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Heterogeneity, legacies, and convergence: forest structure and function on Rocky Mountain landscapes
Dan KashianDepartment of Forest, Rangeland, and Watershed Stewardship
Colorado State University
Defining landscape ecology
YellowstoneNational
Park
TargheeNationalForest
1. Broad spatial scales
2. Spatial heterogeneity
3. Pattern and process
4. The human role/impact
Outline
• Climate change and carbon cycling on the Yellowstone landscape
• Future directions
• Variability and convergence in forest structure
• Post-fire heterogeneity in forest regeneration• The 1988 fires in Yellowstone
Yellowstone: an exciting place!
The Yellowstone landscape
• Stand-replacing fires
• 100-300 year fire interval
• Large,“natural”landscape
Light/Severe Surface Fire
Severe Surface/Crown Fire
Variation in burn severity
Yellowstone Burn Mosaic, 10/88
Serotiny in lodgepole pine
Variation in lodgepole pine serotiny
Variation in regeneration density
>50,000 stems/ha
1,000 stems/ha
0 stems/ha
Mapping regeneration density
Orthorectification Supervised classification
GIS map
Do initially dissimilar stand structures eventually converge?
???
???
Stand structure: Methods
• Chronosequential measurements of unburned standsacross the landscape.
• Analyses of size and age structures and spatial patterns
• Regression analyses to reconstruct past density of stands using tree ring widths.
Variation in Stand Density
11,000 stems/ha
3,000 stems/ha
1,100 stems/ha
Stands shown arein the 50-100 year
age class
Den
sity
(ste
ms/
ha)
0
2000
4000
6000
69000
71000
73000
75000 a
b
cd d
250
0
50
100
150
200
12 50-100 125-175 200-250 300-350
Age class (years)
Coe
ffici
ent o
f var
iatio
n (%
)
bb
c c
a
Variation and change in stand density with age
0
5
10
15
20
25
30
35
40
45
50
0 2 4 6 8 10
Stand A: Initially dense
Age: 130 yearsDensity: 5,400 stems/ha
Stand B: Initially sparse
Age: 130 yearsDensity: 1,020 stems/ha
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 2.0 4.0 6.0 8.0 10.0
0
40
80
120
160
200
4 8 12 16 20 24 28 32 36
DBH Class (cm)
02
6
10
14
18
60 70 80 90 100 110 120 130Age (years)
Num
ber
of T
r ees
Initially densestand
• Unimodal, steepdistributions
• Dead trees common and small
Dead treesLive trees
0
2
4
6
4 8 12 16 20 24 28 32 36
10
15
0
5
55 65 75 85 95 105 115 125
DBH Class (cm)
Age (years)
Num
ber
of T
r ees
Initially sparsestand
• Bimodal or widedistributions
• Dead trees muchless common
Dead treesLive trees
-0.15
-0.05
0
0.05
0.15
1 2 3 4 5
-0.1
0
0.1
0.3
0.5
1 2 3 4 5L(t)
Reg
ular
C
lust
ered
Distance (m)
Initially dense stand: Spatial patterns
Pre-mortality(live + dead):stems clumped at all scales.
Post-mortality:stems random at most scales
Initially sparse stand: Spatial patterns
Stems random at all scales
Distance (m)
L(t)
Reg
ular
Clu
ster
ed
-0.6
-0.2
0
0.2
0.6
1 2 3 4 5
70-year-old stand
120-year-old stand
220-year-old stand
300-year-oldstand
70
70120
70120220
70120220
0
5000
10000
15000
20000
25000
30000
12 70 120
DenseSparse
Stand Age (years)
Stan
d Den
sity
(st
ems/
ha)
Stand Density Trajectory Reconstruction
64248
13048
683 931
5400
1020
Stand Density Trajectory Reconstructions
0
20000
60000
100000
12 70 125
Modeling age (years)
Den
sity
(ste
ms/
ha)
0
20000
60000
100000
12 70Modeling age (years)
Den
sity
(ste
ms/
ha)
12 70 125 220 300Modeling age (years)
12 70 125 220Modeling age (years)
• Rule-based simulation model in Arc based on empirical data.
• Model run on a decadal time step with resolution of 50 meters (0.25 ha).
• Subroutines simulate self-thinning, infilling in young stands, infilling in older stands.
How do stand-level processesaffect landscape pattern?
Landscape patternat 12 years following fire
Relative deviation from median density[(density-median)/median)]
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 2.967 stems/ha
Landscape patternat 50 years following fire
Relative deviation from median density
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 1,091 stems/ha
12 70 125 220 300
Landscape patternat 100 years following fire
Relative deviation from median density
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 866 stems/ha
12 70 125 220 300
Landscape patternat 200 years following fire
Relative deviation from median density
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 619 stems/ha
12 70 125 220 300
Landscape patternat 300 years following fire
Relative deviation from median density
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 627 stems/ha
12 70 125 220 300
Landscape patternat 300 years following fire
(with small fires)
Relative deviation from median density
-1 – -0.25-0.25 – 0.250.25 – 11 - 33 - 5>5
Median density = 700 stems/ha
Structure Conclusions
• Structural variability is likely related to initial variation in postfire density.
• Variation in stand structure across the landscape decreases with time and converges near 200 years.
• Large, infrequent disturbances leave an imprint on the landscape that may endure for two centuries.
What are the implications of dissimilar stand structures for
landscape carbon storage?
• How do large fires affect landscape carbon storage?
• How sensitive is carbon storage of a landscape to changes in climate and/or disturbance regimes?
Landscape carbon storage is affected by:• Balance between carbon accumulating in vegetation/forest floor and carbon lost through decomposition of dead wood.
• Changes in the stand density distribution across the landscape following fires.
• Changes in the standage distribution across the landscape followingfires.
∆C
(g C
/ yr
)
Years Since Stand-Replacing Fire
0
∆C Vegetation
∆C Dead Wood
Total NEP
NEP = C gained (NPP) – C lost (decomposition)
∆C Vegetation(g C/m2/yr)
Total NEP
(g C/m2/yr)
Cumulative NEP
(g C/m2)
0
50
100
Sparse
Dense
-500
50
Age Since Fire0 50 100 150 200 250
-6000
-4000
-2000
0
Do stand structures “replace themselves”?
Sparse pre-fire Sparse post-fire
Dense pre-fire Dense post-fire
Little changein C stored
over fire cycle
Little changein C stored
over fire cycle
=
=
Do stand structures “replace themselves”?
Sparse pre-fire Sparse post-fire
Dense pre-fire Dense post-fire
C lost over
fire cycle
C gainedover
fire cycle
=
=
Stand age distributions affect landscape NEP
50 years
300 years150 years
10 years
Modeling stand age and density effects on landscape C storage
Climate changemodel predicts fire frequency
(EMBYR)
Modeledcomponentsof C stocks
C stocks forage classes
Age distribution (ha)under wet or dry climate
Landscapeestimate of
C stocks
C stocks fordense & sparse
stands
∆C
Stor
ed(g
C/m
2x
103 )
Current climate,50% dense
Wet,50% dense
Dry,50% dense
9
11
13
15Stocks
Wet,90% dense
Dry,90% dense
Wet,10% dense
Dry,10% dense
C St
ocks
(g
C/m
2 x
103 )
Stand age and density effects on C storage
-101
2 Change from current
-2
Modeling future landscape C storage for Yellowstone
NEP forage classes
Age distributions (ha)
Modeledcomponentsof NEP
Pre-fire andpost-fire
vegetation mapsof Yellowstone
Landscapeestimates of NEP
-120
-100
-80
-60
-40
-20
0
20
40
1989 2029 2069 2109 2149 2189 2229
Prefire NEP
Total Landscape NEP
Tota
l Lan
dsca
pe N
EP (g
C/m
2 /yr
- )
-30
-25
-20
-15
-10
-5
0
5
10
Cumulative C Storage
Cumulative C Storage (g/m
2x 100)Long-term changes in C storage for Yellowstone
“Function” Conclusions:
• Equilibrium C storage is resistant to changes in disturbance regimes at landscape scales.
• Large changes in the distribution of stand densities on the landscape are necessary to shift its ability to store carbon.
• The post-1988 Yellowstone landscape will recover all carbon lost within the fire cycle (~230 years), but it is currently a large source of C to the atmosphere.
Current Projects:
• Carbon budgets along replicated age and density chronsequences
• Landscape carbon budget of Yellowstone using Century, EMBYR and Landsat
• Changes in productivity with tree age usingstable carbon isotopes (∆13C/ ∆12C )
• Changes and variability in foliar nitrogen with stand age and denisty.
Future research directions
How have disturbance regimes affected human land use, and how is this linked to climate change?
How do exotic disturbances affect forest structureand function compared to native disturbances?
What are the critical links between terrestrial systems(landscapes) and aquatic systems?
Defining Landscape Ecology
YellowstoneNational
Park
TargheeNationalForest
1. Broad spatial scales
2. Spatial heterogeneity
3. Pattern and process
4. The human role/impact
5. Collaboration