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Land and Watershed Management

Land and Watershed Management Program
Pacific Northwest Research Station
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United States Department of Agriculture Forest Service.

Genetic and Silvicultural Foundations for Management

Oak Studies

Can Oak Trees Respond to Release from Overtopping Conifers?

Connie Harrington, Warren Devine
Pacific Northwest Research Station, Olympia, WA
Email: charrington[at] or wdevine[at]


Oregon white oak or Garry oak (Quercus garryana) woodlands provide unique habitat for many plant and animal species in the Pacific Northwest, but these habitats are rapidly disappearing due to increased urban and agricultural land use and the encroachment of conifers in oak stands. More than a century without frequent fire has allowed conifers (mainly Douglas-fir) to establish in oak savannas and woodlands; over time the conifers overtop the oaks. Many of the largest oak stands remaining in western Washington are on Fort Lewis, but most of these stands have been invaded by Douglas-fir which are now much taller than the oaks and growing more rapidly. The shade-intolerant oaks have survived in these stands due to repeated thinnings that have periodically removed conifers, increasing the amount of sunlight reaching oaks. However, given the current disparity in height between the dominant conifers and the suppressed oaks, we suspect the oaks will not survive for long without appropriate management. This study investigates whether individual suppressed oaks on Fort Lewis will respond to release from overtopping conifers. Based on results from this study, future research may involve release treatments applied at the stand level.


The study’s primary objectives are to determine:

  • Will overtopped oaks respond to release treatments by increasing growth rates or by expanding crowns?
  • What pre-treatment tree and stand characteristics can be used to predict response to release?
  • Does response differ between 3 levels of release?

We are also looking at effects on acorn production, oak regeneration, and understory vegetation.


The experimental design is three levels of release applied to 18 trees in each of 4 stands overtopped by Douglas-fir (total of 90 plots). The treatments include:

Control Treatment-
A stand-level operational thinning designed to reduce competition between overstory Douglas-fir. On average, 2 conifers were cut around each study oak.

Half Release Treatment-
The stand-level thinning described in the control treatment plus the removal of all conifer close to each oak. On average, 6 conifers were cut around each study oak.

Full Release Treatment-
The removal of all conifer competitors within in the vicinity of the oak. On average, 15 conifers were cut around each study oak.

Observations so far:
  • Release treatments were implemented with minimal logging damage to the oaks.
  • There has been no oak mortality or windthrow since release.
  • Diameter growth of oaks is increased by the release treatments.
  • Release treatments increase acorn production (see graph below left).
  • Released oaks are expanding their crowns by sprouting new (epicormic) branches (see graph below right).
  • Growth of naturally established oak regeneration is increased by both release treatments.
  • These early results show overtopped oaks respond to release, and the response is more consistent with full release.

Acorn production in the fourth year after release treatments.

Percentage of trees with new epicormic branches two years after release at four sites.

Oregon white oak in the process of being released from overtopping Douglas-fir. This photo is from a five-acre release at Glacial Heritage Preserve in Thurston County, WA.

Oak trees with a large number of living branches respond to release the fastest. This was one of the fastest growing trees in the study during the first five years after it was released.

Published results from our Oregon white oak release research (Adobe pdf format):
We would like to thank the following people who helped with various stages of this research:

Fort Lewis Forestry (provided funding and logistical support for the oak release study)

  • Gary McCausland (retired)
  • Jeff Foster
  • Jim Rohde (retired)

Hugh Snook (BLM)

Christel Kern (FS Northern Research Station)

The many current PNW employees who assisted



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

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