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

Summary 1.Observational and experimental studies have generally shown that warming is associated with greater growth and abundance of deciduous shrubsin arctic ecosystems. It is uncertain, however, if this trend will persist in the future. 2.Our study examined growth responses of deciduous shrubsto climate change over the late 20th and early 21st centuries near Kangerlussuaq in western Greenland. We combined shrub dendrochronology, stable isotope analysisand weekly measurements of leaf gas exchangeto examine the drivers of secondary growthin two widespread and dominant deciduous shrubspecies: Salix glaucaand 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 ~7°C appeared to be an important threshold. 4.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 exchangeof Salix was also highly sensitive to seasonal variation in moisture availability. Betula growth declined more dramatically than Salix, but leaf gas exchangewas less sensitive to moisture availability and there was less evidence of a ∆13C 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. 5.Synthesis. Our findings contrast with widespread observations of increasing shrubgrowth in the Arctic and instead point to a potential decline in the flux of carbon into a pool with a long mean residence time (wood). 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 in other arctic ecosystems. This article is protected by copyright. All rights reserved.