Latitudinal response of subarctic tree lines to recent climate change in eastern Canada

Abstract

Aim The predictions from biogeographical models of poleward expansion of biomes under a warmer 2 · CO2 scenario might not be warranted, given the non-climatic influences on vegetation dynamics. Milder climatic conditions have occurred in northern Québec, Canada, in the 20th century. The purpose of this study was to document the early signs of a northward expansion of the boreal forest into the subarctic forest-tundra, a vast heterogeneous ecotone. Colonization of upland tundra sites by black spruce (Picea mariana (Mill.) BSP.) forming local subarctic tree lines was quantified at the biome scale. Because it was previously shown that the regenerative potential of spruce is reduced with increasing latitude, we predicted that tree line advances and recent establishment of seedlings above tree lines will also decrease northwards.Location Black spruce regeneration patterns were surveyed across a > 300-km latitudinal transect spanning the forest-tundra of northern Québec, Canada (55°29¢-58°27¢ N).Methods Elevational transects were positioned at forest-tundra interfaces in two regions from the southern forest-tundra and two regions from the northern forest-tundra, including the arctic tree line. The surroundings of stunted black spruce, forming the species limit in the shrub tundra, were also examined. Position, total height and origin (seed or layer) of all black spruce stems established in the elevational transects were determined. Dendrochronological and topographical data allowed recent subarctic tree line advances to be estimated. Age structures of spruce recently established from seed (< 2.5 m high) were constructed and compared between forest-tundra regions. Five to 20-year heat sum (growing degree-days, > 5°C) and precipitation fluctuations were computed from regional climatic data, and compared with seedling recruitment patterns.Results During the 20th century, all tree lines from the southern forest-tundra rose slightly through establishment of seed-origin spruce, while some tree lines in the northern forest-tundra rose through height growth of stunted spruce already established on the tundra hilltops. However, the rate of rise in tree lines did not slow down with latitude. The density of < 2.5-m spruce established by seed declined exponentially with latitude. While the majority of < 2.5-m spruce has established since the late 1970s on the southernmost tundra hilltops, the regeneration pool was mainly composed of old, suppressed individuals in the northern forest-tundra. Spruce age generally decreased with increasing elevation in the southern forest-tundra stands, therefore indicating current colonization of tundra hilltops. Although spruce reproductive success has improved over the twentieth century in the southern forest-tundra, there was hardly any evidence Journal of Biogeography (J. Biogeogr.) (2005) 32, 849-862 ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi

Treelines—Approaches at Different Scales

“…On the other hand, lasting snowpack can limit the growth of species that are usually restricted to snowrich terrain, such as mountain hemlock (Tsuga mertensiana) [111]. At the treeline in eastern Quebec, the expansion of black spruce (Picea mariana) driven by climate warming has slowed down probably due to drought and drying seedbeds on tundra soils [112]. In the southern Lack of available moisture during the growing season may be more critical for tree establishment and alpine plant communities in general than heat deficiency, as for example in some semi-arid or arid inland mountain areas, in the Mediterranean, and on several subtropical islands, where the position and local pattern of the treeline often depend on low precipitation above the trade wind inversion, (e.g., [52,[98][99][100][101][102][103][104][105][106]).…”

“…On the other hand, lasting snowpack can limit the growth of species that are usually restricted to snowrich terrain, such as mountain hemlock (Tsuga mertensiana) [111]. At the treeline in eastern Quebec, the expansion of black spruce (Picea mariana) driven by climate warming has slowed down probably due to drought and drying seedbeds on tundra soils [112]. In the southern Swedish Scandes, mountain birch has been declining in response to summer droughts, while Scots pine expansion will probably continue [3].…”

Recent forest limit changes in south-east Norway: Effects of climate change or regrowth after abandoned utilisation?

“…Sufficient soil moisture is a precondition for germination of mountain birch, and more precipitation could therefore increase seed propagation at the forest limits in continental regions of Scandinavia . Topographically wind-exposed areas will probably experience reduced forest range expansion rates and lower upper forest limits (Gamache & Payette 2005), due to wind damage and snow drift.…”

Bioclimatic effects on different mountain birch populations in Fennoscandia

“…This result is in accordance with the basic assumption that the birch population has developed climatic adaptations to its different habitats that influence survival rates after transplantation (cf. Myking & Heide 1995, Gamache & Payette 2005. At the 3 most oceanic and southernmost sites (NB, GNa and IH), only a few plants had survived by the end of the period at Kilpisjärvi (Fig.…”