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Saturday, March 14, 2015

Got Oaks?! By Dr. Mike White, Conservation Science Director




After spending nearly 7 years at Tejon Ranch, I find myself at times getting a little complacent about our oak woodlands. You see, we got oaks, lots of oaks! The conserved lands at Tejon Ranch support about 80,000 acres of oak woodlands, a significant contribution to the conservation of oak woodlands in the Tehachapi region. With help from our partners we have documented at least 11 oak taxa (species, varieties, and recognized hybrids) in these woodlands. To put this in perspective, this represents 1/3 of all the oak taxa in California on a property that comprises 0.25% of the state’s land area!

Oak taxa documented at Tejon Ranch
Scientific Name
Common Name
Quercus berberidifolia
Scrub oak
Quercus chrysolepis
Canyon live oak
Quercus douglasii
Blue oak
Quercus garryana var. breweri
Brewer’s oak
Quercus john-tuckeri
Tucker’s oak
Quercus kelloggii
California black oak
Quercus lobata
Valley oak
Quercus wislizeni var. frutescens
Interior scrub oak
Quercus wislizeni var. wislizeni
Interior live oak
Quercus xalvordiana
Alvord oak (hybrid)
Quercus xmorehus
Oracle oak (hybrid)

Large expanses of the foothills and high country at Tejon support oaks, and many of these oaks, particularly our valley oaks, are really big trees. In fact, the famous California naturalist Joseph Grinnell, who camped at the site of Fort Tejon in 1904, set up his skinning table under “an immense white oak, said to be the largest in California.” Grinnell wrote “It was 27 feet in circumference at the base, and was only one of many others nearly as large which form a group in front of the rectangle formed by the Fort ruins. In fact the most impressive feature of the Tejon valley (actually Grapevine Canyon) to one entering from the dry barren plains on either side, are the magnificent oak groves, interspersed with green pastures.”

I was out on the Ranch the other day, daydreaming as I drove, when it suddenly struck me, “these are really big trees!” (see photos below). Let me assure you that I am not completely oblivious of my surroundings, nor unaware that we have big trees. In 2010 interns from the Bren School of Environmental Science and Management were measuring oak trees as part of a master’s project and came across a valley oak that was 6.6 feet in diameter at breast height (DBH, about 4 feet off the ground), and found a number of trees that exceeded 5 feet DBH. It’s just after seeing lots of big trees for so many years one can be lulled into complacency! However, it is worth taking the time to note how special our oak woodlands really are.

Conservancy Board Chair Joel Reynolds poses with a large valley oak. Photo by Mike White.


Valley oak estimated at nearly 7-feet diameter a breast height. Photo by Mike White.
Oak woodlands are some of California’s richest wildlife habitat. Acorns provide an important seasonal food source for acorn woodpeckers, squirrels and rodents, mule deer and, unfortunately, wild pigs. Oak woodlands support a rich community of cavity nesting birds, with 17 cavity nesting bird species documented at Tejon Ranch, as well as other nest building species. Large valley and blue oaks at Tejon Ranch and in other parts of the Tehachapi Mountains are considered one of the remaining strongholds for cavity nesting purple martins (Progne subis), a Species of Special Concern in California. At Tejon Ranch, martins like to nest in large valley oaks near the tops of ridges (photo below).


A stand of valley oaks supporting nesting purple martins on Tunis Ridge. Purple martin (above left).  Photos by Mike White and J.J. Cadiz.
In many parts of California, oak woodlands are characterized by lots of mature trees but few young trees and saplings, which can result in a loss of oaks over the long-term as old trees die. This has been called the “oak regeneration problem” and has been attributed to many causes including cattle grazing. While many of the deciduous (i.e., those that lose their leaves in the winter) oak stands at Tejon Ranch show similar size patterns as the rest of California (live oaks seem to show more regeneration than deciduous oaks), there are places at Tejon Ranch that have very high densities of young trees and saplings, even in the face of long-term cattle grazing. Why regeneration occurs at some sites but not others is not clear to us, but it is a welcome sight nonetheless.


Young valley oaks in the Catskins area of Tejon Ranch, an area grazed by cattle. Photo by Mike White
Did you know that blue oaks can be used as rain gauge? Our colleague Dr. Daniel Griffin at the University of Minnesota (https://sites.google.com/a/umn.edu/daniel-griffin/home) and his collaborators have found that the sizes of blue oak annual growth rings are highly correlated with annual precipitation. Dan is a dendroclimatologist, or someone that determines past climates from the patterns in annual tree rings. Tree rings are wider when conditions are wetter and narrower when conditions are dryer. The patterns of ring widths in live trees can be correlated with known precipitation records to develop a statistical relationship that can then be projected back in time using ring width patterns from old dead trees to look at past climates. Using a long period of record developed from both living blue oaks and old dead blue oak, some of which were collected from Tejon Ranch (see photos below), Dan and his colleagues were able to estimate that the magnitude of the 2012-2014 California drought was greater than any such period in the last 1,200 years (https://sites.google.com/a/umn.edu/daniel-griffin/CAdrought).


Pencil-like tree-ring cores are collected non-destructively using a Swedish increment borer. Photo by Daniel Griffin.

Dan Griffin collects a tree-ring sample from a dead tree, which can be used to extend the tree-ring record back in time up to 700 years. Photo by Kevin Anchukaitis.

Coring blue oaks on the Antelope Valley side of the Ranch. Photo by Dan Griffin.
 I always find it good to step back periodically and take stock of things in my life, and for me this includes reminding myself to keep my eyes and mind open to the wonders that we see every day at Tejon Ranch. Our oak woodlands are truly majestic, with many individuals on the Ranch likely having germinated from acorns produced before Columbus arrived in North America. I find it reassuring that these oaks are now permanently protected, that we have the time and space to learn more about them, and, oh yeah, that we can really experience these oaks if we take the time to.