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Stabilising characteristics of NZ riparian plants
Chris Phillips & Mike Marden
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Outline
• Set the scene • The question• What we did• What we found• What it means• Summary
Grant Hunter
http://icm.LandcareResearch.co.nz/
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An aside …… context?Would you ….
• Take on a new breed of sheep or cow on your farm because someone said it would be good?
• Would you want to see numbers on growth performance? Mortality? Costs/benefits?
• Would you match the animal to the farm or accept that it is ok for any farm?
• What about native plants and their introduction back into NZ’s managed landscapes?
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Riparian functions• Filtering of contaminants – bugs, sed., nutr.• Bank stabilization• Nutrient uptake by plants• Denitrification• Shading for temperature• Shading for instream plant control• Input of wood & leaf litter• Enhancing fish habitat• Controlling downstream flooding• Recreation• Aesthetics
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The Issue
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The cure-all?
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The Big Question?Can our New Zealand native plants Can our New Zealand native plants
perform a river bank stabilising function perform a river bank stabilising function as well as introduced willows?as well as introduced willows?
In geotechnical terms, how do we quantify the benefits of In geotechnical terms, how do we quantify the benefits of vegetation to soil stabilisation? vegetation to soil stabilisation?
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What do we want from our plants?
• Root depth - anchor plant• Root spread – overlap with adjacent
plants• Strong surface root mat – hydraulic
protection• High root biomass – more the better• Root occupancy – biggest volume• Root strength – stronger roots more
resistant to external forces
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What do we want from our plants?
• Root depth - anchor plant• Root spread – overlap with adjacent
plants• Strong surface root mat – hydraulic
protection• High root biomass – more the better• Root occupancy – biggest volume• Root strength – stronger roots more
resistant to external forces
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2 strands of recent workRiparian plant trial Cabbage trees
Common name Botanical nameKaramu Coprosma robustaRibbonwood Plagianthus regiusKowhai Sophora tetrapteraLemonwood Pittosporum eugenoidesKohuhu Pittosporum tenuifoliumLacebark Hoheria populneaMapou Myrsine australisFivefinger Pseudopanax arboreusCabbage tree Cordyline australisRewarewa Knightia excelsaManuka Leptospermum scopariumTutu Coriaria arborea
Marden, Rowan, Phillips Czernin (2002)
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Methods – plant trial
• 10 plants / species/ age class – 1 to 5 years• 1 and 2 yr old plants from pots• 3-5 yr old plants extracted from trial plot • measured dbh, root collar, tree height, canopy width• above-ground components - stem, branches & foliage• root system extracted intact - air lance• below-ground - root bole (stump) & roots• roots – diam. size classes measured for length• all components oven dried and weighed• tensile strength of roots tested
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Root spreadPittosporum tenuifolium (kohuhu)
Coprosma robusta (karamu)
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Root spread – 5 year old
400350300
(cm) 250200150100500
mapou
reware
wafiv
e.fing
erman
uka
karam
uko
whai
laceb
arkca
bbag
e.tree
kohu
hurib
bonw
ood
tutu
lemon
wood
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Results - root depth
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Root depth – 5 year old
50
45
40
35
30
25
20
15
10
5
0
mapou
ive.fin
kohu
huko
whai
monw
manuk
eware
laceb
atut
u
f
ger
le
ood
ar
wa rk
karam
ubb
onw
bbag
e
ri
ood
ca
.tree
(cm)
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Root depth – cabbage tree
11 yr
y = 4.8813xr2 = 0.9617
0255075
100125150175200
0 10 20 30 40
Mean root collar diameter (cm)
Roo
ting
dept
h (c
m)
~ AgeCzernin (2002)
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Biomass – 5 year old
4202468101214
mapou
reware
wafiv
e.fing
erma
nukakowh
ailac
ebarkkohuhu
lemonwo
odkaram
ubonw
ood tutu
cabbage.tre
e
Above-ground
(kg)rib
Below-ground
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Czernin (2002)
Growth summary
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Root tensile strength
0
10
20
30
40
50
60
RataLac
ebark
Hard bee
chKowhai
Manuka
-1Man
uka-2
Red bee
chKan
ukaKohuhuKam
ahi
Fivefin
gerRew
arewa
Cabbag
e tree
ntain bee
chDouglas
firRibbonwoodRad
iata p
ineLem
monwood
TutuKara
mu
Mea
n m
ax. t
ensi
le s
tren
gth
(MPa
)
Watson & Marden (submitted)
Exotics
Willows 30 - 75 MPa
Riparian plant trial
(1 - 4 mm diameter)
Mou
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Implications for bank stabilization -small streams
• no limitations, provided that bank height is not more than ~2 m and channel bed is stable
• success depends on density - formation of dense canopy & full root occupancy of the soil
• shallow soil stabilisation after 3-5 years• improvement in deeper slope stabilisation
expected within 7-10 years of establishment• species can withstand breakage and over-topple
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Implications for bank stabilization -large streams
• lack of roots in deeper soil layers limits usefulness in streams where bank undercutting occurs
• ineffective if bank height exceeds effective rooting depth ~ 2 m.
• banks would need to be graded and unstable channel beds artificially regraded prior to planting
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NO….YES….
MAYBE….
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Decisions?Ecologically designed vs functional performance?
• Depth – cabbage tree, ribbonwood• Spread – lemonwood, ribbonwood• Above gd biomass – cabbage tree, tutu• Below gd biomass – cabbage tree, tutu• Tree height – lacebark, ribbonwood, cab. tree• Canopy spread – tutu, karamu• Root strength – lacebark, kanuka, kohuhu
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Summary• NZ natives take longer to grow than exotics – but not slow• Some natives can regenerate, eg cabbage trees - good• On own, natives not as good as willows for stabilising soils• Effective after about 5 years• Change the ecological mix to suit site• Mixed plantings of natives and exotics?• More work needed
– non-woody spp– Mixed exotic/native– modelling
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The end
http://icm.LandcareResearch.co.nz/
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Species list and numbers extracted for partitioning
Number of plants extracted /species/yearCommon name Botanical name
1 2 3 4 5 Species Total
Karamu Coprosmarobusta 7 8Ribbonwood Plagianthus regius 10 10Kowhai Sophora tetraptera 8 8Lemonwood Pittosporum eugenoides 10 10Kohuhu Pittosporum tenuifolium 10 9Lacebark Hoheria populnea 10 10Mapou Myrsine australis 10 10Fivefinger Pseudopanax arboreus 10 8Cabbage tree Cordyline australis 10 10Rewarewa Knightia excelsa 10 10Manuka Leptospermum scoparium 5 0Tutu Coriaria arborea 10 8Annual totals 120 118 110 101 105 554
10 10 10 4510 10 10 5010 8 10 4410 10 10 5010 10 10 4910 10 8 4810 10 10 5010 10 8 4610 10 10 5010 10 9 4910 10 5 3010 10 5 43
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Pseudopanax arboreus (fivefinger)Biomass