Declining growth of arctic shrubs in the warming climate of continental west Greenland

Background/Question/Methods 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. Our study examined growth responses of Arctic shrubs to climate change over the late 20^th and early 21^st 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.

Results/Conclusions 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 ~7°C appeared to be an important threshold. Carbon isotope discrimination (∆¹³C) 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 more limited evidence of a ∆¹³C trend. We hypothesize 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. Our findings contrast with widespread observations of increasing shrub growth and abundance and raise questions about the extent to which greater vegetation productivity may offset soil carbon losses in a warmer Arctic. While our study area is warmer and drier than much of the Arctic, our results may serve as an early indicator of potential effects of rising temperature on other Arctic ecosystems.