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 concurrently to reconstruct the millennial-long developmental sequence of wetland stands associated 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-rinc, 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 accumulation. 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.