The spectacular preservation of middle Eocene (ca. 45 Ma) wood allows for intraseason sampling of stable carbon, oxygen, and hydrogen in order to reconstruct the paleoseasonal environment of this Arctic forest (paleolatitude 78.6° ± 1.6° N; present latitude 80° N). Carbon isotopes in bulk organics and cellulose reveal striking annual patterns interpreted as the seasonal switchover from stored to actively acquired carbon associated with deciduous growth in these unusual conifers. Oxygen and hydrogen stable isotopes of cellulose and cellulose nitrate allow for the calculation of changes in relative humidity and in the isotopic value of plant-available water. Clear annual patterns of increasing relative humidity resulting from tissue growth and concomitant transpiration are apparent, as is the systematic increase in the reconstructed oxygen isotope value of environmental water, reflecting progressively increasing temperatures during the growing season.