Knotweed Invasion in PNW Riparian Forests:Ecological Effects and Restoration Outcomes

Lauren Urgenson, PhDUniversity of WashingtonSchool of Forest Resources

Puget Sound Knotweed Forum Oct 31, 2011

http://www.ex.ac.uk/knotweed/

Japanese Knotweed

(P. cuspidatum)

Bohemian

Knotweed (P. x bohemicum)

Giant Knotweed (P. sachalinense)

Japanese Knotweeds  

Tall herbaceous perennialEarly, rapid emergenceForms dense, thicketsExtensive rhizome systemVegetative regenerationWoody ligneous stemsAllelochemicals

Knotweed Characteristics 

I. Vegetation response to knotweed removal– Is knotweed removal alone adequate to 

support native recovery?– Variation in response?

II. Effects on tree regeneration– Does knotweed inhibit tree seedling 

survivorship and growth?– Why?– Differences across species?

My Research

I. Understory response to bohemian knotweed (Polygonum x bohemicum) 

removal

Introduction

Knotweed removal is a common approach to riparian restoration

Often assume removal alone will promote native recovery

1. How do riparian understories respond to knotweed removal?Deciduous and coniferous tree seedlingsNative and nonnative shrubsNative and nonnative forbs 

2. Do habitat factors and species traits influence           responses to knotweed removal?   

**Native and non‐native species

Study Questions 

Quileute Tribe restoration project (2003)

Study Site 

Dickey RiverOlympic Peninsula

Dickey Photo

25 plot pairs (blocks)

½ knotweed removed

Before, 1 & 2  years after

Knotweed Removed Knotweed Present

10m 10m1m

Methods ‐ Experimental Design 

Habitat factorsHeight above channelLitter depth Canopy lightFlood disturbance

Methods ‐ Habitat Factors

Methods – Species Traits

Plant traitsLife‐formOriginHabitat Life‐span

Results – Knotweed Treatment

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20080

50

100

150

200

250

300

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0

5

10

15

20

Knotweed -Knotweed +

Knotweed Density #/m2 Knotweed  Height (cm) 

Knotweed treatment is effective

P < 0.001 P < 0.001

Paired permutation tests P ≤ 0.05 

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0

4

8

12

16

20

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0.0

0.4

0.8

1.2

Knotweed [-]Knotweed [+]

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0.00

0.02

0.04

0.06

0.08

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0

10

20

30

40

50

Deciduous Seedlings (#/5m2 ) Coniferous Seedlings (#/5m2 )

Native Shrub Cover (%) Nonnative Shrub Cover (%)

Results – Vegetation Response 

P < 0.001 P < 0.001

P < 0.001 P < 0.001

2006 2007 20082006 2007 20080

2

4

6

8

10

12

2006 2007 20082006 2007 2008

0

2

4

6

8

Results – Vegetation Response 

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0

10

20

30

40

50

2006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 20082006 2007 2008

0

10

20

30

40Native Forb % Cover Nonnative Forb % Cover

Native Forb Species Richness Nonnative Forb Species Richness

P < 0.001 P < 0.001

P < 0.001P < 0.001

Knotweed [-]Knotweed [+]

Results – Habitat Factors 

R2 P

Native:Nonnative Cover 0.52 <0.001Native:Nonnative Richness 0.52 <0.001

Knotweed Litter Depth was a Key Factor 

Results – Plant Traits 

χ 2  < 0.001

Natives ‐ Forest associated speciesNonnatives ‐ Ruderals

Native Nonnative

Freq

uenc

y

0

20

40

60

80

100

Forest Ruderal

Habitat Association

1. How do riparian understories respond to   knotweed removal?

Significant and immediate responseNative and nonnative species Base‐line data 

2. Do habitat factors and species traits  influence understory responses – particularly the    responses of native and nonnative species?

YesKnotweed litter benefitted natives Species traits influenced response

Summary and Conclusions 

II. Effects Polygonum x bohemicum on native tree seedlings

Knotweed seedling interactions

Introduction

Shade Tolerance & Seral Stage

Alnus rubraPicea sitchensis

Tsuga heterophylla

Nitrogen Deficiency Tolerance

(Minore 1979)

Multiple mechanisms

Direct competitionLight Soil resources

Allelopathic interference Disruption of fungal symbionts

Knotweed‐seedling interactionsKnotweed effectsNatives response

Introduction

Mycorrhizal Non‐Mycorrhizal

1. What are the effects of knotweed on understory light and soil properties?

2. Does knotweed reduce the survival and growth of native tree seedling?

3. Do tree species differ in their response to knotweed?

Study Questions

9 blocks x 2 treatment plots3 tree species 2 growing seasons Compared survival and growth

10m 1m 10m

Knotweed Removed Knotweed Remaining

Experimental Design

LightSoil Nutrients

Methods ‐Mechanisms

Biomass allocationRoot:shootFoliar C & NMycorrhizal colonization

Results ‐ Light

% A

mbi

ent P

AR

0

10

20

30

40

50

60

Knotweed [+]Knotweed [-]

Strong reductions in lightCoastal hardwood forests—10.8±10% & 16±2%

(Drever 2005, Roburn 1997) 

P = 0.003

7%AmbientLevels

Results – Soil AnalysisB

ulk

Den

sity

(g/c

m3 )

0.0

0.2

0.4

0.6

0.8

1.0Knotweed [+]Knotweed [-]

pH

0

1

2

3

4

5

6

Tota

l % C

arbo

n

0.0

0.5

1.0

1.5

2.0

2.5

Tota

l % N

itrog

en

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Bulk Density (g/cm3) Total % Carbon

pH Total % Nitrogen

P < 0.01

P = 0.02 P = 0.03

P = 0.04

24%Increase

Results ‐ Seedling Survivorship & Growth

Reduced survivorship & growth Differences across speciesGreater mortality in alderReduced height growth in hemlock

Alnus Picea Tsuga

-0.2

0.0

0.2

0.4

0.6

0.8Ln Height Growth (cm)T : P < 0.001S: P < 0.001TxS: P < 0.001

Alnus Picea Tsuga0.0

0.2

0.4

0.6

0.8Ln Diameter growth (cm)

T : P < 0.001S: nsTxS: ns

Alnus Picea Tsuga0.0

0.2

0.4

0.6

0.8

1.0

1.2Knotweed + Knotweed -

Seedling Survivorship (#/10)T :nsS: P < 0.001TxS: P < 0.001

84%Difference

Tota

l Bio

mas

s (g

)

0

50

100

150

200

250

300Knotweed +Knotweed -

Results – Biomass & Allocation

Roo

t:Sho

ot B

iom

ass

(g)

0.0

0.1

0.2

0.3

0.4

0.5

Alnus Picea Tsuga Alnus Picea Tsuga

T : nsS: nsTxS: P = 0.02

T : P < 0.001S: P < 0.001TxS: ns

Tsuga allocates greater resources to roots

Total Biomass (g) Root:Shoot (g)

Results – Seedling Response to Light

Every seedling in knotweed + plotsEarly and mid‐seral species responded to light

Prop

ortio

n Am

bien

t PAR

0.00

0.05

0.10

0.15

0.20

0.25

0.30DeadSurvived

Alnus Picea Tsuga

79% higher light above survivors

P = 0.02

P = 0.01 P = 0.9

Results – Seedling Foliar NutrientsFo

liar C

arbo

n (%

)

0

10

20

30

40

50

60Knotweed + Knotweed -

Picea TsugaFo

liar N

itrog

en (%

) 0

1

2

3

4

5

6

Picea Tsuga

Foliar Carbon % Foliar Nitrogen %T :nsS: nsTxS: ns

T :nsS: nsTxS: ns

No Differences

Results – Ectomycorrhizal Colonization

Picea TsugaAlnus

Roo

t:Sho

ot B

iom

ass

(g)

0.0

0.1

0.2

0.3

0.4

0.5

Lower ECM colonization on shade tolerant TsugaPr

opor

tion

ECM

Col

oniz

atio

n

0.0

0.2

0.4

0.6

0.8

1.0 Knotweed +Knotweed -

Picea Tsuga

T : P = 0.02S: P = 0.001TxS: P = 0.04

64% Difference

Conclusions

1. What are the effects of knotweed on light transmittance and soil physical and chemical   properties?

Strong reductions in lightWeaker soil effects

2.   Does knotweed reduce native tree seedling  performance?

Yes, all three species

3. Do tree seedlings differ?Yes, Alnus ‐ survivorship

Tsuga ‐ height growth

Yes,  early & mid seral – light late seral – below ground resources

Strong implications for forest structure and functionInvasion success ‐modify resources & interfere with fungal mutualisms

Conclusions

Community ResponseEffective restoration approachStrong influence of habitat factors and plant traits

Interactions with tree seedlingsInvasion success – multiple mechanismsReductions in light & plant mutualisms

Final Summary

Alaine SommargrenPatrick Sowers!

Deborah Oaks

M.J. Murdock Charitable Trust National Park Service – North Cascades National ParkNSF IGERT (0333408) Multinational Collaborations on Challenges to the EnvironmentQuileute Natural Resources – Frank GeyerOlympic and North Cascades National ParksRayonier Forks Office –Silvaseed Co. – David GerdesKnotweed management community – OlyPen & Skagit Working Groups

Acknowledgements