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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?