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Yield Implications of Variable Retention Harvesting
VR Team: Mario Di Lucca, Ken Polsson,
Jim Goudie, and Tim Bogle
Research & Timber Supply BranchesB. C. Ministry of Forests, Victoria
Western Mensurationist Meeting
Victoria, July 3, 2003
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In the Fraser TSA
From a Timber Supply Perspective
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Variable Retention (VR)Impacts on Sustainable Harvest Levels
• Will VR reduce harvest levels?• If so, by how much?• What are the ecological merits of aggregated vs. dispersed retention?• What are the G&Y impacts?• What are the economic implications?
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Variable Retention (VR)• Background
– J. Franklin (UW) “New Forestry”– Clayoquot Scientific Panel (1995)– Weyerhaeuser (1998) &– TASS simulations - Goudie (1998)
• Timber supply analysts request VR volume estimates for the Fraser TSA
• Research Branch develops tools to predict VR yields of:– regenerated stands– excluding retained trees
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Strip shelterwood Uniform shelterwood
Group retentionTraditional clearcut
Retained stand age 100 years - Regenerated stand age 10
TASS Simulations (Goudie, 1998)Weyerhaeuser
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1. Simulate Actual Site
TASSTASSCutblockVariables
VRYield Curves
TSRTSR
Methods to Estimate Variable Retention Yield Curves using TASS
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1. Simulate Actual Site
TASSTASS
TSRTSR
CutblockVariables
VRYield Curves
TSRTSR
CutblockVariables
TASS TASS VRAFFunction
TIPSYTIPSY
2. Derive Relationships
VRYield Curves
SimulationVariables
Methods to Estimate Variable Retention Yield Curves using TASS
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Method 1. Simulate Actual Site in the Fraser TSA
TASS layout
60 years old cutblock
after VR harvest
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Cutblock Statistics(ArcInfo)
Cutblock area: 31.02 haRetention area: 4.38 ha
(15 groups ranging from 0.05 to 2.1 ha)Percent retention: 14%Perimeter or edge retained: 111 m/ha
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TASS Simulations
Simulate VR cutblock scenario:• Plant: 4444 trees/ha Fd on site 35 (age 0)• Grow to: age 60• Harvest to mimic cutblock layout• Plant: 1400 Cw trees/ha• Grow to: age 160 & harvest
Simulate comparable clearcut scenario & calculate:VRAF = VR vol/Clearcut vol = 0.82 (age 100)
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VRAF 14% Retention Regenerated Stand Yields
0
400
800
1200
1600
2000
2400
0 20 40 60 80 100 120 140 160 180
Stand Age (years)
Me
rch
Vo
lum
e
Previous Stand
Average Reduction1.00
0.83Clearcut VR
Clearcut vs. Regenerated (VR) Yields
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• Matrix of TASS simulations• Select important variables &• Derive VRAF equations:
VRAF(sp) = f (edge, % retention, SI, overstory age or height, etc.)
Method 2. Derive Relationships
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Method 2. Derive Relationships
Matrix of TASS simulations (1107 runs):• Site Index: 25, 30 & 35• Harvest ages: 70, 130 & 200 years• Retention level: 10, 20 & 30%• 15 rectangular group sizes: 0.01 to 4 ha• Number of groups: 1 to 9• 27 dispersed tree regimes: 20 to 240 trees/ha
Constants:
• Original stand: 5000 trees/ha FDc natural • Regenerated stand: 1200 trees/ha planted &
600 trees/ha natural • Retained groups: rectangularity of 1:6.25
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Run 1: 1 group - 1.5 ha
(15 ha)
Run 2: 152 groups - 0.01 ha
(1 ha)
Run 3: 718 groups (trees) - 0.0015 ha
(1 ha)
Partial matrix of TASS simulationsSI 30, Overstory age 70 & 10% Retention
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Regenerated Merch Volume10% Retention
0
200
400
600
800
1000
10 20 30 40 50 60 70 80 90 100 110 120
Age
Merc
h V
ol
Clearcut
Variable Run 1 Run 2 Run 3No. groups 1 152 718Group Size (ha) 1.5 0.01 0.0015Average Yield Reduction 0.911 0.729 0.718
152 groups 718 groups
Partial matrix of TASS simulations
1 group
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VRAF 10% Retention
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
10 20 30 40 50 60 70 80 90 100 110 120
Age
VR
AF
Clearcut
1 group
152 groups718 groups
Avg. Yield Reduction
Partial matrix of TASS simulations
1.00
0.91
0.72
0.73
VRAF = VR volume/Clearcut volume
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• Matrix of TASS simulations• Select important variables &• Derive VRAF equations:
VRAF(sp) = f (edge, % retention, SI, overstory age or height, etc.)
Method 2. Derive Relationships
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Select important variablesto estimate VRAF using TASS
• Species & Site Index • Overstory retained stand:
Edge length > f (group shape, size & number)
% retentionTop height /age
• Regenerated stand:Top height
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Edge Effect
Uniform shelterwood(69/ha)
Strip shelterwood(1/ha)
Group retention (1/ha)
Traditional clearcut Group retention (4/ha)
Group retention (9/ha)
Retained stand age 100 years - Regenerated stand age 10
0 m edge 118 m edge 235 m edge
200 m edge 910 m edge352 m edge
No trees will grow under the overstory canopy (black areas)TASS Simulations (Goudie, 1998) of Weyerhaeuser treatments
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VRAF declines (< 1.00) as:
• Edge length increases by:– increasing number of groups
– decreasing group size
VRAF as affected by edge length, no. & group size
VRAF Douglas-fir SI 30 @ age 100 10% retention
0.60
0.70
0.80
0.90
1.00
0 100 200 300 400 500 600 700 800Length of Edge (m/ha)
Run 3: (718 Groups)
Run 1: (1 Group)
Run 2: (152 Groups)
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VRAF as affected by % Retention & Overstory Age
VRAF Site Index 35
0.4
0.5
0.6
0.7
0.8
0.9
1
10 20 30
Percent Retention (%)
VR
AF
Age 70 Age 130 Age 200
VRAF declines (< 1.00) as:
• % retention increases
• overstory age decreases
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VRAF as affected by Site Index & Overstory Topht
VRAF 20% Retention
0.6
0.65
0.7
0.75
0.8
20 30 40 50 60 70
Overstory Topht (m)
VR
AF
SI 25 SI 35SI 30
VRAF declines (< 1.00) as:
• SI increases
• overstory top height decreases
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• Matrix of TASS simulations• Select important variables &• Derive VRAF equations:
VRAF(sp) = f (edge, % retention, SI, overstory age or height, etc.)
Method 2. Derive Relationships
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10%
20%
30%
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VRAF Segmented Regression Function
VRAF = 1- (b * Edge + c * (Edge - x0) * d1 + f * (Edge - x1) * d2)
1st Slope: b = b0 + b1 * SI + b2 * retht + b3 * perc + b4 * tophtSlope change: c = c0 + c1 * SI + c2 * retht + c3 * perc + c4 * topht
2nd slope change: f = f0 + f1 * SI + f2 * retht + f3 * perc
Where:Edge = Edge length (m/ha)SI = Site indexRetht = Overstory top heightPerc = % retentionTopht = Regenerated top height
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Fitted VRAF Function
VRAF Douglas-fir SI 30 @ age 100 10% retention
0.60
0.70
0.80
0.90
1.00
0 100 200 300 400 500 600 700 800Length of Edge (m/ha)
R2 = 0.993
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10%
20%
30%
Retention
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Percent retention: 14%Edge length: 111 m/haOverstory height: 30 mOverstory age: 60 yrs.
TIPSY ver. 3.2
VRAF = 0.83
Fraser TSA Cutblock
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Variable retention vs. clearcut yields & value at age 60
Treatment Merch. Volume Site Value m3/ha $/ha
Variable Retention 728 1514Clearcut 873 3181Difference -145 -1667
VRAF = 0.83
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CCCrown Cover % vs. Basal Area %
SI 30
05
101520253035
0 5 10 15 20 25 30
Basal Area (%)
Cro
wn
Co
ver
(%) Overstory Age
70130 200
CC % = b * ba ** c
Where: b = b0 + b1 * rethtc = c0 + c1 * SI
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0
20
40
60
80
100
120
140
0 5 10 15 20 25
decades from now
harvest ('000s
Assume clearcut with WTP reserves
VR accounting for area loss
VR accounting for area loss and regenerated volume impact
Variable Retention Harvesting Effects on Timber Supply
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Variable Retention Summary
VRAF declines (< 1.00) and the relative yield of regenerated stands decreases as:
top height/age of overstory trees
% retention edge length SI top height of regenerated trees
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Variable Retention Summary
The primary factor affecting VRAF is:
the amount and distribution of the
retained trees which will compete for the regenerated growing space
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Current & Future Development
• incorporate VRAF into TIPSY
• address other species
• model impact of windthrow & pests
• incorporate VRAF into TASS III which
is linked to a light model
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TASS with and without light model
TASS II
TASS III
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TASS with and without light model
TASS II
TASS III
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TASS with and without light model
TASS II
TASS III
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TASS with and without light model
TASS II
TASS III
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TASS II vs TASS III Aggregated Retention
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40
% Aggregated Retention
VR
AF
TASS III
TASS II
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TASS II vs TASS III Dispersed Retention
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40
% Dispersed Retention
VR
AF
TASS II
TASS III
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Questions?