USDA Forest Service

Land and Watershed Management

Land and Watershed Management Program
Pacific Northwest Research Station
Olympia Forestry Sciences Laboratory
  3625 93rd Ave. SW
Olympia, WA 98512
(360) 753-7747
Corvallis Forestry Sciences Laboratory
  3200 SW Jefferson Way
Corvallis, OR 97331
(541) 750-7250
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United States Department of Agriculture Forest Service.

Genetic and Silvicultural Foundations for Management

Oak Studies

Oak Root Research

Although much of our oak research is occurring aboveground, we are also interested in what happens below ground because understanding the morphology (size and distribution) of oak roots may help us understand tree responses to environmental factors or to management activities. For example:

  • Where in the soil profile are the roots located?

  • Do root systems differ with age or soil type?

We conducted a study in 2003 to document the root morphology of Oregon white oak growing on the rocky soils common in the south Puget Sound area (our oak were growing on the Spanaway series). In this study we used an Air-Spade® ( to excavate the root systems of oaks ranging in above-ground diameter from 1 to 57 cm.

Most of the oaks had a central taproot, but the size, taper, and orientation of the taproot varied widely among trees. The majority of the lateral roots were located within 40 cm of the surface where the soil contained fewer rocks, was finer in texture, and thus, had greater water-holding capacity than deeper soil layers. The vertical penetration of roots was partially to totally restricted by an extremely gravelly soil horizon that began at a depth of approximately 70 to 80 cm. Very few roots penetrated beyond a depth of 150 cm. Thus, the oaks we studied were competing for water with shallow-rooted trees, shrubs, and herbaceous vegetation. They were likely drawing little water from deeper soil horizons.

For views of roots of some other species of western oaks, see

The roots of a large oak with a decaying Douglas-fir stump in the foreground. The oak taproot tapers off rapidly around a depth of one meter (scale in photo is a meter stick). Most of the large lateral roots are near the surface of the soil where the texture is finer and more soil water is available.

The roots of a 25-year old oak tree. The taproot turns sharply about 80 cm below ground and then grows horizontally, likely in response to a rocky soil layer.

Often, oaks were relatively young sprouts from older, sometimes well-developed root systems. When the aboveground part of an oak is killed by fire, the roots can survive and sprout new stems.

See how the roots grew around the rock in the lower center of the photo.

USDA Forest Service - GenSilv Team
Last Modified: Saturday, 15 October 2016 at 18:38:00 CDT

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