Genetic and Silvicultural Foundations for Management
Natural Regeneration in Thinned Douglas-fir
Belowground Competition Influences Sapling
Growth and Morphology
Warren D. Devine and Timothy
Regenerating Douglas-fir in multi-aged stands
The recent emphasis on managing Pacific Northwest
Douglas-fir forests for multiple objectives, including wildlife
habitat and old-forest aesthetics, has created a renewed interest
in managing stands with trees of multiple age classes. While some
types of commercial thinning in even-aged second-growth Douglas-fir
forests can produce characteristics of uneven-aged stand structure,
long-term development of multiple cohorts requires successful establishment
and continued growth of conifer regeneration. It is well documented
that growth and survival of Douglas-fir regeneration in the understory
require at least a moderate amount of sunlight (40% or more full
sun). However, belowground resources (soil water and nutrients)
also are vital to success of this regeneration, and belowground
competition from overstory trees and understory vegetation in multi-aged
stands are not well understood. This study was designed to learn
how these types of root competition affect Douglas-fir regeneration.
Where small canopy gaps were created by
a previous variable-density thinning treatment,
Douglas-fir has regenerated in the available sunlight.
Over time, crowns of overstory trees will expand to fill these
gaps, and the vigor of saplings will decline.
The study took place in three stands of recently
thinned 60- to 80-year-old Douglas-fir on gravelly glacial-outwash
soils on Fort Lewis Military Reservation near Tacoma, WA. To evaluate
the effects of root competition on naturally regenerated Douglas-fir
saplings, we installed 48 plots where two different types of belowground
competition were either present or excluded.
On a subset of the plots, tree root competition
was excluded by digging a trench around the plot perimeter. On another
subset of plots, the effect of understory root competition was excluded
by clipping all understory vegetation back to ground level every
two weeks (without leaves this vegetation provided virtually no
root competition). By comparing plots with or without each type
of competition, we evaluated the effects of each on saplings and
We used a trencher to trench around selected study plots.
This severed all tree roots, leaving saplings on these plots free
from belowground competition of overstory trees.
On a subset of plots, we removed all understory vegetation.
Saplings on these plots were free of belowground competition from
For Douglas-fir saplings
growing in the partially shaded understory of a thinned Douglas-fir
- Root competition from overstory trees reduced
Douglas-fir sapling height growth by 29% and diameter growth by
22% during the study. Roots from understory vegetation had no
detectable effect on sapling growth.
- Root competition from overstory trees also had
a negative effect on sapling foliar production, bud formation,
bud size, and retention of older needles. However, morphology
of individual needles (length, thickness, and distance between
needles) was influenced by shade, not by root competition.
- Soil water content in mid- to late-summer was
significantly reduced by roots of overstory trees; competing understory
vegetation had a smaller negative effect on soil water.
- Although foliar nitrogen levels were generally
low for the saplings, sapling growth did not increase in those
treatments where foliar N concentration was increased. Thus, in
a partially shaded understory environment, N was not the most
growth-limiting resource: growth of Douglas-fir saplings was more
limited by light and water availability.
- In the low light of the understory (less than
30% full sunlight), understory vegetation provided relatively
little competition. Management of this vegetation would likely
produce little benefit.
- Stand thinning provided enough understory light
and soil moisture for Douglas-fir seedling establishment, but
their continued development depends on future treatments to further
reduce the density of the overstory. This could be in the form
of thinning or creating gaps around young trees.
Stem growth was greater for the saplings
that had no competition from roots of overstory trees.
In addition to the effects of root competition,
the amount of available sunlight was positively related to sapling
A summary of the information on this page is available
for download as a
one-page pdf document.
There are two journal articles describing the results
of this study. The first one, in
Canadian Journal of Forest Research, describes belowground
competition effects on sapling growth, soil water, and sapling N
status. The second one, in
the journal New Forests, describes how sapling morphology
was affected by belowground competition. If you cannot access the
full articles from these links, you can request copies by emailing
We thank Fort Lewis Forestry for providing funding
and logistical support for this research.
Warren D. Devine wdevine[at]fs.fed.us
Timothy B. Harrington tharrington[at]fs.fed.us
Olympia Forestry Sciences Laboratory
3625 93rd Ave. SW, Olympia, WA 98512