Upload
timothy-wilfred-hall
View
217
Download
0
Tags:
Embed Size (px)
Citation preview
Growing season dynamics in high-latitude ecosystems:
relations to soil thermal regimes, productivity, carbon sequestration, and
atmospheric heating
Bonanza Creek LTER Symposium February 2006
(Serreze et al., Climatic Change, 2000)
High Latitude Temperature
Trends
(1966-1995)
Annual data
°C per decade
Outline of this talk:
Part I: Examine changes in growing season length in
high-latitude ecosystems (Based on Euskirchen et al.,
in press, Global Change Biology)
Part II: Relate these changes to changes in energy
Spring: beginning of the growing season:
Increasing temperature and light availability
The snow melts
Thawing of soil organic horizons
Onset of photosynthesis
Fall: end of growing season:
Temperatures and light availability decrease
Soils re-freeze
Photosynthesis slows or ceases
Net ecosystem productivity could increase or decrease in response to changes in
soil freeze-thaw regimes.
Increases could be due to a longer growing season.
However, enhanced productivity could be counter-balanced by increases in respiration from the soil
heterotrophs.
The recent availability of remotely sensed spatially
explicit data from high-latitudes provides an opportunity to
evaluate if a large-scale process-based model captures
changes in snow cover, soil freeze-thaw regimes, and growing season length.
Satellite detection of recent changes in timing of pan-arctic
spring thaw (K.C McDonald et al., Earth
Interactions, 2004) Earlier thaw Later thaw
Change in Day of Thaw (Days/Year)
-3 -2 -1 0 1 2 3
Pan-Arctic Growing Season Change
Slope = -0.25 days per year
23-Apr
28-Apr
3-May
8-May
13-May
18-May
23-May
28-May
1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998
Date of leaf-out in Fairbanks (Chena Ridge) 1974-1998
Data courtesy of J. Anderson
The increase in growing season length over the last 50 years averaged for 8 stations in Alaska having the longest and most consistent temperature records
(Keyser et al., 2000).
slope = 0.33 days/yr; p< 0.0001
Part I: What are the implications of recent observed changes in snow cover, soil freeze-thaw regimes,
and the timing and length of the growing season on terrestrial carbon dynamics, both retrospectively (1960-2000) and prognostically (2001 –2100)?
Terrestrial Ecosystem Model couples biogeochemistry & soil thermal dynamics
Soil Thermal Model (STM)
Vegetation type; Snow pack; Soil moistureSoil temperature
RA RH
LC
LN
Soil
Temps.
at
Different
Depths
Upper Boundary Conditions
Snow Cover
Moss & litter
Frozen Ground
Thawed Ground
Frozen Ground
Lower Boundary Conditions
Heat Conduction
Moving phase plane
Organic Soil
Mineral Soil
Prescribed Temperature
Prescribed Temperature
Snow Depth
Moss Depth
Organic Soil Depth
Mineral Soil Depth
Moving phase plane
Heat balance surface
Lower boundary
Heat Conduction
Terrestrial Ecosystem Model (TEM)
TEM Simulations & Model Validation
-Conducted simulations focusing on terrestrial land areas above 30º N and retrospective decadal trends from the 1960s –2000
-Also conducted prognostic simulations focusing on 2001-2100 using interpolated climate data obtained from a two dimensional climate model (Sokolov and Stone, 1998)
-Performed simulations with transient CO2 and climate data
-Validated the TEM results with several remotely sensed datasets (Dye, 2002; McDonald et al., 2004; Smith et al., 2004)
8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3
Duration of Snow FreePeriod 1972-2000
Based on simulation of the TEM for
north of30o N 1972 1980 1990 2000
-3
-1
1
Sn
ow
Fre
e D
ura
tio
n A
no
ma
ly (
we
eks
)
-4
-2
0
2
4
-3
-1
1
3
Region 1
Region 2
Region 3
D. Dye = White lines TEM = Colored lines
Boreal region
Boreal region
Trends in the Duration of the Snow-Free Period
1972-2000 Anomaly (Weeks)Slope Intercept R2 Correlation
Region 1TEM 0.07 -1.05 0.14
0.36Dye* 0.03 -0.47 0.20
Region 2TEM 0.04 -0.64 0.12
0.73Dye 0.03 -0.70 0.23
Region 3TEM 0.03 -0.39 0.04
0.57Dye 0.01 -0.21 0.05
*D. Dye, Hydrological Processes, 2002
8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3
Duration of Snow FreePeriod 1972-2000
Growing season length (GSL)
change(days per year)
1960-2000 2001-2100
Shorter GSL Longer GSL
<-2 -1 -0.5 0 0.25 0.5 1 2 >3
Region
(Years)
Change in spring thaw (days earlier per year)
TEM McDonald et al.(1) Smith et al. (2)
North America
(1988 – 2000)0.22 0.92 0.09
Eurasia
(1988 – 2000)0.15 0.34 0.36
Pan-Arctic
(2001 – 2100)0.36
(1) Earth Interactions, 2004(2) Journal of Geophysical Research, 2004
Net primary productivity
Heterotrophic respiration
9.1 g C m-2 yr-1
day-1
3.8 g C m-2 yr-1
day-1
18.3 g C m-2 yr-1
day-1
8.8 g C m-2 yr-1
day-1
-250
0
250-550
0
550
-75
0
75-150
0
150
-8 -6 -4 -2 0 2 4 6 8 -30 -20 -10 0 10 20
Growing season length anomaly (days)
1960-2000 2001-2100A
no
ma
ly (
g C
m-2 y
r-1)
[R2] = 0.40-0.87[p] < 0.0001
9.5 g C m-2 yr-1 day-1
Anomaly (g C m-2 yr-1)Soil C
Vegetation C
8.9 g C m-2 40 yr-1 33.8 g C m-2 100 yr-1
Growing season length anomaly (days)
-300
0
300
-75
0
75
[R2] = 0.30-0.88[p] < 0.0001
Net ecosystem productivity
-8.1 g C m-2 40 yr-1
-1000
0
1000
-30 -20 -10 0 10 20
-300
0
300-75
0
75 5.3 g C m-2 yr-1
day-1
-100
0
100
-8 -6 -4 -2 0 2 4 6 8
-13.2 g C m-
2 100 yr-1
22.2 g C m-2 100 yr-1
1960-2000 2001-2100
8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3
Trends in growing season length, productivity and respirationGreatest
increases in GSL. Smallest increases in
productivity and respiration
Similar increases in GSL to Region
2. Greatest overall increases in
productivity and respiration
Similar increases in GSL to Region 3. Intermediate
increases in productivity and
respiration.
Duration of snow-free period
1960197019801990
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
(c)
0
0.5
-0.5
-1
1
-1.5 2000 20102020 20302040 20502060 20702080 2090
(d)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2000 20102020 20302040 20502060 20702080 2090
1960197019801990
(a) (b)
Cu
mu
lati
ve N
EP
(P
g C
re
gio
n-1) 2
3
1
0
-1
-2
-3
4 1960-2000 2001-2100
J F M A M J J A S O N D J F M A M J J A S O N DMonth
Bo
real
& t
un
dra
re
gio
ns
(60
– 90
° N
)T
emp
erat
e re
gio
ns
(30
– 60
° N
)
Sou
rce
Sou
rce
Sin
kS
ink
Part I: Conclusions
Model simulations indicate strong connections between decreases in snow cover and changes in growing season length.
These dynamics substantially influence carbon fluxes, including enhanced respiration and productivity in our analyses.
Increases in productivity and respiration at high latitudes are not as large as those in lower latitudes.
It is important to improve our understanding of the relative responses of photosynthesis and respiration to changes in atmospheric CO2 and climate.
Part II – What are the relative responses of changes in high-latitude carbon uptake due to growing season length increase versus changes in albedo on the climate system?
-1.5
-1
-0.5
0
0.5
1
1.5
2
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
50-55 55-60 60-65 65-70
19401910
1970 - 20001970 2000
Air temperature anomaly (ºC, five year running mean)
Year
Air temperature anomaly (ºC)
Five-Degree Latitudinal band (ºN)
0.0078 – 0.0354 ºC year-1 0.0381 – 0.0388 ºC year-1
1910 - 1940
The snow – albedo feedback loop
Decreases in albedo
Decreases in snow cover
The greenhouse gas-ecosystem metabolism
feedback loop
Decreases in temperature
Enhancements in productivity greater than
enhancements in respiration
Increases in temperature
Increase in heat absorption
Increases in greenhouse
gases
Enhancements in respiration greater than
enhancements in productivity
Increases in growing season
length
8.0 –18.0 Weeks – Region 118.0 – 28.0 Weeks – Region 228.0 –37.0 Weeks – Region 3
Duration of Snow FreePeriod 1972-2000
Based on simulation of the TEM for
north of30o N 1972 1980 1990 2000
-3
-1
1
Sn
ow
Fre
e D
ura
tio
n A
no
ma
ly (
we
eks
)
-4
-2
0
2
4
-3
-1
1
3
Region 1
Region 2
Region 3
D. Dye = White lines TEM = Colored lines
1970-2000
1910-1940
Change in autumn snowfall
Change in springsnowmelt
Total change in snow cover duration
Days per year earlier (or shorter for the ‘total’)
Days per year later (or longer for the ‘total’)
<- 0.4 -0.4 -0.3 -0.2 -0.05 0.01 - 0.1 >0.1
Anomaly
-1.4 days
decade-1
-2.5 days
decade-1
SNOW:-Compiled seasonal data on surface energy balance (sensible-plus-latent heat flux to the atmosphere as a proportion of net radiation) by vegetation type.
-Calculated monthly mean incoming shortwave radiation in TEM.
-Estimated daily atmospheric heating depending on snow surface conditions.
-Multiplied changes in snow cover (days per year) by changes in atmospheric heating (Chapin et al. 2005).
ECOSYSTEM METABOLISM:A 4.4 W m-2 atmospheric heating change with a doubling of [CO2].
311 g C m2 increase/decrease for each 1 W m-2 increase/decrease
Translation of changes in snow cover and ecosystem metabolism to changes in radiative forcing
Houghton et al., 2001
springautumn spring & fallChanges in energy (W m-2) due to snow cover changes in:
1970-2000
1910-1940
< 0 0 - 0.5 0.5 - 1 1 - 2 2 - 3 ≥ 3 W m-2
Cooling Heating
< 0 0 - 0.5 0.5 - 1 1 - 2 2 - 3 ≥ 3 W m-2
Cooling Heating
1970-2000
1910-1940
Changes in energy (W m-2) due to changes in:
snow coverecosystem metabolism
For boreal forests,
changes in ecosystem metabolism(CO2 + climate): 1910-1940: 0.00 W m-2
1970-2000: -0.06 W m-2
changes in snow cover:1910-1940: +0.56 W m-2
1970-2000: +1.04 W m-2
Change in energy (W m-2)due to changes in:
Time period snow coverecosystem metabolism
1910-1940 1.1 ~ -0.05
1970-2000 1.9 ~ -0.10
(Negative sign represents negative feedback for the sink term, positive sign is positive feedback for a source term)
Changes in snow cover had a much greater effect on energy than did changes in ecosystem metabolism
Suggests the importance of considering other factors that may alter albedo.
Pan-Arctic: north of 50° N
Foley et al., 2003
Reduced growing season albedo and increased spring energy absorption
Part II Conclusions:
The effects of a longer snow-free season on atmospheric energy balances should considered in studies of climate change, particularly with respect to associated shifts in vegetation between forests, grasslands, and tundra.
We should also consider other factors that play an important role in altering surface albedo, such as changes in fire regime, insect defoliation, timber harvest, and conversion to/from agriculture.
And finally, how do these factors interact with changes in growing season length?
AcknowledgementsFunds were provided by:
The NSF for the Arctic Biota/Vegetation portion of the Climate of the Arctic: Modeling and Processes project within International Arctic Research Center at the University of Alaska Fairbanks
The USGS ‘Fate of Carbon in Alaska Landscapes’ project