Andrzej Bytnerowicz, Pacific Southwest Research Station; Mark Fenn, Pacific Southwest Research Station; and Chuck Sams, USFS Region 8; with assistance from Linda Pardo and Molly Robin-Abbott, Northern Research Station.
An archived version of this topic paper is available.
Climate change and air pollution are closely linked, although in scientific research and often in policy they have been largely separated. Many of the common air pollutants and greenhouse gases not only have common sources, but may also interact physically and chemically in the atmosphere, causing a variety of environmental impacts at local, regional and global scales. The combined effects of numerous climate change and air pollution factors may significantly differ from the sum of separate effects due to various interactions (1).
Linkages between climate change and air pollution extend to the terrestrial environment, with impacts that are complex and highly variable in time and space. The air pollutants of most importance for their effects on forest growth and health are tropospheric ozone (O3), and various components of reactive nitrogen (Nr)* and its deposition. Consequently in this synthesis we focus on O3 and N deposition as the air pollution factors that have the most pronounced direct and indirect effects on US forests and other terrestrial ecosystems. Air pollutants that play a major role in climate warming but that are not discussed in depth here are carbon dioxide (CO2), methane (CH4), and atmospheric aerosols (including black carbon, soot). Other important pollutants include sulfur dioxide (SO2) which still contributes significantly to acidic deposition, and mercury (Hg) which is an important environmental toxin especially in its methylated form (CH3Hg+)(2, 3).
Table 1. Symbols and names of chemical compounds discussed in this Topic Page.
||Particulate matter of diameter < 2.5mm
||Particulate matter of diameter < 10mm
Ozone is a greenhouse gas that has the 3rd largest positive radiative forcing, or warming effect, after CO2 and CH4 (4). It is also considered the most important phytotoxic (directly toxic to plants) air pollutant affecting growth and health of forests and agricultural crops (5; 6). Negative O3 impacts on plants include chlorophyll damage, lowered stomatal conductance, premature senescence of foliage, and lower root mass (7), all of which result in decreased photosynthesis and CO2 sequestration by vegetation, which also indirectly increases climate warming (8). Ozone is produced via a complex suite of photochemical reactions between nitrogen oxides (NOx) and hydrocarbons in sunlight. High ambient O3 caused massive dieback of mixed conifer forests, specifically sensitive ponderosa and Jeffrey pines, in the San Bernardino Mountains (9, 10) and western slopes of the Sierra Nevada Mountains (11) in the 1960s and 1970s. Although ozone damage directly predisposed trees to mortality, other factors, such as drought and bark beetles were the ultimate cause of tree death (12; 13). Ambient O3 also caused foliar injury of various sensitive forest tree species throughout most of the eastern US until the mid-1990s (14).
While combustion processes are a major source of NOx, agricultural activities are primarily responsible for ammonia (NH3) and nitrous oxide (N2O). As with O3, certain N compounds can influence the climate system directly. Nitrogen oxides can increase O3 formation (warming effect) and also increase the removal of CH4 (cooling effect). Additionally, both NOx and NH3 can produce a cooling effect by enhancing light-scattering aerosols (15). Reactive N compounds are also deposited onto the landscape and influence ecosystems directly. This deposition affects the sources and sinks of greenhouse gases such as N2O, CH4, and CO2, but its dominant effect is changes in ecosystem carbon (C) stocks (16). Nitrogen is an essential nutrient required for proper development of plants, however high N deposition may have serious ecological consequences in forests when elevated above critical loads**. Elevated N and sulfur deposition contributes to acidification of terrestrial and aquatic ecosystems, resulting in release of toxic heavy metals in soils that affect forest trees and fish populations (2; 3). Currently, more widespread are exceedances of the N critical loads causing nutrient-N or eutrophication effects in various US ecoregions. This may lead to changes in species composition and richness (especially in lichen and understory plant communities), invasion of exotic plant species, water and soil contamination by NO3-, and predisposition of trees to drought and pests (17; 18, 19; 20; 21). Changes to forest fungal communities may also occur (22, 23; 21). Tree survival is variable as N increases, and is not necessarily tied to growth (24). However, growth for many eastern tree species has been negatively correlated with exceedances of N, and sulfur (S) critical loads for acidity (25).
Since a period of very high O3 concentrations during the 1950s through the 1980s, ambient O3 concentrations, especially peak values, have been declining in the US (26; 27). Trends of decreasing O3 phytotoxicity indices***, which are a measure of toxic effects of O3 on plants, were observed all over the US (28), including western locations like the San Bernardino Mountains of southern California (29). This trend is expected to continue due to improved air pollution control strategies, although future O3 concentrations will greatly depend on technological advancements in the industrial, energy and transportation sectors. However, it is also projected that increasing air temperatures will result in higher rates of O3 generation that will counteract the declining trends in O3 concentration (30, 31). O3 is also generated by wildfires, which are expected to become larger and more frequent with climate change (32). In addition, at a more regional scale, O3 concentrations in the western US during the springtime may continue to increase due to long-range transport of O3 precursors from Asia (33). If that happens, the recently observed improvement of the health of forests could be reversed, as expressed by diminished leaf damage to sensitive plants in the eastern (Dr. Howard Neufeld, personal communication) and western US forests (Dr. Andrzej Bytnerowicz, unpublished).
Concentrations of NOx and HNO3 will most likely continue to decline due to the implementation of air pollution control measures. However, uncontrolled NH3 emissions, mostly from agricultural activities and automobiles with 3-way catalytic converters, will most likely continue and will result in continuously high nitrogen deposition in the Midwest and some areas in California and the eastern US (34). Exceedances of critical loads of nutritional N in sensitive ecosystems will therefore continue for the foreseeable future.
The interactions between climate change and air pollution will likely introduce additional complexity. For example, climate changes such as increased air temperature and changing precipitation patterns could significantly impact responses of forest ecosystems to N and S deposition (35). Higher temperatures can significantly increase the capacity of forest soils to process and neutralize acidifying atmospheric deposition (36).
Increased CO2 levels can enhance the water use efficiency of plants by reducing stomatal conductance, or the exchange of CO2 and water vapor between a plant and the atmosphere. However, elevated O3 may negate this effect due to reduced photosynthetic gain (37). Elevated CO2 may reduce plant O3 uptake, diminishing the pollutant’s phytotoxic effects (Ainsworth et al. 2012). However, O3 can also damage plant stomates (38; 39; 40). Unrestricted opening of stoma could then cause higher and uncontrolled losses of water from plants via evapotranspiration, leaving less water in soils and less water to route to streams (41), leading to reduced streamflow and higher frequency and severity of droughts.
These complex interactions illustrate the challenges in projecting the combined effects of increased temperature, elevated CO2, changing water and nutrient availability, O3 and N deposition, and other abiotic and biotic factors. In addition, any projections of future air pollution and climate changes bear a high degree of uncertainty because of the difficulty in predicting new developments in energy production and use.
Options for Management
Informing management decisions - research & monitoring needs
In order to be able to recommend and apply management strategies addressing the complex interactive effects of climate change and air pollution, it is imperative that we improve our understanding of spatial and temporal changes in distribution of the most important air pollutants, as well as their effects on overstory and understory plants. There is an urgent need to improve monitoring of ambient O3 and Nr compounds in remote forested areas, especially in complex terrain where existing models do not adequately perform. In this context passive samplers and portable battery/solar power operated O3 monitors could be more widely used. Such monitoring activities could be linked to the US Forest Service Experimental Forests and Ranges network. Modern remote sensing techniques for air pollution measurements could be very helpful in understanding large-scale pollutant distribution in remote forested areas and could be further explored.
Robust holistic models able to evaluate the interactive effects of multiple air pollutants, climate change, long-range transport of air pollution, ecosystem nutrient status, management practices, and various other biotic and abiotic stressors are also needed. Regional and global models of hydrologic cycles and related ecosystem functions should consider potential interactions of O3 with future climate change. Maintaining experimental forest watersheds where researchers can gather information and test models is essential.
Air quality indices and thresholds can be used to help forest managers understand the risks that forests face from air pollutants and to plan effective management responses. For O3, research is in progress to develop models that more accurately describe physiological plant responses to O3 (flux-based models) which could improve future indices (e.g., DO3SE, 42). Currently however, using an index based on overall vegetation exposure to O3 appears to be the most practical measure for relating ambient air quality standards to vegetation response (43). Recently the US EPA has recommended that the exposure-based W126 index become a new O3 secondary standard for protection of vegetation. Use of this newly proposed standard may greatly help resource managers to understand potential O3 risks to forests and to plan future management measures.
Better understanding of spatial and temporal distributions of N dry deposition, cloudwater deposition, and of key drivers of deposition (such as NH3, HNO3 and NO2, and NO3- and NH4+ ions and organic N) are needed to identify pollution sources and to develop options for their control. There is also a need for better models to describe the deposition of Nr in remote forests (especially complex mountain terrain). Likewise, air quality thresholds for Nr, such as critical loads for ecological effects, should continue to be developed and applied towards the protection and restoration of ecosystems exposed to chronic air pollution (3).
Forest health monitoring is needed, including evaluation of impacts of O3 on sensitive plants and changes in lichen communities caused by N deposition. These continuous and long-term records of biological changes can show trends and the extent of negative impacts of air pollution and other interactive stresses (e.g. drought, pests and diseases) on forests. The USDA Forest Service Forest Health Monitoring program offers the opportunity for nation-wide assessments.
Thinning dense forest stands, especially in the western US, improves their resilience and ability to cope with multiple stressors, including climate change and air pollution (44; 45; 46; 13). This can be done with consideration of differences between O3 sensitivity of various tree species. The use of prescribed fires can be a desirable tool in forest thinning. Such fires could also help in removing accumulated N on the forest floor and upper soil layers, effectively decreasing risks of N saturation (47). Although emissions from prescribed fires have the potential to exceed air quality standards especially for particulate matter (PM2.5 and PM10), prescribed burning and the potential impacts to people can be managed more easily than those for wildfire. The acceptance of fire as a management tool may require comprehensive large-scale monitoring of smoke emissions and the use of models that are able to predict spatial and temporal distribution of toxic pollutants resulting from fires, and could be used for better management response and planning (48).
In planning for forest restoration, decision makers could consider differences in responses to single air pollutants, such as O3, and also complex interactive effects of air pollution and climate change. For instance, plantation forests could be planted using genotypes with lower sensitivity to O3.
Linkages between projected climate changes and their effects on ecosystem processes, atmospheric circulation, air chemistry, atmospheric deposition, and other stressors, highlight the difficulties in addressing the effects of air pollution in on-the-ground management. Clearly, a close collaboration between scientists and land managers is the key for maintaining healthy and resilient forests facing complex future effects of air pollution and climate change.
*definition of reactive nitrogen (Nr): all biologically active, chemically reactive, and radiatively active nitrogen compounds in the atmosphere and biosphere of the earth. Reactive nitrogen includes inorganic chemically reduced forms of N (NHx) [e.g., ammonia (NH3) and ammonium ion (NH4+)], inorganic chemically oxidized forms of N [e.g., nitrogen oxides (NOx), nitric acid (HNO3), nitrous oxide (N2O), nitrogen pentoxide (N2O5), nitrous acid (HONO), peroxy acetyl compounds such as peroxyacetyl nitrate (PAN), and nitrate ion (NO3-)], as well as organic compounds (e.g., urea, amines, amino acids, and proteins). [see pdf]
**definition of critical loads: "the threshold deposition of pollutants at which harmful effects on sensitive receptors begins to occur according to present knowledge" [see pdf]. For more information see Pardo 2006 (49).
***explanation of ozone phytotoxicity indices commonly used in the US and Europe is comprehensively described in (43).