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Oregon Ash
Fraxinus latifolia
Information about this taxon
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Green Ash
Fraxinus pennsylvanica		   Border Privet
Ligustrum obtusifolium
  Owner:   C. Michael Hogan    
Compound leaves of F. latifolia, Sonoma County, California
Oregon Ash
Fraxinus latifolia

C. Michael Hogan PhD
October 24, 2008

Oregon Ash is a common deciduous woodland tree species in the canyon forests of the Pacific Northwest through Central California. This tree is most frequently found in mixed oak or conifer forests in riparian settings or other bottomland. A number of fungi attack this species, causing effects ranging from leaf discoloration to plant death. The wood has certain utility, having been originally used by Native Americans for harpoon shafts, tobacco pipes and other purposes.


DISTRIBUTION

F. latifolia along the banks of Buckeye Creek, Sonoma County, California
F. latifolia occurs below elevations of 1700 meters from northeastern Washington to Central California in canyons riparian areas and woodlands. In the Pacific Northwest Oregon ash is found along the shores of Puget Sound, in the western Cascade Range, and along the southwestern coast, but is absent in the Olympic Mountains. In the Columbia River watershed it is found from the coast to The Dalles, while in western Oregon, it occurs from the coast into the western Cascades, and is prominent in the Williamette Valley. There are over 2.5 million cubic meters of Oregon Ash standing timber in the Pacific Northwest. In California, Oregon ash occurs within the North Coast Ranges extending as far south as Santa Clara County . It also grows in the Sierra Nevada and in the delta region of the California Cental Valley. The species is common in the Sacramento and Pit River catchments, and is found on the Modoc Plateau. The Oregon Ash is found in many riparian areas of Northern and Central California. (Hogan) F. latifolia intergrades with F. velutina from southern California eastward into Arizona.

F. latifolia sometimes occurs within its northern range in small pure stands, but it is characteristically associated with species such as Alnus rubra, Populus trichocarpa and Quercus garryana. In its drier regions, it also is found with conifers such as Abies grandis. Associated trees in southwestern Oregon and northern California are Alnus rhombifolia, Platanus racemosa, Pinus ponderosa and P. sabiniana. Oregon ash is a prominent tree in Red Alder, Port Orford-Cedar, Black Cottonwood-Willow and White Oak forest types.



MORPHOLOGY
This diocieous tree attains heights of up to 25 meters and has cylindric twig structures. The compound pinnate leaf occurs as five to seven leaflets, each widely elliptical to narrowly ovate. The leaflets are entire to somewhat serrate and are sparsely puberulent on the lower side. (Jepson) Individual leaflets are two to ten cm in length and are either sessile or of abbreviated petiole. The wind dispersed samara fruits are 12 to 33 mm long and five to eight mm wide.



ECOLOGY
In Valley Oak dominant riparian forests, Oregon Oak is a common associate, surveyed to comprise 13 percent of the tree cover in the Consumnes River basin. (Sands) Preferred soils are those lacking good drainage and moist bottom land with deep humus rich soil . Oregon Ash is often concentrated along ribbonlike fringes of streams and wetlands,(Collingwood) and is even found in seasonally flooded habitats Specific soil types supporting F. latifolia are mollisols, alfisols, inceptisols and Ultisols. Fibrous rooting is somewhat shallow but vigorously laterally spreading.(Sterrett) The species is somewhat tolerant of shade, with shaded trees undergoing rapid self-pruning (cladoptosis).

Throughout the range of Oregon Ash the minute weevil Thysanocnemis can destroy three fifths of an annual seed crop. Other insects that cause foliage or twig damage harmful to ornamentals but are not considered forest pests are: Leptoypha minor, Tropidosteptes pacificus, Oecanthus fultoni and Hyphantria cunea. Leperisinus oregonus produces no real damage but is often found in harvested wood.

A variety of fungi and other parasites occur on Oregon Ash. Phyllosticta innumera, Mycosphaerella effigurata, Cylindrosporium fraxini, Piggotia fraxini, Mycosphaerella fraxinicola, and Cylindrosporium californicum create leaf spot. The mildew Phyllactinia guttata occurs on Oregon ash. Common fungi found on twigs are Hysterographium fraxini, Cytospora ambiens, and Nectria cinnabarina. The mistletoe Phoradendron longispicum often occurs in Oregon ash, while the rot, Perenniporia fraxinophilus, manifests in older Oregon Ash and produces severe forest damage.



REFERENCES

* C. Michael Hogan (2008) California Buckeye: Aesculus californica, GlobalTwitcher.com, ed. N. Stromberg
* Willis Linn Jepson (1993) Jepson Manual, University of California Press, Berkeley, Ca.
* Anne Sands (1981) Riparian Forests in California: Their Ecology and Conservation: a Symposium, University of California, Davis Institute of Ecology, ANR Publications, 122 pages
ISBN 0931876419
* G.H. Collingwood and Warren D. Brush (1978) Knowing your trees, ed. Devereux Butcher, American Forestry Association, Washington, DC. 392 p.
* W.D. Sterrett (1915) The ashes: their characteristics and management, U.S. Department of Agriculture, Bulletin 299, Washington, DC. 88 p.