Today, the area north of the Arctic Circle is a challenging environment, home to only the hardiest of plants, animals and people. Beyond 66 °N, forests are sparse and primarily consist of conifers. However, 50 million years ago, during the warm Eocene epoch, the Canadian High Arctic was rife with lush forests that wouldn't seem out of place in modern temperate regions. Arctic temperatures during the early to middle Eocene were certainly more hospitable to forests — average temperatures during the coldest months probably didn't drop much below freezing, if at all — but the plants still had to cope with three months of near-total darkness.

Credit: © THOMAS NORTHCUT / THINKSTOCK

This unusual set of growing conditions has made it difficult to identify modern analogues to these high-latitude forests. Based on average annual temperature, the amount of biomass and overall productivity, the forests of the Pacific Northwest seemed a relatively good fit. However, taking into account seasonal patterns in fossilized tree rings, Brian Schubert of the University of Hawaii, Honolulu, and colleagues suggest that the forests of eastern Asia may be a better match (Geologyhttp://doi.org/hwr; 2012).

Unlike the brightly coloured pieces of fossil wood that decorate the walls of gem and mineral shops, some fossil wood from the Eocene Arctic was not permineralized, meaning that the original wood is intact. This includes the tree rings, which record each growing season. Using fossil wood samples from two sites, Schubert and his team divided the tree rings into subsamples of no more than 78 μm and measured the δ13C values of each. The measured variability of δ13C across each ring could then be used to estimate the ratio of summer to winter precipitation.

They find that summer precipitation was about three times as high as winter precipitation, with an estimated 1,130 mm of rain falling in the summer. This seasonal pattern of precipitation is similar to that of eastern Asia. However, unlike in Asia, the growing season above the Arctic Circle is tightly constrained by available sunlight. It was therefore the high summer precipitation that allowed photosynthetic organisms to flourish during the sunlit months, and supported such high productivity in such an extreme environment.