THE INFLUENCE OF STAND

CONDITIONS ON TREE FORM

Sean M. Garber and Aaron R. Weiskittel

Oregon State University

June 21, 2004

INTRODUCTION

• A goal of silvicultural research is to detect changes in growth.

• Changes in stand conditions affect diameter and sometimes height:– Density (spacing and thinning)– Social position within the stand– Control of competing vegetation– Fungal diseases

INTRODUCTION

• Consequently stem volume also responds to these stand conditions.

INTRODUCTION

• Existing volume and taper equations:– Most are functions of only DBH and HT;

– Regional development;

– May not include intensively managed stands or other dramatically altered stand conditions;

– Averages lose subtle differences.

INTRODUCTION

• Questions:– Is there an affect of stand conditions

above and beyond the effect on DBH and height?

– Does this influence the ability to discern growth differences among treatments in silvicultural studies?

INTRODUCTION

DBH

HT

METHODS

• Analytically assessed stem form on four separate studies:– Two vegetation management studies

– A western larch LOGS thinning study

– Two mixed-species spacing trials

– Study of Swiss needle cast

METHODS

• Kozak variable-exponent taper model applied in all studies:

d = γ1DBHγ2 XC + ε

Represents shape

METHODS

• Shape component (XC)–X varies from 0 (top), 1 (p), > 1

below p.– C = linear function of:

• Z = stem position (h/HT)• DBH/HT• Treatment variables

d/DBH

Z

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.2

0.4

0.6

0.8

1.0

Model dynamics

Positive term

Negative term

METHODS

• Sample trees were climbed or felled:– Sampled across range of within

treatment DBH;– DBH, HT, HCB, and CW measured;– Upper stem diameters (ib and ob) and

height measured.

METHODS

• Autocorrelation accounted for in two ways:– Model fit by tree and parameters modeled

using SUR.

– Model fit using GNLS with CAR(1)

• Multicollinearity was also a problem– Screened to reduce VIF

Douglas-Fir Stem Taper Under Early Vegetation

Control

Study part of the Vegetation Management Research Cooperative, Oregon State University.

INTRODUCTION

• As trees develop under intensive management:– Form differs from a cone (/12)D2H,

– Trees large enough for existing volume or taper equations.

Summit

Marcola

Vegetation Management sites

• Study description– Planted in 1993,– Completely randomized design,– 8 treatments,– 3 replicates,– Plot area = 0.112 ac,– 49 seedlings planted at 9.8 ft square spacing.

METHODS

• Treatments– No herbicide,– 4 ft2 full control,– 16 ft2 full control,– 36 ft2 full control,– 64 ft2 full control,– 100 ft2 full control,– 100 ft2 woody vegetation control,– 100 ft2 herbaceous vegetation control.

METHODS

• Diameter outside bark was collected at tree base, breast height, 8 ft, and every 4 ft above 8 ft.

• Observed DOB’s were modeled using variable-exponent model using GNLS.

• C was a function of site, area treated, and target vegetation.

RESULTS

• Modeling– Model fits were excellent (R2 > 0.95);– Impact of autocorrelation eliminated w/

CAR(1);– Multicollinearity present, albeit small.

• Treatment variables– Area treated and herb control significant

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.2

0.4

0.6

0.8

1.0

Check2-ft5-ft

d/DBH

h/H

0.0 0.2 0.4 0.6 0.8 1.0 1.2

CheckHerbWoody

Stem Taper in a Western Larch Levels-of-Growing-

Stock Thinning Study

From Lennette A.P. 2000. Twenty-five-year responses of Larix occidentalis stem form to five stand density regimes in the Blue Mountains of eastern Oregon. M.S. Thesis, Oregon State University. 59 p.

METHODS

• Study description– Established in 1933,

– Completely randomized design,

– 5 treatments,

– 2 replicates,

– Plot area = 0.4 ac,

– Thinned in 1966, 1975, and 1985

METHODS

• Treatments based on bole surface area (BSA) targets:

BSA (ft2/ac) SDI Relative density

5,000 65 0.16

10,000 109 0.27

15,000 150 0.37

20,000 193 0.47

25,000 235 0.57

METHODS

• Data analysis:–Parameters fit by tree;

–Parameters tested among treatments by MANOVA;

–Parameters modeled using SUR.

RESULTS

• Stem form responded to thinning:– C = a1sin-1(Z) + a2Z2;– MANOVA suggested differences among

the treatments;– Parameters increased w/ thinning

intensity:• a1 = f (DBH/HT)• a2 = f (CR)

BSA: 15,000 to 25,000

BSA 10,000

BSA 5,000

RESULTS

• CR improved model fit after accounting for DBH/HT

• Treatment variables did not account for additional variation beyond DBH/HT and CR.

Increasing thinning intensity

Stem Taper in Two Mixed-Species Spacing Trials

From Garber, S.M. and Maguire, D.A. 2003. Modeling stem taper of three central Oregon species using nonlinear mixed-effects models and autoregressive error structures. For. Ecol. Manage. 179: 507-522.

Study sites

• Two study sites located at Pringle Falls EF on the Deschutes National Forest:– Pringle Butte (34 years)

• Lodgepole pine and Ponderosa pine• 4500 feet elevation, west aspect, and dry• SI100 = 100 ft (Barrett 1978)

– Lookout Mountain (27 years)• Grand Fir and Ponderosa pine• 5100 feet elevation, east aspect, and “more mesic”• SI100 = 110 ft (Barrett 1978)

Experimental Design

• Completely randomized split-plot design– Whole plot factor: spacing

– Split-plot factor: species composition

• Replacement series setup– Only one mixture 50:50

GF

MX

PP

Whole plot

Split plot

P P

P

PP

F

F

F

F

MX

INTRODUCTION

• Mixed-species stand development:– Spacing

• More spacing, less stratification

– Species composition• PP over GF

• LP over PP

From Garber, S.M. and Maguire, D.A. 2004. Stand productivity and development in two mixed-species spacing trials in the central Oregon Cascades. For. Sci. 50: 92-105.

02

46

81

01

2(a) 1.8-m

LPPP

(b) 3.7-m

LPPP

(c) 5.5-m

LPPP

0 10 20 30

02

46

81

01

2

(d) 1.8-m

GFPP

0 10 20 30

(e) 3.7-m

GFPP

0 10 20 30

(f) 5.5-m

GFPP

Top

hei

ght (

m)

Stand age (yr)

Top height growth across spacing

RESULTS

• Modeling– Model fits were excellent (R2 > 0.95);– Impact of autocorrelation eliminated w/ random

effects and CAR(1).

• Profiles– Showed differences among spacing;– Slight differences between pure and mixed

stands.

SPACING

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.83.75.5

Pinus contorta

d/DBH

h/H

0.0 0.2 0.4 0.6 0.8 1.0 1.2

1.83.75.5

Abies grandis

SPECIES COMPOSITION

Mix PP (subordinate)

Pure PP

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.8-m spacing

Pure PPMix PPPure LPMix LP

d/DBH

h/H

0.0 0.2 0.4 0.6 0.8 1.0 1.2

5.5-m spacing

Pure PPMix PPPure LPMix LP

SPECIES COMPOSITION

Pure PP

Mix PP(dominant)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

0.2

0.4

0.6

0.8

1.0

1.8-m spacing

Pure PPMix PPPure GFMix GF

d/DBH

h/H

0.0 0.2 0.4 0.6 0.8 1.0 1.2

5.5-m spacing

Pure PPMix PPPure GFMix GF

DOUGLAS-FIRSTEM TAPER & SNC

Part of the Swiss Needle Cast Cooperative, Oregon State University.

From Weiskittel, A.R. 2003. Alterations in Douglas-fir crown structure, morphology, and dynamics imposed by Swiss needle cast in the Oregon Coast Range. M.S. Thesis, Oregon State University. 389 p.

INTRODUCTION

• SNC disrupts normal needle physiology, leading to premature loss of foliage

• 10 yrs of extensive defoliation has dramatically altered Douglas-fir crown structure & morphology– modified age class structure and vertical location of

foliage

– reduced branching

– increased crown recession rates

HYPOTHESES

• loss of foliage has decreased stem increment in the crown

• increased crown recession rates modified stem form

• stem taper equations require additional SNC covariates

METHODS

• 105 trees from 31 plots destructively sampled– DBH; 12 – 65 cm – Relative density; 21.5 - 73.6– Site index; 27.7 – 47.4 m @ 50-yr– Foliage retention; 1.2 – 4.4 yrs

• 3 to 4 trees/plot & 10 to 15 discs/tree

ANALYSIS

• utilized modified Kozak’s equation– includes Z, DBHZ, DBH/HT, FOLRET– little correlation between covariates– continuous AR1 error structure

• tree volume predicted using the developed equation and Bruce & Demars (1974) equation– results compared graphically and statistically

RESULTS

• R2 of 0.95

• parameters all highly significant (p<0.001)

• likelihood ratio test also suggested that foliage retention significantly improved model fit (p<0.0001)

• for a given DBH/HT, foliage retention significantly reduced dibs throughout the stem, except below BH

RESULTS

DIB (cm)

0 10 20 30 40

h/H

0.0

0.2

0.4

0.6

0.8

1.0

HIGH SNC LOW SNC

DISCUSSION

• Does relative stem form change w/ stand conditions?– Differences in profiles after accounting

for DBH and HT.– Treatments variables significant after

accounting for DBH/HT:• Vegetation treatment;• Thinning level;• Foliage retention.

DISCUSSION

Study Shape change Consequences

↑ Veg. control More Neoloid Overestimate

↑ Thinning More Neoloid Overestimate

↑ Spacing More Neoloid Overestimate

↑ Dominance More Neoloid Overestimate

↑ SNC More Neoloid Overestimate

• Does relative stem form change w/ stand conditions?

DISCUSSION

• Do these changes affect results of silvicultural studies?– Significant difference between volumes

estimated from SNC equations and Bruce-DeMars;

– Bias a function of top height, DF QMD, FOLRET, and CLSA;

– Mean volume losses due to SNC were 31%.

DISCUSSION• Mechanisms

– Differential basal area growth rate along the stem differences among treatments.• Stem growth rate responds to increased

resources;• Response increases w/ decreasing stem height.

– Swiss needle cast• Reduces tree leaf area (thins crown)• Decreases upper stem growth

Curves diverge

DISCUSSION

• Measure upper stem diameters.– Assess existing volume or taper

equations.– Develop new site-specific equations.

• Incorporate crown ratio into the model• Include stand or treatment variables

into models.

THE END

Any questions?