Space-Based Photosynthetic
Trends in High Latitude
Mountains: Greening or
browning?
Andy Bunn
Huxley College, Western Washington Univ.
Scott Goetz
Woods Hole Research Center
High latitude global change
~0.1°C/decade from
60°N-90°N
~0.06 °C/decade for
lower latitudes
Projected rise by late
21st century
60°N-90°N: 3.7°C
Globally: 1.9°C
NASA Goddard Institute for Space Studies
High latitude change: Impacts
Global
Albedo, Ocean circulation,
Carbon cycle, Biodiversity
Vegetation
Forest structure, Increase fire,
Insects, Forest production
Animals
Range shifts, Fisheries
Coastal communities
Erosion, Sea level rise,
Flooding, Relocation
Infrastructure
Frozen period length,
Destabilization of structures
Marine transport
Shipping, Drilling, Sea ice
movement, Geopolitics
Indigenous communities
Physical and cultural livelihood,
uncertainty
Elevated UV
People, flora, fauna
Multiple influences interact to cause impacts to people and ecosystems
Carbon pools
Upland boreal
forest soils:
90-290e+15 g C
Peatland soils:
120-460e+15 g C
Boreal forest:
100e+15 g C
Global Atmospheric Pool:
730e+15 g C
Boreal lakes:
120e+15 g C
Tundra soils:
60-190e+15 g C
Upper permafrost:
10,000e+15 g C (!)
Marine permafrost:
10,000e+15 g C (!)
Arctic Ocean:
450e+15 g C
Adapted from ACIA 2004
Land surface change
Evidence of treeline expansion (Lloyd, 2005, Ecology)
Ubiquitous advancement but idiosyncratic and dependent
on drainage
Rates of change are slow
• 1m/year in permafrost-affected areas
• 10m/year in permafrost-unaffected areas (far fewer sites)
Feedbacks from forest expansion will be small in near
future
Evidence of shrub expansion (Sturm, 2005, Bioscience)
Woody shrubs invading tundra
Feedbacks involving snow, albedo, and microbial activity
Has lead to a complicated system of ecological
propagation
Land surface change
Role of land surface changes (Chapin, 2005, Science)
“Pronounced terrestrial summer warming in arctic Alaska
that correlates with a lengthening of the snow-free
season has increased atmospheric heating by about 3
w/m2/decade”
“The continuation of current trends in shrub and tree
expansion could further amplify this atmospheric heating
by two to seven times.”
Massive positive feedbacks that could acerbate warming
A negative feedback….
Several studies of the high latitudes
indicate massive “greening” of the
boreal forest as seen from satellites
Greening is an increase in NDVI over time
• Normalized Difference Vegetation Index
Well correlated to chlorophyll abundance
Used as a proxy for gross photosynthesis
1981-present
15-day composites
8-km pixels
0.2
Pg
0.4
0.6
Myneni et al. 1997 Nature
1982
1984
1986
Time
1988
1990
1992
1980
1982
1984
1986
Time
1988
1990
1992
Seasonal Pg
0.56
0.60
0.64
1980
Average of land surfaces above 50° N
Early evidence for greening
Negative feedback
could ameliorate high
latitude C cycling
IPCC 2001
ACIA 2004
0.2
Pg
0.4
0.6
The full record 1981-2003
1980
1985
1990
1995
2000
2005
1995
2000
2005
Seasonal Pg
0.56
0.60
0.64
T
Time
1980
1985
1990
T
Time
Goetz, Bunn, et al. 2005 PNAS
0.55
0.50
0.45
Evergreen needle-leaf forest
Tundra
0.40
Seasonal Pg
0.60
0.65
Subset by vegetation type
1980
1985
1990
1995
2000
2005
Time
Goetz, Bunn, et al. 2005 PNAS
Time series analysis of AVHRR-NDVI 1982-2003
RAN
yt
AR
β2 = 0
ADF
DET
lm
β2 ≠ 0
yt = β1 + β 2t + ut , H 0 : β 2 = 0, H1 : β 2 ≠ 0
Trends in North America
Goetz, Bunn, et al. 2005 PNAS
Trends in North America: Climate
Using gridded climate data
Bunn & Goetz, 2005, GRL
Tundra responses key on maximum summer
temperature
Forest responses are more complicated
• Mostly key on previous year’s conditions
• Spring minimum temperatures (+) and
precipitation (+)
• Summer max temperature (-)
Time to move beyond North America
Circumpolar Trends
Bunn & Goetz, 2006, Earth Interactions
Circumpolar Trends
Bunn & Goetz, 2006, Earth Interactions
Circumpolar Trends
Slope Magnitude
Overall
Forest
Tundra
Negative
4.8%
7.3%
3.6%
Not Significant
86.3%
90.5%
83.4%
Positive
8.9%
2.3%
13.1%
Trends in High Latitude Mountains
Slope Magnitude
Overall
Forest
Tundra
Negative
5.7%
11.4%
4.4%
Not Significant
81.4%
86.5%
79.2%
Positive
13.0%
2.0%
16.4%
Trend Comparison
Most areas show no significant trends
81% mountain vs. 86% circumpolar
Mountain trends better expressed
Forests show more ‘browning’
• 11.4% mountain vs. 7.3% circumpolar
Tundra shows more ‘greening’
• 16.4% mountain vs. 13.1% circumpolar
Conclusions
Most areas show no significant trend
Greening largely confined to tundra
Browning in largely confined to “forest”
Early summer greening gives way to late
summer browning (not shown)
Mountains show slightly stronger results
than entire circumpolar
Prospectus
What accounts for browning in forests?
Tree-ring studies show evidence for
temperature induced drought stress
• Barber; Lloyd; Wilmking; and others
Physiological temperature thresholds
• D’Arrigo; Jacoby
Prospectus: tree-rings
Vast network of tree-ring
sites
Integrative measure of
growth that overlaps the
space-based record
Tree-ring sites have long
records
Satellite record is spatially
continuous
Many records incomplete
• E.g., End in the 1980’s
Needs updating
Prospectus: tree-rings
0.6
-0.2
0.0
0.2
0.4
Integrate statistically and
via process modeling
(LEAP)
-0.4
Species
Landscape setting
Incorporate long term
variability
ACF
Preliminary results are
exciting
Climate-growth
relationships
Ring width and July NDVI
-4
-2
0
Lag
2
4