The study includes 19 bogs between 45° 07′ N. and 51° 59′ N. (Fig. 1), spaced at about 50 mile intervals from the St. Lawrence valley across the Laurentian Shield to James Bay (Jack River). Plane service made possible selection of excellent bogs for boring in wilderness regions. The plane landed on lakes near the bogs selected. Up to lat. 47° N. five major climatic changes are recognized for Quebec and are referred to as Q-1 to Q-5. The pollen profiles suggest that an initial, pronounced warm period (Q-1) (correlative with the Lake Timiskaming retreat) followed by cooling (Q-2), very likely also accompanied by local glaciation (correlative with the Cochrane halt), prevailed from the St. Lawrence valley to Lac Soscumica bog (50° 39′ N.). The initial warm period is marked by prominent pine peaks accompanied by an impressive minor oak peak. During the major xerothermic period (Q-3), all bogs record a very long and prominent pine climax, with replacement of Pinus banksiana by the white–red pine group up to the Lacroix River bog (49° 02′ N). From Clova (48° 07′ N.) to Jack River bog (51° 59′ N.) jack pine replaced red white pine, while the upper half of the profile accumulated. This shows jack pine with a striking bimodal pattern of representation. Also, north of Clova, jack pine formed an important association with Picea mariana during the more recent past, introducing the forest type which prevails up there today. From the St. Lawrence valley (45° 07′ N.) to the lower edge of the Shield at Saint-Lin (45° 55′ N.) white–red pine held an important place in the forest cover up to the present (represented by the close of the bog mat). The single most striking feature of the study is that red–white pine penetrated as important forest associates to the Rupert River (51° 28′ N.). No doubt white–red pine extended their range northward during the prominent warm–dry period (Q-3) because their highest representation appears in the lower levels of bogs. The great change in vegetation type, with more emphasis on boreal species (Picea mariana and Pinus banksiana), from Clova (48° 07′ N.) northward to James Bay suggests the Cochrane oscillation influence and subsequent retreat during the ensuing major xerothermic period (Q-3). This period had probably waned by the time the forests were able to invade the James Bay region, thus giving rise to a rather monotonously changeless forest history from lat. 50° 54′ northward (Q-5). Local glaciation is indicated in the bog from Mont Tremblant (Bog 14), where forest history began during the xerothermic period. Fig. 2 presents the highest percentage attained by species at a given latitude, which at a glance divides the species according to latitudinal preference, suggesting temperature control.Since the region about James Bay was covered by the sea following northward wasting of the ice, forest history here begins later than in the areas located above elevation of submergence. Radiocarbon dating of the peat from the bottom level of a bog near Rupert River (51° 27′ N., 78° 32′ W.) places the beginning of organic sediment accumulation at 2350 ± 200 years. This marks the time when marine waters withdrew from the region. Hemlock must have been quite abundant northward to Lac Shaw bog (46° 19′ N.), but reached a pollen representation of 7% even at Lac Mazanaskwa bog (47° 07′ N.). The decline of the white–red pine group on the northern half of the Shield suggests that the muskeg condition north of Clova is due to more recent paludification and more moist climate following the warm–dry period (Q-3). This conclusion is also supported by the prominent rise of Picea mariana. Evidence of tundra conditions was found only in bogs 1, 3, 6, 9, 10, and 12.
Response of Suppressed Conifer Seedlings to Release from an Aspen-pine Overstorey