Climate and Picea mariana seed maturation relationships: A multi-scale perspective

One of the most important ways by which northern forests will respond to anticipated climate change is through variations in seed maturation. In this study, the relationship between growing degree-days (DD) >5°C and seed maturity was evaluated at three spatial scales. At the continental scale, the development of female gametophytes and embryos was evaluated as a function of the heat sums obtained from 11 sites distributed across the Canadian range of black spruce. At the regional scale, cone size and the percentage of germinated seeds formed in 1998, 1999, and 2000 were analyzed from seven sites situated along a latitudinal gradient in northern Quebec. At the local scale, cones were collected along perilacustral and insular transects according to their exposure to large water bodies, and from 10 islands located within a 2835-km2 hydroelectric reservoir. Our results confirm the 800—940 DD thermal sum threshold necessary for the complete maturation of black spruce embryos at several populations distributed across the total range of the species. Along the regional south-to-north climatic gradient, the percentage of germination can be predicted by a sigmoid function of a thermal sum (y = 2.8 + 25.1/[1 + e(x-896.6)/84.6]; r2 = 0.85, P < 0.0001) that attains a plateau at around 800—940 DD. Once the 800—940 DD threshold is attained, variations in the percentage of seed germination are mainly associated with inter-tree differences and local site factors (thickness of organic matter, tree density, tree height, tree age, and fetch). In the springtime, cold enclaves are created by the presence of the hydroelectric reservoir. The seed germination percentages in these enclaves varied from 0.6% ± 0.7% to 14.9% ± 19.1% (mean ± SD) according to the site (compared to 22.7% ± 15.1% for a site not exposed to the reservoir), which was equivalent to the germination percentages for sites at latitudes 1—3° farther north. These data suggest that the potential for black spruce regeneration increases strongly beyond the 800 DD isotherm, which evokes the possibility that subarctic open forests may become more dense under the current anticipated climate changes.