©Verlag Ferdinand Berger & Söhne Ges.m.b.H., Horn, Austria, download unter www.biologiezentrum.at
Phyton (Austria)
Special issue:
"Eurosilva"
Vol. 39
Fasc. 4
(241)-(252)
15.7. 1999
Evaluation of the Impact of Ozone on Conifers in
the Alps: A Case Study on Spruce, Pine and Larch
in the Austrian Alps
By
G. WlESER 1}
Key
antioxidants.
words:
Conifers, Alps, Europe, ozone, exposure, uptake, gas exchange,
Sum m a r y
WIESER G. 1999. Evaluation of the impact of ozone on conifers in the Alps: a case study
on spruce, pine and larch in the Austrian Alps. - Phyton (Horn, Austria) 39 (4): (241) - (252).
For coniferous forests in alpine regions unambiguous evidence of ozone (O3) induced
visible injury does not exist, although ambient O3 concentrations often exceed values which caused
injury in laboratory and glass-house studies. Given such uncertainties field experiments using twig
chambers were conducted at several forest sites within the European Alps in order to determine the
impact of O3 on mature Norway spruce (Picea abies (L.) Karst.), Cembran pine (Pinus cembra L.)
and European larch (Larix decidua Mill.) trees under field conditions. After fumigation throughout
one growing season only mean O3 concentrations higher than 100 ppb caused a pronounced decline
gas exchange. This observed lack in symptom expression at mean O3 concentrations lower than 100
ppb can be attributed to modifications in the amount of O3 entering the needles. In all the three
species investigated. O3 uptake was mainly limited by high leaf to air vapour pressure difference
(Aw) and/or soil water deficits. This is because of the co-occurrence of high O3 concentrations and
high Aw, the latter causing stomatal closure and thus avoiding high O3 uptake rates. The potential
toxicity of ambient O3 concentration is not only modified by the stomatal control of O3 uptake but
also by the detoxification capacity. In twigs exposed to O3 concentrations between zero and 137 ppb
O3 did not cause a depletion of the ascorbate pool. Therefore, it is concluded that ambient O3
concentrations presently do not constitute an acute danger for conifers in the central European Alps.
Forstliche Bundesversuchsanstalt, Institut für Immissionsforschung und Forstchemie,
Abt. Forstpflanzenphysiologie, Rennweg 1, A 6020 Innsbruck, Austria. Fax: ++ 512 573933 5250;
e-mail: [email protected]
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Introduction
Forest ecosystems cover large regions within the European Alps. In
Austria forests cover approximately 47 % of the total land area, and 80% of the
forests are located within the alpine region (BÜCHSENMEISTER, FBVA pers.
comm.). Among them coniferous forests are widely distributed and account for 83
% of the total alpine forests, mainly being stands of spruce (61 %), pine (9%) and
larch (8%) (SCHADAUER 1994).
During the last century the atmosphere over Europe has changed in the
composition of trace gas components. Recent analysis indicates that tropospheric
ozone (O3) concentrations in Europe have at least doubled over the last 100 years
(VOLZ & KLEY 1988). In the northern hemisphere enhanced photochemical O3
production in the lower troposphere appears to be responsible for the observed
increase in O3 during the last decades (LOGAN 1985). This increase is believed to
be due to an increase in the anthropogenic emission of precursor substances in
industrialised regions (FISHMAN & al. 1979). Nowadays alpine forests in central
Europe can experience O3 episodes above 100 ppb. Mean annual O3 levels are
around 25 ppb at elevations lower than 650 m a.s.l. and around 40 ppb at higher
elevations up to 1500 m altitude. Mean annual O3 levels of 50 ppb coincide with
the altitude of montane and subalpine forests in the Alps of central Europe (SMIDT
& GABLER 1994).
In the late 1970s foresters in central Europe began to observe a decline in
the health of trees and stands, defined in terms of crown thinning and yellowing of
the foliage. Since 03-induced damage on the foliage of forest ecosystems in
southern California has been reported (MILLER 1983), O3 was examined as a
causative factor for tree injury or „forest decline" (PRINZ 1984). The O3-hypothesis
was supported by the fact that crown transparency in Austrian high-altitude
protection forests increased in parallel with ambient O3 concentration from 1984
throughout 1987 (AMT der TIROLER LANDESREGIERUNG 1985-1998). For
coniferous forests however, unambiguous evidence of O3 injury did not exist, not
even at the alpine timberline, where ambient O3 concentrations often exceed values
which clearly caused injury in chamber experiments (SANDERMANN & al. 1997).
Results obtained from chamber experiments can not be used without
reservation to predict responses of mature trees in the field because of
morphological and physiological differences between seedlings and mature trees.
Furthermore, seedlings in chambers do not experience such a variety of interacting
stress as will commonly occur in the field (REICH 1987). Therefore, twig chamber
systems have been developed in order to determine the effects of ambient and
above ambient O3 levels on forest trees under site conditions (HAVRANEK &
WIESER 1994).
Since O3 is ubiquitous and tree response is altered by many other site
factors, it has been proven difficult to determine whether or not O3 significantly
affects conifers in the field. In order to quantify the effects of ambient O3
concentrations, this article will summarise what is known about the response of
mature conifers to O3 exposure. The focus will be on field studies with mature
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(243)
Norway spruce (Picea abies (L.) Karst.), Cembran pine {Pinus cembra L.) and
European larch (Larix decidua Mill.) trees using realistic ecophysiologically
relevant O3 concentrations. The main aim however, is to synthesise the current
understanding of O3 effects on mature field grown conifers in the European Alps.
Quantification
of o z o n e
exposure
Tree response to O3 can be quantified in terms of concentration, external
dose and uptake. Concentration is an instantaneous measure. The external „total
dose" (SUMO) is an integrated measure, defined as total hours of exposure
multiplied by mean concentration using no threshold value. Another external
exposure index is the AOT40 value (i.e. the accumulated exposure over a threshold
of 40 ppb O3 based on a 24 h period (UN/ECE 1994). Mean O3 concentration,
SUMO and AOT40 are commonly used measures of ambient air quality and
experimental O3 exposure. Furthermore, as it is generally agreed that the
phytotoxic action of O3 occurs after the passage of O3 through the stomata into the
leaf interior (TlNGEY & TAYLOR 1982), cumulative O3 uptake (CU) is a third
measure of O3 exposure. Except for concentration all these measures of O3
exposure are integrated over time.
Visible
symptoms,
chlorophyll
hardiness
content
and
frost
Field surveys of mature conifers in the European Alps failed to detect
visible symptoms of O3 injury (MATYSSEK & al. 1997, RENNENBERG & al. 1997)
comparable to those reported to occur in California (cf. MILLER & al. 1997). The
sensitivity of mature trees to ambient and above ambient O3 concentrations has
been examined only in a few studies at different forest sites of the European Alps.
These experiments indicated that even O3 exposure for 23 weeks with mean
concentrations up to 137 ppb did not cause any visible injury nor affected the 100needle dry weight, specific leaf area or shoot properties as compared to Oß-free
controls (WIESER & HAVRANEK 1996). Chlorophyll contents also remained
unaffected as shown for Cembran pine in Fig. 1.
At higher elevations frost can also be an important stress factor and there is
evidence that summer exposure to elevated O3 may reduce winter hardiness of
conifer seedlings (BROWN & al. 1987, BARNES & DAVISION 1988, CAPE & al.
1989). In field grown mature Norway spruce trees even mean O3 concentrations up
to 137 ppb given for up to 23 weeks failed to cause needle loss or any injury during
the subsequent winter or impaired growth of the new flush in the following spring
(WlESER & HAVRANEK 1994). Furthermore, measurements of frost hardiness at the
alpine timberline suggested, that the maximum frost resistance was related to
minimum air temperature throughout the winter and not to summer O3 exposure
(Table 1).
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Pinus cembra
i 1 2 l+l
I 1
ü
1950 m
I\
50
100
AA
O, Treatment [ppb]
Fig. 1. Contents of total chlorophyll (a + b) in current (open bars) and 1-year-old needles
(hatched bars) of cembran pine (Pinus cembra) after exposure to different O3 treatments throughout
one growing season in twig chambers as compared to twigs grown outside the chambers (AA). n = 3
+ SD.
Table 1. Maximum frost resistance during winter dormancy of Norway spruce (Picea
abies) at the alpine timberline (Klimahaus, 1950 m a.s.l.) related to minimum air temperature during
the winter (December throughout February) and to mean and peak ambient ozone concentration of
the preceding growing season. Frost resistance and temperature: after data from PISEK & SCHIESSL
1947' and GROSS 19892. Ozone data: after HAVRANEK & al. 1989 and AMT der TIROLER
LANDESREGIERUNG 1985-1989.
Winter
42/43 '
43/44 '
85/86 2
86/87 2
87/88 2
88/892
Maximum frost
resistance (°C)
-38
-38
-44
-43
-24
-38
Effects
winter minimum air
temperature (°C)
= -18
s-18
-22
-28
-14
-16
of o z o n e
mean and (peak) ambient ozone
concentration (ppb)
65(110)
64 (120)
66(119)
74 (142)
o ng a s exchange
Fig. 2 summarises 8 years of experimental research on O3 effects on the
gas exchange of mature conifers. After up to 23 weeks of fumigation no distinct
effects on light saturated net photosynthesis and stomatal conductance were
detectable in the current flush of Norway spruce (HAVRANEK & al. 1989, WIESER
& HAVRANEK 1994, 1996), European larch (HAVRANEK & WIESER 1993) and
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cembran pine trees (HAVRANEK & WIESER unpubl.) at mean O3 concentrations
ranging from zero to 100 ppb. Corresponding SUMO, AOT40 and CU values were
200 ppm-h, 100 ppm-h, and 20 mmol m"2 of total needle surface area, respectively
(data not shown). A higher O3 exposure however, induced a significant decline in
net photosynthesis of 20-40%. Long term studies of KOCH & LAUTENSCHLAGER
1988 and GÖTZ 1996 in Bavaria also failed to find significant differences in gas
exchange between spmce twigs provided with O3-free air and twigs exposed to
ambient air and ambient air plus 15 ppb of O3, respectively.
Piceas abies, Pinus cembra
Larix decidua
40
20
0
-20
\
-40
40
20
f
0
-20
m
-40
-60
30
60
90 120
Mean ozone
concentration [ppb]
Fig. 2. The relationship between the percentage loss of net photosynthesis (Pn, top) and
stomatal conductance (gH2o> bottom) of current year needles of Norway spmce {Picea abies),
cembran pine {Pinus cembra) and European larch {Larix decidua) and mean O3 concentration, as
compared to O3-free controls. Measurements were conducted in situ under controlled conditions
(light saturation, 21° C needle temperature, a Aw of 10 Pa kPa"1 and 350 umol mol"1 CO2) after 28
to 141 days of fumigation. Results are mean values ± SD of 4 - 8 individual twigs. Mean ambient O3
concentrations during the vegetation periods Were 25 - 65 ppb.
Observed loss in net photosynthesis of needles continuously exposed to O3
concentrations higher than 100 ppb were greater than reductions in stomatal
conductance (Fig. 2), a behaviour of gas exchange reflecting a decline in the wateruse-efficiency. Stomatal narrowing which occurred after fumigation with the
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highest O3 concentrations was probably a function of increased intercellular CO2
partial pressure resulting from a decline in the efficiency of CO2 uptake (WIESER &
HAVRANEK 1996) as previously shown by others (REICH & AMUNDSON 1987, PELL
& al 1992, WALLIN & al. 1992).
Although stomatal response to increasing O3 exposure was proportionally
lower than that of the net CO2 uptake rate, there was no difference in the regulatory
capacity of the stomata after a shift from light to dark (Fig. 3). Furthermore, when
monitoring gas exchange by tracking ambient conditions throughout an
assimilation period there were no differences in the regulatory capacity of the gas
exchange between O3 free controls and twigs that experienced ambient or twice
ambient O3 levels, neither in spruce (KOCH & LAUTENSCHLAGER 1988), nor in
larch (HAVRANEK & WIESER 1993) and in cembran pine (HAVRANEK & WIESER
unpubl.).
Picea abies
0
1000 m
30 60 90 120 150
Time [min]
Fig. 3. The time course of stomatal conductance after a shift from light to dark of current
year needles of Norway spruce {Picea abies) after O3 exposure throughout 110 days. Data are given
in % of maximal stomatal conductance under saturating irradiation, n = 3 to 5.
Control of ozone
uptake
In mature trees under field conditions several factors prevented or masked
long term effects of O3 concentrations below 100 ppb as compared to chamber
experiments (REICH 1987, SKÄRBY & al. 1995). As O3 effects result from
alterations in biochemical and physiological processes occurring in the needle
interior (TINGEY & TAYLOR 1982), this observed lack in symptom expression in
mature field grown trees at mean 03 concentrations lower than 100 ppb can mainly
be attributed to modifications in the amount of O3 entering the needles.
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O3 flux into the needles is dependent on both O3 concentration and
stomatal conductance. High O3 concentrations generally occur during periods of
high irradiance and temperature (KRUPA & MANNING 1988). In general, O3
concentration in the ambient air is positively correlated with leaf to air vapour
pressure difference (Aw) which similar to O3 increases with irradiance and air
temperature (GRÜNHAGE & JÄGER 1994, WIESER & HAVRANEK 1993a, 1995). Field
data collected from canopies of mature conifers have emphasised Aw as the
environmental factor most likely to control stomatal conductance and, thus, O3 flux
into the needles (WIESER & HAVRANEK 1993a, 1995).
Canopies of forests and trees are not homogenous. Forest stands have
complex vertical and horizontal environmental gradients which are created by
differences in crown architecture and topography of the site and thereby,
influencing the microenvironment. Temperature, humidity, wind speed and O3
concentrations vary within the canopy and differ significantly from the conditions
outside. As a result, it has to be expected that both stomatal conductance and O3
flux varies in relation to the position of the foliage within the canopy.
Based on total leaf surface area maximum leaf conductance varies between
species and even between individual leaves of a tree. Larch displayed a higher
stomatal conductance relative to Norway spruce and cembran pine (Fig. 4A).
Different microclimatic conditions within a tree affect leaf morphology and hence
the potential for O3 uptake. Stomatal conductance has been shown to decrease with
increasing depth of the canopy (LEVERENZ & al. 1982, DANG & al. 1997). After
exposure to similar light levels sun-exposed needles of conifers are known to
exhibit a higher stomatal conductance than needles in the shade of the canopy (Fig
4A; c.f. also HÄSLER 1991, HAVRANEK & WIESER 1994, WIESER & HAVRANEK
1993a, b, 1995). Stomatal conductance also seems to be greater in young and small
trees compared with old or large individuals (KOLB & al. 1997). Needles of
fertilised Norway spruce trees also showed a tendency to higher stomatal
conductance than needles of trees grown under nutrient deficiency (WlESER &
HAVRANEK 1993b).
In general, the sensitivity of the stomata to increasing Aw increased with
maximum stomatal conductance whether the differences in maximal conductance
are dependent on the tree species or due to the position within the crown of an
individual tree (Fig. 4A). On a relative scale however, all these responses were
similar and an increase in Aw from 10 to 20 Pa kPa"1 resulted in a 30% reduction in
stomatal conductance (Fig. 4B).
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Larix decidua, Picea abies,
Pinus cembra
g
o
«
1 g
o
o
0
10
20
30
Aw[PakPa' 1 ]
Fig. 4. Absolute (A) and relative values (B) of stomatal conductance versus leaf-air
vapour pressure difference (Aw). Values are means of 10 to 45 0.5-h intervals collected in situ from
canopy access towers under conditions of light saturation. Relative values were obtained by setting
stomatal conductance to 100 % at a Aw of 10 Pa kPa"' for each tree. Open symbols represent sun
needles and closed symbols shade needles from mature trees of Norway spmce (Picea abies, • ) in
1000 m, cembran pine (Pinus cembra, • ) in 1950 m and European larch (Larix decidua) in 1000 m
(T) and in 1950 m (A), a.s.l., respectively.
Detoxification
capacity
Beside the amount of O3 entering the needles O3 injury depends also on the
presence and the efficiency of defence mechanisms available inside the tissue
(RUNECKLES & VAARTNOU 1997). As O3 is a strong oxidant and will give rise to
several reactive oxygen species in the aqueous phase (HOIGNE & BADER 1975,
GRIMES & al. 1983) antioxidative systems may provide an important protection
from O3 injury.
The capacity to detoxify oxygen radicals is enhanced in plants growing at
high altitudes (RENNENBERG 1988, WILDI & LUTZ 1996), where plants are
generally exposed to higher oxidative stress due to a lower temperature, a higher
radiation as well as a higher O3 concentration compared to low altitudes. Foliar
contents ascorbate and glutathione increase with increasing altitude (POLLE &
RENNENBERG
TAUSZ & al.
1992,
POLLE & al.
1995,
BERMADINGER-STABENTHEINER
1995,
1996). Furthermore, antioxidant contents show pronounced seasonal
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fluctuations as well as light dependent diurnal rhythms with high concentrations
during midday and low concentrations during the night (ESTERBAUER & al. 1980,
GRILL & al. 1987, SCHUPP & RENNENBERG 1988, WIESER & al. 1996).
Although in conifer needles antioxidants are naturally present at high
concentrations, the crucial question is whether or not these pools are available
when O3 enters and whether they can be regenerated at a sufficient rate in order to
remove O3 effectively. In a field grown Cembran pine exposure to elevated
concentrations of O3 did not cause a depletion in total ascorbate (Fig. 5).
Pinus cembra
1,
•
i rh
T
!
1950 m
U
•
:
0
50
100
AA
O3 Treatment [ppb]
Fig. 5. Contents of whole needle ascorbate in current (open bars) and 1-year-old needles
(hatched bars) of cembran pine (Pinus cembra) after exposure to different O3 treatments throughout
one growing season in twig chambers as compared to twigs grown outside the chambers (AA).
Circles refer to reduced ascorbate. n = 3 ± SD.
Apoplastic ascorbate significantly increased in Norway spruce needles
after short term O3 exposure (17 h) to 600 to 800 ppb (POLLE & al. 1995). This
suggests that apoplastic ascorbate in spruce needles can be rapidly regenerated and
might constitute a defence mechanism against oxidative stress (RENNENBERG & al.
1997). However, only 5 to 10 % of the apoplastic ascorbate interacted with O3
taken up (POLLE & al. 1995). The other reactions with O3 remain unknown.
Conclusions
In conclusion, these findings indicate that tropospheric O3 presently does
not represent a dominating stress on coniferous forests in the central European Alps
because long lasting O3 levels below 100 ppb did not cause visible injury or
impairment in gas exchange of mature conifers. In Norway spruce, cembran pine
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and European larch stomatal conductance provides the limiting factor for O3 uptake
and, therefore, determines the "physiological threshold dose".
Increases in tropospheric O3 are accompanied by a steady rise in
atmospheric CO2 levels. Future higher CO2 concentrations are expected to enhance
photosynthesis, to alter water use efficiency and to augment carbon storage in
forest ecosystems unless future climate change has no negative effects on processes
down regulating photosynthesis in response to a higher CO2 partial pressure
(AMTHOR 1995a). An enhanced above ground biomass resulting from elevated CO2
might also increase the water use of woody species (AMTHOR 1995b). Increasing
concentrations of CO2 and other green house gases might also cause an increase the
frequency and severity of drought and thus affecting the distribution and
productivity of woody plants (WlGLEY & al. 1984). The behaviour of conifers
however, is difficult to predict in scenarios where factors have contrasting effects
on carbon gain and water availability. Hence, further research on possible
interactions of O3 and other environmental factors is necessaiy in order to
determine the responses of forest ecosystems to O3 in a changing environment and
their long term consequences for the development of conifers in the Alps.
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