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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