Tended intimate mixtures (tending stands or individual spruce)

• Advantages– Nursing effects are evenly distributed

(better protection from radiation frost and better control of understory)

Retaining some aspen cover may reduce resprouting vigour – but may require 2 or more entries

• Disadvantages– Requires use of selective manual or

herbicide treatments (ground based) - $$$– Growth of most of the conifers in the

stand may be reduced, if sufficient density is retained (with uniform spacing)

– If densities are too low – wood quality of broadleaves may be reduced

– Harvesting of dispersed broadleaves without damaging understory conifers may be difficult (commercial thinning or 2-stage systems)

Tend patches

• Tend in patches (“group planting” – Sutton 1974)

• Concerns:– Growth of spruce near

the patch edges

– Frost protection in opening

Transfer of N in red alder litterfall(Lavery, unpubl.)

0 5 10 15 20 25 30 350

50

100

150CH1alder conifer

0 5 10 15 20 25 30 350

50

100

150CH2 alder

conifertotal

0 5 10 15 20 25 30 350

50

100

150

Tot

al N

fro

m li

tter

fall

(kg

ha-1

yr-1

)

MK1

0 5 10 15 20 25 30 350

50

100

150MK2

0 5 10 15 20 25 30 35

Position along transect (m)

0

50

100

150NM1

0 5 10 15 20 25 30 350

50

100

150NM2

Light levels in gaps

• Stand boundaries are at –10 and +10 m (strip is oriented going north-south)

• Light levels at 1 m height reach 80% within 2 m from the edge

Predicted light levels across a strip clearing 20 m wide through a simulated 7 m tall aspen stand

0

20

40

60

80

100

-20 -15 -10 -5 0 5 10 15 20

Distance east of center (m)

% T

rans

mitt

ance

Edge orientation and distance effects

0102030405060708090

100

0 0.5 1 1.5 2 2.5 3

Proportional Distance

% t

ran

sm

itta

nc

e

90

180

270

360

0

10

20

30

40

0 10 20 30 40 50

Age (ye a rs)

Str

ip w

idth

(m

)60% at 50%

60% at 66%

60% at 100%

40% at 50%

40% at 66%

40% at 100%

F igure 15. E stim ated changes in strip w idth required to reach 60% and 40% of fu ll sunlight at 50% , 66% and 100% of spruce height over 60% of the w idth of the strip (strip oriented east to w est). E stim ates are based on w idths described in F igure 13 and m odels of spruce and aspen height grow th.

Tending patches

• Advantages– Can use broadcast or non-selective patch treatments (e.g.

aerial application of glyphosate herbicide) – reducing costs

– Potential for better growth of conifers - those in the open have the potential to grow at close to 100%

– Wood quality of broadleaves (eg. Birch) is likely to be higher and harvesting may be easier

• Disadvantages– Nursing effects are not evenly distributed– Frost protection will require narrow strips

Temporal mixtures

• Understory protection harvest –”Natural shelterwood” system– Harvest aspen at 60 years,

release understory spruce and harvest spruce at 120-140 years

– Navratil, S., L.G. Brace, E.A. Sauder, and S.Lux. 1994. Silvicultural options to favor immature white spruce and aspen regeneration in boreal mixedwoods. Can. For. Serv, Edmonton. Information Rep. NOR-X-337.

Temporal mixtures

• Advantages– Provides substantial shelter for frost protection during early

growth of understory conifers– Reduces vigor of understory vegetation – notably

Calamagrostis canadensis– Telescoping of rotations of two species – spruce are well

established when aspen are harvested (negative regen delay!)

• Disadvantages– Harvesting costs to protect understory– Need for planning – and land-swapping between conifer and

deciduous landbases

Yield implications of 7 options Based on TIPSY(Sw) and VDYP (Aw)

model runs (harvest at 90 years)

150

288357

218286

355

491260

158107

208156

104

0

0

100

200

300

400

500

600U

ntre

ated

200

Sw

/ha

x 2.

5 m

300

Sw

/ha

x 2.

5 m

20%

herb

icid

ed(p

atch

es)

40%

herb

icid

ed(p

atch

es)

60%

herb

icid

ed(p

atch

es)

Her

bici

de

Yie

ld (

tota

l) (m

3/ha

)

Sw Vol Aw Vol

Natural development of mixedwood stands

• Spatial pattern– Variability in aspen

stocking – patchiness

• Temporal pattern• Fire vs harvesting?

Some challenges in mixedwood management(Lieffers and Beck 1994)

1. Unless the understory species is exceptionally shade tolerant – its establishment and survival may be poor during early stages of stand development

2. Mixing species that mature at different times creates challenges to harvesting – “understory protection”

3. Yield prediction is very difficult (virtually impossible)4. Estimating future economic values of different species is a challenge.

[Establishment costs differ for different species – should we always accept the cheapest? What will we get if we allow industry to use the least expensive option – without restrictions on species?]

5. Different operations and mills have different demands for different species. Overlapping tenures create management challenges and can create conflict.

6. It is far easier to plan and manage for single species than for mixtures, but mixtures might be more FUN and are certainly more challenging!

7. Mixedwoods comprise a wide range of species structures and compositions – complexity and confusion

8. Regulations promote monocultures.

Conclusions

• Competition in mixedwood systems is dynamic

• Potential exists for broadleaves to seriously reduce survival and growth of conifers during the first 20 to 40 years after regeneration – but a number of factors influence the intensity and impact of these interactions

• Tending (intimate or patchy mixtures) can increase spruce yield while retaining a substantial component of deciduous

• Initial stand composition and tending practices applied to the stand will influence the end result

• Excellent opportunity to apply Adaptive Management