and some
Acknowledgements
This study was funded by the Polish-Norwegian Programme of the EEA Norway Grants (WICLAP, project 198571), by the Research Council of Norway (225006/E10), and by FRAM–High North Research Centre for Climate and the Environment through its terrestrial flagship program (project 362222). We thank Geraldine Mabille, Sidsel Grønvik, Trond V. Johnsen, John K. Bjerke and Laura Stendardi for assistance in the field.
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