Effects of Land Use Change on Forest Carbon Budgets Throughout the Southern USA from 1900 to 2050 Peter B. Woodbury Crop and Soil Sciences Department, Cornell University James E. Smith, and Linda S. Heath USDA Forest Service, Durham, NH Third USDA Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry March 22-24, 2005, Baltimore, Maryland

Goal: Improve US national estimates of terrestrial carbon flux from the forest sector. Such estimates needed annually for the US Greenhouse Gas Inventory required under the United Nations Framework Convention on Climate Change (UNFCCC). The UNFCCC reporting period begins in 1990.

Land Use Transitions: Model focuses on afforestation and deforestation Forest Pasture/ Grassland Urban/ Developed Plowed Cropland

Forest Carbon Pools Down dead wood Understory Live trees Standing dead trees Soil organic carbon (to 1 m) Forest floor

How to estimate land use change effects on forest carbon cycling? For tree and down dead wood pools, use forest inventory (FIA) data and FORCARB model (see talks by James Smith and by Linda Heath this afternoon) For soil and forest floor, we developed a model to predict effects from 1900 to )Quantify land use transition rates (based on work by R. Birdsey) 2)Develop equations to predict effect of each land use transition on soil and forest floor carbon.

Model Regions: For today, South-Central and Southeast regions

Key model inputs Historical land use transition matrix: F orest inventory data and reports, National Resource Inventory data, and Census of Agriculture. Soil and forest floor carbon density for each land use and each forest type: Soil from STATSGO soil database, overlaid with forest type groups, from Johnson and Kern Forest floor, from Smith and Heath 2002.

Land Use Transition Example: Historical afforestation and deforestation in South-Central

Key Model Equations Afforestation effects on soil carbon Deforestation effects on soil carbon Afforestation effects on forest floor carbon Deforestation effects on forest floor carbon

Change in soil carbon after afforestation for Loblolly pine forest type Woodbury et al. draft May 2004

Changes in forest floor carbon for pine forest types Based on Smith and Heath 2002

RESULTS: Effects of afforestation are damped and lagged, for example in the South-Central region

In Southeast, net emission until 1940’s, then generally sequestration Woodbury et al. draft June 2004

In South-Central, greater effects with later peak Woodbury et al. draft June 2004

Down dead wood Understory Live trees Standing dead trees Soil organic carbon (1m) Forest floor But wait, soil and forest floor are only part of the story!

From 1990 to 2004, sequestration in trees in Southern US was 6-fold greater than that in soil and forest floor

Conclusions From 1900 to 1990, land use change in the Southern US caused net carbon emission of 270 Tg C from soil and forest floor. Conversely, from 1990 to the present, land use change caused net carbon sequestration of 39 Tg C. Changes in forest floor carbon were similar to changes in soil carbon, despite much less total carbon. Changes in tree carbon stocks during this period were 6-fold greater than effects on soil and forest floor carbon stocks. Therefore, estimates of forest carbon sequestration are dominated by effects on tree carbon stocks, which can be estimated based on forest inventory data.

Acknowledgements Rich Birdsey, USDA Forest Service, PA Jeff Kern & Mark Johnson, USEPA, OR Steve Prisley & Beyhan Amichev, Virginia Tech, VA Mike Nichols, USDA Forest Service, NH Ken Skog, USDA Forest Service, WI Tom Wirth, USEPA, Washington D.C. Funding provided by USEPA and the USDA Forest Service

Draft results for all regions of USA

Why model land use change effects? Land use change is an important type of global change in its own right! Land use change affects terrestrial carbon flux, and may also affect climate. Our immediate goal: Improve US national estimates of terrestrial carbon flux, with a focus on forests. Such estimates needed annually for the US Greenhouse Gas Inventory required under the United Nations Framework Convention on Climate Change (UNFCCC).

Main data sources for estimating land use transitions National Resource Inventory (measures actual transitions on survey plots, only started 20 years ago, doesn’t cover all lands). Forest Inventory data (systematic surveys since about 1935) Forest survey reports (used for early 1900s) Census of Agriculture (used for early 1900s)

Model Parameter Example: Forest soil carbon density values derived from STATSGO

Just a little jargon … AFFORESTATION: Conversion of land that has been non- forested for years into forest. REFORESTATION: Growth of forest on previously forested land (not considered a land use change in our model). DEFORESTATION: Conversion of forest to another land use. FOREST FLOOR: All material above the mineral soil, including woody material < 7.5 cm in diameter. FOREST TYPE: A broad classification of forests by dominant species, from USDA Forest Service (for example “Maple/Beech/Birch”.

Change in Soil Carbon with Deforestation

Test of Forest Floor Model: Change in forest floor carbon with replacement of a 55 y old mixed conifer / hardwood stand with a loblolly pine plantation

Key Model Calculations All calculations are by region, by forest type, and by “cohort” (cell in the transition matrix). For each time period, calculate: Effect of each land use transition on soil carbon flux. Effect of each land use transition on forest floor carbon flux. Store results for each “cohort” for each time period, generate graphs and tables.

How we estimate forest and wood carbon stocks FIA data, volume-biomass  Tree carbon, equations in the past ATLAS and FORCARB  Tree carbon, Future forests modelspresent, future Soil survey data, land use  Soil carbon model (FIA P3, coming soon!) Intensive research site data and models  Forest floor, (FIA P3, coming soon!) Down dead, Understory

Tree Carbon Density in 2002 (t/ha) Woodbury et al (in US Greenhouse Gas Inventory

Annual Change in Tree Carbon in 2002 (t/ha) Based on Woodbury et al (in US Greenhouse Gas Inventory)

Estimating changes in forest carbon (stock change approach) Estimate forest carbon at Time 1 = C 1 Estimate forest carbon at Time 2 = C 2 Annual change = (C 2 – C 1 ) / years

Land use change effects during the UNFCCC reporting period are very different than effects over the last century

A CONCLUSION: Sequestration in the rest of the forest sector is much greater than that in soil and forest floor

Some Key Assumptions Because the model focuses on transitions to and from forest, not all lands and land use changes are modeled. All land throughout a large region is assumed to respond in the same way to land use changes. All carbon lost from forest soil and forest floor is emitted to the atmosphere. For example, no carbon is assumed to be stored in sediments.

More Key Assumptions There is no net change in soil carbon due to transitions between forest and pasture, but there is loss of forest floor carbon. Disturbances such as fire are not explicitly included in the model except as they are captured by differences in soil and forest floor carbon between land use types. Changes in soil bulk density are not explicitly accounted for, but the parameter selected for the total change in soil carbon with deforestation implicitly accounts for higher bulk density in agricultural than in forest soils.