Interactions Between Increasing CO 2 and Temperature in Terrestrial Ecosystems

1. Interactions Between Increasing CO2 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 CO2 and Temperature in Terrestrial Ecosystems A Conceptual Framework Richard J. Norby Oak Ridge National Laboratory

4. Why do we care about “Interactions Between Increasing CO2 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 CO2 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 CO2 effects are said to be uncertain. Projections based solely on CO2 effects also lead to provocative conclusions… but feedbacks, interactions, and scale issues are ignored. “… a 300-ppm increase in atmospheric CO2 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”

7. IPCC Assessment Reports 1995 2001 1990 Temperature and CO2 interact to affect photosynthesis and growth….the general response for C3 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 CO2 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 CO2

8. Climate Change Impacts on the United States 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

9. Prediction of NPP with Three Biogeochemical Models • Predictions are for 2025-2034 (425 ppm CO2) • All three models predict increased NPP with climate change and increased CO2 for both climate simulations • Increases are smaller (or become decreases) when CO2 is not included • Increases are less with the Canadian climate simulation

10. Overview of Model Assumptions about Responses to CO2 and Temperature

11. What do we know about CO2 x temperature interaction? Strong, mechanistic understanding of CO2 x temperature interactions in the biophysics and biochemistry of photosynthesis and photorespiration From SP Long (1991) PC&E 14:729 • Most additional information comes from • case studies • elevated CO2 studies in relation to natural temperature variation • combining results of single factor studies in models • Is this the best approach?

12. CO2 x Temperature Interactions: a case study with maple trees Elevated CO2 increased growth of maples trees by 73%

13. CO2 x Temperature Interactions: a case study with maple trees Elevated temperature reduced growth by 35% because of increased stress

14. CO2 x Temperature Interactions: a case study with maple trees Positive effects of CO2 and negative effects of temperature were additive

15. Null Hypothesis Responses to CO2 and temperature are additive; therefore… We can best understand the combined effects of elevated CO2 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 CO2 and temperature effects, and the way we study them must also be different • CO2 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 CO2 and temperature variables are characterized Temperature varies widely over a single day and seasonally; CO2 is relatively constant Decadal changes in temperature are small relative to short-term variation Mean, minimum, maximum, range, extreme temperature events are all important treatment CO2 (ppm) Temperature (oC) control Day of year Day of year

19. CO2 Response Temperature Response ? DT (oC) CO2 (ppm) 1900 2000 ------2100--- year 2000 -------------2100----------- -year • Temperature responses depend on the initial conditions of the system • Temperature increases in the future have wider uncertainty • Increases will not be uniform • Responses to CO2 are relatively simple • CO2 will increase uniformly across the planet

20. CO2 (ppm) 1900 2000 ------2100----- year ? We cannot specify the CO2 and temperature conditions for a future ecosystem CO2 Response • When combining CO2 and temperature effects, we must rely on scenario testing • uncertainty in combination of CO2 and temperature increase for a given date • different response geographically • different ways to express the warming treatment • Take care in generalizing from model systems! Temperature Response DT (oC) 2000 -------------2100--------------- year

21. N mineralization 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 Mean annual temperature Plant response 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 CO2 x temperature interaction?

23. Sample questions about CO2 x temperature interaction? • If more C enters an ecosystem due to elevated CO2, will it simply be respired faster due to elevated temperature? • If higher temperature increases C turnover, is this response ameliorated by elevated CO2? • If higher temperature extends the length of the growing season, does this present an opportunity for a larger effect of elevated CO2? • Over the longer-term, will vegetation patterns that are currently defined by temperature regimes be modified in the future by elevated CO2?

24. Experimental studies and approaches • CO2 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 • CO2 springs • Vegetation patterns • Flux networks

25. CO2 A conceptual framework for CO2 x temperature effects on NPP and NEP Shaver et al. (2000) BioScience 50:871

26. Conceptual framework for analysis of CO2 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 CO2 x temperature interactions in ecosystems (continued) CO2 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 CO2 x temperature interactions in ecosystems (continued) • Multi-factor (CO2 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 CO2 x temperature interactions in ecosystems (continued) • Multi-factor (CO2 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”