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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
40
60
80
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
30
40
50
60
70
Broadleaf Mixed Conifer
Axis 1
Axis
2
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
Sheelah Griffiths: MSc thesis
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
Sheelah Griffiths: MSc thesis
Managing mixedwoods: managing aspen – white spruce densities
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
Sheelah Griffiths: MSc thesis
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
Legend
Aw/sw 1000 500 0
Natural
4000
1500
500
200
0
Managing mixedwoods: managing aspen – white spruce densities
Variation in understory composition – with aspen density
Sheelah Griffiths: MSc thesis
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
500
200
0
Managing mixedwoods: managing aspen – white spruce densities
Explanatory variables for understory composition
Understory Composition
Sheelah Griffiths: MSc thesis
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
Managing mixedwoods: managing aspen – white spruce densities
Variation in understory composition – with aspen density
Pure aspen - natural Pure aspen – managed Spruce dominated – managed Mixedwood - managed
Sheelah Griffiths: MSc thesis
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
20
30
40
1-Pass 2nd of 2-Pass 0
10
20
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
10
15
20
25
15 10 5
# Sp
ecie
s/Pl
ot
Years since planting
0-1m from spruce
1-2m from spruce
aspen control
Erica Graham: MSc thesis
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
0
20
40
60
80
100
120
0-1 m 1-2 m 2-3 m 3-4 m unplantedSp
ecie
s ric
hnes
s pe
r plo
t
0
5
10
15
20
25
0 – 2 m from planted spruce there were differences in: litter depth, soil temperature, pH, some nutrients, microbial biomass nitrogen, light
Erica Graham: MSc thesis
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