Mixed Species Silviculture
• Pros vs. Cons
Pure Stand
Mixed Stand
vs.
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Oliver (1978)
Mixed Species Silviculture
• Objectives of Mixed Species
Silviculture
– Diversify timber products
– Increase nutrition of target crop species
– Maximize resource use and, thereby,
overall yield
• Resource partitioning
– Increase biodiversity
2
Mixed Species Silviculture
• How common is mixed
species silviculture?
– Not very common at all,
especially on large, industrial
plantations
– More common on private
lands and small woodlots
– Why?
3
Nichols et al. (2006)
Mixed Species Silviculture
• Economic Considerations
– Financial analysis indicates that, in worst case scenario, a 11% increase in
yield is necessary with mixed species to offset increased cost
– Do mixed species stand increase yield by ≥11%?
Nichols et al. (2006)
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Mixed Species Silviculture
• Do mixed species forests work?
– If objective is to increase timber crop
yield, then in general yes
• Many field trials have
demonstrated ≥10% increase in
yield in mixed species stands over
monocultures
– However, there are certainly
exceptions
• Depends strongly on the ecology
of the individual species, and their
ecological interactions
5
Erskine et al. (2006)
Mixed Species Silviculture
• Do mixed species forests work?
– If objective is to increase
nutrition of target crop, then
results are mixed
– Only 1 case where mixtures
caused a ↓
– Many cases, mixture ≈ single
species (or exceed)
– As before, ecological
interactions are the key
Jose et al. (2006)
6
Mixed Species Silviculture
• Ecological Interactions
in mixed species forests
– Competition
• Light, H2O, & nutrients
• interspecific vs.
intraspecific
– Complementary resource
use (Resource use
partitioning; or niche
separation)
– Facilitation
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Mixed Species Silviculture
• Competition
a) Inter = Intra for
both
b) Inter < Intra for A
Inter > Intra for B
c) Inter > Intra for
both
d) Inter < Intra for
both
Interspecific = Between species
Intraspecific = Within a species
Harper (1977)
8
Mixed Species Silviculture
• Complementary resource use
– Aboveground ≈ competition
for light
Kelty (2006)
9
Mixed Species Silviculture
• Complementary resource use
– Belowground ≈ competition
for nutrients and/or water
Jose et al. (2006)
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Mixed Species Silviculture
• Facilitation
– Typically involves the use of
N-fixing species to enhance
nutrition of target timber
species
Kelty (2006)
11
Mixed Species Silviculture
• Ecological Interactions in
mixed species forests
– Key to mixed species
silviculture → species that
differ in ecological
characteristics
•
•
•
•
•
•
Shade tolerance
Height growth rate
Crown structure
Foliar phenology
Root depth
Nutrition
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Case Study: Quercus and Fagus in Germany
• Target species: Quercus sp. from natural regeneration
• Problem: in pure stands, Quercus branches form epicormic
sprouts, reducing value of timber
• Solution: natural regeneration of shade tolerant Fagus sylvatica
which grows densely in Quercus understory
– Typically, Fagus is eliminated (non-desireable species that competes with
target species)
Lupke (1998) 13
Case Study: Quercus and Fagus in Germnany
• Outcome: After 60 years, Fagus grew too well, competed
strongly and overtopped Quercus
• Competition
• Quercus needs to have large age advantage over Fagus
Lupke (1998)
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Case Study: Pseudotsuga and Alnus in PNW
• Two sites: Wind River, WA (low N) and Cascade Head, OR
(high N)
• Two treatments at each site: Pure conifer (Douglas-fir) and
conifer + N-fixer (alder)
– Douglas-fir (conifer) is the target timber species
• Followed stand and tree growth for >70 years
• Only long-term study (rotation length) of mixed-species
plantations in temperate regions
Binkley (2003)
15
Case Study: Pseudotsuga and Alnus in PNW
• Low N site
– Stem mass ↓ compared to
high N site
– Stem mass ↑ at conifer +
alder site
• High N site:
– Total stem mass still ↑ at
both sites
– Much less stem mass at
conifer + alder site
• ???????
16
Binkley (2003)
Case Study: Pseudotsuga and Alnus in PNW
• Low N site:
– Stem mass increment shows
same pattern
• High N site:
– Stem mass increment much
higher at pure conifer site
• ???????
17
Binkley (2003)
Case Study: Pseudotsuga and Alnus in PNW
• Clear cases of both facilitation and competition with the same
species mixture
• Facilitation occurred where N was limiting
• Competition occurred where N was abundant
– No added benefit of alder on nutrition, so alder simply competes with
conifers for other limiting resources and lowers overall stand yield
• Highlights importance of knowing site and species
characteristics before implementing a silvicultural prescription
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Case Study: Eucalyptus and Facaltaria in HI
• Hamakua Coast, Big Island of Hawaii
• Eucalyptus is the target crop, Facaltaria is a N-fixer
• Pure Eucalpytus, pure Facaltaria, and 5 mixtures of the two (11
to 75% Facaltaria)
• Same spacing and total stand density (2,500 trees/ha)
• Pure Eucalyptus was heavily fertilized at establishment
• Followed stand development for 20 years
• Can mixed species stands with a N-fixer increase yield of
Eucalyptus over monocultures of fertilized Eucalyptus?
19
Case Study: Eucalyptus and Facaltaria in HI
• 10 years-old
• Eucalpytus comprises majority of
stand biomass in all but pure
Facaltaria stand
• Largest annual increment of
Eucalyptus found at highest % of
Facaltaria
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Binkley et al. (1992)
Case Study: Eucalyptus and Facaltaria in HI
• Relative Yield = Eucalyptus Mix /
Eucalyptus Pure (or Facaltaria)
• Eucalyptus biomass relative yield
approaches pure Eucalyptus stands
at high % of Facaltaria
• Eucalyptus increment relative yield
at highest % of Facaltaria
EXCEEDS that of pure stands
Binkley et al. (1992)
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Case Study: Eucalyptus and Facaltaria in HI
• Means trees in mixed stands grew
faster and were larger than trees in
pure stands
• Stem mass does not differ, but
average stem diameter is larger in
mixed stands
• Important because larger trees tend
to be more valuable
Binkley et al. (2003)
22
Case Study: Eucalyptus and Facaltaria in HI
• Benefit of mixed species stands on
Eucalyptus growth even more
pronounced after 20 years of stand
development
• Mixed stands increase yield of
Eucalyptus, but not Facaltaria
• Because Eucalyptus dominates
stand biomass, mixtures increase
overall stand biomass
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Binkley et al. (2003)
Case Study: Eucalyptus and Facaltaria in HI
• Increased Eucalyptus yield in mixed stands was a result of:
– Increased soil N cycling and supply with Facaltaria
(facilitation)
– Increased dominance of Eucalyptus trees over interplanted
Facaltaria (complementary resource use)
Binkley et al. (2003)
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Case Study: Eucalyptus and Acacia in Australia
• Eucalpytus globulus (target tree)
and Acacia mearnsii (target tree
+ N-fixer)
• Same spacing and total stand
density (~900 trees/ha)
• Mixtures of 100E, 75E:25A,
50E:50A, 25E:75A, 100A
• Grown for 11 years
• Can mixed species stands with a
N-fixer increase total yield over
monocultures?
Forrester et al. (2006)
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Case Study: Eucalyptus and Acacia in Australia
• For both species, diameter
growth increased more in
mixtures than when grown alone
26
Forrester et al. (2004)
Case Study: Eucalyptus and Acacia in Australia
• Total stand biomass was
always higher in the
mixtures at the end of 11
years
• Relationship changed over
time
Forrester et al. (2004)
27
Case Study: Eucalyptus and Acacia in Australia
• Aboveground biomass
increment highest in mixed
stands
• Increment in pure Eucalyptus
stands ½ that in 50E:50A
mixtures
Forrester et al. (2004)
28
Case Study: Eucalyptus and Acacia in Australia
• Mechanisms?
• Both N and P in litterfall increase
in mixtures
• Exclusively due to high N and P
in Acacia litter → increased
nutrient cycling and availability
• Works because both species are
target timber crop
Forrester et al. (2004)
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Case Study: Eucalyptus and Acacia in Australia
• Both species benefit from
mixture
• For Eucalyptus → facilitation
• For Acacia → compensatory
resource use? Facilitation?
– appears to be result of a
reduction in light competition
• Works because both species are
target timber crop
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Forrester et al. (2004)
Case Study: Eucalyptus and Acacia in Australia
• As before, site plays overriding role
in explaining benefits of mixed
species forests
Low N
• Increase in stand biomass only
occurs at low N
High N
• “Mixtures will only improve
productivity if the processes and
interactions between species reduce
competition or increase availability
of a growth limiting resource…”
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Forrester et al. (2006)
Mixed Species Forests - Conclusions
• Advantages to mixed species forests
• Disadvantages to mixed species forests
• End results of mixed species forests
will depend on ecological interactions
between species, taken together with
site characteristics
• Mixed species forests can increase
yield over monocultures, but not
always
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