Thinning mixed-species stands of Douglas-fir and western hemlock in the presence of Swiss needle cast

Junhui Zhao, Douglas A. Maguire, Douglas B. Mainwaring, Alan Kanaskie

Background

• Young Douglas-fir plantations (≤40 yr) are tremendously important to the economic and environmental health of Oregon and Washington due to their extent and productivity (Campbell et al. 2004, Gray et al. 2005).

• Over the past 20 years, coastal forests in this region have been suffering from an epidemic of Swiss needle cast (SNC).

Premature loss of older foliage,Needle longevity 1-4 years

(Alan Kanaskie, 2011)

Swiss Needle Cast affect Douglas-fir

Needle on the left showing rows of black fruiting bodies of Swiss needle cast.

197019801983 1961

2008:1984

Direction of growth

The trees’ growth between 1984 and 2008 was packed into just a millimeter.

(Photo by Bryan Black)

Current plantation strategy

• Due to its historically greater value, Douglas-fir continues to be an important component of planted stands.

• Increasing proportions of Douglas-fir are generally planted stands from west to east within the Coast Ranges of Oregon, with western hemlock making up most of the remainder.

(Beth Fitch, pers. comm)

Pre-commercial thinning• Infected stands

respond positively to thinning.

• But Douglas-fir growth remains lower than its potential in absence of SNC.

WH

DF

Growth market value

2000 2002 2004 2006 2008 2010 2012$200

$300

$400

$500

$600

$700

$800 Douglas-fir western hemlock

NO. 2 Saw log

Aims of this study

1. to develop distance-independent individual tree diameter growth models for Douglas-fir and western hemlock growing in plots established across a gradient in SNC severity;

2. to compute the implied relative basal area growth of Douglas-fir and western hemlock trees of varying initial diameter as a function of foliage retention;

3. to develop a field chart to help managers select trees for removal and retention during thinning of mixed Douglas-fir and hemlock stands .

Study plots• 10-30-yr old

Douglas-fir plantation

• 76 stands• 0.2 acre• Measured in

1998, 2000, 2002, 2004, and 2008.

Method

• Develop diameter increment models for Douglas-fir and western hemlock by testing:– Tree size: DBH, CR, HCB– Competition: TPA, D40, H40, QMD, AGE, BA, CCF,

SDI, SI, BAL, CCFL– Site: ELEV, SL, LONG, LAT– SNC: FR

The models

• Douglas-fir (R2=0.733)

• Western hemlock (R2=0.766)

Residuals

40.4% less

85.0% more

Model application• TPA=400 ( DF250, WH150). CR=0.7.• Diameter of western hemlock: 2-12 inches• Diameter of Douglas fir: 0-5 inches larger than

western hemlock.• All Douglas-fir trees have the same DBH, and

all Western hemlock have the same DBH.

Model application• The ratio of basal area growth of the Douglas-

fir and western hemlock trees was computed as a function of foliage retention and the difference between the diameters of the two species.

• For a given foliage retention, the diameter difference between the species where the growth ratio equals one implies a diameter difference threshold during a thinning.

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0D_wh = 4 inch

1.52.02.53.03.5reference line

i=DBHDF-DBHWH

BAG

RO_W

H/B

AG

RO_D

FFR

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 2 inch1.52.02.53.03.5ref-er-ence line

i

BAGR

O_W

H/BA

GRO

_DF

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 4 inch1.52.02.53.03.5ref-er-ence line

i

BAGR

O_W

H/BA

GRO

_DF

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 6 inch1.52.02.53.03.5ref-er-ence line

i

BAGR

O_W

H/BA

GRO

_DF

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 8 inch1.52.02.53.03.5ref-er-ence line

i

BAGR

O_W

H/BA

GRO

_DF

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 10 inch1.52.02.53.03.5ref-er-ence line

i

BAGR

O_W

H/BA

GRO

_DF

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5

3.0 D_wh = 12 inch1.52.02.53.03.5ref-er-ence line

iBA

GRO

_WH/

BAGR

O_D

F

i=DBHDF-DBHWH

Conclusion

1. The foliar losses of Douglas-fir imposed by SNC diminish Douglas-fir crown density and diameter increment, thereby enhancing diameter increment of western hemlock, the most common associate of Douglas-fir in coastal forests of Oregon.

2. Application of Douglas-fir and western hemlock diameter increment models to simulate indicates that the relative basal area growth of the two species in young, mixed stands varies directly with foliage retention.

Conclusion

3. When thinning in mixed stands where foliage retention is as low as 1.5 yrs, western hemlock trees will grow more in basal area than Douglas-fir tree that are 2-3 inches larger in diameter.

4. These results can be useful for forest managers who can prescribe “D+x” thinning where x represents the diameter advantage that Douglas-fir must have over an adjacent western hemlock to be selected as the leave tree. In this approach, “x” would be selected as a function of SNC intensity as measured by foliage retention.

Acknowledgements

• This project was funded by the Swiss Needle Cast Cooperative (SNCC) hosted at Oregon State University.

• We gratefully acknowledge field work performed by many different field crews working for the SNCC and the Oregon Department of Forestry.

THANK YOU FOR YOUR ATTENTION!