Estimating critical acid loads across the lower 48

Estimating critical acid loads across the lower 48 US: Opportunities and challenges Steven Mc. Nulty USDA 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 km 2 resolution 2. Assess how a changing climate could impact forest soil critical acid loads 3. Discuss why historic definitions of a “healthy” forest may need to change

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

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. 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

Simple Mass Balance Equation for Forest Soils CL(S+N) = BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit) BCdep Cldep BCw BCu = Base Cation Deposition = Chloride Deposition = Base Cation Weathering = Base Cation Uptake Ni = Nitrogen Immobilization Nu = Nitrogen Uptake Nde = Nitrogen Denitrification ANCe(crit) = Acid Neutralizing Capacity

Simple Mass Balance Equation for Forest Soils CL(S+N) = BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit) BCdep Cldep BCw BCu = Base Cation Deposition = Chloride Deposition = Base Cation Weathering = Base Cation Uptake Ni = Nitrogen Immobilization Nu = Nitrogen Uptake Nde = Nitrogen Denitrification ANCe(crit) = Acid Neutralizing Capacity

Data Wet Deposition Ca, Mg, K, Na, Cl, N, SO 4, NH 4, NO 3 – Eastern US Grimms and Lynch, 2003 Incorporates elevation into calculation ~300 m 2 resolution – Western US USGS NADP/NTN 6. 25 km 2 resolution Climate – – – Spatial Climate Analysis Service Prism Temperature, Precipitation 4 km 2 and 16 km 2 resolution

Data National Forest Cover Dataset – USGS/USFS – 25 tree classes – 1 km 2 resolution Soil – Miller and White, 1998 – Soil fraction, depth to bedrock, soil unit – 1 km 2 resolution

Data Resolution – 1 km 2 across all forested soils Temporal Extent – Average 1994 - 2000 Spatial Extent – Conterminous United States

• BC = Ca + K + Mg + Na

BC = Ca + K + Mg + Na

BC = Ca + K + Mg

Acid Neutralizing Capacity

Estimated Forest Soil Critical Acid Load

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

For more details on this study see Mc. Nulty, Steven G. ; Cohen, Erika C. ; Moore Myers, Jennifer A. ; Sullivan, Timothy J. ; and Li, Harbin. 2007. Estimates of critical acid loads and exceedences forest soils across the conterminous United States. Environmental Pollution 149: 281 -292

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

Simple Mass Balance Equation for Forest Soils CL(S+N) = BCdep – Cldep + BCw – BCu + Ni +Nu +Nde – ANCle(crit) BCdep Cldep BCw BCu = Base Cation Deposition = Chloride Deposition = Base Cation Weathering = Base Cation Uptake Ni = Nitrogen Immobilization Nu = Nitrogen Uptake Nde = Nitrogen Denitrification ANCe(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 cation uptake increases (reducing ANC the CAL) - as air temperature increases, forest evaportanspiration increases and runoff decreases (reducing ANC and reducing CAL)

Gross ecosystem productivity change

Implications Changing climate could signficantly impact the amount of forest soil In exceedance of the critical acid load - total forest area in exceedance could decrease by 6% However, most of the exceedance occurs in New England where The 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 purchase Cadillacs 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

Background Western NC experienced a moderate three year drought from 1999 -2002. In 2001, red spruce (Picea rubens) trees began to die in large numbers in and around Mt. Mitchell NC, USA. The initial evidence suggested that the affected trees 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 the SPB attack.

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

Sampling damaged Southern Appalachian red spruce stand

Drought Stress Drought, Temperature, Insect, & N deposition Stresses

Residual Tree Foliar Chemistry 0. 12 Dead Plots Live Plots 1. 3 0. 1 1. 2 0. 08 1. 1 0. 06 1. 0 Foliar N Foliar Mg: N 0. 04 Foliar Mg: N Ratio Foliar N Concentration (%) 1. 4

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?

Hypothesis for mortality • The area in and around Asheville received elevated nitrogen deposition, but these levels are below that considered critical acid load • The ratio between above ground growth (i. e. , stem wood, branches and foliage) and below ground growth (i. e. coarse and fine roots) increased • The increased level of nitrogen inputs likely had a fertilization impact

Hypothesis for mortality (cont. ) • The larger more vigorously growing trees had a higher AG/BG ratio than the small trees • The drought conditions reduced available water, carbohydrate reserves for the production of secondary carbon compounds such as oleoresin. The lack of oleoresin (especially in large trees) allowed for the colonization of and large scale forest mortality witnessed during that time.

Stress interactions Elevated nitrogen deposition Causing altered tree physiology Forest Mortality Climate Change Reducing carbohydrate reserves Insects Causing tree mortality through colonization and tree girdling by larval feeding

Conclusions As 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 = 0 Mortality = 0% + 3 yr Drought Stress N dep = 10 kg/ha/yr + 3 yr Drought Stress + insects + temperature N leaching = 10 Mortality = 10% N leaching = 25 Mortality = 100% Critical N > 10 kg Load N leaching = 1 Mortality = 5% Critical = 10 kg Load N dep = 10 kg/ha/yr Critical = 8 kg Load Critical < 5 kg Load

For more details on this study see Mc. Nulty, Steven G. , and Johnny Boggs. 2009. A Conceptual Framework for Redefining Forest Soil Critical Acid Loads under a Changing Climate. Environmental Pollution (In Press).

Solutions Coarse 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.