TRULY OREGON’S TREE: REASSESSMENT OF THE IMPRESSION FOSSIL RECORD OF PSEUDOTSUGA (DOUGLAS FIR) IN WESTERN NORTH AMERICA

Dozens of fossil occurrences of Pseudotsuga (Pinaceae) have been reported from as early as the Cretaceous in western North America, but reevaluation of the impression fossil record of the genus finds that the majority of these are either incorrectly identified or too poorly preserved to be identified. Only nine occurrences have been confirmed at present, all from winged seeds and ovulate cones, and all from within the current range of the genus. Co-occurring dispersed foliage and foliated and defoliated axes, although not diagnostic, are consistent with Pseudotsuga, as well. The verified fossils form three groups and four morphotaxa distinguishable by the dimensions of ovulate cones and winged seeds. The oldest of these, Pseudotsuga laticarpa Lakhanpal from the ~ 32 Ma Rujada Flora, Oregon, resembles the extant P. macrocarpa (Vasey) Mayr. in cone and seed dimensions, as do the long needles of associated foliage. The newly described Pseudotsuga cassiana (Axelrod) Schorn from the ~ 13 Ma Trapper Creek Flora, Idaho, represents an intermediate-sized group, with ovulate cones and seeds that are consistently smaller than those of P. laticarpa. Pseudotsuga taxifolioides Arnold from the 13 Ma Trout Creek Flora, SE Oregon, and P. sonomensis Dorf from the ~ 3 Ma Neer’s Hill locality of the Sonoma Volcanics, resemble the extant P. menziesii (Mirb) Franco in their smaller cone and seed dimensions. Limited initial data suggest that the big-coned form of Pseudotsuga originated in the humid-mesic, mesothermal, coastal Pacific Northwest during the early Oligocene, and that the intermediate and small-coned forms first appeared in the middle Miocene of the interior west under cooler, summer dry climatic conditions. In all of the assemblages in which the occurrence of fossil Pseudotsuga has been verified, it formed a sub-dominant component in broadleaved vegetation. Modern Douglas Fir-dominated coniferous forest of the coastal Pacific Northwest does not appear to have developed before latest Pliocene time. The record of Douglas Fir thus confirms the geologically recent evolution of modern west coast plant communities in response to the expansion of summer drought, resulting from strengthening and cooling of the North Pacific Gyre with the growth of north polar ice after ~ 3 Ma.