Modern and Future Forest Ecosystems Richard J. Norby Environmental Sciences Division Oak Ridge National Laboratory Snowbird, Utah December 8, 2001
Trees that are planted today will be growing in a higher CO2 concentration tomorrow
Trees respond to atmospheric [CO2] • The rate of photosynthesis in tree leaves increases with increasing [CO2], as with any C3 plant • Myriad secondary and tertiary effects follow
Physiological adjustments, biogeochemical feedbacks, ecological interactions, and the influence of other environmental factors will alter the primary response to elevated [CO2] • Net effect on forests is uncertain • Implications of any effect are unclear • Detection of effects is difficult Aber et al. 2001
- Paul Kramer, 1981 We cannot make reliable predictions concerning the global effects of increasing CO2 concentration until we have information based on long-term measurements of plant growth from experiments in which high CO2 concentration is combined with water and nitrogen stress on a wide range of species.
- Boyd Strain & Fakhri Bazzaz, 1983 The initial effect of elevated CO2 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.
Forestry Issues with Rising Atmospheric CO2 Global Carbon Cycle flux storage Climatic Change “greenhouse effect” Vegetation Response “CO2 fertilization” Net primary productivity Forest composition and biomass Deforestation, reforestation
Has increasing [CO2] influenced today’s forests? • Evidence • Tree-ring chronologies • Forest inventory • 13C analysis • Stomatal density
Tree-Ring Evidence for Past Effects of [CO2] LaMarche et al. (1984) claimed that increased growth of subalpine conifers since 1960 could be attributed to increasing [CO2]. Science 225:1019 (1984) Graumlich et al. (1989) attributed recent increases in tree growth in western Washington to increased temperature – CO2 effects were unimportant Jacoby and D’Arrigo (1997) concluded “The present tree-ring evidence for a possible CO2 fertilization effect under natural environmental conditions appears to be very limited.” PNAS 94:8350 Ecology 70:405 (1989)
Inventory analysis suggests limited influence of CO2 on C accumulation in eastern forests • 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. • 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 CO2, N deposition, ozone, etc. • Forest regrowth is by far the dominant factor governing the rate of C accumulation Caspersen et al. (2000) Science 290:1148
Forests in a future atmosphere • Forest physiology, productivity, and ecology in a future, CO2-enriched atmosphere can be addressed through: • direct observation • manipulative experiments • modeling The most important problems concern issues of scaling across time and space
Tree-ring analysis at a CO2 spring Ring-width chronologies of Quercus ilex and Q. pubescens show no effect of [CO2] Geothermic CO2 spring in Tuscany Cumulative basal area was lower for Q. ilex in high CO2 and higher (ns) in Q. pubescens. Other species showed no effect at all. Tognetti et al. (2000). New Phytologist 146: 59
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
Tree species respond to elevated CO2 like other plants • 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
Does Tree Growth Increase in High CO2? • 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 CO2? • Expressing annual growth per unit leaf area adjusts for exponential growth pattern • Response is uniform across species and conditions: 29% increase at ~650 ppm CO2 • 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
Can we predict the responses of the future forest from the responses of individual saplings? 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?
Questions for new experiments at the forest stand scale • Will maximum stand leaf area increase with increasing [CO2]? • 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?
Oak Ridge Experiment on CO2 Enrichment of Sweetgum Goal • To understand how the eastern deciduous forest will be affected by CO2 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.
CO2 Enrichment of a Deciduous Forest: The Oak Ridge Sweetgum Experiment • Liquidambar styraciflua plantation started in 1988 • 2 elevated, 3 control plots (2 with blowers) • Each plot is 25 m diameter (20 m diameter inside buffer) • Full year of pre-treatment measurement in 1997 • CO2 exposure (560 ppm) started spring, 1998 • Exposure is 24 hours per day, April through October • Brookhaven exposure system
Oak Ridge Experiment on CO2 Enrichment of Sweetgum Calculation of NPP Coarse root Allometry: DM = f(BA) Stem Allometry : DM = f(BA, H, taper, density) Leaf Litter traps Fine root Minirhizotrons and in-growth cores Understory Harvest
Oak Ridge Experiment on CO2 Enrichment of Sweetgum Net Primary Productivity • CO2 effect on NPP was: 27% in 1998, 17% in 1999, 19% in 2000. • GPP was 27% higher in elevated CO2. • Allocation of the extra C changed from stem to fine root.
Oak Ridge Experiment on CO2 Enrichment of Sweetgum Aboveground Woody Increment • No difference in growth prior to treatment (1997). • CO2 significantly increased growth in 1st year of treatment (35%), but not in 2nd (15%) or 3rd (7%). • The CO2 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 CO2 Enrichment of Sweetgum 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 CO2 efflux - subtract root respiration - add litter decomposition
Oak Ridge Experiment on CO2 Enrichment of Sweetgum Net Ecosystem Productivity • NEP was positive each year. • CO2 effect on heterotrophic respiration was less than effect on NPP.
Dynamics of Carbon Storage NPPe NPPa NEPe NEPa NPP gain NEP gain wood gain Short-term C sequestration (NEP) was enhanced by CO2 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
Interactions between CO2 and Water Elevated CO2 often reduces stomatal conductance and leaf-level transpiration and increases water-use efficiency Common assumption is that this will confer increased drought resistance, but evidence generally is lacking CO2 effects on leaf-level transpiration are mitigated as the scale increases to a whole canopy, stand, and region
Interactions between CO2 and Warming • 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 CO2 is likely to ameliorate negative effects of warming
CO2 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 CO2 and negative effects of temperature were additive
Prediction of NPP with Three Biogeochemical Models • Predictions are for 2025-2034 (425 ppmv 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
Predicted Shift in Distribution with Climate Change Sugar maple Red maple 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.
Dominant Forest Types 1960-1990 2070-2100 Hadley scenario Canadian scenario
Conclusions • Increasing atmospheric CO2 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 [CO2] -- 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