This is a geologically ancient subregion, the mountaintops of which were formerly islands in the Pacific Ocean. It is underlain by both sedimentary rocks of the Franciscan Complex and older metamorphic and igneous formations that include serpentine, diorite, and granite. It stretches from Oregon south to the North Coast Ranges and east to the Cascades and northern Central Valley. Its variety of substrate and topography is mirrored in its exceptional plant diversity.
Typical insect and disease problems
• Root diseases caused by native and non-native pathogens: blackstain root disease in Douglas-fir, Heterobasidion root disease, Port Orford-cedar root disease
• Native bark beetles including Ips, Scolytus, and Dendroctonus species
• Flatheaded fir borer
• Wood rots in old-growth conifers
• Anthracnose, canker rots, true mistletoes, and Armillaria root disease in oaks
• Dwarf mistletoes in numerous conifer species