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Estimating c ritical acid loads across the lower 48 US: Opportunities and challenges Steven McNulty USDA Forest Service Southern Research Station Raleigh,

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Presentation on theme: "Estimating c ritical acid loads across the lower 48 US: Opportunities and challenges Steven McNulty USDA Forest Service Southern Research Station Raleigh,"— Presentation transcript:

1 Estimating c ritical acid loads across the lower 48 US: Opportunities and challenges Steven McNulty USDA Forest Service Southern Research Station Raleigh, NC

2 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 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

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

4 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.

5 Simple Mass Balance Equation for Forest Soils CL(S+N) = BC dep – Cl dep + BC w – BC u + N i +N u +N de – ANC le(crit)‏ BC dep = Base Cation Deposition Cl dep = Chloride Deposition BC w = Base Cation Weathering BC u = Base Cation Uptake N i = Nitrogen Immobilization N u = Nitrogen Uptake N de = Nitrogen Denitrification ANC e(crit) = Acid Neutralizing Capacity

6 Simple Mass Balance Equation for Forest Soils CL(S+N) = BC dep – Cl dep + BC w – BC u + N i +N u +N de – ANC le(crit)‏ BC dep = Base Cation Deposition Cl dep = Chloride Deposition BC w = Base Cation Weathering BC u = Base Cation Uptake N i = Nitrogen Immobilization N u = Nitrogen Uptake N de = Nitrogen Denitrification ANC e(crit) = Acid Neutralizing Capacity

7 Data  Wet Deposition Ca, Mg, K, Na, Cl, N, SO 4,NH 4,NO 3 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

8 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

9 Data  Resolution –1 km 2 across all forested soils  Temporal Extent –Average  Spatial Extent –Conterminous United States

10 BC = Ca + K + Mg + Na

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14 BC = Ca + K + Mg

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16 Acid Neutralizing Capacity

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18 Estimated Forest Soil Critical Acid Load

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

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

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

22 Simple Mass Balance Equation for Forest Soils CL(S+N) = BC dep – Cl dep + BC w – BC u + N i +N u +N de – ANC le(crit)‏ BC dep = Base Cation Deposition Cl dep = Chloride Deposition BC w = Base Cation Weathering BC u = Base Cation Uptake N i = Nitrogen Immobilization N u = Nitrogen Uptake N de = Nitrogen Denitrification ANC e(crit) = Acid Neutralizing Capacity

23 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)

24 Gross ecosystem productivity change

25 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%

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29 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....

30 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”

31 Picea rubens (red spruce) mortality near Asheville, NC

32 The loss of the red spruce in the southern Appalachian Mountains

33 Western NC experienced a moderate three year drought from 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. Background

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

35 Sampling damaged Southern Appalachian red spruce stand

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

37 Foliar N Concentration (%) Dead Plots Live Plots Foliar Mg:N Ratio Foliar NFoliar Mg:N Residual Tree Foliar Chemistry

38 Basal Area Growth as a Tree Water Stress

39 Questions  What conditions allowed the red spruce to be colonized?  Why did only the larger, more vigorous trees on some plots die?

40 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

41  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

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

43 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

44 If any one of the three environmental stresses were removed, the mortality would not likely have occurred

45 How a different critical nitrogen load could be determined within the same ecosystem N dep = 10 kg/ha/yr N leaching = 0 Mortality = 0% Critical N > 10 kg Load N dep = 10 kg/ha/yr N leaching = 1 Mortality = 5% Critical = 10 kg Load + 3 yr Drought Stress N dep = 10 kg/ha/yr N leaching = 10 Mortality = 10% Critical = 8 kg Load + 3 yr Drought Stress + insects N dep = 10 kg/ha/yr N leaching = 25 Mortality = 100% Critical < 5 kg Load + 3 yr Drought Stress + insects + temperature

46 McNulty, Steven G., and Johnny Boggs 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

47 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.


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