The biogeographically correct eastern border of California's Klamath bioregion

In formulating the now well known Jepson bioregions for California, Jim Hickman made one significant error: he mis-mapped the geographic setting that differentiates the Klamath Ranges (KR) region from the Cascade Ranges (CaR) region.  Hickman’s boundary concept was correctly formulated: the Cascade region he characterized as “this volcanic region..” which contrasted with the Klamath Range region geologically,  which is also correlated with strong biogeographic pattern (along an aridity gradient from toward the interior).  However, in the 1989 edition (Hickman 1989), the map depicts the border that, in the color map and text in the 1993 Jepson Manual (Hickamn 1993), is “...more or less along of Interstate 5”. 

This border is geologically and biogeographically wrong.

Both geologically and biogeographically, the Klamath Ranges region includes about 250,000 acres of non-volcanic mountainous terrain comprising the McCloud arm, Squaw Creek arm, Pit River arm of Shasta reservoir, including the highest peaks in the reigon Grizzly Peak (6252 ft) and Bald Mountain (5536 ft), a region of metamorphic rocks.  The vegetation of the region has more in common with the Klamath Ranges than the Cascade Ranges, being forests largely dominated by Douglas-fir.  The region in question is home to two endemics I have described: Shasta boneset (Agartina shastensis) and Shasta Snow-wreath (Neviusia cliftonii), both with high fidelity for calcareous, not volcanic rocks.

The first figure shows the proposed, corrected border between the KR and CaR regions (red line) imposed on the map that appears in the inside front cover of both editions of the Jepson Manual.

Essentially, the border I suggest is exactly coincident with the extent of the Eastern Klamath terrane (Irwin 1994), which to the east is bordered by Quaternary and Tertiary volcanic rocks of the true Cascade ranges (yellow eastern border of the second figure).

References:

Hickman, J.C. 1989.  Introduction to the Jepson Manual.  Jepson Herbarium, University of California, Berkeley.

Hickman, J.C.  [Ed.].  1993.  The Jepson Manual, Higher Plants of California.  University of California Press.

Irwin, W.  1994.  Geologic Map of the Klamath Mountains, California and Oregon.  USGS Map I-2148.