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Land and Watershed Management
Genetic and Silvicultural Foundations for Management
Quercus garryana Acorn Production Study
David Peter and Constance Harrington
We thank Fort Lewis for financial and logistical support and all the volunteers who helped with data collection.
Special thanks to Gary McCausland and Jeff Foster at Fort Lewis Military Reservation, Washington, for financing and supporting this project.
Larger views of most photos on this page are available by clicking on the photos.
Why Study Oak?
Oak/prairie ecosystems are rapidly disappearing
The main factors causing oak and prairie ecosystems to disappear are forest succession, agricultural conversion and urbanization. There are very few places where oaks can not be succeeded by Douglas-fir and although oaks can invade prairies that are not frequently burned, so can Douglas-fir.
Remaining oak systems are changing rapidly due to introduction of exotics and fire suppression
Many exotic herbs and shrubs have been introduced that thrive in open oak stands or savannas. Scots broom is probably the most noticeable introduction but exotic herbs and grasses have almost completely replaced natives in many places. Besides having different values for wildlife, exotic species can change the fire/fuels equation substantially. Scots broom for example is a much taller fuel than what was naturally present. Thus when the understory burns flames reach higher into the oak canopy than before when the understory was grass dominated. Most exotics are not shade tolerant and disappear from dense, shady stands, but dense shady stands were probably never a conspicuous part of the landscape due to aboriginal burning.
Oaks are a rich source of food for many wildlife species including the western gray squirrel (a state threatened species), deer, elk, bear, birds, many insects.
Western gray squirrels in western Washington maintain populations only where oak communities exist. Acorns are a critical, but not sole food source in winter.
Oak is monoecious which means that male and female flowers are separate though with oaks both are born on the same plant. The male (staminate) flowers are long, thin greenish-yellow catkins. The female (pistilate) flowers are very small and reddish. They appear in the axils of developing leaves. Flowering is fullest when the first leaves are about half size (March-June depending on the location). A more detailed description of flowering can be found in volume 2 of Silvics of North America (Agricultural Handbook 654). Click here for more (go to North American Flora)
This Quercus garryana illustration comes from:
Hitchcock, C.L., A.R. Cronquist. 1973. Flora of the Pacific Northwest.
Univ. of Washington Press, Seattle, WA. 730 p.
acorns typically measure from 1-3 cm in length. These acorns were photographed
at Fort Lewis on 9/27/99 (#66). The brown one is insect infested. Damaged
acorns often turn brown prematurely and fall from the tree. Acorns in
this stage of development are viable though they will continue to ripen
on the tree for several weeks. Once healthy acorns ripen to a brown color
they fall very rapidly leaving the cap on the tree.
Range of Quercus garryana
garryana, commonly called Oregon white oak or Garry oak, ranges from mid-way
on Vancouver Island to southern California. It has the longest north-south
distribution of any western oak. Northward it is increasingly restricted
to topographic rain-shadows and dry soils. Southward it is found more
on moister windward sides of mountain masses. Oak extends east of the
coastal and Cascade mountains in several places, most notably the Columbia
Gorge where moist maritime air is able to flow farther east.
Succession is an important concept for explaining the current distribution of oak and why it is disappearing from the landscape. This diagram tracks the successional pathways for the oak/prairie ecosystems typical of Oregon, Washington and parts of British Columbia.
Given time and lack of management or wildfire, oak will be replaced by conifers (mostly Douglas-fir). The landscape was originally burned extensively by aboriginal peoples which arrested succession so there were many prairies and oak savannas. When aboriginal burning ended, oaks and Douglas-fir seeded onto the prairies and savannas so woodlands and forests became more common. Douglas-fir can enter the successional sequence at any point provided fire does not kill its seedlings and thin-barked saplings. It grows taller than the oaks and shades them out. As succession proceeds towards forest, fuels accumulate so fires are likely to be catastrophic, that is trees are killed and succession starts over. The new stand will succeed directly to another Douglas-fir forest unless fire is reintroduced on a short cycle.
The best acorn producing trees are usually found in the early successional stages of the oak pathway but more trees producing fewer acorns per tree are found in the later stages of the oak pathway. We don't yet know where in the succession maximum acorn production per area occurs, but early indications point to oak savanna or woodlands.
oak and Douglas-fir trees are at Johnson Prairie in Fort Lewis, Washington.
Oak commonly occurs along the forest-prairie ecotone. These ecotones are
not stable in the absence of fire or other management. One representative
oak sampled at this location was 119 years old at breast height (#32).
The green shrubs in front of the oaks are Scots broom. Scots broom is
an alien species which rapidly invades open areas west of the Cascades.
oak forest with occasional Douglas-fir bordering the Scatter Creek prairie
near Olympia, Washington (#13-15). One representative tree in this stand
was 106 years at breast high. This stand was probably part of the prairie
in the 1800's.
have a very wide ecological amplitude. They are able to grow in wet sites
such as the Oregon ash/slough sedge wetland on the left as well as very
dry sites like the one on the right. It is not clear if oak can reproduce
on wet sites like thisin the absence of disturbance but none of the trees
that grow with oak on these sites are capable of overtopping it. In 1999
the wetland oaks at this site did not produce acorns. The tree in the
center of the photo is about 140 years old at breast height (#118).
The trees on the right are growing on south-facing slopes
in the southern Olympic Mountains (South Fork Skokomish River). Fire history
studies indicate that the last major fire occurred about 1701 in this
area. This may be the only kind of habitat that oak could be considered
a climax species.
Purpose of Acorn Study
Area of Observation
most productive oaks in the survey come from this riparian grove in eastern
Stand Density Relations
Typical flame lengths for the Fort Lewis prescribed burns in 2002 (left) and in 2003 (right).
Characteristic Tree Shapes
Tree shape is a good predictor of productivity because it is largely determined by crown competition from surrounding trees. The mushroom shape is characteristic of open grown trees with full illumination from all sides. Trees with this shape are the best acorn producers on a per tree basis. The columnar tree is typical of sites where illumination is reduced from several sides. These trees do not produce as well as mushroom shaped trees but are better than inverted vase shaped trees on a per tree basis. The inverted vase shape tree is characteristic of forest stands. They typically have 100% crown contact. Only upper limbs with narrow branch angles can grow tall enough to reach sunlit areas. The vigor of these trees is reduced by competition and acorn productivity on a per tree basis suffers.
The external surface area of each of these shapes in 3 dimensions is a useful number for comparing tree productivity. Acorns are produced mostly in the very external, sunlit portions of the canopy—essentially on the external "surface" of the crown's shape. Therefore acorns per square meter of external crown surface is a very descriptive term for comparing tree productivity. It is also descriptive of what a person sees when surveying for acorns. In calculating this figure only the upper and lateral surfaces are included. The bottom surface is too shaded to produce many acorns.
grown trees adopt large mushroom shaped crowns such as this one in a hayfield
near Winlock (#115). This tree is about 126 years old. It has grown much
faster than trees on the gravelly outwash prairies to the north because
it is growing on a finer textured soil with higher fertility and water
holding capacity. This was one of the best acorn producers in the 1999
Oak commonly forms clumps of stems in open prairies. These may be clones
caused by root-sprouting. One way this can be stimulated is when trees
are killed by fire. The clump adopts the characteristic mushroom shape
of an open grown tree but each stem's crown is either columnar or
inverted vase shaped due to competition from its neighbors. This clump
is at Scatter Creek Preserve south of Olympia, Washington (#11).
USDA Forest Service - GenSilv Team