Effects of Land Use Change on Forest Carbon Budgets Throughout the Southern USA from 1900 to 2050

Peter B. WoodburyCrop and Soil Sciences Department, Cornell University

James E. Smith, and Linda S. HeathUSDA 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:Land Use Transitions:Model focuses on afforestation and deforestationModel focuses on afforestation and deforestation

Forest

Pasture/Grassland

Urban/Developed

PlowedCropland

Forest Carbon Pools

Down dead woodDown dead wood

UnderstoryUnderstory

Live Live treestrees

Standing Standing dead dead treestrees

Soil organic Soil organic carbon (to 1 m)carbon (to 1 m)

Forest floorForest floor

How to estimate land use change How to estimate land use change effects on forest carbon cycling? 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 2050.

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

Key model inputsKey model inputs

Historical land use transition matrix:

Forest 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 2003.

Forest floor, from Smith and Heath 2002.

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

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0

1000

2000

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4000

1900 1925 1950 1975 2000

Year

An

nu

al A

rea

Ch

an

ge

(1

00

0 h

a)

Total Defor.

Total Affor.

Pine Defor.

Pine Affor.

Key Model EquationsKey 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

0102030405060708090

100

0 25 50 75 100 125 150 175 200Years Since Land Use Change

So

il C

to

1 m

Dep

th (

t/h

a)

-20

-15

-10

-5

0

5

10

15

20

0 10 20 30 40 50

Years Since Land Use Change

Car

bon

Flu

x fr

om F

ores

t F

loor

(t/

ha)

Afforestation (negative)

Deforestation (positive)

Changes in forest floor carbon for pine forest typesChanges in forest floor carbon for pine forest types

Based on Smith and Heath 2002

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

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-1

0

1900 1925 1950 1975 2000 2025 2050Year

An

nu

al C

ha

ng

e in

C S

toc

ks

(T

g/Y

)

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0A

nn

ua

l Are

a A

ffore

ste

d

In Southeast, net emission until 1940’s, then generally sequestration

Woodbury et al. draft June 2004

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0

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1900 1925 1950 1975 2000 2025 2050Year

Cu

mu

lati

ve

Ch

an

ge

in C

Sto

ck

s (

Tg

)

Forest Floor

Forest Floor

Soil

Soil

TOTAL

In South-Central, greater effects with later peak

Woodbury et al. draft June 2004

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0

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1900 1925 1950 1975 2000 2025 2050Year

Cu

mu

lati

ve

Ch

an

ge

in C

Sto

ck

s (

Tg

)

Forest Floor

Forest Floor

Soil

Soil

TOTAL

Down dead woodDown dead wood

UnderstoryUnderstory

Live Live treestrees

Standing Standing dead dead treestrees

Soil organic Soil organic carbon (1m)carbon (1m)

Forest floorForest floor

But wait, But wait, soil and forest floor are only part of the story!soil and forest floor are only part of the story!

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

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0

50

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Tre

es

So

ila

nd

Fo

r.F

loo

rTo

tal C

ha

ng

e in

Ca

rbo

n (

Tg

)

-39-240

ConclusionsConclusions

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.

AcknowledgementsAcknowledgements

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 USADraft results for all regions of USA

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1900 1925 1950 1975 2000 2025 2050

Year

Cum

ulat

ive

Cha

nge

in C

Sto

cks

(Tg)

TOTAL

SE

NE

PC

GP

SCRMNC

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 Main data sources for estimating land use transitionstransitions

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:Model Parameter Example:

Forest soil carbon density values derived from STATSGO Forest soil carbon density values derived from STATSGO

0

50

100

150

200

250

Oak-p

ine

Oak-h

ickor

y

Loblol

ly-sh

ortle

af p

ine

Map

le-bee

ch-b

irch

Oak-g

um-cy

press

Spruce

-fir

Aspen

-birc

h

Soi

l Car

bon

Den

sity

(t/

ha,

1 m

dep

th)

Just a little jargon …

AFFORESTATION: Conversion of land that has been non-forested for 20-50 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 DeforestationChange in Soil Carbon with Deforestation

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0

20

40

0 5 10 15 20 25 30 35 40 45 50 55

Years after Deforestation

Ch

ange

in S

oil C

(%

)

Houghton and Hackler (2001)

Murty et al. 2002

FORLUCModel

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

0

5

10

15

20

25

30

35

0 5 10 15 20

Years Since Land Use Change

For

est

Flo

or C

(t/

ha)

Model (Smith & Heath 2002)

Key Model CalculationsKey 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 stocksHow we estimate forest and wood carbon stocks

FIA data, volume-biomass Tree carbon, equations in the past

ATLAS and FORCARB Tree carbon, Future forests models present, 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. 2004 (in US Greenhouse Gas Inventory

Annual Change in Tree Carbon in 2002 (t/ha)

Based on Woodbury et al. 2004 (in US Greenhouse Gas Inventory)

Estimating changes in forest carbonEstimating changes in forest carbon(stock change approach)(stock change approach)

Estimate forest carbon at Time 1 = C1

Estimate forest carbon at Time 2 = C2

Annual change = (C2 – C1) / years

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

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RL

UC

1990

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RL

UC

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00 t

o 20

04

Tot

al C

han

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C (

Tg)

Ove

r P

erio

d

-58 299

Soil

ForestFloor

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

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US

GH

G19

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FO

RL

UC

1990

to

2004

Source of Estimate

Tot

al C

han

ge in

C (

Tg)

Ove

r P

erio

d

Soil

ForestFloor

Tree

HarvestedWood

Down Woody Debris

Soil

ForestFloor

Some Key AssumptionsSome 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 AssumptionsMore 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.