1. Modern and Future Forest Ecosystems Richard J. Norby Environmental Sciences Division Oak Ridge National Laboratory Snowbird, Utah December 8, 2001

2. Trees that are planted today will be growing in a higher CO 2 concentration tomorrow

3. The rate of photosynthesis in tree leaves increases with increasing [CO 2 ], as with any C 3 plant Myriad secondary and tertiary effects follow Trees respond to atmospheric [CO 2 ]

4. Net effect on forests is uncertain Implications of any effect are unclear Detection of effects is difficult Aber et al. 2001 Physiological adjustments, biogeochemical feedbacks, ecological interactions, and the influence of other environmental factors will alter the primary response to elevated [CO 2 ]

5. We cannot make reliable predictions concerning the global effects of increasing CO 2 concentration until we have information based on long-term measurements of plant growth from experiments in which high CO 2 concentration is combined with water and nitrogen stress on a wide range of species. - Paul Kramer, 1981

6. The initial effect of elevated CO 2 will be to increase NPP in most plant communities... A critical question is the extent to which the increase in NPP will lead to a substantial increase in plant biomass. Alternatively, increased NPP could simply increase the rate of turnover of leaves or roots without changing plant biomass. - Boyd Strain & Fakhri Bazzaz, 1983

7. Forestry Issues with Rising Atmospheric CO 2 Global Carbon Cycle flux storage Net primary productivity Vegetation Response “CO 2 fertilization ” Forest composition and biomass Deforestation, reforestation Climatic Change “greenhouse effect ”

8. Has increasing [CO 2 ] influenced today’s forests? Evidence Tree-ring chronologies Forest inventory 13 C analysis Stomatal density

9. LaMarche et al. (1984) claimed that increased growth of subalpine conifers since 1960 could be attributed to increasing [CO 2 ]. Graumlich et al. (1989) attributed recent increases in tree growth in western Washington to increased temperature – CO 2 effects were unimportant Jacoby and D’Arrigo (1997) concluded “The present tree-ring evidence for a possible CO 2 fertilization effect under natural environmental conditions appears to be very limited.” PNAS 94:8350 Science 225:1019 (1984) Ecology 70:405 (1989) Tree-Ring Evidence for Past Effects of [CO 2 ]

10. From forest inventory analysis, growth and mortality rates in the 1980’s were calculated and compared to realized biomass Discrepancies are assumed to be due to changes in growth rates over time. Inventory analysis suggests limited influence of CO 2 on C accumulation in eastern forests Caspersen et al. (2000) Science 290:1148 The fraction of regional ANEP due to growth enhancement is no more than 7.0% (average = 2.0 ± 4.4%) Growth enhancement is net effect of increased CO 2, N deposition, ozone, etc. Forest regrowth is by far the dominant factor governing the rate of C accumulation

11. Forests in a future atmosphere Forest physiology, productivity, and ecology in a future, CO 2 -enriched atmosphere can be addressed through: direct observation manipulative experiments modeling The most important problems concern issues of scaling across time and space

12. Tree-ring analysis at a CO 2 spring Geothermic CO 2 spring in Tuscany Ring-width chronologies of Quercus ilex and Q. pubescens show no effect of [CO 2 ] Cumulative basal area was lower for Q. ilex in high CO 2 and higher (ns) in Q. pubescens. Other species showed no effect at all. Tognetti et al. (2000). New Phytologist 146: 59

13. The scale of experimental studies has been gradually increasing It is not possible to create a simulation of a future forest – rather, we rely on experimental systems

14. photosynthesis increases, and the enhancement is sustained higher photosynthesis  faster growth Result: plants are bigger at the end of the experiment N concentration usually is lower stomatal conductance often is reduced no large changes in allocation Tree species respond to elevated CO 2 like other plants

15. Wide range of response of tree seedlings and saplings in different experiments Dry matter increase is confounded by increasing leaf area and exponential growth These results do not predict the response of a forest, and they do not represent fundamental differences between species Does Tree Growth Increase in High CO 2 ?

16. Expressing annual growth per unit leaf area adjusts for exponential growth pattern Response is uniform across species and conditions: 29% increase at ~650 ppm CO 2 Growth per unit leaf area separates the functional response from the structural response This approach may allow for prediction after canopy closure, but it breaks down if allocation changes

17. Response of tree productivity is likely to be less for mature trees in a closed forest How can data from relatively short-term experiments be used to address the long-term responses of a forest? Can we predict the responses of the future forest from the responses of individual saplings?

18. Questions for new experiments at the forest stand scale Will maximum stand leaf area increase with increasing [CO 2 ]? Will growth per unit leaf area increase after canopy closure? Will fine root turnover increase after standing crop not reaches a maximum? Is growth downregulated through feedbacks from the N cycle? Is stand water use predicted from effects on stomata?

19. Oak Ridge Experiment on CO 2 Enrichment of Sweetgum To understand how the eastern deciduous forest will be affected by CO 2 enrichment of the atmosphere, and what are the feedbacks from the forest to the atmosphere. This goal will be approached by measuring the integrated response of an intact forest ecosystem, with a focus on stand-level mechanisms. Goal

20. Each plot is 25 m diameter (20 m diameter inside buffer) Full year of pre-treatment measurement in 1997 CO 2 exposure (560 ppm) started spring, 1998 Exposure is 24 hours per day, April through October Brookhaven exposure system CO 2 Enrichment of a Deciduous Forest: The Oak Ridge Sweetgum Experiment Liquidambar styraciflua plantation started in 1988 2 elevated, 3 control plots (2 with blowers)

21. Calculation of NPP Oak Ridge Experiment on CO 2 Enrichment of Sweetgum Stem Allometry : DM = f(BA, H, taper, density) Fine root Minirhizotrons and in-growth cores Coarse root Allometry: DM = f(BA) Understory Harvest Leaf Litter traps

22. Net Primary Productivity CO 2 effect on NPP was: 27% in 1998, 17% in 1999, 19% in 2000. GPP was 27% higher in elevated CO 2. Allocation of the extra C changed from stem to fine root. Oak Ridge Experiment on CO 2 Enrichment of Sweetgum

23. Aboveground Woody Increment No difference in growth prior to treatment (1997). CO 2 significantly increased growth in 1st year of treatment (35%), but not in 2nd (15%) or 3 rd (7%). The CO 2 effect on stem growth also has disappeared in the Duke prototype FACE ring, but it has been sustained for 5 years in the fully replicated study. Oak Ridge Experiment on CO 2 Enrichment of Sweetgum

24. Fine Root Productivity Delay in fine root response may be related to accumulation of CHO in coarse roots. Fine root C turns over faster than wood C through soil processes. Analyze soil heterotrophic respiration: - integrate CO 2 efflux - subtract root respiration - add litter decomposition Oak Ridge Experiment on CO 2 Enrichment of Sweetgum

25. Net Ecosystem Productivity NEP was positive each year. CO 2 effect on heterotrophic respiration was less than effect on NPP. Oak Ridge Experiment on CO 2 Enrichment of Sweetgum

26. NPPe NPPa NEPe NEPa NEP gain wood gain NPP gain Short-term C sequestration (NEP) was enhanced by CO 2 enrichment With an increasing fraction of the additional C storage allocated to short-term pools rather than to wood, the gains are likely to be short-lived Dynamics of Carbon Storage

27. Common assumption is that this will confer increased drought resistance, but evidence generally is lacking CO 2 effects on leaf-level transpiration are mitigated as the scale increases to a whole canopy, stand, and region Elevated CO 2 often reduces stomatal conductance and leaf- level transpiration and increases water-use efficiency Interactions between CO 2 and Water

28. Temperature affects all biological processes Effects are non-linear, time-dependent, and highly dependent on initial conditions Warming can stimulate productivity through increased photosynthesis and longer growing season Warming can decrease productivity through increased stress Elevated CO 2 is likely to ameliorate negative effects of warming Interactions between CO 2 and Warming

29. CO 2 x temperature interactions in maple Red maple and sugar maples leafed out earlier in warmer chambers, increasing growth. Photosynthesis was higher in elevated CO2 and warmer air, also increasing growth However, a high temperature stress event caused a net negative effect on growth Positive effects of CO 2 and negative effects of temperature were additive

30. Predictions are for 2025- 2034 (425 ppmv 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

31. Prasad, A. M. and L. R. Iverson. 1999-ongoing. A Climate Change Atlas for 80 Forest Tree Species of the Eastern United States [database]. http://www.fs.fed.us/ne/delaware/atlas/index.html, Northeastern Research Station, USDA Forest Service, Delaware, Ohio. Sugar maple Red maple Predicted Shift in Distribution with Climate Change

32. 1960-1990 2070-2100 Hadley scenario Canadian scenario Dominant Forest Types

33. Forest Productivity

34. Increasing atmospheric CO 2 concentration has almost certainly affected modern forests and will affect forests in the future -- but the effects may be difficult to detect Net primary productivity of forests is likely to increase with increasing [CO 2 ] -- but the additional carbon may not accumulate in the ecosystem Physiological and process understanding should be used to inform ecosystem models -- but many responses are moderated as the scale increases from a tree to a forest Regional scale analyses should not be taken as predictions of the future – but they can help to define the sensitivities and vulnerabilities of forests to future environmental conditions Conclusions