Declining growth of deciduous shrubs in the warming climate of continental western Greenland

Abstract

1. Observational and experimental studies have generally shown that warming is associated with greater growth and abundance of deciduous shrubs in arctic ecosystems. It is uncertain, however, if this trend will persist in the future.2. Our study examined growth responses of deciduous shrubs to climate change over the late 20th and early 21st centuries near Kangerlussuaq in western Greenland. We combined shrub dendrochronology, stable isotope analysis and weekly measurements of leaf gas exchange to examine the drivers of secondary growth in two widespread and dominant deciduous shrub species: Salix glauca and Betula nana.3. Betula showed a dramatic growth decline beginning in the early 1990s, when correlations between growing season air temperature and growth shifted from neutral to strongly negative. Salix also showed a growth decline, but it began slightly later and was more pronounced among older stems. May-August mean air temperature of c. 7°C appeared to be an important threshold. 4. Carbon isotope discrimination (∆ 13 C) in α-cellulose of Salix growth rings declined strongly during the period of reduced growth, suggesting drought-induced stomatal closure as a possible cause. Leaf gas exchange of Salix was also highly sensitive to seasonal variation in moisture availability. Betula growth declined more dramatically than Salix, but leaf gas exchange was less sensitive to moisture availability and there was less evidence of a ∆ 13 C trend. We hypothesize that the dramatic Betula growth decline might reflect the combined effects of increasing moisture limitation, repeated defoliation during recent moth outbreaks and greater browsing by a growing muskoxen population. Synthesis.Our findings contrast with widespread observations of increasing shrub growth in the Arctic and instead point to a potential decline in the flux of carbon | 641Journal of Ecology GAMM et Al.

Uniform shrub growth response to June temperature across the North Slope of Alaska

“…Within glacial landscapes, variability in local soil conditions has further consequences for vegetation response to climate. For example, while shrub growth often responds positively to growing season temperature, temperature-induced moisture limitation may constrain growth during particularly hot years at drier locations such as uplands (Ackerman et al 2017, Gamm et al 2017. Such local variability in soil conditions interacts with regional glacial geology and broad-scale climate patterns to create a hierarchical structure of controls on tundra vegetation growth (Myers-Smith et al 2015a).…”

“…Dendroecology, the study of annual growth increments in woody plants, has become a popular method within the last two decades for analyzing climate response of arctic vegetation (See studies included in Myers-Smith et al 2015a, Young et al 2016, Ackerman et al 2017, Gamm et al 2017. This method is particularly well-suited for arctic tundra, since many plant species are long-lived and interannual climate is highly variable.…”

Annual ring growth of a widespread high arctic shrub reflects past fluctuations in community‐level plant biomass

“…More surprisingly, recent dendrochronology studies from Greenland and Svalbard report on a deviation from the aforementioned clear and positive summer temperature responses (Forchhammer, ; Gamm et al, ; Opała‐Owczarek et al, ). In a coastal site in Svalbard, with twice as high precipitation levels as our inland site, Opała‐Owczarek et al () proposed that soil moisture and soil structure have a larger influence on S. polaris ring growth than temperature .…”

“…polaris growth in our study, with the possible exception that the extremely rainy summer in 2013 resulted in a prominent reduction in heath habitat's above‐ground biomass. Furthermore, in spite of increasing summer temperatures, Gamm et al () observed a temporal decline in ring growth in continental Western Greenland for Betula nana and Salix glauca . They suggested this could be due to increased defoliation from herbivory and increasing moisture limitation.…”

“…More surprisingly, recent dendrochronology studies from Greenland and Svalbard report on a deviation from the aforementioned clear and positive summer temperature responses (Forchhammer, 2017;Gamm et al, 2017;Opała-Owczarek et al, 2018 Secondary growth in biomass production of shrubs is often overlooked. Especially shrubs' below-ground growth-the "hidden part of the iceberg"-has received little attention so far (Bret-Harte et al, 2002;Iversen et al, 2015).…”