DYNAMICS OF SUBARCTIC WETLAND FORESTS OVER THE PAST 1500 YEARS

Boreal forests at high latitudes are climate-sensitive ecosystems that respond directly to environmental forcing by changing their position according to latitude or by changing their abundance at local and regional scales. South of the arctic treeline, external forcing (warming, cooling, drought, fire) necessarily results in the changing abundance of the impacted forests; in particular, the deforestation of well-drained sites through fire is the most important factor. In this study, we examined the changing abundance of wetland forests located at the arctic treeline (northern Quebec, Canada) during the last 1500 years, a period of known contrasting climatic conditions. Black spruce (Picea mariana) trees submerged in small lakes and peatland ponds and soil-peat stratigraphy were used con- currently to reconstruct the millennial-long developmental sequence of wetland stands as- sociated with moisture changes and fire disturbance. Changing lake levels from AD 300 to the present were identified based on radiocarbon-dated submerged paleosols and tree- ring cross-dating of submerged trees distributed in three wetlands from the same watershed. Dead and living trees in a standing position below and above present water level of a small lake (LE Lake) showed direct evidence of past water levels from the 12th century to the present day. Submerged subfossil trees from another lake (LB Lake) and two peatland ponds (PB Peatland) also responded synchronously to changes in soil moisture during the last 1500 years. Regional-scale catastrophic flooding around AD 1150, inferred from paleosol and subfossil tree data, eliminated riparian peat and wetland trees growing at least since AD 300. Also, the coincidence of events such as the mass mortality of wetland spruce and post-fire deforestation of a small hill surrounding LE Lake during the late 1500s suggests the impact of local-scale flooding, probably attributable to greater snow transportation and accumulation on the lake surface after fire disturbance. Massive tree mortality climaxed at ca. 1750, when all wetland trees at LB Lake and PB Peatland died because of permafrost disturbance and soil upthrusting. Lower water levels from AD 300 to 1750 were associated with drier conditions, possibly caused by greater evaporation and/or reduced snow accu- mulation. Permafrost development in shallow waters occurred during the Little Ice Age, after 1600. It is concluded that the climate at the eastern Canadian treeline was warmer and drier from AD 300 to the onset of the Little Ice Age and promoted tree establishment. The highest water levels were recorded recently (19th and 20th centuries), causing lake and peatland expansion. Any future moisture changes at these subarctic latitudes will result in important spatial rearrangements of wetland ecosystems.

[1]  S. Payette,et al.  The circumboreal tundra-taiga interface: late Pleistocene and Holocene changes. , 2002, Ambio.

[2]  H. Almquist-Jacobson.  Lake-level fluctuations at Ljustjärnen, central Sweden and their implications for the Holocene climate of Scandinavia , 1995 .

[3]  L. Filion,et al.  Relationship of Temperature and Light Ring Formation at Subarctic Treeline and Implications for Climate Reconstruction , 1993, Quaternary Research.

[4]  J. C. Ritchie,et al.  Past and Present Vegetation of the Far Northwest of Canada. , 1985 .

[5]  S. Payette,et al.  Black spruce growth forms as a record of a changing winter environment at treeline, Quebec, Canada , 1992 .

[6]  P. Richard,et al.  Écologie des tourbières du Québec-Labrador , 2003 .

[7]  Crowley,et al.  Atmospheric science: Methane rises from wetlands , 2011, Nature.

[8]  Annika Hofgaard,et al.  Plant distribution pattern across the forest-tundra ecotone: The importance of treeline position , 2002 .

[9]  R. Spear.  The palynological record of Late-Quaternary arctic tree-line in northwest Canada , 1993 .

[10]  Cohmap Members.  Climatic changes of the last 18,000 years: observations and model simulations. , 1988, Science.

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

[12]  J. Rowe,et al.  Forest regions of Canada. , 1972 .

[13]  J. Eischeid,et al.  Precipitation Fluctuations over Northern Hemisphere Land Areas Since the Mid-19th Century , 1987, Science.

[14]  W. Rouse,et al.  Impacts of Hudson Bay on the Terrestrial Climate of the Hudson Bay Lowlands , 1991 .

[15]  Digerfeldt Gunnar.  Studies on past lake-level fluctuations , 1986 .

[16]  A. Hofgaard,et al.  Natural causes of the tundra-taiga boundary. , 2002, Ambio.

[17]  D. Foster.  The history and pattern of fire in the boreal forest of southeastern Labrador , 1983 .

[18]  J. Knox.  Large increases in flood magnitude in response to modest changes in climate , 1993, Nature.

[19]  H. Fritts,et al.  Tree Rings and Climate. , 1978 .

[20]  Y. Bégin,et al.  Tree-Ring Dating of Extreme Lake Levels at the Subarctic–Boreal Interface , 2001, Quaternary Research.

[21]  The Little Ice Age , 1989 .

[22]  P. Zetterberg,et al.  Holocene humidity changes in northern Finnish Lapland inferred from lake sediments and submerged Scots pines dated by tree-rings , 1999 .

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

[24]  S. Payette,et al.  LANDSCAPE CHANGE FOLLOWING DEFORESTATION AT THE ARCTIC TREE LINE IN QUEBEC, CANADA , 1997 .

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

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

[27]  L. Filion,et al.  Holocene plant succession in a dune-swale environment of southern Québec: A macrofossil analysis , 1996 .

[28]  S. Payette,et al.  Holocene water-level fluctuations of a subarctic lake at the tree line in northern Québec , 1993 .

[29]  S. Zoltai,et al.  The High Subarctic Forest-Tundra of Northwestern Canada: Position, Width, and Vegetation Gradients in Relation to Climate , 1992 .

[30]  B. Cumming,et al.  Greater drought intensity and frequency before AD 1200 in the Northern Great Plains, USA , 1996, Nature.

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

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

[33]  Y. Bégin,et al.  Tree-ring Dating of Extreme Water Level Events at Lake Bienville, Subarctic Quebec, Canada , 1996 .

[34]  Y. Bergeron,et al.  An 802-year tree-ring chronology from the Quebec boreal forest , 1992 .

[35]  F. K. Hare,et al.  Late-Quaternary Vegetation and Climate Near the Arctic Tree Line of Northwestern North America , 1971, Quaternary Research.

[36]  S. Payette,et al.  Recent Permafrost Dynamics in a Subarctic Floodplain Associated with Changing Water Levels, Quebec, Canada , 2000 .

[37]  S. Stine,et al.  Extreme and persistent drought in California and Patagonia during mediaeval time , 1994, Nature.

[38]  Serge Payette,et al.  Chronologie des cernes pâles de l’épinette noire ( Picea mariana [Mill.] BSP.) au Québec subarctique : de 706 à 1675 ap. J.-C. , 2002 .