compensatory effects of overstory and understory
TRANSCRIPT
Compensatory effects of overstory and understory competition attenuate conifer
seedling growth responses in 40- to 70-year-old 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 Kimmins1977, 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 overstorydensities established in summer 2000
• conifer seedlings planted in early 2001 at 3-m spacing
Special thanks to:
Site 3
Site 2Site 1
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.
Thinned stand
ShelterwoodClear-cut
3 levels of overstorycompetition were selected at each site (nine 1.5-ha plots)
18 seedlings per conifer species were randomly selected in buffer areas of each plot Douglas-fir
(Pseudotsugamenziesii)
western hemlock (Tsuga heterophylla)western redcedar (Thuja plicata)
6 seedlings per species and plot were randomly assigned 1 of 3 levels of understory competition, defined by area of vegetation control (AVC):
3 msprayed areanon-sprayed area
3 m
conifer seedling
4.5 m2
9 m2
sprayed0 m2
0% AVC 50% AVC 100% AVC
100% AVC
50% AVC0% AVC
3 levels of understorycompetition (area of vegetation control)
Variables measured (2002-04):• seedling growth• overstory & understory covers• soil water content• D. fir needle nitrogen
vertical densitometer for overstory cover
hydrosense for soil water content
Experiment comprised a total of 162 seedlings per conifer species:
• 6 seedlings per understorylevel
• 3 understory levels• 3 overstory levels• 3 sites
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:
161total146error AB
1initial size covariate4overstory x understory interaction
2understory competition level (B)4error A2overstory competition level (A)2blocksdfSource
field computer for data collection
Overstory density
clearcut shelterwood thinned
Ove
rsto
ry tr
ee c
over
(%)
0
10
20
30
40
50
60
70
20022004
Measurement year
BA (m2/ha) 2 12 26
• Overstory cover (light availability) varied systematically among the manipulated levels
0
20
40
60
80
100
0
1
2
3
50
100
Und
erst
ory
vege
tatio
n co
ver (
%)
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Understory vegetation
Clearcut
ShelterwoodThinned
• Initially, understory cover varied among overstorylevels
• Later, it decreased linearly with AVC and was not related to overstory level
Douglas-fir responded only to specific combinations of overstory and understorycompetition:
• significant overstory x understory interaction
• small growth increase for shelterwood vs. thinned stands for all levels of AVC (attenuated response)
• superior performance in clear-cuts if AVC ≥ 50%
0
100
200
300
400
500
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Douglas-fir
Clearcut
Shelterwood
Thinned
Western hemlock responded to a range of overstory and understory densities:
• additive overstory and understory effects
• good performance in shelterwoods if AVC ≥50% (attenuated response)
• superior performance in clear-cuts if AVC ≥ 50%
0
300
600
900
1200
1500
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Western hemlock
Clearcut
Shelterwood
Thinned
Western redcedar responded primarily to reductions in understory density:
◦ marginal effects of overstory level
◦ growth peaked at AVC = 50% in shelterwoodstands0
20
40
60
80
100
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Western redcedar
Clearcut
Shelterwood
Thinned
Soil water
MonthApr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan
Soil
wat
er c
onte
nt (%
)
10
20
30
0 50100
area of veg. control (%)
200420032002
• soil water during mid-summer increased linearly with AVC
• no relationship with overstorycompetition level
Area of vegetation control (%)0 50 100N
eedl
e N
con
cent
ratio
n (%
)
1.2
1.4
1.6
1.8
2.0
2.2
clearcutshelterwood
thinned
• greater in clear-cuts than in shelterwood or thinned stands
• increased linearly with AVC
• explained 78% of variation in 3rd-year seedling size
Douglas-fir needle nitrogen
Douglas-fir seedling size correlated with needle nitrogen
0
100
200
300
400
500
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Douglas-fir
Clearcut
Shelterwood
Thinned
Douglas-fir seedling size correlated with needle nitrogen
Area of vegetation control (%)0 50 100N
eedl
e N
con
cent
ratio
n (%
)
1.2
1.4
1.6
1.8
2.0
2.2
clearcutshelterwood
thinned
0
100
200
300
400
500
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Douglas-fir
Clearcut
Shelterwood
Thinned
Douglas-fir seedling size correlated with needle nitrogen
Area of vegetation control (%)0 50 100N
eedl
e N
con
cent
ratio
n (%
)
1.2
1.4
1.6
1.8
2.0
2.2
clearcutshelterwood
thinned
0
100
200
300
400
500
0
1
2
3
0
50
100
Stem
vol
ume
(cm
3 )
Year
s sin
ce tr
eatm
ent
Area of veg. control (%)
Douglas-fir
Clearcut
Shelterwood
Thinned
Needle nitrogen concentration (%)1.4 1.6 1.8 2.0D. f
ir 3r
d-ye
ar v
olum
e (c
m3 )
0
100
200
300
400
500r2=0.78clear-cutshelterwoodthinned
Conclusions:
• Douglas-fir growth was intolerant of overstorycompetition, while hemlock and cedar demonstrated some tolerance
• All species responded to reductions in understorycompetition (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?