E-mail : IGamache@rncan.gc.ca

Genetic homogeneity and ecological heterogeneity of Québec subarctic forests in relation to climatic change 
Supervisor : Serge Payette 

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In this project, we evaluated the reforestation of tundra hilltops by black spruce forests (Picea mariana) under a 2 x CO2 scenario across a forest tundra transect in northern Québec. Black spruce populations distributed along a north-south transect extending from below the forest tundra to the northern limit of the species were sampled. The genetic diversity of the subarctic black spruce populations, which are possibly reduced compared to boreal populations following the rapid post-glacial migration and subsequent forest fragmentation, was estimated using molecular methods, i.e., mitochondrial DNA markers that are transmitted both by seed and by pollen. The response of tree establishment by seed on the tundra hills and the growth of black spruce situated at the forest tundra interface (subarctic tree limit), to more clement 20th century climatic conditions were evaluated using dendrochronological, topographical, and meteorological data. Subarctic black spruce forests are generally very homogenous at the genetic level. At the mitochondrial level, there is a total absence of genetic diversity. However, at the nuclear level, they show an elevated genetic diversity equivalent to that of trees found in the boreal forest. Black spruce pollen, which are easily transported by the wind over large distances, would have re-distributed the nuclear alleles of the most northern populations, thus erasing the genetic imprint of the colonizing events still present within the mitochondrial genome. Black spruce populations in the forest tundra, which will likely eventually expand in response to 20th century global warming, should therefore contain nuclear alleles equally diverse as those of the boreal populations. Tree establishment by seed and tree height growth for the various black spruce forest tundra stands in the study responded variably to 20th century warming. Below the forest tundra, black spruce regenerates by seed, thus permitting limited local tree line advances, as well as a colonization of tundra hills since the 1970s. In contrast, black spruce situated beyond the tree limit south of the forest tundra showed greatly reduced height growth due to wind exposure, suggesting that the reforestation of tundra hills predicted under the 2 x CO2 scenario may be delayed. North of the forest tundra, black spruce propagate exclusively by layering. An acceleration in the height growth of arbustive black spruce during the 1960-1970s allowed for a local advance in the tree limit of the same order as that produced by seeds south of the forest tundra. The future transformation of arbustive black spruce into arborescent forms as suggested by the tendency for an increase in annual elongation of the principal stems should be associated with an increased reproductive potential north of the forest tundra. In nearly all of the regions, the growth of black spruce established beyond the tree limit seems to be controlled strongly by the estival temperature (sum of degree-days) than those black spruce established below the tree limit. Beyond the regional tendencies in the responses of marginal black spruce populations to climatic warming, local topographical factors sometimes produced very variable responses between the sites.

Publications

Gamache, I., Payette, S., 2005. Latitudinal response of subarctic tree lines to recent climate change in eastern Canada. Journal of Biogeography, 32: 849-862.


Gamache, I., Payette, S., 2004. Height growth response of tree line black spruce to recent climate warming across the forest-tundra of eastern Canada. Journal of Ecology, 92: 835-845.


Gamache, I., Jaramillo-Correa, J., Payette, S., Bousquet, J., 2003. Diverging patterns of mitochondrial and nuclear DNA diversity in subarctic black spruce: imprint of a founder effect associated with postglacial colonization. Molecular Ecology, 12: 891-901.


Payette, S., Fortin, M.-J., Gamache, I., 2001. The subarctic forest-tundra: the structure of a biome in a changing climate. BioScience, 51: 709-718.