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Land and Watershed Management

Land and Watershed Management Program
Pacific Northwest Research Station
Olympia Forestry Sciences Laboratory
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Corvallis Forestry Sciences Laboratory
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Corvallis, OR 97331
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United States Department of Agriculture Forest Service.

Genetic and Silvicultural Foundations for Management

Oak Studies

Acorn Survey Results

A survey of Quercus garryana (Oregon white oak or Garry oak) acorn production was started in Washington and Oregon with 284 trees in 1999. By 2005, the survey has expanded to 1399 trees and includes British Columbia (Vancouver Island) as well as many geographic regions in Washington and Oregon.

The Acorn coding system was:

1 = no acorn production

2 = 1 to a few acorns, need to observe tree carefully to detect acorns

3 = acorns readily observed, most acorns occur individually (not in clusters)

4 = acorns occur in clusters at 2 or more all around the tree

(For more detail on acorn coding method, see Graves, W.C. 1980. Annual oak mast yields from visual estimates. Pp 270-274 in Plumb, T.R., tech. coord. Proceedings of a symposium, Ecology, Management, and Utilization of California Oaks. PSW-GTR-44 (see for ordering information).



Mean acorn classes by year and location. A downloadable excel spreadsheet of these data with sample sizes is also available on this website.



Sample Size

The total number of trees sampled in the project as of March, 2006 over 7 years is 1605. But, not every tree has been sampled in every year. This table shows the number of trees sampled by year, and by the number of times they have been sampled. Thus, there are 232 trees that have a continuous 7 year record.

Number of trees sampled by frequency of sampling
Year Total
this year
1 time 2 times 3 times 4 times 5 times 6 times 7 times
1999 283 283 0 0 0 0 0 0
2000 521 298 253 0 0 0 0 0
2001 827 347 276 244 0 0 0 0
2002 1114 476 263 257 243 0 0 0
2003 1281 251 416 251 250 238 0 0
2004 1430 250 174 404 246 248 237 0
2005 1399 186 134 165 404 238 246 232

Location of study sites and an overview of mean acorn productivity by region

Mean acorn classes over 7 years (1999-2005) from British Columbia, Washington and Oregon by major analysis groupings. Notice that 2004 was the peak acorn production year nearly everywhere. This was a year with above average late summer precipitation on a regional basis.






Riparian areas include all trees growing adjacent to, or in a wetland of any kind. Trees west of the Cascade crest are in the maritime group and those east of the Cascade crest are in the interior group. Cultivated areas include lawns, gardens, managed pastures, etc. These areas receive competition control at a minimum, but may also be irrigated or fertilized. Juvenile trees are trees less than 10 cm. diameter at breast height (4.5 ft or 1.3 m).

Acorn Class by Geographic and Analysis Grougps
Grand mean acorn class over the entire period of record for the major analysis groups. Included are the groups depicted in the graphs above as well as several more that were too small or had too short of a record to be graphed.
Mean AcornClass
Years in Record
Number of trees
British Columbia
Vancouver Island woodland (north)
Vancouver Islandwoodland (south)
Bald Hills
Columbia Gorge upland
southern Oregon savanna
southern Oregon woodland
Willamette Valley savanna
Willamette Valley woodland
Bald Hills
Chehallis woodlands
Columbia Gorge forest
Columbia Gorge woodland
Everett soil
Klickitat River upland
north Puget Sound
north Puget Sound salt marsh
rocky bald
Spanaway soilsforest
Spanaway soils savanna
Spanaway soils woodland
cultivated areas
riparian, interior
riparian, maritime
Percent crown contact affects acorn productivity

Percent crown contact is a good predictor of acorn productivity because it measures crown competition. It is defined as the percent of the circumference of the canopy in contact with trees of similar or greater height. In 1999 we counted all acorns for 18 trees. We divided the rees into two equal groups. The first group had 0-20% crown contact and the second group had 30-99% crown contact. Although the sample size is not large, trees with high crown contact had much lower mean acorn production than trees with low crown contact.

Percent crown contact by acorn code

The sample of trees coded for acorn production is much larger than the sample of trees where acorns were also counted. Here the average percent crown contact for trees assigned codes 1, 2, and 3 is given. Lower percent crown contact is found in code 3 trees than code 1 or 2.

Effects of age

Tree age influences acorn production. We found no acorns on trees less than 20 years old at breast height. From age 20 up to 60-80 years we see in the data a decline in the trees in code 1 (non-producing trees) and a rise in trees rated acorn code 2 and 3; thus, average productivity increases from age 20 to 80. Beyond age 80 there is no apparent change in productivity with increasing age. The oldest tree in our sample in 1999 (>300 years) produced no acorns. We need to sample more old trees to know if acorn production declines with age in old trees.

Breast height age is used because that is the age of the tree where it is increment cored (1.3 m).

Site description

Most of the oak sites are in flat to gentle topography. Mean area elevations range from 2 meters at Oak Bay to over 400 meters in the Columbia Gorge (OR and WA) and S. F. Skokomish River (WA). Individual trees in the Columbia Gorge area (south of Mosier) and in the Klickitat are over 600 meters high. Precipitation ranges from a low of 33 cm near Yakima, WA, to a high of 318 cm. in the South Fork Skokomish River (WA). Details about site descriptions.

Tree dimensions

Sampled oak trees ranged in diameter, height, and crown characteristics.

Water availability

Most of our trees grow in natural, non-irrigated settings. These trees are dependent on precipitation and soil water storage for their water so climate and soil characteristics are especially important to their productivity. The second largest group of trees comes from wetland edges where trees have adequate drainage, but also have access to year-around water. Twelve trees grew in wetlands where water is always available, but soil drainage is poor. So far our sample trees in wetlands have produced few acorns even though some of them are large and appear to be healthy. We have also included trees growing in cultivated circumstances such as city parks and golf courses. These trees are irrigated and some are fertilized. They have generally out-produced trees growing on the same soil type, but on non-irrigated sites. Trees in parks which were irrigated weekly or on the edge of wetlands had the highest average acorn codes. Details about water availability.

Ecological setting

Most of our trees grow in natural settings such as grassland, savanna, woodland or forest, but 94 come from urban or agricultural settings. So far, it is difficult to see an effect of these categories on acorn production. The urban and agricultural classes alert us to the possibility that cultural practices such as irrigation, fertilization or grazing or competition control may have been applied in the past or are currently being used. Most of these practices would probably benefit the tree and might be expected to increase acorn production. Urban trees might also suffer negative impacts from pollution, or digging or paving over root systems. Details about ecological settings.

Dominance class

Dominance class is a way foresters use to express the relative crown position in the stand that the tree is currently growing under. It is a fairly good index of competition with other trees. A dominant tree has neighbors, but it is clearly larger or taller than any of the surrounding trees. A codominant has neighbors about the same size as it is. An intermediate tree is surrounded by larger trees, but has not yet been overtopped by them. Suppressed trees have been overtopped by their neighbors and thus are growing in the shade. Understory trees have regenerated under the tree canopy and so might be thought of as a special category of suppressed trees. Open grown trees have had no neighbors in contact with its crown throughout its life span. Open grown, dominant and codominant trees out-produced the other categories in 2000. Details about dominance class.

Health class

Trees that were judged to be unhealthy in 2000 had a lower mean acorn code than healthy or very healthy trees. Unhealthy trees have one or more of much visible rot, sparse foliage, major dead or broken limbs. In general acorn productivity does not seem much affected until the tree shows extreme health problems, but as we get more data we may see new relationships depending on the kind and degree of damage or disease. Details about health class.

Mistletoe abundance

Mistletoe is not present in Washington and northernmost Oregon. Our sample of trees with mistletoe is still too small to be able to say much about its effect on acorn productivity. Our data do not show a clear impact of increasing number of brooms. Details about mistletoe.

Crown shape

Tree crowns can be classified into 3 major shapes: columnar, inverted vase, and mushroom (see the Background section for sketches and examples of these shapes). Shape class appears to be a good predictor of acorn productivity. Mushroom shaped trees commonly produce more acorns than the other classes and inverted vase shaped trees usually produce the least. Mushroom shaped trees grow in open situations. Inverted vase shaped trees grow in forests where light competition has eliminated all but the uppermost branches. Columnar shaped trees come about in two ways. Most are in a growing situation between open-grown and forest. They are still able to get some side light but not as much as an open grown tree. Also young trees with strong apical dominance are often columnar shaped even in open grown situations. Only after height growth begins to slow do the trees expand out into the mushroom shape. Details about crown shapes.

Understory vegetation

The understory vegetation or condition was coded for each sampled tree. Sampled trees were growing in several different vegetative settings; grass and shrubs were the most common understory codes. Understory components are listed in order of dominance, but frequently two or more components (e.g. grass and shrubs) are present in large quantities. Grass was listed first as a dominant understory plant in most of our sample trees. Shrubs dominated understories are also very common. Bare understories usually indicate disturbance. Details about understory vegetation.

Soil series

The Washington trees were distributed across several soil series (Oregon sites have not yet been classified by soil series). In the table above the soil series are arranged by the percentage of trees in acorn code 1 so the soils at top of the list had the lowest percentage of non-producing trees and soils at the bottom had the highest percentage of non-producing trees. The abbreviations are: si=silt or silty, CL=clay, lo=loam, sa=sand or sandy, gr=gravelly, st=stony, ex=extremely. These are 1999 data.

The finer-textured soil (Prather) and the soil without gravel (Nisqually) sorted out near the top (highest productivity). Sequim very gravelly sandy loam is coarse textured but in an irrigated setting so it also appears near the top of the list. Carstairs is a coarse textured soil but is located in an area of higher precipitation than Everett and Spanaway and also sorts out above them (higher acorn productivity). The extreme habitats all appear at the bottom of the list. The wetlands sites produced poorly in 1999 as did the extremely dry sites with Fricaba and Waketickeh (steep, south facing, very rocky sites in the Olympic Mountains). The site with San Juan soil occurred about 2-3 meters above sea level and the trees may be influenced by salty soil water. Details about the soil series.

Effects of Underburning

We had information on date of last fire for 83 trees. These fires were all underburns but we don't know their severity in many cases other than they did not kill the trees. In general there is a high percentage of trees given acorn code 1 (non-producing trees) in the year after the burn. Trees appear to recover to better than average productivity in subsequent years. The data for burned and unburned trees come from trees on Spanaway, Everett and Fitch soils which are very similar. These data are for 1999.

In 1999 many trees had produced a crop before being underburned. Even so there appears to be a decrease in productivity. There appears to be a large drop in productivity for trees underburned in 1998 (and sampled 1 year later in 1999). Data for 1995 and 1997 underburns show recovery to average levels. Data for 1994 still shows a depression of productivity. Data for 1993 and 1989 show recovery to better than average conditions.

Percent of trees in each acorn code by date of last fire
Acorn code 1989 1993 1994 1995 1997 1998 1999 Unburned
1 17 18 60 36 40 71 54 48
2 33 45 40 18 40 29 36 36
3 50 36 0 45 20 0 11 16
n 6 11 15 11 5 7 28 119
Acorn productivity higher in upper crown
Mean acorns counted by acorn code
Acorn code*  n  Mean counted acorns top Mean counted acorns bottom Mean counted acorns Range of counts
1 9 5 4 10 0-88
2 5 25 10 36 4-110
3 4 350 135 485 215-729
4 0 -- -- -- --

A subsample of trees that were given acorn codes were carefully counted. Four trees were counted with the aid of a lift truck ("cherry picker"). Others were counted using binoculars. Both methods are very time consuming. These are the results of those counts. One tree with 88 acorns was miscoded code 1 however all the acorns were born on the top of that tree and the tree was fairly tall making counting difficult. This serves to underscore the need to observe very carefully when coding the acorn productivity. The other 8 trees given code 1 had no acorns.

Counts of acorns on code 2 trees ranged from 4-110 in 1999.

Counts of trees rated 3 had 215-729 acorns in 1999.

We found no code 4 trees in 1999.

Acorn density related to code and position in tree crown

The impression of productivity an observer forms relates more to acorn density than actual acorn numbers. For this table the the number of acorns counted was divided by the external crown surface area to give a measure of acorn density. These data also show that more acorns are produced near the top of the canopy than the bottom and that trees rated code 1 typically have a smaller external surface canopy than trees rated code 3. Not only do trees which were rated code 3 produce more acorns per square meter of canopy, they also produced more canopy making them much more productive on a per tree basis.

Readers should bear in mind that the sample size was small and that these numbers may change with more data. Even so this illustrates the magnitude of the difference between acorn codes. These data are from 1999.

Acorns/square meter by acorn code
Acorn code  n  Mean acorns/sq. m top Mean acorns/sq. m bottom Mean acorns/sq.m Mean tree surface area (m)
1 9 0.04 0.03 0.03 180
2 5 0.30 0.08 0.19 233
3 3 2.64 0.72 1.60 368
4 0 -- -- -- --

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USDA Forest Service - GenSilv Team
Last Modified: Monday, 16 December 2013 at 14:18:50 CST

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