Early to Late Cenozoic Changes in North American High-Latitude Lowland-Forest Biomass

I evaluated secular changes in the ecology of high-latitude fossil swamp-forests using methods by which, forest structure, biomass and productivity could be reconstructed using excavated fossil tree remains. Taxodiaceous swamps (prevalent in the far north during the early Cenozoic) had large amounts of standing biomass (>500 Mg per ha) and at least moderate primary productivity (4-5 Mg per ha per yr). Given the size of the fossil trees in the eastern Canadian Arctic (up to 2 m diameter and 40 m tall), and the geographic extent of these coal deposits; the Arctic forests probably represent the maximum forest biomass and productivity achievable near the poles. By middle Miocene time, the high-latitude Taxodiaceae-dominated swamp forests were replaced by Pinus-dominated forests, which sequestered less biomass (< 260 Mg per ha) but were moderately productive (3-4 Mg per ha per yr) although they were smaller in stature (canopy heights of 16-21 m) than their early Cenozoic counter parts. By the late Cenozoic, fossil floras indicate a transition to lowland forests dominated by Picea and Larix. The Pliocene forests were smaller in stature than Miocene forests. Trees with an average diameter of 21 cm and average heights of 12 m comprised the Pliocene fossil forests. Stem-dimensional analysis of fossil trees indicates the Pliocene-age trees were substantially larger than modern taiga vegetation and may have sequestered as much at least 80 to 100 Mg of biomass per ha depending on the forest tree density. Interestingly, Cenozoic era high-latitude forests do not exhibit any structural features that reflect growth in a high-latitude light regime and their biomass and productivity fall within the range of productive forests growing on Earth today. Nevertheless, the decline in forest biomass and stature together with cooling Cenozoic climate may indicate important vegetation-carbon-climate feedbacks at work over geologic time.