Non-pollutant ecosystem stress impacts on defining a critical load

Or why long-term critical loads estimates are likely too high

Steven McNultyUSDA Forest Service

Raleigh, NC

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

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.

Examples of Critical Load Variables

Nitrogen dioxide (NOx)Ammonia (NH3)Sulfur dioxide (SO2)Ozone (O3)

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

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?

The Problem

Non-Critical Load Environmental Stresses

- Insects (native and invasive)- Disease- Fire- Drought- Flood- Wind- Extreme Temperature (hot and cold)- Poor Ecosystem Management

How do these environmental stresses impact ecosystem critical load thresholds?

A Case Study

The loss of the southern red spruce and white pine forest

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

Background

Southern Pine Beetle Induced White Pine Mortality Near Asheville, NC

Southern Pine Beetle Damage in Southern Appalachian Red Spruce Stand

Questions

• What conditions allowed the red spruce and white pine trees to be colonized?

• Why did only the larger, more vigorous trees 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 loads”

• 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

DroughtReducing carbohydrate

reserves

InsectsCausing tree mortality through

colonization and tree girdling by larval feeding

• 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 = 10%

Critical = 10 kgLoad

+ 3 yr Drought Stress

N dep = 10 kg/ha/yr

N leaching = 15Mortality = 75%

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

S dep = 10 kg/ha/yr

Revised critical loads model that includes other environmental factors

Critical Load

Ecosystem structure and function

Abiotic Stress Biotic Stress

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

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