The Blue Oak Enigma

For years people have been worried about these iconic trees; stands throughout their range consist almost entirely of mature trees with few smaller specimens to indicate that reproduction was replacing ones that died.

I, and others, have been looking at blue oaks in different environments on and adjacent to the Big Chico Creek Ecological Reserve (BCCER) and have observed that they don't seem to be replacing themselves in most areas where they grew freely one or more centuries ago but are reproducing, sometimes sparsely, occasionally abundantly, in limited areas. Palmerlee et al. made similar observations on more than 20 cattle ranches in Northern California (Palmerlee, Alex, personal communication.) Comparing areas where oaks currently reproduce with areas where they formerly reproduced may provide insight into their general lack of reproduction. The BCCER provides an excellent site for blue oak studies, having 3107 acres of blue oaks in densities from sparse savanna to dense woodland; about half on soils derived from Chico Formation and half on soils from the Tuscan Formation and half grazed, half free of livestock for 24 years.

 In the BCCER there was also no size continuum to suggest continuing reproduction so we cut “cookies” from representative dead trees and counted rings.

Although there was little correlation between diameter and age, it was apparent  that the typical blue oak in the reserve sprouted from an acorn more than a century ago when they were reproducing regularly as suggested by the observed variation in age.

We ran belt transects counting live and dead trees, seedlings (defined as <30 cm tall), and saplings (30 cm to 2 m tall) through most of our blue oak stands and a few on neighboring properties for a total of 6224 square meters. The biggest differences, as expected, were seen between the new part of the reserve which still experiences cattle grazing and the original BCCER where cattle have been excluded for 24 years. (Grazing was not the only difference between grazed and not grazed sites; the grazed site was on soils derived from Tuscan Mudflows while the not grazed site was primarily on soils derived from Chico Sandstone.)

The difference between tree density results primarily from ecosystem structure; the grazed zone was more savanna while the ungrazed was more woodland, probably as a function of underlying soils. Grazing probably reduces the number of acorns that become seedlings and the young trees in the grazed zone showed much more browse damage, yet the ratio of surviving saplings was about the same. What became very apparent was that the saplings survived only in special locations that were quite predictable and that did not correspond to areas protected from grazers.

In both soil types, seedlings were highly clumped but the clumps were scattered more or less randomly and were not noticeably more abundant where saplings were surviving.

In some areas of the reserve we have used fire, mowing, and seeding to recreate an ecosystem dominated by native perennials. Prior to restoration, oak seedlings were common but saplings were essentially non-existent. Now saplings of both valley and blue oaks  are quite common even though we did not plant them and they are not close to adult trees.

Our past restoration efforts became unintended experiments in oak reproduction but they took about 20 years. We are starting intentional research using black tarps to kill the annual weeds, suppress seed production, and possibly allow a more rapid transition to perennials. The ultimate test of the deep soil compaction hypothesis will be if blue oaks begin to mature in those restoration areas.

We also intend to try a “quick fix” using a power auger to break up compacted soil, then plant acorns there and on adjacent sites to compare survival and growth.