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Effect and Risk Assessment of Ozone Air Pollution on
Forest Vegetation in Switzerland
By: Marcus Schaub*
Abstract
Many studies investigating the
negative impacts of ozone on biomass production and physiological functions
have demonstrated the relationships between ozone exposure and reductions in
both growth and physiological gas exchange. Such studies have led to an
increasing interest in effects of ozone exposure expressed as a critical
cumulative exposure of 10’000 ppb hrs above the threshold of 40 ppb (AOT40).
Ozone effects on plants mainly depend on atmospheric transport and stomatal
uptake. Thus ozone risk assessments should not only use measured ozone
concentrations, but should also account for the influence of atmospheric
conditions and soil moisture on stomatal conductance and non-stomatal ozone
deposition. Following the Level II approach, our on-going studies aim to
provide a model to estimate the ozone flux for forest ecosystems throughout
Switzerland.
Introduction
Ozone pollution leaves no
elemental residue that can be detected by analytical techniques. Therefore,
visible injury on the foliage of broad leaf and needle bearing species is
basically the only easily detectable evidence of ozone-induced injury in the
field. The evidence we have today strongly suggests that ozone occurs at
concentrations which cause visible foliar injury to sensitive plants. Even
though visible injury does not include all the possible forms of injury to
trees and natural vegetation (e.g. pre-visible physiological changes,
reduction in growth, etc.), observation of typical symptoms on aboveground
plant parts in the field has turned out to be a valuable tool for the
assessment of the impact of ambient ozone on sensitive plant species.
During the 1990s, the potential
impact of ground-level ozone on plants and human health has come into focus
within the European co-operation on reductions of air pollution emissions
within the United Nations Economic Commission for Europe (UNECE) and the
European Union (EU). This has led to a request from policy-makers to the
scientific community for quantitative information concerning ozone effects.
The International Co-operative Programme on Assessment and Monitoring of Air
Pollution Effects on Forests’ (ICP-Forests) operating under the UNECE
Convention on Long-range Transboundary Air Pollution (LRTAP) requires the
assessment, validation, and mapping of visible ozone injury on the Long-term
Forest Ecosystem Research plots (Level II plots). Furthermore, the Task
Force of ICP-Forests agreed that data on air
concentrations of tropospheric
ozone need to be implemented in the ICP-Level II monitoring.
International framework
In 1997, based on ten years of
monitoring forest condition in Europe for estimating direct effects by air
pollution on forest trees, the UN/ECE with the LRTAP has concluded that
ozone is the main air pollutant to be considered. The Task Force of ICP-Forests
further agreed in June 1999 – based on a report prepared by the working
group for ‘Ambient Air Quality’ of the expert panel on deposition – that
data on air concentrations of ozone need to be implemented in the ICP-Level
II monitoring.
In December 1999, ministers
from more than 20 countries signed the UN/ECE Gothenburg Protocol to abate
acidification, eutrophication and ground-level ozone. A revision on the UN/ECE
Gothenburg protocol is expected to take place around 2004-2005. The
Sub-Manual of ‘Ambient Air Quality’, Part II on The Assessment of Ozone
Injury on European Forest Ecosystems, was adopted at the 17th
Task Force meeting in Ennis, Ireland in May 2001 and first coordinated
assessments of visible ozone injury were conducted across Europe during the
season of 2001.
State of the Art
Current levels of tropospheric
ozone have been shown to cause damage to forest trees, agricultural crops
and semi-natural vegetation. A rise in ozone concentrations has steadily
occurred over the past decades, and as a result of the likewise continuing
rise in the emission of precursor pollutants of nitrogen oxides and reactive
hydrocarbons further increases in ozone are expected in many parts of the
world. In order to address this, the UN/ECE has adopted an effects-based
approach, using the critical loads/levels concept. This effects-based
research has resulted in the establishment for critical levels for meeting
the Level I standard. The current critical levels are expressed as
cumulative ozone exposures using the index AOT40, i.e. the sum of hourly
ozone concentrations above a cut off of 40 ppb (nl l-1) during
daylight hours when global radiation exceeds 50 Wm-2. The
critical level for ozone above the threshold of 40 ppb has been established
in the range of 10 ppm.h for a seasonal period of 6 months during the
daylight hours to protect ozone-sensitive forest tree species from a 10%
biomass loss. This critical level is based on a significant regression of
biomass decrease in 0-3 year-old Fagus sylvatica L. versus AOT40,
established by normalizing data from three separate open-top chamber
projects. However, it remains to be determined, whether other deciduous tree
species show a similar biomass decrease. Furthermore, it needs to be tested
whether a possible biomass decrease with other tree species is indicated by
visible foliar injury on a macroscopic as well as on a micro-morphological
level, and by changes within their physiological gas exchange process.
It is widely acknowledged that
the impacts of ozone are more closely related to the ozone dose absorbed
through the stomata than to ozone exposure in the atmosphere. The Level I
standard is attractive because of its simplicity, but it is limited by the
fact that any factor which may influence a plant’s response to ozone is
largely ignored. These include but are not limited to plant and
site-specific characteristics such as soil moisture content, vapor pressure
deficit, wind speed, radiation, and temperature. Hence, a realistic estimate
of the actual ozone dose and its impact is not feasible using the Level I
approach. In 1999, it was agreed that ozone flux, the instantaneous rate at
which surfaces such as leaf- and soil surfaces, and stomatal openings absorb
ozone, would lead to the biologically more relevant estimates of ozone
risks. This Level II approach proposed several additional environmental
parameters to be included due to their modifying influence on tree
responses. It was also stated that there was a scarcity of data upon which
to base an ozone dose-response function, especially for Level II plots.
Thus, the development of a realistic flux-based approach should be the
long-term goal of our Level II investigations in order to gain additional
data for an accurate ozone risk assessment.
There is evidence that ambient
ozone concentrations as they are monitored in Europe can cause a variety of
effects to vegetation such as visible foliar injury, growth and yield
reductions, and altered sensitivity to biotic and abiotic stresses. Our
surveys at the Level II plots throughout Switzerland and within the WSL
open-top chamber research facility at Lattecaldo, southern Switzerland have
recorded ozone-like symptoms on numerous native tree, shrub, and forb
species since 1997.
Open-top chamber studies
(filtration studies) and research using Continuously Stirred Tank Reactors (CSTR’s)
(fumigation studies) have confirmed that ozone is the cause of the visible
injury seen on seedlings of a variety of plant species. We have found that
seedlings varied markedly in symptom severity and several species were found
to develop ozone-induced symptoms at exposures below the current short- and
long-term European air quality standards. However, there is still a lack of
adequately documented evidence of ozone-induced foliar injury to tree and
shrub species across the whole of Europe. Using the biomonitoring approach
while looking for symptoms on native ozone-sensitive plant species, is the
main objective of the ICP-Forests Sub-Manual to provide information on the
distribution of ozone injury within the European forest ecosystems.
Recent studies have shown that
increasing ozone concentrations not only have a negative effect on wood
production, but may also lead to unstable conditions in forest ecosystems,
which could result in a lowered adaptive capacity to new stresses in the
future. Long-term effects of elevated ambient ozone concentrations on trees
may weaken the function of forest ecosystems with respect to water and
energy balances and soil protection against erosion particularly in the
alpine regions. Some of the most important impacts may be the possible
shifts in species composition and loss of biodiversity especially in areas
with large numbers of endemic plant species with unknown sensitivity to
ozone. However, much more detailed and defined site and species
exposure/response research is required in order to confirm such hypotheses.
Environmental factors
The negative impacts of ozone
on biomass production and physiological functions have been investigated in
a number of experiments. Many of these experiments demonstrated
relationships between ozone exposure and reductions in both growth and
physiological gas exchange. Studies investigating physiological sensitivity
to ambient ozone concentrations indicate that ozone stress may reduce carbon
fixation, increase foliar and root respiration, shift the partitioning of
carbon into different chemical forms, and disrupt carbon and nutrient
allocation patterns. There is general agreement that ozone must enter the
leaf interior through stomata to cause leaf tissue injury. Therefore,
stomatal regulation must be an important factor in controlling ozone
sensitivity of plants. The stomata also control carbon uptake, a crucial
process for plant growth. When plants close their stomata, thereby avoiding
further ozone uptake, they are additionally stressed through the detrimental
effects on both plant growth and photosynthate allocated towards the repair
of cells injured during ozone exposure. Therefore, it has been suggested
that the ratio of net photosynthesis and stomatal conductance is a better
indicator of plant sensitivity to ozone than either parameter alone.
Changes in environmental
conditions such as light, temperature and humidity influence stomatal
aperture and hence also the potential for ozone flux. It has been
demonstrated that foliar injury responses were associated with site
differences; trees on wetter sites experienced higher visible foliar injury
compared to trees on drier sites with two thirds of the trees on wet sites
being consistently more symptomatic than the trees on the dry sites. Various
other studies have shown greater visible ozone injury on plants grown in
moist soil over those grown in drier soil. Differences in the soil water
regimes within an OTC-study with four-year-old seedlings of Prunus
serotina L., Fraxinus americana L., and Acer rubrum L.
seems to be a controlling factor in affecting interactions with ambient
ozone and subsequent physiological differences leading to alterations in
ozone uptake. Stomatal conductance has also been reported to be affected by
tree age and tree size. However, these findings support the hypothesis that
negative correlation between ozone uptake and net photosynthesis indicates
that water stress and nutrient deficiency may prevent ozone injury by
reducing stomatal conductance and ozone uptake. Making matters even more
complicated, these findings also confirmed the hypothesis that greater
visible foliar injury in the lower versus upper crowns of dominant black
cherry trees and saplings was a result of higher ratios of ozone uptake and
net photosynthesis when measured within the lower-crown which had shaded
leaves.
In conclusion, it is widely
accepted that the general receptor-specific maximum leaf conductance is
modified by environmental and phenological parameters: soil factors, such as
soil moisture deficit and irrigation, plant development factors such as
phenological stages, and factors influencing the instantaneous ozone uptake
by plants, including temperature, leaf-to-air vapor pressure deficit, global
radiation, wind speed and positions within mature tree crowns. In order to
improve our ability to identify areas potentially at risk for ozone impacts,
an extension of the existing database and a more detailed understanding of
the flux-response relationships are needed.
Below-ground plant response
A number of
studies have demonstrated that ozone would adversely affect net
photosynthetic rates. Consequently, physiological imbalances in the
above-ground parts of the trees exposed to elevated ozone may alter
below-ground carbon processes. However, quantification of below-ground pools
and fluxes of C has proven to be a difficult task, because of the high
spatial heterogeneity and difficulty of
observation within the soil.
Decreased allocation of
photosynthates to the roots is assumed to be responsible for a reduced root
growth, a change in the root-to-shoot ratio and decreased fine root
production in various tree species under elevated ozone
exposures. Furthermore, a reduced supply of photosynthate to the
roots was suggested as a consequence of decreased soil respiration
of field-grown aspen trees under ozone exposure. A
decline of root exudation of organic compounds due to reduced
translocation of photosynthate to the roots under ozone exposure could
result in a reduced nutrient supply to soil microorganisms, leading to a
lower microbial metabolism and a lower total soil respiration.
However, it is still unclear whether the decrease in
total soil respiration is mainly attributed to the reduced heterotrophic
(microbial) respiration or to the reduced autotrophic (root) respiration, or
to both respectively or interactively. It is difficult to distinguish
between heterotrophic and autrotrophic respiration, but both components are
dependent on the carbon sources transported from the above-ground biomass
into the soil.
Since the utilization rate of
photosynthates is reflected in the respiration rates above and below ground,
more research should be directed towards a better understanding of
respiration in the below-ground portions. The soil including roots and soil
microorganisms not only play a major role in the carbon budgeting of the
ecosystem but also in regulating water and nutrient supplies to the tree.
Large-scale risk assessment
It is often difficult to assess
the sensitivity of mature trees to ozone, especially when considering the
diversity of sites occupied by a species across its entire natural range. In
addition, the accuracy of using seedlings to predict responses of mature
trees and forest ecosystems to ozone is questionable, considering the long
life span of trees and the phenological differences. Although controlled
experiments with seedlings have been valuable for revealing the ‘principles’
of ozone action on woody plants, these findings would lack ecological
significance as long as validation at forest sites is missing. Differences
in canopy structure and microclimate, exposure to ozone, gas exchange, and
ransport of water and carbohydrates, nutrient allocation, and recurrent
flushing between seedlings and mature trees are some of the complications
that must be taken into account.
The current Level I critical
level values, which are being used in developing European ozone control
strategies both by UN/ECE and the EU, are intended to protect the most
sensitive vegetation types under the most sensitive conditions. Exceedance
of these critical levels, however, only provides an indication that some
risk exists of negative effects to vegetation from ozone; the degree of
exceedance cannot be used to provide a measure of the relative risk of
negative effects to vegetation in different areas of Europe.
A risk assessment based on
ozone flux to receptor sites within the leaf, rather than ozone exposure,
could provide an improved estimate of the relative degree of risk of ozone
damage to vegetation across Europe. Existing models of ozone flux require
detailed micrometeorological information, and are applicable to only a
limited number of species, and cannot be readily applied at a large
geographical scale. Alternatively, models designed to model ozone deposition
on a regional scale have a limited description of the stomatal responses,
which are not species-specific and which do not consider the effects of
vapor pressure deficit and soil moisture deficit. By limiting the existing
data pool to hardwood species, 81% of the variation in observed
photosynthetic response could be explained by cumulative ozone uptake alone.
Given the strength of this relationship and uncertainties surrounding
additional sources of variation, examining its implications across natural
forested landscapes represents a valuable way to advance current
understanding of ozone effects, even while recognizing the limitations of
its simplicity.
Summary and conclusions
It is widely accepted that
tropospheric ozone concentrations are expected to increase in the near
future. While ozone effects and the underlying mechanisms on plant tissue
and single plants are reasonably well understood, there is still a wide gap
of knowledge and understanding as to how air pollution in general and ozone
in specific, affects forest ecosystems. Furthermore, considering the
expected increase of the global average surface temperature and the
simultaneous alteration in precipitation patterns, the air pollution
effects, and in particular the effects ascribed to tropospheric ozone, may
change as well. The application of mechanistic models combined with the
respective future findings based on detection, monitoring and evaluation
should allow us a more profound understanding to estimate a better risk
assessment for forest ecosystems.
*
Swiss Federal Institute for Forest, Snow and Landscape Research WSL, 8903,
Birmensdorf, Switzerland -
e-mail: [email protected] |
