Compensatory effects of overstory and
understory competition attenuate conifer
seedling growth responses in 40- to 70-yearold Douglas-fir stands
Timothy B. Harrington, USDA Forest Service,
PNW Research Station, Olympia, WA
Douglas-fir seedling growing in the
absence of overstory competition
Previous research:
• Douglas-fir seedlings require 40% of full
sunlight to survive, but maximum development
occurs under full sun (Mailly and Kimmins
1977, Drever and Lertzman 2001)
• Western redcedar seedlings require only 10%
of full sunlight to survive, and their maximum
development occurs at 30% of full sunlight
(Wang et al. 1994, Drever and Lertzman 2001)
• Recent research to determine overstory effects
in stands of western hemlock (Mitchell 2001)
and Douglas-fir (Barg and Edmonds 1999,
Brandeis 1999) indicated that availability of
belowground resources differed little among
overstory densities
• As a result of these and other findings, Mitchell
(2001) concluded that light was the primary
factor limiting growth of conifer seedlings
Objectives:
• Characterize relationships of conifer seedling growth to
various levels of overstory and understory competition.
• Determine how differences in availability of light, soil water,
and nitrogen facilitate observed responses.
Douglas-fir seedling growing in the
presence of overstory competition
The experiment was
conducted at the
Overstory Density Study,
Capitol Forest, Olympia,
WA:
• wide range of overstory
densities established in
summer 2000
• conifer seedlings planted
in early 2001 at 3-m
spacing
Special thanks to:
Site 1
Site 2
3 sites:
• 40- to 70-yr-old D. fir
• 180-550 m elev.
• D. fir SI50 41 m
• silty loam soils
• 200 cm annual ppt.
Site 3
Shelterwood
Clear-cut
3 levels of overstory
competition were
selected at each site
(nine 1.5-ha plots)
Thinned stand
western redcedar (Thuja plicata)
18 seedlings per
conifer species were
randomly selected in
buffer areas of each
plot
western hemlock (Tsuga heterophylla)
Douglas-fir
(Pseudotsuga
menziesii)
6 seedlings per species and plot were randomly
assigned 1 of 3 levels of understory competition, defined
by area of vegetation control (AVC):
3m
0% AVC
0m
2
3m
non-sprayed area
50% AVC
4.5 m
2
sprayed area
100% AVC
2
9m
sprayed
conifer seedling
0% AVC
50% AVC
3 levels of understory
competition (area of
vegetation control)
100% AVC
Variables measured (2002-04):
• seedling growth
• overstory & understory covers
• soil water content
• D. fir needle nitrogen
vertical densitometer for overstory cover
Experiment comprised a total
of 162 seedlings per conifer
species:
• 6 seedlings per understory
level
• 3 understory levels
• 3 overstory levels
• 3 sites
hydrosense for soil water content
Statistical analysis:
• seedling=experimental unit
• RCB with 3 blocks (sites)
• split-plot (understory levels within overstory levels)
• covariate: initial size
• orthogonal polynomials to test for linear & quadratic effects
ANOVA for seedling growth:
field computer for data collection
Source
df
blocks
2
overstory competition level (A)
2
error A
4
understory competition level (B)
2
overstory x understory interaction
4
initial size covariate
1
error AB
146
total
161
Overstory density
Overstory tree cover (%)
70
60
50
Measurement year
2002
2004
• Overstory cover (light
availability) varied
systematically among the
manipulated levels
40
30
20
10
0
2
BA (m /ha)
clearcut
2
shelterwood
thinned
12
26
)
tation cover (%
e
g
e
v
ry
to
rs
e
Und
Understory vegetation
100
• Initially, understory cover
varied among overstory
levels
80
60
Thinned
Clearcut
1
50
Area
of ve
g. co
n
trol (
%)
100
0
tre
at
m
2
si
nc
e
0
Shelterwood
Ye
ar
s
20
3
en
t
40
• Later, it decreased linearly
with AVC and was not
related to overstory level
Douglas-fir
Douglas-fir responded only to
specific combinations of
overstory and understory
competition:
Clearcut
• significant overstory x
understory interaction
Shelterwood
400
Thinned
300
200
Ye
ar
ss
in
ce
tre
at
m
en
t
3)
m
(c
e
m
lu
o
v
Stem
500
3
100
2
0
100
1
50
Area
of ve
g. co
ntrol
(%)
0
0
• small growth increase for
shelterwood vs. thinned
stands for all levels of AVC
(attenuated response)
• superior performance in
clear-cuts if AVC ≥ 50%
Western hemlock
Western hemlock responded
to a range of overstory and
understory densities:
Clearcut
1500
Shelterwood
• additive overstory and
understory effects
Thinned
900
sin
ce
tre
at
m
en
t
600
3
300
2
0
100
1
50
Area
of ve
g. co
ntrol
(%)
0
0
Ye
ar
s
m3 )
Stem volume (c
1200
• good performance in
shelterwoods if AVC ≥
50% (attenuated
response)
• superior performance in
clear-cuts if AVC ≥ 50%
Western redcedar
Shelterwood
Clearcut
100
80
Thinned
60
◦ marginal effects of
overstory level
tre
at
m
en
t
40
3
20
2
100
1
50
Area
of ve
g. co
ntrol
(%)
0
0
sin
ce
0
Ye
ar
s
3)
m
(c
e
m
lu
o
v
Stem
Western redcedar responded
primarily to reductions in
understory density:
◦ growth peaked at AVC =
50% in shelterwood
stands
Soil water content (%)
Soil water
30
area of veg. control (%)
0
50
100
• soil water during mid-summer
increased linearly with AVC
• no relationship with overstory
competition level
20
10
2002
2003
2004
Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan
Month
Needle N concentration (%)
Douglas-fir needle nitrogen
2.2
2.0
1.8
• greater in clear-cuts than in
shelterwood or thinned stands
clearcut
shelterwood
1.6
1.4
thinned
1.2
0
50
100
Area of vegetation control (%)
• increased linearly with AVC
• explained 78% of variation in
3rd-year seedling size
Douglas-fir
Clearcut
Shelterwood
400
Thinned
300
3
100
2
0
100
1
Area
o
50
f veg
. con
trol (
%)
0
0
en
t
200
Ye
ar
ss
in
ce
tre
at
m
Douglas-fir seedling size
correlated with needle nitrogen
m3 )
Stem volume (c
500
Douglas-fir
Clearcut
Shelterwood
400
Thinned
300
3
100
2
0
100
1
Needle N concentration (%)
Area
o
50
f veg
2.2
2.0
1.8
en
t
200
Ye
ar
ss
in
ce
tre
at
m
Douglas-fir seedling size
correlated with needle nitrogen
m3 )
Stem volume (c
500
. con
trol (
%)
0
0
clearcut
shelterwood
1.6
1.4
thinned
1.2
0
50
100
Area of vegetation control (%)
Douglas-fir
Clearcut
Shelterwood
400
Thinned
300
3
100
2
0
100
1
2
r =0.78
400
300
clear-cut
shelterwood
thinned
200
100
0
1.4
1.6
1.8
2.0
Needle nitrogen concentration (%)
Needle N concentration (%)
D. fir 3rd-year volume (cm3)
Area
o
500
50
f veg
2.2
2.0
1.8
en
t
200
Ye
ar
ss
in
ce
tre
at
m
Douglas-fir seedling size
correlated with needle nitrogen
m3 )
Stem volume (c
500
. con
trol (
%)
0
0
clearcut
shelterwood
1.6
1.4
thinned
1.2
0
50
100
Area of vegetation control (%)
Conclusions:
• Douglas-fir growth was intolerant of overstory
competition, while hemlock and cedar demonstrated
some tolerance
• All species responded to reductions in understory
competition (Douglas-fir: only in clear-cuts)
• Overstory competition effects were determined primarily
by differences in light and soil nitrogen
• Understory competition effects were determined primarily
by differences in soil water and nitrogen
Any questions?