interactions between rising CO₂ levels and increasing global temperatures in terrestrial ecosystems

The presentation explores the interactions between rising CO₂ levels and increasing global temperatures in terrestrial ecosystems. It emphasizes that both factors significantly shape ecosystem productivity, carbon storage, and climate feedbacks. While CO₂ fertilization enhances plant growth, higher temperatures can counteract these benefits through increased respiration, altered nutrient cycling, and water stress. Studies indicate that C3 plants exhibit higher photosynthetic efficiency at elevated CO₂, but their net primary productivity (NPP) may not always increase due to temperature-induced physiological stress.

Model simulations suggest that climate scenarios impact vegetation carbon stocks differently, with some regions experiencing up to 20% carbon loss while others gain due to altered precipitation patterns. A case study on maple trees found that CO₂ enrichment boosted growth by 73%, but higher temperatures reduced it by 35%, showing additive effects. The presentation underscores the complexity of multi-factor experiments, highlighting that temperature fluctuates widely, while CO₂ levels remain relatively stable. It concludes that understanding CO₂-temperature interactions is crucial for predicting ecosystem responses to climate change and that single-factor experiments may not fully capture the complexity of future environmental shifts.

1. Interactions Between Increasing CO
2 and
Temperature in Terrestrial Ecosystems
Lake Tahoe, California
April 27-30, 2003

2. Organizing Committee
Claus Beier
Jeff Dukes
Sune Linder
Yiqi Luo
Dave McGuire
Rich Norby
Bill Parton
Diane Pataki
Lou Pitelka
Lindsey Rustad
Gus Shaver
Ben Smith
…and special thanks to
Tracey Walls

3. Interactions Between Increasing CO
2 and
Temperature in Terrestrial Ecosystems
A Conceptual Framework
Richard J. Norby
Oak Ridge National Laboratory

4. Why do we care about “Interactions
Between Increasing CO
2
and
Temperature in Terrestrial Ecosystems”?
“The ecosystems of the world are critical foundations
of human society.”
and…
“…ecosystems participate in the shaping of weather,
climate, atmospheric composition, and climate
change”
Global Environmental Change: Research Pathways for the Next Decade.
National Academy Press, 1999.

5. Increasing atmospheric CO
2 concentration
and increasing global air temperature are two
of the most important environmental
influences that will impact future ecosystems

6. Projections based
solely on warming lead
to provocative
conclusions… but CO
2

effects are said to be
uncertain.
“… a 300-ppm increase in atmospheric CO
2

concentration produces a 182% increase in the mean
productivity of the world’s forests, which is the same
as the growth response of the sour orange trees”
Projections based solely on CO
2 effects also lead
to provocative conclusions… but feedbacks,
interactions, and scale issues are ignored.

7. Temperature and CO
2
interact to affect
photosynthesis and
growth….the general
response for C
3

plants is that the
optimum temperature
increases for net
photosynthesis.
Although increasing
temperature may lead
to higher NPP, NEP
may not increase and
may even become
negative. However, the
direct effect of
increasing CO
2 may
partly offset or reverse
this effect
It is no longer useful
to examine the
impacts of climate
change absent their
interactions with
rising atmospheric
CO
2
IPCC Assessment Reports
1990 1995 2001

8. Projections of relative changes in
vegetation carbon between 1990 and
the 2030s for two climate scenarios.
Under the Canadian model scenario,
vegetation carbon losses of up to
20% are projected in some forested
areas of the Southeast in response
to warming and drying of the region
by the 2030s.
Under the same scenario, vegetation
carbon increases of up to 20% are
projected in the forested areas in the
West that receive substantial
increases in precipitation.
Output from TEM) as part of the
VEMAP II
Climate Change Impacts on the United States

9. Hadley simulation
0
5
10
15
20
Biome-BGC CENTURY TEM
%

c
h
a
n
g
e

f
r
o
m

c
u
r
r
e
n
t
climate+CO2
climate
Canadian simulation
-15
-10
-5
0
5
10
15
Biome-BGC CENTURY TEM
%
c
h
a
n
g
e

f
r
o
m

c
u
r
r
e
n
t
climate+CO2
climate
•Predictions are for
2025-2034 (425 ppm
CO
2)
•All three models predict
increased NPP with
climate change and
increased CO
2
for both
climate simulations
•Increases are smaller
(or become decreases)
when CO
2 is not
included
•Increases are less with
the Canadian climate
simulation
Prediction of NPP with Three Biogeochemical Models

10. Biome-BGC CENTURY TEM
Plant responses
CO
2
Ci ↑
production ↑
canopy conductance ↓
leaf N ↓
 
potential production ↑
transpiration ↓
leaf N ↓
Ci ↑
production ↑

Temperature
Pn optimum
Rm ↑
Rg ↑ with Pn
production optimum GPP optimum
Rm ↑
Rg ↑ with GPP
Soil responses
CO
2
soil moisture ↑
litter N ↓
soil moisture ↑
decomposition ↓ with
leaf N ↓
 
decomposition ↓ with
leaf N ↓

Temperature
decomposition ↑
soil moisture ↓
N mineralization ↑
decomposition ↑
soil moisture ↓
N mineralization ↑
decomposition ↑
soil moisture ↓
N mineralization ↑
Overview of Model Assumptions about Responses to
CO
2
and Temperature

11. What do we know about CO
2 x
temperature interaction?
Strong, mechanistic understanding
of CO
2 x temperature interactions in
the biophysics and biochemistry of
photosynthesis and
photorespiration
Most additional information comes from
•case studies
•elevated CO
2 studies in relation to natural temperature
variation
•combining results of single factor studies in models
Is this the best approach?
From SP Long (1991) PC&E 14:729

12. CO
2
x Temperature Interactions: a
case study with maple trees
Elevated CO
2
increased
growth of
maples trees
by 73%
0
1
2
3
4
5
6
7
8
amb CO2 elev. CO2 amb. CO2 elev. CO2
ambient temperature elevated temperature
k
g

d
r
y

m
a
s
s

13. Elevated
temperature
reduced
growth by 35%
because of
increased
stress
0
1
2
3
4
5
6
7
8
amb CO2 elev. CO2 amb. CO2 elev. CO2
ambient temperature elevated temperature
k
g

d
r
y

m
a
s
s
CO
2
x Temperature Interactions: a
case study with maple trees

14. Positive
effects of CO
2
and negative
effects of
temperature
were additive
0
1
2
3
4
5
6
7
8
amb CO2 elev. CO2 amb. CO2 elev. CO2
ambient temperature elevated temperature
k
g

d
r
y

m
a
s
s
CO
2
x Temperature Interactions: a
case study with maple trees

15. Null Hypothesis
Responses to CO
2
and temperature are additive;
therefore…
We can best understand the combined effects of
elevated CO
2
and temperature by gaining a
thorough understanding of their separate effects in
single-factor experiments

16. Multi-factor experiments
Expensive
Substitute factors for replications
Difficult to constrain hypotheses
Results often difficult to interpret
Conceptually confusing
Useful for reminding us that the future is uncertain!

17. There are important differences between
CO
2 and temperature effects, and the way
we study them must also be different
CO
2 primarily stimulates photosynthesis – most other responses
are secondary
Temperature affects all biological processes
•photosynthesis
•respiration
•cell division
•phenology
Changing temperature implies changing water

18. There are important differences in how CO
2 and
temperature variables are characterized
Temperature varies widely over a single day and seasonally; CO
2
is relatively constant
Decadal changes in temperature are small relative to short-term
variation
Mean, minimum, maximum, range, extreme temperature events
are all important
300
400
500
600
700
-30
-20
-10
0
10
20
30
T
e
m
p
e
r
a
t
u
r
e

(
o
C
)
C
O
2

(
p
p
m
)
control
treatment
Day of year Day of year

19. •Responses to CO
2 are
relatively simple
•CO
2
will increase
uniformly across the
planet
•Temperature responses
depend on the initial
conditions of the system
•Temperature increases in the
future have wider uncertainty
•Increases will not be uniform
200 300 400 500 600 700
CO
2 (ppm)
1900 2000 ------2100--- year
?
-2 -1 0 1 2 3 4 5
2000 -------------2100----------- -year
T (

C)
CO
2
Response Temperature Response

20. -2 -1 0 1 2 3 4 5
?
2000 -------------2100--------------- year
T (

C)
CO
2 Response
Temperature Response
When combining CO
2
and
temperature effects, we
must rely on scenario
testing
•uncertainty in combination
of CO
2
and temperature
increase for a given date
•different response
geographically
•different ways to express
the warming treatment
Take care in generalizing
from model systems!
200 300 400 500 600 700CO
2 (ppm)
1900 2000 ------2100----- year
We cannot specify the CO
2 and
temperature conditions for a
future ecosystem

21. -9 -6 -3 0 3 6 9
Mean annual temperature
-.5
0.0
0.5
1.0
1.5
2
E
f
f
e
c
t

s
i
z
e
N mineralization
-9 -6 -3 0 3 6 9
Mean annual temperature
-1
0
1
2
3
E
f
f
e
c
t

s
i
z
e
Plant Response
Why do we need
ecosystem
studies?
Synthesis of warming studies
shows why:
Hypothesis: elevated T
stimulates Nmin which leads
to increased NPP
Problem: some studies
measured soil processes
others looked at
aboveground productivity,
few looked at both
Result: the hypothesis could
not be tested
N mineralization
Plant response
Mean annual temperature
Mean annual temperature

22. Scale considerations are
paramount
Biochemical and physiological responses do not necessarily
predict plant, community, ecosystem, region
•Some responses become less important (e.g., stomatal
conductance)
•Other processes increase in importance (structural changes)
Short-term responses to experimental perturbations do not
necessarily predict long-term responses to gradual
environmental change
•acclimation
•pool turnover
•natural variation – a surrogate for global change?
Are there any special scaling considerations with
CO
2
x temperature interaction?

23. Sample questions about CO
2 x
temperature interaction?
•If more C enters an ecosystem due to elevated CO
2
, will it
simply be respired faster due to elevated temperature?
•If higher temperature increases C turnover, is this
response ameliorated by elevated CO
2?
•If higher temperature extends the length of the growing
season, does this present an opportunity for a larger
effect of elevated CO
2?
•Over the longer-term, will vegetation patterns that are
currently defined by temperature regimes be modified in
the future by elevated CO
2
?

24. Experimental studies and approaches
CO
2
enrichment experiments
•Salt marsh vs. tundra
•FACE experiments
•Analyze results in relation to natural temperature variation
Warming studies
•Soil warming
•Infrared warming
•Experiments cover a wider range of temperature zones
Multi-factor studies
•Small stature systems
•Components of large-stature ecosystems
Observations of nature
•CO
2
springs
•Vegetation patterns
•Flux networks

25. CO
2
Shaver et al. (2000) BioScience 50:871
A conceptual framework for CO
2
x temperature
effects on NPP and NEP

26. Conceptual framework for analysis of CO
2
x
temperature interactions in ecosystems
Projections of ecosystem responses to environmental changes must
recognize and incorporate the reality of multiple factor influences
We cannot experimentally duplicate a future ecosystem and the
multiple influences on it, and we cannot generalize from case studies
Models need to be informed by single-factor experiments; in the
absence of specific evidence of interactions, assume additivity
between factors

27. Conceptual framework for analysis of CO
2
x
temperature interactions in ecosystems
(continued)
CO
2 enrichment will affect ecosystem metabolism primarily by
increasing C input through photosynthetic stimulation and growth, as
modified by N, water, and other environmental factors
Warming will influence ecosystem metabolism through effects on C
processing rates that regulate NPP, microbial respiration, and
ecosystem structure (population and community responses)
Responses to warming are dependent on initial conditions and are the
net effect of multiple responses, possibly in opposite directions
Analyses of ecosystem responses must be sensitive to scale
considerations, especially in regard to fluxes between pools with
different rate constants

28. Conceptual framework for analysis of CO
2
x
temperature interactions in ecosystems
(continued)
Multi-factor (CO
2 x temperature)
experiments are important
•for testing concepts (looking for
non-additivity)
•demonstrating the reality of
multiple-factor influence
•reminding us that “predicting the
future is …

29. Conceptual framework for analysis of CO
2
x
temperature interactions in ecosystems
(continued)
Multi-factor (CO
2 x temperature)
experiments are important
•for testing concepts (looking for
non-additivity)
•demonstrating the reality of
multiple-factor influence
•reminding us that “predicting the
future is … fraud with uncertainty”