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

Abstract 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 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. 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. 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 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. 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 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

Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems

Abstract Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (>40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw