Modern and Future Forest Ecosystems

Richard J. NorbyEnvironmental Sciences DivisionOak Ridge National Laboratory

Snowbird, UtahDecember 8, 2001

Trees that are planted today will be growing in a higher CO2 concentration tomorrow

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•The rate of photosynthesis in tree leaves increases with increasing [CO2], as with any C3 plant

•Myriad secondary and tertiary effects follow

Trees respond to atmospheric [CO2]

•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 [CO2]

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.

- Paul Kramer, 1981

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.

- Boyd Strain & Fakhri Bazzaz, 1983

Forestry Issues with Rising Atmospheric CO2

Global Carbon Cycle

flux storage

Net primary

productivity

Vegetation Response

“CO2 fertilization”

Forest composition

and biomass

Deforestation,

reforestation

Climatic Change

“greenhouse effect”

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CO

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Atmospheric Carbon Dioxide

Has increasing [CO2] influenced today’s forests?

Evidence•Tree-ring chronologies

•Forest inventory•13C analysis•Stomatal density

LaMarche et al. (1984) claimed that increased growth of subalpine conifers since 1960 could be attributed to increasing [CO2].

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

Science 225:1019 (1984)

Ecology 70:405 (1989)

Tree-Ring Evidence for Past Effects of [CO2]

• 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 CO2 on C accumulation in eastern forests

Caspersen et al. (2000) Science 290:1148

Forest Land Percentages

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Pacific NW

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Heartland

Alaska

West

Great Plains

•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

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 Geothermic CO2 spring

in Tuscany

Ring-width chronologies of Quercus ilex and Q. pubescens show no effect of [CO2]

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

• 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 CO2 like other plants

• 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

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Does Tree Growth Increase in High CO2?

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

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Dry matter increment

Growth per unit leaf area

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?

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

• 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.

Goal

• 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

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)

Calculation of NPP

Oak Ridge Experiment on CO2 Enrichment of Sweetgum

Stem

Allometry : DM = f(BA, H, taper, density)

Fine rootMinirhizotrons and in-growth cores

Coarse rootAllometry: DM = f(BA)

UnderstoryHarvest

LeafLitter traps

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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

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Understory

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CoarseRoot

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

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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

Fine root production

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Net Ecosystem Productivity

• NEP was positive each year.

• CO2 effect on heterotrophic respiration was less than effect on NPP.

Oak Ridge Experiment on CO2 Enrichment of Sweetgum

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Respiration

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NEP gain

wood gain

NPP 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

Dynamics of Carbon Storage

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

Elevated CO2 often reduces stomatal conductance and leaf-level transpiration and increases water-use efficiency

Interactions between CO2 and Water

• 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

Interactions between CO2 and Warming

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CO2 x temperature interactions in maple

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•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

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Hadley simulation

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Biome-BGC CENTURY TEM

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Canadian simulation

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• 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

Prediction of NPP with Three Biogeochemical Models

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

1960-1990

2070-2100

Hadleyscenario Canadian

scenario

Dominant Forest Types

Forest Productivity

• 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

Conclusions