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 19952001

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. 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-BGCCENTURYTEM 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 optimumGPP 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%

13. Elevated temperature reduced growth by 35% because of increased stress CO 2 x Temperature Interactions: a case study with maple trees

14. Positive effects of CO 2 and negative effects of temperature were additive 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 Temperature (oC) CO 2 (ppm) control treatment 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 CO 2 (ppm) 1900 2000 ------2100--- year ? 2000 -------------2100----------- -year  T (  C) CO 2 ResponseTemperature Response

20. ? 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! CO 2 (ppm) 1900 2000 ------2100----- year We cannot specify the CO 2 and temperature conditions for a future ecosystem

21. 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

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”