Monoterpene emissions and photoinhibition of “living fossil” trees grown under CO₂enrichment in a simulated Cretaceous polar environment

Polar forests populated high‐latitude landmasses during the Cretaceous and early Paleogene when the climate was warm and the atmospheric CO 2 concentration ([CO 2 ]) was at least double that of the present‐day. We investigated emission rates and composition of monoterpenes in relation to photoinhibition and photoprotection of two evergreen and three deciduous “living fossil” tree species representative of these ancient forests, after growth in a simulated Cretaceous polar (75°N) environment at either ambient (400 ppm) or elevated (800 ppm) [CO 2 ] for four years. Total monoterpene emission rates peaked during the polar summer, when prolonged periods of 24 h light would have been conducive to enhancing the formation and chemical lifetime of ozone, and hence its radiative forcing. During most of the growing season, α ‐pinene was the main compound emitted, although the magnitude and chemical speciation of emissions was highly species specific. Growth at elevated [CO 2 ] changed the composition of monoterpene emissions in evergreen and deciduous species, but only altered total leaf‐scale emission rates of the deciduous species. The continuous daylight (photon flux: 300–400 μ mol PAR m −2 s −1 ) of the simulated polar summer (14.5 weeks) did not result in leaf photoinhibition or increased photoprotection via the xanthophyll cycle. Nevertheless, species with high α ‐pinene emission rates showed also high values of photochemical efficiency at ambient [CO 2 ], suggesting a possible function of this monoterpene in protecting the photosynthetic apparatus. We suggest that high monoterpene emissions from forests during the long polar summers may constitute a regional high‐latitude climatic feedback through alteration of tropospheric composition, and secondary organic aerosol formation. These biotic effects may be important to represent in Earth system models of ancient “greenhouse” climates.