The Arctic forests of the Eocene, which thrived under elevated CO2, a temperate climate, high precipitation and annually extremely different daylengths, represent a quite spectacular no‐analogue habitat of Earth's greenhouse past. The aim of this study was to improve our understanding of the ecophysiology of Arctic broad‐leaved deciduous forests of the Eocene, by analyzing leaf photosynthesis and tree productivity based on gas exchange modeling for two fossil Eocene sites, Svalbard and Ellesmere Island. For this, a single‐leaf photosynthesis model that includes heat transfer and leaf senescence was derived. Environmental conditions were based on available palaeoclimate data and a CO2 level of 800 μmol/mol. Additionally, different light regimes (diffusivity and transmissivity) were considered. With this model, annual photosynthesis was calculated on the basis of annual temperature and day lengths (derived by celestial mechanics). To obtain productivity of a whole deciduous broad‐leaved tree, the single leaf data were then upscaled by a canopy model. The results indicate that productivity was enhanced at both high latitude sites by elevated CO2, temperature of the growing season and high maximum daylength (24 hr) during late spring and early summer. With productivity values about 30%–60% higher as for a mid‐latitude continental European forest, the results indicate a potential for high productivity at the Eocene polar sites which is in the range of extant tropical forests. In contrast to speculations, no evidence for a selective advantage of large leaf size—as shown by various fossil leaves from high latitude sites—could be found.
N2 - Plain Language Summary: Greenhouse conditions of the past allowed forests to thrive in the Arctic. The productivity of early Eocene broad‐leaved trees, growing about 55–45 million years ago within the Arctic circle, was studied by applying physiological models to the climate conditions of the past. The Arctic environment during that time was non‐analogous, meaning that today there is no spot on Earth showing the same conditions which included a temperate climate, ample precipitation, 24 hr of daylight during early summer and a CO2 concentration twice as high as today's CO2 concentration. It was found that under these conditions, tree productivity is up to about 30%–60% higher as that of an extant temperate mid‐latitude forest. Knowledge on the ecology of high‐latitude environments of past greenhouse periods can provide valuable information for possible climate change scenarios of the future. N2 - Key Points:Lush forests thrived in the Arctic during the Eocene under non‐analogous climatic conditions
Tree productivity of Eocene forests was studied for two exemplary fossil Arctic sites based on a photosynthesis model
Compared to extant deciduous forests, productivity of Arctic Eocene forests was in the range of modern tropical forests