Wind & Soil – Roots &
Douglas-fir
by Steve Minta
Douglas-fir is a species providing a good example of
integrating knowledge of trees, soil, roots, and wind.
These interacting variables need to be considered when
assessing construction activities or any other habitat
modification. Kala Point soils, particularly the Cassolary
series, have forest land grades of very good to excellent
for eastern Jefferson County, more because of favorable pH
and high drainage characteristics than because of soil
fertility. These sandy loam soils promote robust and
extensive root structure.
In an evolutionary sense, the root system required for
water and nutrient uptake exceeds the requirements
necessary for wind resistance, and this has resulted in
redundancy of root structures. Throughout the species
range, Douglas-fir evolved to withstand high winds,
season-long snow loads, ice storms, steep slopes, poorly
drained soils, and seasonal flooding. In contrast, the
conditions of Kala Point are very benign for all these
variables. In addition, a forested environment has additive
benefits because trees in groups result in a dramatically
increased wind-firmness.
Growth
Rates
The species’ vertical growth is slow during the first
decade, while root growth is rapid and extensive. Under
good conditions, vertical growth typically reaches its peak
during the 3rd to 5th decade (up to several feet/year),
when it begins slowing down to about 6 inches near the end
of its first century, and 3 or 4 inches/year by age 120.
Radial growth (diameter) continues throughout its life,
which is why you cannot correlate tree rings with vertical
growth or canopy growth.
Roots
and Soils
Soil constraints strongly influence the development of
Douglas-fir root systems (e.g., wet soils vs.
well-drained). The taproot is very strong and massive, but
there are also secondary taproots, along with many skeletal
roots and sinkers. There is often a rather compact and
dense root system in the immediate vicinity of the stem,
and rope-like lateral roots penetrating for a long distance
in the soil (10-13 ft.). The maximum depth recorded for
Douglas-fir roots is around 10 ft. Lateral extension is
very fast: lateral roots of 4-year-old trees can reach 12
ft. The figure below represents root systems produced by a
variety of Douglas-fir tree conditions and ages.
When people come across a fallen tree in the forest, they
most often observe it has a plate-like root system instead
of deep tap roots and descending oblique roots. This
happens when trees grow in poorly drained soils, especially
with a high water table or an abrupt, saturated soil
horizon that is shallow. The roots resist penetrating such
soil conditions, therefore the roots resembles a flattened
rootwad or plate of knotted and gnarled roots that weaken
the tree’s stability. Most Kala Point soils are
favorable to deep and wide root penetration.
Roots are frequently damaged by construction activity, and
it is nearly always one of the many lateral roots that is
severed. The main concern is that root wounding invites
infection, so if the root is torn, it should be sawn
cleanly and perpendicular to the root length. With a few
exceptions, the majority of tree species are reported to be
susceptible to wound infections. However, relatively low
susceptibility to wound decay has been noted for pines,
Douglas-fir and several species of true firs. Once the root
system has refilled with soil, there is little that can be
inferred about the damage. Soil probing and coring (e.g.,
Bohm’s sector method) is very labor and time
intensive, and it reveals little.
Trees,
Roots and Wind
Coastal Douglas-fir ranks among the most wind-throw
resistant of commercial conifers. Critical factors behind
these differences is the lower drag coefficient of
Douglas-fir foliage, the dampening characteristics of the
crown branches, and favorable ratios of crown with tree
height and basal width. Wind-throw is greatest on sites
with poorly drained soils, in over-dense stands, or where
winds are concentrated by topography. Indeed, the most
common management regime in wind-prone areas of coastal
Oregon, Washington and British Columbia is to plant
Douglas-fir following a clearcut harvest.
Soil conditions that restrict rooting depth/growth (e.g.,
bedrock, water-table) or decrease soil shear strength
(e.g., waterlogging) are more prone to wind damage. Deeper
well-drained soils that are less prone to wind-throw
increase management flexibility in much the same way as
exposure to high wind risk across a region. A more closed
canopy seems to improve wind resistance by increasing the
damping effect of swaying as a result of the crowns being
in contact with each other and provides a more favorable
ratio between the aerial parts and the roots. Studies
confirm that wind-throw results from trees that have been
long protected from wind are suddenly being exposed, rather
than from excessively strong or unusual winds. If trees are
already unstable, thinning increases the risk of wind
damage by allowing more wind to penetrate the canopy,
creating more turbulent airflow and reducing the sway
dampening effect of neighboring trees.
It has been observed in studies of wind-throw occurrence
that trees grown in more open stands are more wind-firm
than those grown in dense stands. For wind vulnerability,
the most important consideration for a silviculturist would
be an analysis of height to diameter ratio with
consideration of crown and canopy characteristics.
Key
Sources from Primary Literature
Canham, C. D., M. J. Papaik, and E. F. Latty. 2001.
Interspecific variation in susceptibility to windthrow as a
function of tree size and storm severity for northern
temperate tree species. Canadian J. Forest Research.
31(1):1-10.
Curt, T., E. Lucot, and M. Bouchaud. 2001. Douglas-fir root
biomass and rooting profile in relation to soils in a
mid-elevation area. Plant and Soil 233:109-125.
England, C. J. Baker, and S. E. T. Saunderson. 2000. A
dynamic analysis of windthrow of trees. Forestry 73:
225-238.
Flewelling, J. 2001. Height-age curves for planted stands
of Douglas-fir, with adjustments for density. SMC Working
Paper No. 1, College of Forest Resources, Univ. Washington,
Seattle.
Han, H. S., and L. D. Kellogg. 2000. Damage characteristics
in young Douglas-fir stands from commercial thinning with
four timber harvesting systems. Western J. Applied Forestry
15(1):27-33.
Kramer, M. G., A. J. Hansen, M. L. Taper, and E. J.
Kissinger. 2001. Abiotic controls on long-term windthrow
disturbance and temperate rain forest dynamics in southeast
Alaska. Ecology 82(10):2749-2768.
Kuiper, L. C., and M. P. Coutts. 1992. Spatial disposition
and extension of the structural root system of Douglas-fir.
Forest Ecology & Management 47:111-125.
Mason, W. L. 2002. Are irregular stands more windfirm?
Forestry 75(4):347-355.
Mitchell, S. J. 2000. Stem growth responses in Douglas-fir
and Sitka spruce following thinning: Implications for
assessing wind-firmness. Forest Ecology & Management
135(1-3): 105-114.
Moore, J. R., and D. A. Maguire. 2004. Natural sway
frequencies and damping ratios of trees: concepts, review
and synthesis of previous studies. Trees - Structure &
Function 18:195-203.
Moore, J., and B. Gardiner. 2001. Relative windfirmness of
New Zealand-grown Pinus radiata and Douglas-fir: A
preliminary investigation New Zealand Journal of Forestry
Science 31:208-223.
Quine, C. P. 2000. Estimation of mean wind climate and
probability of strong winds for wind risk assessment.
Forestry 73: 247-258.
Rowan, C. A., S. J. Mitchell, and H. Temesgen. 2003.
Effectiveness of clearcut edge windfirming treatments in
coastal British Columbia: Short-term results. Forestry
76(1):55-65.
Ruel, J.-C., D. Pin, and K. Cooper. 2001. Windthrow in
riparian buffer strips: Effect of wind exposure, thinning
and strip width. Forest Ecology & Management
143(1-3):105-113.
Silen, R. R., D. L. Olson, and J. C. Weber. 1993. Genetic
variation in susceptibility to windthrow in young
Douglas-fir. Forest Ecology & Management 61:17-28.
Sundstrom, E., and M. Keane. 1999. Root architecture, early
development and basal sweep in containerized and
bare-rooted Douglas fir. Plant and Soil 217:65-78.
Vasiliauskas, R. 2001. Damage to trees due to forestry
operations and its pathological significance in temperate
forests: a literature review. Forestry 74:319-336.
Wilson, J. S. 2004. Vulnerability to wind damage in managed
landscapes of the coastal Pacific Northwest. Forest Ecology
& Management. 191(1-3):341-351.
Wilson, J. S., and C. D. Oliver. 2000. Stability and
density management in Douglas-fir plantations. Canadian J.
Forestry Research 30:910-920.
Wilson, J. S., and P. J. Baker. 2001. Flexibility in forest
management: Managing uncertainty in Douglas-fir forests of
the Pacific Northwest. Forest Ecology & Management
145:219-227.
Zhu, Jiao-jun, Liu Zugen, Takeshi Matsuzaki, and Gonda
Yutaka. 2004. Review: effects of wind on trees. J. Forestry
Research 15(2):153-160.