Estimating critical acid loads across the lower 48 US: Opportunities and challenges

Steven McNultyUSDA Forest Service

Southern Research Station Raleigh, NC

Steve_mcnulty@ncsu.edu

Three Parts to the Presentation

1.Development of a simple mass balance equation of critical acid loading and exceedances to forest soils across the conterminous US at a 1 km2 resolution

3. Discuss why historic definitions of a “healthy” forest may need to change

2. Assess how a changing climate could impact forest soil critical acid loads

Part 1. Development of a simple mass balance equation of critical acid loading and exceedances to

forest soils across the conterminous US

When pollutant loads exceed the critical load it is considered that there is risk of harmful effects. The excess over the critical load has been termed the exceedance. A larger exceedence is often considered to pose a greater risk of damage.

UK Centre for Ecology and Hydrology

Standard definition of a critical load

A critical load can be defined as  a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.

Simple Mass Balance Equationfor Forest Soils

CL(S+N) =

BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit)

BCdep = Base Cation DepositionCldep = Chloride DepositionBCw = Base Cation WeatheringBCu = Base Cation Uptake

Ni = Nitrogen ImmobilizationNu = Nitrogen UptakeNde = Nitrogen DenitrificationANCe(crit) = Acid Neutralizing Capacity

Simple Mass Balance Equationfor Forest Soils

CL(S+N) =

BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit)

BCdep = Base Cation DepositionCldep = Chloride DepositionBCw = Base Cation WeatheringBCu = Base Cation Uptake

Ni = Nitrogen ImmobilizationNu = Nitrogen UptakeNde = Nitrogen DenitrificationANCe(crit) = Acid Neutralizing Capacity

Data Wet Deposition Ca, Mg, K, Na, Cl, N, SO4,NH4,NO3

– Eastern US Grimms and Lynch, 2003 Incorporates elevation into calculation ~300 m2 resolution

– Western US USGS NADP/NTN 6.25 km2 resolution

Climate– Spatial Climate Analysis Service Prism– Temperature, Precipitation– 4 km2 and 16 km2 resolution

Data

National Forest Cover Dataset– USGS/USFS – 25 tree classes– 1 km2 resolution

Soil– Miller and White, 1998– Soil fraction, depth to bedrock, soil unit– 1 km2 resolution

Data

Resolution– 1 km2 across all forested soils

Temporal Extent– Average 1994 - 2000

Spatial Extent– Conterminous United States

• BC = Ca + K + Mg + Na

eq/ha

600 - 728

400 - 600

200 - 400

100 - 200

50 - 100

< 50

Base Cation Wet DepositionAverage: 1994 - 2000

Data Source:USGS NADP/NTNGrimms and Lynch, 2003

eq/ha

400 - 555

200 - 400

100 - 200

50 - 100

< 50

Chloride Wet DepositionAverage: 1994 - 2000

Data Source:USGS NADP/NTNGrimms and Lynch, 2003

Mean Depth to Bedrock

meter

0 - 0.34

0.35 - 0.7

0.71 - 1.03

1.04 - 1.34

1.35 - 1.52

No Bedrock Detected

Caution:"Many STATSGO component table entries for depth to bedrock used 60 inches (1.52 m) to indicate that bedrock was not encountered within this distance of the surface. As a result, the mean depth to bedrock values in this dataset should be used primarily to identify mapunits in which bedrock may be enountered at depths shallower than 1.52 m" (Miller and White, 1998)

BC = Ca + K + Mg + Na

eq/ha/yr

3,000 - 3,739

2,000 - 3,000

1,000 - 2,000

27 - 1,000

Base Cation WeatheringSoil Type Texture Method

Data Source:SASC ClimateSoil Data, Miller and White, 1998

BC = Ca + K + Mg

eq/ha/yr

252

613

Base Cation Uptake

Data Source:Hall et al., 1998

eq/ha/yr

278

279

Nitrogen Uptake

Data Source:Hall et al., 1998

eq/ha/yr

-250 - 0

-500 - -250

-1000 - -500

-2000 - -1,000

-3,664 - -2,000

Acid Neutralizing Compacity

Data Source:USGS NADPGimms and Lynch, 1998USGS/USFS Forest Type

Acid Neutralizing Capacity

eq/ha

600 - 1,388

400 - 600

200 - 400

100 - 200

50 - 100

< 50

Sulfur + Nitrogen Wet DepositionAverage: 1994 - 2000

Data Source:USGS NADPGimms and Lynch, 1998

Estimated Forest Soil Critical Acid Load

Estimated Areas of Forest Soil in Exceedence of the Critical Acid Load

McNulty, Steven G.; Cohen, Erika C.; Moore Myers, Jennifer A.; Sullivan, Timothy J.; and Li, Harbin. 2007. Estimates of critical acid loads and exceedences for forest soils across the conterminous United States. Environmental Pollution 149: 281-292

For more details on this study see

Define environmental conditions impacting forest soil critical acid loads that will likely change with climate

Assess the directions an magnitude of change

Recalculate critical acid loads based on updatedparameters

Part 2. Climate change Impacts on forest soil critical acid loads

Simple Mass Balance Equationfor Forest Soils

CL(S+N) =

BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit)

BCdep = Base Cation DepositionCldep = Chloride DepositionBCw = Base Cation WeatheringBCu = Base Cation Uptake

Ni = Nitrogen ImmobilizationNu = Nitrogen UptakeNde = Nitrogen DenitrificationANCe(crit) = Acid Neutralizing Capacity

Climate change factors

Base cation weathering - as air temperature increases, BCW will increase(increasing the CAL)

Nitrogen uptake – as air temperature increases, productivity increases and nitrogen uptake will increase (increasing the CAL)

Acid Neutralizing capacity (2 components)- as air temperature increases, productivity increases and base cationuptake increases (reducing ANC the CAL)

- as air temperature increases, forest evaportanspiration increases and runoff decreases (reducing ANC and reducing CAL)

Gross ecosystem productivity change

ImplicationsChanging climate could signficantly impact the amount of forest soilIn exceedance of the critical acid load

- total forest area in exceedance could decrease by 6%

However, most of the exceedance occurs in New England whereThe impacts of climate change will be felt the most.

- Therefore, the amount of area in the highest catogory of Exceedance (i.e. 750 eq/ha/yr) could decrease by over 20%

HOWEVER!

Before we all go rushing out turn up our thermastats and purchaseCadillacs to do our part to contribute to global warming, we should listen to the last part of the talk....

Part 3. Beyond altering the forest soil critical acid loading on a forest, can climate change

Impact how healthy forests respond to stress?

A Case Study entitled

“The Rise of the Mediocre Forest”

Picea rubens (red spruce) mortality near Asheville, NC

The loss of the red spruce in the southern Appalachian Mountains

Western NC experienced a moderate three year droughtfrom 1999-2002. In 2001, red spruce (Picea rubens) treesbegan to die in large numbers in and around Mt. MitchellNC, USA. The initial evidence suggested that the affectedtrees were killed by the southern pine beetle (SPB). This insect species is not normally successful at colonizing these tree species. Subsequent investigations revealed an interesting pattern where trees died or survived theSPB attack.

Background

12.0

12.5

13.0

13.5

14.0

14.5

1996-2000

1991-1995

1986-1990

1981-1985

1976-1980

1971-1975

1966-1970

1961-1965

1956-1960

1951-1955

Periods

Me

an

Air

Te

mp

era

ture

o C

200

300

400

500

600

700

800

Me

an

Pre

cip

itatio

n (

mm

)

Mean Air Temperature

Precipitation

Five Year Averaged Climate (1951 – 2001), Mt. Mitchell, NC, USA

Sampling damaged Southern Appalachian red spruce stand

0

4

8

12

16

20

1996-2000

1991-1995

1986-1990

1981-1985

1976-1980

1971-1975

1966-1970

1961-1965

1956-1960

1951-1955

Years

Bas

al A

rea

Gro

wth

(c

m2/

5 ye

ar a

vera

ge)

Dead

Live

Drought Stress

Drought, Temperature, Insect, & N deposition

Stresses

1.0

1.1

1.2

1.3

1.4

Fol

iar

N C

once

ntra

tion

(%)

Dead Plots

Live Plots

0.04

0.06

0.08

0.1

0.12

Fol

iar

Mg:

N R

atio

Foliar N Foliar Mg:N

Residual Tree Foliar Chemistry

y = 1.67x - 23.0R2 = 0.84

y = 3.70x - 48.9R2 = 0.75

0

4

8

12

16

20

15.5 16 16.5 17 17.5 18 18.5 19

Wood tissue Δ C12/13(‰)

Bas

al A

rea

Tre

e G

row

th

(cm

2 / 5ye

ar a

vera

ge)

Dead

Live

Basal Area Growth as a Tree Water Stress

Questions

What conditions allowed the red spruce to be colonized?

Why did only the larger, more vigorous trees on some plots die?

• The ratio between above ground growth (i.e., stem wood, branches and foliage) and below ground growth (i.e. coarse and fine roots) increased

Hypothesis for mortality

• The area in and around Asheville received elevated

nitrogen deposition, but these levels are below that considered critical acid load

• The increased level of nitrogen inputs likely had a

fertilization impact

The lack of oleoresin (especially in large trees) allowed for the colonization of and large scale forest mortality witnessed during that time.

Hypothesis for mortality (cont.)

• The drought conditions reduced available water, carbohydrate reserves for the production of secondary carbon compounds such as oleoresin.

• The larger more vigorously growing trees had a higher AG/BG ratio than the small trees

Stress interactions

Forest Mortality

Elevated nitrogen depositionCausing altered tree

physiology

Climate ChangeReducing carbohydrate

reserves

InsectsCausing tree mortality through

colonization and tree girdling by larval feeding

ConclusionsAs the climate warmers, insects become more active

(no big surprise)

As the climate warms, trees use more water and the potential for soil drying and tree desiccation increases

Interactions between air pollutants and climate change can exacerbate the insect stress by altering the tree physiology and morphology

Insect damage is likely to occur in new ways and more frequently in the future

If any one of the three environmental stresses were removed, the mortality

would not likely have occurred

How a different critical nitrogen load could be determined within the same ecosystem

N dep = 10 kg/ha/yr

N leaching = 0Mortality = 0%

Critical N > 10 kgLoad

N dep = 10 kg/ha/yr

N leaching = 1Mortality = 5%

Critical = 10 kgLoad

+ 3 yr Drought Stress

N dep = 10 kg/ha/yr

N leaching = 10Mortality = 10%

Critical = 8 kgLoad

+ 3 yr Drought Stress+ insects

N dep = 10 kg/ha/yr

N leaching = 25Mortality = 100%

Critical < 5 kgLoad

+ 3 yr Drought Stress+ insects+ temperature

McNulty, Steven G., and Johnny Boggs. 2009. A Conceptual Framework for Redefining Forest Soil Critical Acid Loads under a Changing Climate. Environmental Pollution (In Press).

For more details on this study see

SolutionsCoarse scale application of simple mass balance equation predictions of forest soil critical acid loading can provide some guidance on potential areas of forest acid load exceedence, continue to use them

Climate change will likely reduce the red spruce growth and acid uptake. Account for these reduction by reducing the ecosystems critical acid load level.