title: Different parts, different stories: climate sensitivity of growth is stronger in root collars vs. stems in tundra shrubs.
abstract: Shrub densification has been widely reported across the circumpolar arctic and subarctic biomes in recent years. Long-term analyses based on dendrochronological techniques applied to shrubs have linked this phenomenon to climate change. However, the multi-stemmed structure of shrubs makes them difficult to sample and therefore leads to non-uniform sampling protocols among shrub ecologists, who will favor either root collars or stems to conduct dendrochronological analyses. Through a comparative study of the use of root collars and stems of Betula glandulosa, a common North American shrub species, we evaluated the relative sensitivity of each plant part to climate variables and assessed whether this sensitivity is consistent across three different types of environments in northwestern Québec, Canada (terrace, hilltop and snowbed). We found that root collars had greater sensitivity to climate than stems and that these differences were maintained across the three types of environments. Growth at the root collar was best explained by spring precipitation and summer temperature, whereas stem growth showed weak and inconsistent responses to climate variables. Moreover, sensitivity to climate was not consistent among plant parts, as individuals having climate-sensitive root collars did not tend to have climate-sensitive stems. These differences in sensitivity of shrub parts to climate highlight the complexity of resource allocation in multi-stemmed plants. Whereas stem initiation and growth are driven by microenvironmental variables such as light availability and competition, root collars integrate the growth of all plant parts instead, rendering them less affected by mechanisms such as competition and more responsive to signals of global change. Although further investigations are required to determine the degree to which these findings are generalizable across the tundra biome, our results indicate that consistency and caution in the choice of plant parts are a key consideration for the success of future dendroclimatological studies on shrubs.
authors.affiliation: Chaire de recherche du Canada en biodiversité nordique and Département de biologie, chimie et géographie, Université du Québec à Rimouski, 300 allée des Ursulines, Rimouski, QC, G5L 3A1, Canada.|Centre d'études nordiques, 2405 av. de la Terrasse, Québec, QC, G1V 0A6, Canada.
authors.forename: Pascale
authors.identifier: 0000-0002-1602-167X
authors.initials: P
authors.lastname: Ropars
authors.affiliation: Centre d'études nordiques, 2405 av. de la Terrasse, Québec, QC, G1V 0A6, Canada.|School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3FF, UK.
authors.forename: Sandra
authors.initials: S
authors.lastname: Angers-Blondin
authors.affiliation: Centre d'études nordiques, 2405 av. de la Terrasse, Québec, QC, G1V 0A6, Canada.|Département de biologie, Université Laval, 1045 av. de la Médecine, Québec, QC, G1V 0A6, Canada.
authors.forename: Marianne
authors.initials: M
authors.lastname: Gagnon
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3FF, UK.
authors.forename: Isla H
authors.initials: IH
authors.lastname: Myers-Smith
authors.affiliation: Centre d'études nordiques, 2405 av. de la Terrasse, Québec, QC, G1V 0A6, Canada.|Département des sciences de l'environnement, Université du Québec à Trois-Rivières, 3351 boul. des Forges, Trois-Rivières, QC, G9A 5H7, Canada.
authors.forename: Esther
authors.initials: E
authors.lastname: Lévesque
authors.affiliation: Centre d'études nordiques, 2405 av. de la Terrasse, Québec, QC, G1V 0A6, Canada.|Département de biologie, Université Laval, 1045 av. de la Médecine, Québec, QC, G1V 0A6, Canada.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Boudreau
mesh_terms: D001110:Arctic Regions; D002170:Canada; D002980:Climate; D057231:Climate Change; D017753:Ecosystem; D063245:Plant Development; D018517:Plant Roots / Q000254:growth & development; D011792:Quebec; D065929:Tundra
keywords: 
  Betula glandulosa
  ; climate sensitivity; dendrochronology; dwarf birch; shrub densification; subarctic; tundra