The supply of regeneration micro-sites and segregation of tree species in a hardwood/boreal forest transition zone

We examine the role that demand for, and supply of, micro-sites for seedling regeneration plays in preserving segregation of species into boreal and hardwood communities in the Lake Superior region of northern Ontario. The sensitivity of tree seedlings of the two dominant species in each community to micro-environmental variations was measured in transitional zones, where all species were present, and these responses compared to the availability of micro-sites in adjacent 'pure' communities. Acei saccharum Marsh seedlings exhibited the largest ecological amplitude, and were found growing on the deepest litter layers and where the greatest mass of hardwood litter had accumulated. Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, and Betula alleghaniensis Britton seedlings were found growing where hardwood litter layers were thinnest, more moss was present, and light levels were higher, all conditions most frequent in the boreal community. The results imply that Acer should regenerate well in both community types, while the other three species should regenerate well only in boreal communities. The abruptness of the hardwood/boreal community boundary in this area is explained by the abrupt population boundary of Acer saccharum, the potential regional dominant; this is tentatively attributed to temperature limitation on this species at its northern range limit.