Contrasted dynamics of northern Labrador tree lines caused by climate change and migrational lag.

The northern Québec-Labrador tree lines are the most climatically stressed tree ecosystems of eastern North America. In particular, white spruce (Picea glauca) tree line populations distributed between 56 degrees N and 58 degrees N and 61 degrees W and 66 degrees W show contrasted responses to recent changes in climate according to their geographic position relative to the Labrador Sea. Along the coast, the northernmost latitudinal and altitudinal tree lines responded positively to warming over the last 50 years with invading spruce several tens of meters above the current tree line. In contrast, white spruce tree lines across the wind-exposed Labrador plateau are located much higher in altitude and have receded a few tens of meters beginning around AD 1740-1750 and have not yet recovered. Whereas no field evidence of recent fire and insect damage was found, all inland tree line stands were progressively damaged likely due to mechanical defoliation of wind-exposed trees. Massive tree death in the 19th century caused a reduction in the number of seed-bearing trees, and declining tree lines were not replenished by seedlings. Recent warming reported for northern latitudes has not been strong enough to change the regressive tree line trajectory in interior Labrador. However, white spruce expansion above coastal tree line in the northernmost forest site in Labrador is in line with current climatic trends. It is hypothesized that the species is still advancing toward its potential tree line higher in altitude due to delayed postglacial migration. The slow advance of white spruce in northernmost coastal Labrador is likely caused by the rugged topography of the Torngat-Kaumajet-Kiglapait mountains.

[1]  Dan Binkley,et al.  Expansion of forest stands into tundra in the Noatak National Preserve, northwest Alaska , 1999 .

[2]  J. C. Ritchie,et al.  Postglacial Vegetation of Canada , 1988 .

[3]  Ola M. Johannessen,et al.  The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism , 2004 .

[4]  G. MacDonald,et al.  Response of the Central Canadian Treeline to Recent Climatic Changes , 1998 .

[5]  I. Hustich.  On the recent expansion of the Scotch pine in northern Europe , 1958 .

[6]  L. Kullman.  20th Century Climate Warming and Tree-limit Rise in the Southern Scandes of Sweden , 2001, Ambio.

[7]  H. Lamb.  Modern pollen spectra from Labrador and their use in reconstructing Holocene vegetational history , 1984 .

[8]  Gordon C. Jacoby,et al.  Tree ring width and density evidence of climatic and potential forest change in Alaska , 1995 .

[9]  G. MacDonald,et al.  The patterns of post-glacial spread of white spruce , 1986 .

[10]  D. Cooper.  White Spruce above and Beyond Treeline in the Arrigetch Peaks Region, Brooks Range, Alaska , 1986 .

[11]  Gian-Reto Walther,et al.  Plants in a warmer world , 2003 .

[12]  E. J. Jäger.  Rousseau, Camille, Géographie floristique du Québec-Labrador. Distribution des principales espèces vasculaires. (Travaux et Documents du Centre d'Études Nordiques 7). 799 S., 5 Übersichtskarten, 1020 Verbreitungskarten. Les Presses de l'Université Laval. Québec 1974 , 1975 .

[13]  G. Yohe,et al.  A globally coherent fingerprint of climate change impacts across natural systems , 2003, Nature.

[14]  J. Houghton,et al.  Climate change 2001 : the scientific basis , 2001 .

[15]  M. Fortin,et al.  The Subarctic Forest–Tundra: The Structure of a Biome in a Changing Climate , 2001 .

[16]  H. Nichols,et al.  Holocene Pollen Diagrams from Subarctic Labrador-Ungava: Vegetational History and Climatic Change , 1977 .

[17]  G. MacDonald,et al.  A 403-year record of July temperatures and treeline dynamics of Pinus sylvestris from the Kola peninsula, northwest Russia. , 2000 .

[18]  L. Cwynar,et al.  REVERSION OF FOREST TO TUNDRA IN THE CENTRAL YUKON , 1991 .

[19]  Fritz H. Schweingruber,et al.  Large‐scale treeline changes recorded in Siberia , 2004 .

[20]  A. Lloyd,et al.  Recent changes in treeline forest distribution and structure in interior Alaska , 2003 .

[21]  G. MacDonald,et al.  Recent White Spruce Dynamics at the Subarctic Alpine Treeline of North-Western Canada , 1995 .

[22]  D. Cairns,et al.  Herbivory influences tree lines , 2004 .

[23]  E. Holsten.  Insects and diseases of Alaskan forests , 1985 .

[24]  Bruce P. Finney,et al.  Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress , 2000, Nature.

[25]  A. Morin,et al.  Expansion récente du mélèze à la limite des forêts (Québec nordique) , 1984 .

[26]  S. Payette,et al.  White spruce expansion at the tree line and recent climatic change , 1985 .

[27]  L. Kullman.  LITTLE ICE AGE DECLINE OF A COLD MARGINAL PINUS SYLVESTRIS FOREST IN THE SWEDISH SCANDES , 1987 .

[28]  H. Lamb.  Palynological Evidence for Postglacial Change in the Position of Tree Limit in Labrador , 1985 .

[29]  B. Efron.  Bootstrap Methods: Another Look at the Jackknife , 1979 .

[30]  S. Payette,et al.  Light Rings in Subarctic Conifers as a Dendrochronological Tool , 1986, Quaternary Research.

[31]  R. Sutton,et al.  Atlantic Ocean Forcing of North American and European Summer Climate , 2005, Science.

[32]  G. MacDonald,et al.  A 538-Year Record of Climate and Treeline Dynamics from the Lower Lena River Region of Northern Siberia, Russia , 1998 .

[33]  D. Elliott-Fisk.  The Stability of the Northern Canadian Tree Limit , 1983 .

[34]  L. Kullman.  Rapid recent range‐margin rise of tree and shrub species in the Swedish Scandes , 2002 .

[35]  F. Chapin,et al.  Modeling the Influence of Topographic Barriers on Treeline Advance at the Forest-Tundra Ecotone in Northwestern Alaska , 2001 .

[36]  L. Kullman.  Change and stability in the altitude of the birch tree-limit in the southern Swedish Scandes 1915-1975 , 1979 .

[37]  L. Mysak,et al.  Sea-ice anomalies observed in the Greenland and Labrador seas during 1901–1984 and their relation to an interdecadal Arctic climate cycle , 1990 .

[38]  Robert Baxter,et al.  How will the tundra-taiga interface respond to climate change? , 2002, Ambio.

[39]  John E. Walsh,et al.  Recent Variations of Sea Ice and Air Temperature in High Latitudes , 1993 .

[40]  L. Kullman.  TREE-LIMITS AND MONTANE FORESTS IN THE SWEDISH SCANDES : SENSITIVE BIOMONITORS OF CLIMATE CHANGE AND VARIABILITY , 1998 .

[41]  R. Holmes.  Computer-Assisted Quality Control in Tree-Ring Dating and Measurement , 1983 .

[42]  S. Payette,et al.  Height growth response of tree line black spruce to recent climate warming across the forest‐tundra of eastern Canada , 2004 .

[43]  E. Russell,et al.  Postglacial Vegetation of Canada. , 1988 .