| Three well-preserved in situ fossil forests presented an opportunity to reconstruct the physical dimensions of Tertiary floodplain forests that grew at polar latitudes. I reconstructed the structure, biomass, productivity, and stand development history of two Eocene-aged forests (“N” and “HR”) on Axel Heiberg Island and a Paleocene-aged forest on Ellesmere Island, Nunuvut, Canada. I excavated fossil tree trunks and treetops to developed equations that describe the height, structure, and mass of the aboveground components of Metasequoia trees. I combined those data with surveys of in situ stumps to determine the structure, biomass and productivity. I validated my field methods on modern Metasequoia forests and found them sufficient to obtain estimates of fossil tree size and biomass with errors of 15–20%. Two of the fossil forests were of tall and of high biomass. We calculated a canopy height of 40 ± 3 m for the N forest and 25 m ± 2 m for the HR forest. The Paleocene forest was 34 ± 2 m tall. Buried knots in dissected logs and abundant branch-free stem wood indicate that the Eocene forests were closed canopy forests. Stem tapers indicate that the overstory was of uniform height. In contrast, the Paleocene fossils exhibited external branch anatomy characteristic of more open forests.; Stem biomass was 600 Mg ha−1 in the N forest. Biomass in the HR forest was much smaller, 63.3 Mg ha−1. Paleocene forest biomass was estimated at 489 Mg ha−1, but may have exceeded 576 Mg ha−1 if non-preserved sapwood is accounted for. I estimated branch wood and foliar biomass using the diameter of branch stubs on fossil tree tops and allometric equations derived from modern Metasequoia. Estimated stand-level branch biomass was 13 and 6.7 Mg ha−1 in the N- and HR-forests, respectively. Branch biomass in the Paleocene forest was 18 Mg ha−1. Standing foliar biomass was estimated to be 3.2 and 2.1 Mg ha−1 in the N- and HR-forests, respectively. Foliar biomass in the Paleocene forest was 4 Mg ha−1. We calculated annual wood production to be 2.3 Mg ha−1 yr−1 for the N forest, 0.55 Mg ha−1 yr−1 for the HR forest and 3.8 Mg ha−1 yr−1 for the Paleocene forest. If the ancient Metasequoia were deciduous like their living relatives, annual aboveground net primary productivity was 5.5 Mg ha−1 yr−1 for the N-forest, 2.8 Mg ha−1 yr−1 for the HR-forest and 5.8 Mg ha−1 yr−1 for the Paleocene forest. Our estimated biomass and productivity values are similar to those of modern old-growth forests of the Pacific Northwest (USA) and old-growth coastal Cordillera forests of southern Chile. |
