Non-pollutant ecosystem stress impacts on defining a critical load

1. Non-pollutant ecosystem stress impacts on defining a critical load Or why long-term critical loads estimates are likely too high Steven McNulty USDA Forest Service Raleigh, NC

2. Examples of Critical Load Variables Nitrogen dioxide (NOx) Ammonia (NH3) Sulfur dioxide (SO2) Ozone (O3) 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 exceedance is often considered to pose a greater risk of damage. UK Centre for Ecology and Hydrology

3. Estimates of Critical Loads Based on Direct Ecosystem Impacts N & S Deposition Stream acidification - loss of aquatic biodiversity Soil Acidification - forest mortality - nitrate leaching - reduced biological diversity Episodic Ozone - reduced forest productivity - impaired human health

4. The Problem There are well established relationships between the exceedance of a critical load, and ecosystem degradation. However, what about the ecological interaction between critical load pollutants and other non-pollutant environmental stresses? Could other environmental stresses either increase or decrease an ecosystems sensitivity to other critical loads?

5. Non-Critical Load Environmental Stresses - Insects (native and invasive) - Disease - Fire - Drought - Flood - Wind - Extreme Temperature (hot and cold) - Poor Ecosystem Management

6. How do these environmental stresses impact ecosystem critical load thresholds? A Case Study

7. The loss of the southern red spruce and white pine forest

9. Background Western NC experienced a moderate three year drought from 1999-2002. In 2001, white pine and spruce trees began to die in large numbers in and around the Ashville NC area. 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 that only the larger and more vigorously growing trees were killed by the SPB.

10. Southern Pine Beetle Induced White Pine Mortality Near Asheville, NC

11. Southern Pine Beetle Damage in Southern Appalachian Red Spruce Stand

12. Questions • What conditions allowed the red spruce and white pine trees to be colonized? • Why did only the larger, more vigorous trees die?

13. Hypothesis for mortality • The area in and around Asheville received elevated • nitrogen deposition, but these levels are below that • considered “critical loads” • 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

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

15. Stress interactions • Elevated nitrogen deposition • Causing altered tree physiology Forest Mortality • Drought • Reducing carbohydrate reserves • Insects • Causing tree mortality through colonization and tree girdling by larval feeding

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

17. N dep = 10 kg/ha/yr N dep = 10 kg/ha/yr N dep = 10 kg/ha/yr N dep = 10 kg/ha/yr S dep = 10 kg/ha/yr N leaching = 0 Mortality = 0% + 3 yr Drought Stress + insects + 3 yr Drought Stress + insects + 3 yr Drought Stress Critical N > 10 kg Load N leaching = 1 Mortality = 10% N leaching = 25 Mortality = 100% N leaching = 15 Mortality = 75% Critical = 10 kg Load Critical = 8 kg Load Critical < 5 kg Load How a different critical nitrogen load could be determined within the same ecosystem

18. Revised critical loads model that includes other environmental factors Ecosystem structure and function Critical Load Abiotic Stress Biotic Stress

19. Implications - There should be no single set of environmental criteria determining the critical load • By definition a critical load can change over • both space and time • Computer models will be the only viable method of • calculating critical load over a wide range of • environmental stresses and ecosystems • An a priori approach to field monitoring will be • required to develop and test critical load models

20. Benefits • Well validated critical load models would be • temporally and spatially dynamic • Alternative management and pollutant scenarios could • be conducted on at the ecosystem level to determine • when or if critical load levels would be exceeded • The use of standardized models would lead to more • standardized assessments of critical load levels