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Valley Oaks and Blue Oaks: California’s Disappearing Oaks?
When Europeans first wandered into the valleys and foothills of California, many remarked on the open, park-like nature of the oak woodlands. These forests reminded people of formal parks in Europe, where the lower branches of the trees were trimmed and little but grass grew under widely spaced trees. Two species of massive oaks grew in the level valley floors and hillsides and ridgetops; the Valley Oak, Quercus lobata and the Blue Oak, Quercus douglasii. Valley oaks leaves are gracefully sculpted into a series of deep, rounded lobes, much like those hanging from our ears. Leaves on Blue Oaks have scalloped edges, but the lobes are comparatively gentle waves. Both can grow straight and tall, some have a classic vase shape while others spread wide, wandering horizontal branches. These graceful additions to the landscape are slowly dropping out of the pictures. About 15% of the canopy of blue oaks are lost each year due to old age, storms, and other causes of mortality.
The Problem:
Forest Clearing, Rare Saplings, Disease
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The eight species of tree oaks in California
cover an estimated 4.5 x 106 ha, or about 10% of the state [a], and provide
shelter and food
for a vast array of wildlife species including over 300 vertebrates
and an estimated 5,000 species of insects [b]. Despite this, and
the fact that California's oak woodlands include some of the largest
remaining old-growth forest in the United States [c], relatively
little of these woodlands are protected, with over 80% in private
ownership [b]. Thus, land conversion of oak woodlands for range
improvement and development continues to be a major conservation issue,
with
losses being on the order of 10,000 ha per year [a]. As the trees
are cleared, if they are not left in sufficient density, isolated
trees may fail to reproduce because sources of pollen are too far
away (>100m) (f).
Beyond this, oak woodlands in California face at least two major threats. First
is inadequate regeneration, which appears to be affecting at least three of the
eight species (blue oak Quercus douglasii, valley oak Q. lobata, and Engelmann
oak Q. engelmannii) throughout much if not all their range. Whether poor regeneration
is a result of overgrazing, fire suppression or altered landscape composition
including the almost universal replacement of native perennial grasses with introduced
European annual grasses is unclear, although all probably contribute to the problem
at some level [c, e, f].
A second, more recent threat is from the disease “sudden oak death,” or
SOD. SOD was first detected in 1995 and eventually identified as caused by a
species of Phytophthora—the same genus of fungus-like organism responsible
for the Irish potato famine as well as a wide range of other forest and crop
diseases. Two widespread species of California oaks, the coast live oak Q.
agrifolia and California black oak Q. kelloggii, are particularly susceptible
to SOD as
is tanoak Lithocarpus densiflorus, a closely-related species that grows primarily
in coastal forests throughout the state. SOD has thus far killed thousands
of trees in coastal areas of the state both north and south of the San Francisco
Bay region. A variety of other species, including coast redwoods Sequoia sempervirens
and Douglas-fir Pseudotsuga menziesii, are also susceptible to the disease,
but
appear to suffer primarily foliar damage and have not yet been reported to
suffer significant mortality.
Together, poor regeneration and SOD threaten at least five of eight species
and virtually all oak habitats in California. Unless these problems are confronted
and solved, a significant fraction of California’s unique landscape is
likely to be irrevocably altered within the foreseeable future.
Organizations are working
to protect oaks in California. Each part of the life cycle of oaks must
be studied to see where things have gone wrong. The cycle of oak trees
is a circle- you can join in at any time, go around and learn, then spin
out to a new direction. It really doesn’t make any difference where
we start studying the cycle, so let’s start with the small parts
and go from there.
References:
a Bolsinger, C.L. (1988)
The Hardwoods of California's Timberlands, Woodlands, and Savannas.
USDA Forest Service, Pacific Northwest Research Station,
Resource Bulletin PNW-RB-148
b Standiford, R.B. (2002)
California's Oak Woodlands. in Oak Forest Ecosystems, (McShea,
W.J. and Healy, W.M., eds), pp. 280-303. The Johns Hopkins University
Press
c Stahle, D.W. (2002) The unsung ancients. Nat.
Hist. 111(2), 48-53
d Adams, T.E. et al. (1992) Oak seedling establishment on California
rangelands. J. Range Manage. 45, 93-98
e Mensing, S. (1992) The impact of European settlement on blue oak (Quercus
douglasii) regeneration and recruitment in the Tehachapi Mountains, California.
Madroño 39, 36-46
f Koenig, W. D. and M. V. Ashley. 2003. Is Pollen Limited? The answer
is blowin' in the wind. TRENDS in Ecology and Evolution 18(4):157-159