Land and Watershed Management

Genetic and Silvicultural Foundations for Management

Oak Studies

Quercus garryana Acorn Production Study

David Peter and Constance Harrington
PNW Research Station
Olympia Forestry Sciences Laboratory

Acknowledgments

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.

Oak 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

Quercus 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

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.

This savanna grove is at 13th Division Prairie on Fort Lewis (#88). Underburning has been frequent enough to maintain a grassy understory. Stands like this were common prior to European settlement.

These 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.

An 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.

Oaks 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

• Determine how common good and bad acorn crops are.
• Determine what regional climatic factors influence acorn production in the range of oak.
• Determine how much variation there is in the acorn crop between places and trees.
• Describe the conditions contributing to high or low tree and stand acorn productivity.
• Determine what local environmental factors influence acorn production.
• Determine how biological interactions (e.g., succession, predation) influence acorn production.
• Determine how much variation there is in the acorn crop between places and trees.

Preliminary Observations

Area of Observation

• As of January 2004 observations on acorn production are now being made from Vancouver Island to Medford, Oregon and east to Goldendale, WA and Mosier, OR. To date 1406 trees have been surveyed at least once. Fort Lewis, WA and the South Puget Sound Region are the most heavily sampled areas.

Moisture Relations

• Acorns are larger and more numerous from moist well-drained sites. Oaks growing in wetlands (wet, poorly drained sites) or very dry, rocky sites produce poorly.
• Acorn crops correlated positively with summer precipitation in most areas surveyed.
• There appears to be a strong association of good acorn crops with a dependable summer water source. Riparian trees out-produced upland trees everywhere in the range of the survey.
• Warm, moist growing conditions produced the best acorn crops. Easter Washington and souther Oregeon riparian sites were more productive than Puget/Willamette Trough riparian sites. Dry sites at the northernmost edge of oad distribution on Vancouver Island appear to be poor producers.
• Trees growing in cultivated situations that were irrigated, fertilized or receive competition control produced well.
• Cool dry sites sucah as northern Spanaway soil series (South Puget Sound and Fort Lewis) were consistent, but relatively light producers.

The most productive oaks in the survey come from this riparian grove in eastern Washington.

Stand Density Relations

• Open grown trees produced more consistently and heavily on a per tree basis than forest and woodland trees. Crowded oak trees produced acorns in small areas near the top of the crown. Individual tree production was poor but collective forest production may be high. Loss of productivity (on a per tree basis) due to crowding is mostly accounted for in trees with greater than 80 percent crown contact with surrounding trees or growing in stands with at least 15 m²/has basal area.

Fire Relations

• Internal bud temperatures of 50°C cause damage to flower primordia. Longer exposures or higher temperatures result in bud mortality.
• Consistency of acorn production is related to stand conditions and fire history.
• Underburns temporarily reduce acorn production but trees recovered in 2-5 years do better than pre-burn acorn producitivity.
• Hot fires reduced or elminated the acorn crop for 1 or more years.
• Underburned stands produced acorns more consistently and in larger numbers than non-underburned stands for 10 or more years after recovery from fire damage.
• Oak buds are well insulated, requiring up to 90 seconds at 150°C or 30 seconds at 300°C to be killed. Sub-lethal damage may occur, but has not been quantified.
• The highest temperatures recorded in the canopies of 12 trees in two prescribed burns in the summer of 2002 were 70°C, which lasted only a few seconds. Flame lengths in these burns were mostly less than 1 meter. Lower canopy temperatures up to 600°C were recorded in a prescribed burn in 2003. Flame lengths in this burn were 1-3 meters. Temperatures were at lethal levels (over 50°C) for up to 30 seconds at a time. However, most lower canopy temperatures remained below 100°C even in this fire. Foliar damage was heavy, but bud damage is not expected to be serious.

Typical flame lengths for the Fort Lewis prescribed burns in 2002 (left) and in 2003 (right).

Other Observations

• The best acorn crop from 1999-2003 in the South Puget Sound area occurred in 2001.
• Young and at least some old trees produced poorly if at all.
• Filbert moths and weevils infested over 20 percent of the 1999 acorn crop (data only collected in 1999).

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.

Open 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 sample.

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).

If you would like to volunteer, please download the instructions and survey form [pdf format or MS Word format].

USDA Forest Service - GenSilv Team
Last Modified: Wednesday, 14 March 2012 at 21:31:35 EDT