managing canopy composition in boreal mixedwood forests ......managing mixedwoods: managing aspen...

Managing canopy composition in boreal mixedwood forests: effects on understory plant

communities

Ellen Macdonald and Phil Comeau Dept. of Renewable Resources

University of Alberta Edmonton, AB, Canada

Baldwin et al. 2012 - Level 4 map, version 1, Canadian component of the Circumboreal Vegetation Map (CBVM).

Boreal Mixedwood zone in Canada

Western boreal: • Trembling aspen, balsam

poplar, paper birch, • White spruce

Dry Short growing season Natural disturbance – large, frequent wildfire

Varying canopy dominance: broadleaf /conifer trees

Canopy composition affects understory communities: Competition: • light • soil water • nutrients Indirect effects: • soil pH • litter • microenvironment • regeneration microsites

Macdonald/Purdy

A. Craig

A. Craig

Understory communities in mixedwood forest STANDS of varying composition

Broadleaf-dominated Mixed Conifer-dominated

Macdonald & Fenniak: For Ecol & Mgmt 2007

Axis 1-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Axi

s 2

-2

-1

0

1

2

3

(Conifer = Mixed) ≠ Broadleaf

Obs

erve

d To

tal R

ichn

ess

0

20

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Broadleaf Mixed Conifer

Understory Richness Understory Composition

Variation in understory communities among patches WITHIN mixed stands

Chavez & Macdonald For Ecol & Mgmt 2010; 2012

Canopy patch type:

Conifer

Broadleaf

Mixed

Tota

l Spe

cies

Ric

hnes

s pe

r pat

ch ty

pe

0

10

20

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Broadleaf Mixed Conifer

Axis 1

Axis

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NMS ordination

(Conifer = Mixed) ≠ Broadleaf

Understory Richness Understory Composition

Understory plant communities vary with canopy composition of mixedwood forests: • Between-stand • Between patches within stand

Mixed patches/stands: greatest total richness Conifer and Mixed stands/patches: • similar to one another • different than Broadleaf stands/patches Mixedwood stands are a mesocosm of the mixedwood landscape The presence of white spruce is particularly important

So what happens when we manage mixedwoods?

Sheelah Griffiths: MSc thesis

Managing mixedwoods: manipulating aspen – white spruce densities

http://www.wesbogy.rr.ualberta.ca/

Pre-commercial thinning in regenerating mixedwood stands Partial or complete removal of aspen overstory Objectives: • Increase white spruce growth • Manage white spruce and aspen as a mixture


Managing mixedwoods: managing aspen – white spruce densities WESBOGY long-term sites; 3 replicate blocks Mixed stands harvested & planted with white spruce Year 5: Aspen and white spruce thinned to prescribed densities Assessed at year 10 post-thinning

Aspen Density

Spruce Density

0 200 500 1500 4000 Natural

1000

500

0 X X X

Managing mixedwoods: managing aspen – white spruce densities

Aspen Density (Trees/Ha)0 200 500 1500 4000 20000

Perc

ent C

over

0

20

40

60

80

100BaregrounMossesGraminoidForbsShrubsTotal

b

a ab ab ab ab

With increasing aspen density: • Total & graminoid cover declined • No effects on richness

Understory Cover



Spruce Density (Trees/Ha)

0 500 1000

Perc

ent C

over

0

20

40

60

80BaregroundMossesGraminoidsForbsShrubsTotal

c

d cd

a b ab

With increasing spruce density: • Graminoid cover increased – but low spruce density had only high aspen • No effects on richness

Understory Cover


Axis 1 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Axis

2

-0.6

-0.4

-0.2

0.0

0.2

0.4

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1.0

Legend

Aw/sw 1000 500 0

Natural

4000

1500

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0


Variation in understory composition – with aspen density


Axis 1 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Axis

2

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Aspen BA

Aspen density

Spruce density

Light

Aw Sw 1000 500 0

Natural

4000

1500

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0


Explanatory variables for understory composition

Understory Composition


Axis 1 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Axis

2

-0.6

-0.4

-0.2

0.0

0.2

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1.0


Variation in understory composition – with aspen density

Pure aspen - natural Pure aspen – managed Spruce dominated – managed Mixedwood - managed


Conclusions: Managing mixedwoods: managing aspen – white spruce densities

Manipulating aspen and white spruce densities influenced: - understory cover - composition - but not richness Reducing density increases cover (graminoids, shade intolerant) Influence of aspen density depended on spruce density – and vice versa Managed spruce-dominated vs aspen-dominated differed in composition; mixedwoods highly variable Weak evidence that higher spruce densities were associated with higher cover of closed forest species

Macdonald/Purdy

Managing mixedwoods: understory protection harvest

Three studies in Alberta: 1- or 2-pass: • Calling Lake (3 yrs post-cut) • Hotchkiss (8 yrs post-cut, 3 yrs post- 2nd pass) High or Moderate retention: 13 yrs post-cut

Reduce/remove aspen overstory Retain white spruce understory (for later harvest)

Christina Mourelle, Beth Dankert

0

10

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30

40

1-Pass 2nd of 2-Pass 0

10

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30

40

High retention

Moderate retention

Understory Protection Harvest: Effects on understory species richness (per plot)

Hotchkiss (8, 3 yrs post-cut) ‘High-Intermediate’ (13 yrs post-cut)

a a b

ab a b

1st of 2- PASS

Calling Lake (3 yrs post-cut)

0

10

20

30

40

Control-1 1-PASS

a a b c

Control-2

Control Control

-1.0 -0.5 0.0 0.5 1.0 -1.0

-0.5

0.0

0.5

1.0 Calling Lake Control

2-pass

1-pass

Hotchkiss

-1.0 -0.5 0.0 0.5 1.0 -1.0

-0.5

0.0

0.5

1.0 Control

1-pass

2nd of 2-pass

Control

High-Moderate

-1.0

-0.5

0.0

0.5

1.0

-1.0 -0.5 0.0 0.5

HighRet

ModRet

shade intolerant species Shade tolerant, closed-forest species

Understory Protection Harvest: Effects on understory species composition

Short term: Richness post-harvest Subsequent disturbance (2nd pass): richness Short term changes in composition: shade intolerants old-forest understory species Longer term / higher retention: Richness and composition of harvest – more similar to unharvested control

Conclusions: Effects of Understory Protection Harvesting on understory communities

Erica Graham: MSc thesis

Creating mixedwoods: underplanting white spruce

Mature aspen canopy Not too dense Low shrub cover Underplant white spruce After 15 - 20 years years: Understory Protection Harvest of aspen overstory Stand regrows as a mixedwood

Sampled understory communities: • In stands that were 5, 10, 15, 48 years post-planting • Two (younger) or four (oldest) distances away from planted white spruce • In unplanted control areas

Creating mixedwoods: underplanting white spruce

0

5

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25

15 10 5

# Sp

ecie

s/Pl

ot

Years since planting

0-1m from spruce

1-2m from spruce

aspen control


Creating mixedwoods: 48 years post-underplanting planted white spruce height ~ 12 m NOTE: these are older than planned

0-1 m 1-2 m 2-3 m 3-4 m unplanted

Und

erst

ory

cove

r

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0-1 m 1-2 m 2-3 m 3-4 m unplantedSp

ecie

s ric

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0 – 2 m from planted spruce there were differences in: litter depth, soil temperature, pH, some nutrients, microbial biomass nitrogen, light


Creating mixedwoods: 48 years post-underplanting

Constrained ordination (RDA) Erica Graham: MSc thesis

Creating mixedwoods: 48 years post-underplanting

Constrained ordination (RDA) Erica Graham: MSc thesis

0-1 m from spruce 1-2 m from spruce 2-3 m from spruce 3-4 m from spruce unplanted

Creating mixedwoods: Conclusions

No influence of planting spruce in the short term Longer term (48 years?) planted spruce influence the understory community - Reduced richness - Reduced cover - Different composition

Also affect: edaphic properties, understory environment What will happen when underplanted stands are subject to understory protection harvesting?

Conclusions: Effects of managing mixedwoods on understory communities

In unmanaged mixedwood stands and patches understory cover, richness and composition vary with canopy composition (aspen, mixed, conifer) Managing aspen-white spruce densities affects understory cover and composition Notable differences between aspen-dominated vs spruce-dominated Creating spruce-dominated stands won’t necessarily support spruce- or mixedwood – associated understory species

Conclusions: Effects of managing mixedwoods on understory communities

Understory protection harvest – in the longer term will support understory communities more similar to unharvested mixedwoods Underplanting spruce – in the long term planted spruce influence understory community and environment (only within 2 m) Any reduction in canopy cover results in increased understory cover – particularly shade intolerant species and graminoids By leaving canopy cover, increasing the spruce component we can perhaps facilitate development of understory communities more like those found in natural mixed and conifer-dominated stands Patience is required

Thanks!!

Alberta Conservation Association Alberta-Pacific Forest Industries Alberta Sustainable Resource Development Canadian Forest Products Ltd. Canadian Forest Service Canadian Circumpolar Institute Daishowa-Marubeni International Ltd. Natural Sciences and Engineering Research Council Mixedwood Management Association Sustainable Forest Management Network of Centres of Excellence Weyerhaeuser MANY field assistants

managing canopy composition in boreal mixedwood forests ......managing mixedwoods: managing aspen...

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