Persistent reduction of segment growth and photosynthesis in a widespread and important sub‐Arctic moss species after cessation of three years of experimental winter warming

Abstract

Summary Winter is a period of dormancy for plants of cold environments. However, winter climate is changing, leading to an increasing frequency of stochastic warm periods (winter warming events) and concomitant reductions in snow cover. These conditions can break dormancy for some plants and expose them to freeze‐and‐thaw stress. Mosses are a major component of high‐latitude ecosystems, yet the longer‐term impacts of such winter warming events on mosses remain unknown. In order to determine the longer‐term legacy effects of winter warming events on mosses, we undertook a simulation of these events over three consecutive winters in a sub‐Arctic dwarf shrub‐dominated open woodland. The mat‐forming feather moss, Hylocomium splendens (the most abundant cryptogam in this system), is one of the most widespread Arctic and boreal mosses and plays a key functional role in ecosystems. We studied the ecophysiological performance of this moss during the summers of the experimental period (2007–2009) and in the following years (2010–2013). We show that the previously reported warming‐induced reduction in segment growth and photosynthesis during the experimental years was persistent. Four years after the last event, photosynthesis and segment growth were still 30 and 36% lower than control levels, which was only a slight improvement from 44 and 43% 4 years earlier. Winter warming did not affect segment symmetry. During the years after the last simulated event, in both warmed and control plots, chlorophyll fluorescence and segment growth, but not net photosynthesis, increased slightly. The increases were probably driven by increased summer rainfall over the study years, highlighting the sensitivity of this moss to rainfall change. Overall, the legacy effects shown here demonstrate that this widespread and important moss is likely to be significantly disadvantaged in a future sub‐Arctic climate where frequent winter warming events may become the norm. Given the key importance of mosses for soil insulation, shelter and carbon sequestration in high‐latitude regions, such persistent impacts may ultimately affect important ecosystem functions. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12703/suppinfo is available for this article.

Monitoring Winter Stress Vulnerability of High-Latitude Understory Vegetation Using Intraspecific Trait Variability and Remote Sensing Approaches

“…The study focuses on the evergreen dwarf shrub species Empetrum nigrum and Vaccinium vitis-idaea and the mat-forming moss Hylocomium splendens. These species are abundant, co-occur in boreal ecosystems, and are linked to browning [9,14,33]. Our continuous monitoring of plant vitality in the study area shows that these species have not been exposed to severely stressful events since 2012, as reported in Bjerke et al [15].…”

“…To our knowledge, our plots did not suffer from any major stress (browning) events during the last 3 years prior to our measurements, except that Vaccinium myrtillus in the area had been partly defoliated by larvae of geometrid moths [51], but this species was rare or absent in our plots. In the early growing season, plants are especially vulnerable to winter-related stress and are showing accumulated stress responses from the previous years [13,14]. Hence, we monitored the natural range of trait variabilities from start of the growing season (DOY 130) onwards.…”

“…Schimp., deciduous shrubs, such as Vaccinium myrtillus L., evergreen horsetails (Equisetum spp. ), as well as small cushion plants show reduced growth following exposure to winter warming [9,11,13,14]. The other major factor causing browning are pest outbreaks.…”

How does spatial heterogeneity affect inter‐ and intraspecific growth patterns in tundra shrubs?

“…In recent years, however, this trend has been recognized as highly heterogeneous and complex, with the direction of change still poorly understood (Abbott et al., 2016; Elmendorf, Henry, Hollister, Björk, Bjorkman, et al., 2012; Macias‐Fauria et al., 2020; Myers‐Smith et al., 2020; Nielsen et al., 2017; Phoenix & Bjerke, 2016). As growing seasons are lengthening (Epstein et al., 2017), snow‐cover conditions are changing (Xu et al., 2013) and extreme winter warming events are more frequently causing snowmelt in midwinter (Bjerke et al., 2017), questions are rising about the ability of dominant tundra species to adapt within the necessary timeframe to survive under these changing conditions.…”

Impact of Multiple Ecological Stressors on a Sub-Arctic Ecosystem: No Interaction Between Extreme Winter Warming Events, Nitrogen Addition and Grazing

“…Recent experimental field studies of WW events in the sub-arctic have shown great vulnerability of evergreen dwarf shrubs to such events while deciduous plants and cryptogams are less affected ( Bjerke et al, 2011 , 2017a ; Bokhorst et al, 2015 ), which may govern future sub-arctic vegetation changes ( Bokhorst et al, 2015 ). Differences in vulnerability between plant types are partly determined by differences in exposure of overwintering tissue to freezing, and in part by physiological adaptations, such as winter dormancy and changes in the membrane fatty acid composition ( Bokhorst et al, 2018 ).…”