Reproductive development and seed ripening in Betula papyrifera along an altitudinal thermal gradient in eastern Appalachia (Canada)

Abstract

In several gymnosperm tree species of the circumboreal forest, reproductive development is closely associated with the accumulation of degree-days during the growing season. We wanted to verify whether this pattern holds for a widespread angiosperm species such as paper birch ( Betula papyrifera Marsh), a broadleaf tree distributed throughout the North American boreal forest. Stations to simultaneously measure the thermal sum (degree-days > 5 °C, DD) and the reproductive development (anatomical development and seed germination) were installed in four sites along an altitudinal thermal gradient totalling 541 m a.s.l. in the eastern Appalachian Mountains (Canada). In 2004, sites 1, 2, 3 and 4, received 973, 1099, 1266 and 1359 DD, respectively. A strong relationship was found between thermal sum and reproductive development (anatomical development: r2 > 0.90; seed germination r2 > 0.72). This relationship seems to be based on the absolute value of heat-sum received because (i) rates of reproductive development are twice as high in colder sites (1 and 2) than in warmer sites (3 and 4), and (ii) total seed germination did not differ among the sites. Along with this, we noted a lesser production of full seeds in colder sites, coupled with a higher success of germination in the coldest site (site 1), which appears to be a mechanism of phenotypic adaptation to cold.