COMPARISON OF PALEOBOTANICAL OBSERVATIONS WITH EXPERIMENTAL-DATA ON THE LEAF ANATOMY OF DURMAST OAK [QUERCUS-PETRAEA (FAGACEAE)] IN RESPONSE TO ENVIRONMENTAL-CHANGE

To test whether stomatal density measurements on oak leaf remains are reliable tools for assessing palaeoatmospheric carbon dioxide concentr ation [CO2], under changing Late Miocene palaeoenvironmental condition s, young seedings of oak (Quercus petraea, Liebl.) were grown at eleva ted vs. ambient atmospheric [CO2] and at high humidity combined with a n increased air temperature. The leaf anatomy of the young oaks was co mpared with that of fossil leaves of the same species. In the experime nts, stomatal density and stomatal index were significantly decreased at elevated [CO2] in comparison to ambient [CO2]. Elevated [CO2] induc ed leaf cell expansion and reduced the intercellular air space by 35%. Leaf cell size or length were also stimulated at high air humidity an d temperature. Regardless of a temperate or subtropical palaeoclimate, leaf cell size in fossil oak was not enhanced, since neither epiderma l cell density nor length of the stomatal apparatus changed. The absen ce of these effects may be attributed to the phenological response of trees to climatic changes that balanced temporal changes in environmen tal variables to maintain leaf growth under optimal and stable conditi ons. Quercus petraea, which evolved under recurring depletions in the palaeoatmospheric [CO2], may possess sufficient phenotypic plasticity to alter stomatal frequency in hypostomatous leaves allowing high maxi mum stomatal conductance and high assimilation rates during these phas es of low [CO2]. (C) 1998 Annals of Botany Company.