HOW DID ATMOSPHERIC CO2 ENRICHMENT AND LEAF HABIT IMPACT TREE CARBON BALANCES IN HIGH LATITUDE PALEOFORESTS?

Forests have been a common feature of high latitude terrestrial environments since the late Paleozoic. These forests likely influenced regional and global climates via changes in albedo, the production of hydrocarbons, the formation of organic matter, and the recycling of water. Unfortunately, no natural analog exists for these forests today, and thus their relationship with the atmospheric environment and polar light regime remains poorly known. For example, why are these paleoforests dominated by deciduous leaf forms? Also, how do these forests respond to high atmospheric CO₂? Using four fully replicated growth rooms, we have grown five plant species with long fossil records in a simulated high latitude (68 °) environment typical of the Cretaceous and early Tertiary at ambient and elevated (2X) levels of CO₂. After two years of growth in these conditions, we began a year-long study using a replicated system of partitioned whole-plant chambers to quantify diurnal and seasonal patterns of above-ground carbon sequestration and transpiration, and below-ground root carbon exchange.

After eight months of measurements, plants grown at elevated CO₂ have fixed more carbon and have higher whole-plant water-use efficiencies than their ambient CO₂ counterparts. This suggests that the productivities of these forests are stimulated under high CO₂ conditions. During the dark polar winter, the evergreen species consumed slightly more carbon than the deciduous species, but fixed much more carbon during the early spring before the flush of leaf growth in the deciduous plants. Results indicate that, at this latitude, photoperiod is not the dominant filter for leaf habit, but that CO₂ enrichment may have played an important role.