©

Standard Article - doi: 10.3832/ifor0573-004

iForest – Biogeosciences and Forestry

A comparison between stomatal ozone
uptake and AOT40 of deciduous trees inJapan
Hoshika Y, Shimizu Y, Omasa K
A comparison of the maps of stomatal ozone uptake (AF st0) and concentrations
exceeding 40 ppb (AOT40) for dominant temperate deciduous tree species(Quercus serrata, Fagus crenata, Betula ermanii) was conducted in Japan. Es­
timations of stomatal ozone uptake were accomplished using estimated ozone
concentration, climate data, and vegetation data. Key parameters such as sto­matal conductance parameters for each species were collected from scientific
literature in Japan. Stomatal closure induced by vapour pressure deficit af­
fected the AFst0 values in warmer part of Japan. For this reason, the areas
with high AOT40 did not always correspond to the areas with high AF st0. The
result showed that ozone risk assessment using AOT40 is VPD-constrained incentral Japan, which implies an overestimation of risk compared to AF st0.
While in Europe AOT40 is higher where water stress is recurrent, AOT40
peaked in the cool and humid climate region of central-eastern Japan where
also stomatal ozone uptake reached maximum values.
Keywords: Deciduous forest trees, Ozone uptake modeling, Stomatal conduc­tance, Japan, Ozone

Introduction

Phytotoxic nature of ozone has been wellknown for decades (e.g., Nies 1980, Nies
1984). Surface ozone concentrations are in­creasing in East Asia because of rapid in­creases in emissions of the main criticalozone precursors, Nitrogen oxides and vola­tile organic compounds (Naja & Akimoto2004). Ohara & Sakata (2003) reported thatthe annual average concentration of photo­chemical oxidant, mainly ozone, increased
with high rate (0.33 ppb year -1) from 1985 to1999 in Japan. Therefore, assessments ofozone impacts on plants have become verysignificant in Japan.
In Japan, effects of ozone on the growth offorest tree species have been investigated byusing AOT40, which represents the cumula­
tive exposure above a hourly threshold con­
Graduate School of Agricultural and Life
Sciences, The University of Tokyo, 1-1-1
Yayoi, Bunkyo-ku, 113-8657 Tokio (Japan)@ Yasutomo Hoshika(ahoshika@mail.ecc.u-tokyo.ac.jp)
Received: Feb 15, 2011 - Accepted: Mar 31,
2011
Citation: Hoshika Y, Shimizu Y, Omasa K,
2011. A comparison between stomatal ozone
uptake and AOT40 of deciduous trees inJapan. iForest 4: 128-135 [online 2011-0601] URL: http://www.sisef.it/iforest/show.php?id=573© SISEF http://www.sisef.it/iforest/

centration of 40 ppb during daylight hours(e.g., Izuta et al. 2001, Kohno et al. 2005).
Kohno et al. (2005) suggested that 8-15
ppm·h AOT40 for sensitive species (e.g.,
Fagus crenata), and 15-30 ppm·h AOT40
for moderately sensitive species (e.g., Quer­cus serrata) from April to September as cri­tical level for forest species, are able to in­duce a 10% growth reduction. Critical levels
for forest trees are set at 5 ppm·h AOT40
corresponding to a 5 % growth reduction in
Europe (UNECE 2004). An assessment ap­
proach using AOT40 has the advantage ofbeing simple because only atmosphericozone concentration data are needed. How­ever, ozone damage to plants depends not
only on atmospheric ozone concentrations
but also on stomatal ozone uptake into
leaves. Several studies suggested that sto­matal ozone uptake was closely related toozone damages (e.g., Omasa et al. 2002,
Paoletti & Manning 2007). For this reason, astomatal flux-based approach using stomatalozone uptake is expected to provide better
assessments of ozone damage to plants in
Europe (Emberson et al. 2000). Criticallevels for ozone risk using the stomatal fluxbased indices were set for potato and wheat,and provisionally for sensitive forest species(beech, birch - UNECE 2004). Karlsson etal. (2007) reported that the stomatal fluxbased indices were superior to AOT40 forozone-sensitive species, based on a reanaly­
sis of published ozone exposure-response
data.

128

Very little has been done to develop thestomatal flux-based approach for assessing
ozone damage to plants in Japan. The assess­ment approach using AOT40 was suggested
to be appropriate for Japan because of much
precipitation and limited water stress in Ja­pan (Watanabe et al. 2010). However, thisassumption has not been verified yet. In thisstudy, we developed a model of cumulative
stomatal ozone uptake (AFst0) and assess thespatial distribution of AFst0 compared to
AOT40 for deciduous forest tree species inJapan.

Materials and methods
Vegetation data
Deciduous forest tree species were classi­
fied into three forest types within two clima­
tic zones (cool-temperate and warm-tempe­
rate zones) in Japan (Nakashizuka & Iida
1995). Cool-temperate zone and warm-tem­
perate zone are mainly in northern Japan andwestern Japan, respectively. Regarding theforest types, cool-temperate mixed broad­leaf/conifer forest and cool-temperate de­ciduous forest are distributed in the cooltemperate zone. Cool-temperate deciduous
forest is just in central-eastern Japan, with ahumid climate throughout the year andheavy snow in winter. In the cool-temperate
deciduous forest, beech (Fagus crenata) isthe dominant species. The cool-temperate
mixed forest is dominated by deciduous oak(e.g., Quercus crispula) and birch (Betula
ermanii). In the warm-temperate zone, theforest type is classified as warm-temperate
deciduous forest, and is dominated by de­ciduous oak (e.g., Quercus serrata). Threedominant species (Quercus serrata; Fagus
crenata; Betula ermanii) were selected forcomparison between AFst0 and AOT40. The
distributions of the species targeted by thisstudy are shown in Fig. 1. A distribution wasdetermined based on the vegetation data of
the National Survey on the Natural Environ­ment, investigated by the Ministry of the En­
vironment (http://www.biodic.go.jp/J-IBIS.
html). Spatial resolution of these data was 1
× 1 km.

Input data
Climate data (air temperature, air humidity,
solar radiation and wind speed) and ozone
concentration data for the year 2000
(Takigawa et al. 2007) were input into ourmodel. These data were estimated using
CHASER (Sudo et al. 2002) and the
WRF/Chem model (Grell et al. 2005)
provided at 6h temporal resolution, a spatial
resolution of 40 × 40 km across Japan. We
used these data as input data near groundsurface at 20 m high. Validation for themodel estimates of ozone concentration was
conducted at several monitoring sites in Ja­

iForest (2011) 4: 128-135

Hoshika Y et al. - iForest 4: 128-135
pan (Takigawa et al. 2007). Although further
validation and model improvement may beneeded, the model estimates were used for
estimation of stomatal ozone uptake.
Input data of distributions for deciduoustree species were derived from identifying
whether the grid space (40 × 40 km) inclu­
ded the distribution of vegetation data shownin Fig. 1 or not. The estimation of stomatalozone uptake was conducted in each gridsquare (40 × 40 km) with 6 h time steps.

Estimation of AOT40
AOT40 was estimated using ozone concen­
tration at the top of the canopy as recom­
mended for ozone risk assessment (UNECE
2004). We used ozone concentration data at
20 m high as C (z 1). AOT40 was calculated
as follows (eqn. 1):n

AOT40= ∫ max (C (z1 )i−40.0 )⋅dt
i=1

where C (z1)i is averaged ozone concentra­tion for 6 hours at the top of the canopy(ppb), n is the number of data for ozone con­centrations. AOT40 was calculated over day­light hours with a solar irradiation higher
than 50 W m-2 from April to September (e.g.,
Kohno et al. 2005).

Stomatal conductance model
Leaf-level stomatal conductance of water
vapor (gsw) was estimated using the multi­
plicative model (Jarvis 1976, Emberson et al.
2000 - eqn. 2):
g SW =g max⋅f phen⋅f light⋅max { f min , ( f temp⋅f VPD⋅f SMD)}

Fig. 1 - Distribution of dominant forest species in Japan (A: Quercus serrata; B: Fagus
crenata; C: Betula ermanii).

iForest (2011) 4: 128-135

129

where gmax is the maximum stomatal con­ductance. The other functions are limiting
factors of gmax and are scaled from 0 to 1. fminis the minimum stomatal conductance and is
set to 0 in this study because we could not
get data of gmin from the literatures. fphen is thevariation in stomatal conductance with leafage, and flight, ftemp, fVPD, and fSMD are functionsof photosynthetically photon flux density at
the leaf surface (PPFD, μmol photons m-2 s1), temperature (T, °C), vapor pressure defi­cit (VPD, kPa), and volumetric soil watercontent (θ, m3 m-3), respectively.
The variation in stomatal conductance withleaf age (fphen) modifies gmax as a function oftime within the leaf duration (Emberson etal. 2000). Maruyama & Honda (1993) repor­
ted that it took about 1 month for gsw toreach its peak from leaf onset, and to decline
due to its senescence until leaf fall for Japan­
ese beech and deciduous oak species. There­
fore, we assumed that fphen increased linearly
from 0 to 1 during the first 30 days after leaf
onset and decreased linearly from 1 to 0 dur­ing the 30 days prior to leaf fall.
The functions (flight, ftemp and fVPD) have been
expressed in various forms (e.g., Jarvis 1976,© SISEF http://www.sisef.it/iforest/

Comparison between ozone uptake and AOT40 of deciduous trees in Japan
Tab. 1 - Parameters of the stomatal conductance model for each dominant deciduous tree species in Japan.
Deciduous oak(Q. serrata)

Parameter

Beech(F. crenata)

Value

References

Value

gmax
(mmol m-2 s-1)

480

425

flight_0.5(μmol m-2 s-1)
Topt (°C)
Tmin (°C)
Tmax (°C)
FVPD_0.5 (kPa)a145

Tanaka et al. (1998),
Sirisampan et al. (2003),
Yamazaki et al. (2006)
Tanaka et al. (1998),
Sirisampan et al. (2003)
Tanaka et al. (1998),
Yamazaki et al. (2006)

24
0
48
2.1
3.4

Yamazaki et al. (2006)

References

Birch(B. ermanii)
Value

References

Iio et al. (2004)

680

Muraoka & Koizumi (2005),
Yamazaki et al. (2006)90

Iio et al. (2004)

110

Yamazaki et al. (2006)

25
0
40
2.4
1.9

Iio et al. (2004)

25
0
45
1.2
2.5

Yamazaki et al. (2006)

Iio et al. (2004)

Yamazaki et al. (2006)

Sirisampan et al. 2003, Emberson et al.
2000). In this study, the following formulas
were selected because they have been fre­quently used in modeling studies in Japan(e.g., Sirisampan et al. 2003). These func­
tions are expressed as follows (eqn. 3, eqn.
4, eqn. 5):f light =PPFD(PPFD + f light )
0.5

(

( )
[ ( )]

T −T minT max−Tf temp=⋅T opt −T min T max−T opt
f VPD= 1+

VPD
f VPD

T max −T opt
T opt −T min

)

a −1

0.5

Fig. 2 - Plots of the functions flight (A); ftemp (B); fVPD (C) of the stomatal conductance modelfor three deciduous tree species. flight, ftemp and fVPD are functions of photosynthetically photon
flux density at the leaf surface (PPFD, μmol photons m -2 s-1), temperature (T, °C), and vapor
pressure deficit (VPD, kPa), respectively.
© SISEF http://www.sisef.it/iforest/

130

where flight_0.5 is the value of PPFD when flight= 0.5, and Topt, Tmin, and Tmax represent the
optimum, minimum, and maximum tempe­
ratures for stomatal conductance, respec­tively. fVPD_0.5 is the value of VPD when fVPD
= 0.5, and a is a constant.
Tab. 1 lists the parameters used in the sto­matal conductance model. Parameters forflight, ftemp, fVPD, and gmax were determined froma review of scientific literature on stomatal
conductance of temperate deciduous forest
tree species in Japan: Quercus serrata(Tanaka et al. 1998, Sirisampan et al. 2003,
Yamazaki et al. 2006), Fagus crenata (Iio etal. 2004), Betula ermanii (Muraoka & Koi­
zumi 2005, Yamazaki et al. 2006). Fig. 2
shows a plot of these functions in the sto­matal conductance model.
Parameters of fSMD could not be included in
the model because of insufficient publisheddata about fSMD for temperate deciduous tree
species in Japan. The fSMD values were notcalculated in this study (fSMD=1), although
this assumption may lead to an overestima­tion of stomatal ozone uptake. However, Sir­isampan et al. (2003) reported that the soil
water content had no effect on the stomatal
conductance for six tree species in central

iForest (2011) 4: 128-135

Hoshika Y et al. - iForest 4: 128-135
Japan. Therefore, the results would still al­
low a comparison of the maps between sto­matal ozone uptake and AOT40.

Estimation of stomatal ozone uptake
Estimation of stomatal ozone uptake (Fst;
nmol O3 m-2 s-1) were calculated based on theassumption that the ozone concentration at
the top of the canopy [C (z1)] represented aconcentration near sunlit leaves at the top of
the canopy (Emberson et al. 2007). Then sto­matal ozone uptake was calculated as fol­lows (eqn. 6):(g SW /1.65)
1F st =C (zi )⋅⋅(r b+rc ) (g SW / 1.65+ g ext )where rb is the leaf boundary layer resistance(s m-1), rc is the leaf surface resistance [= 1/(gsw/1.65+gext); s m-1], 1.65 accounts for thedifference in diffusivity of water in air com­pared with ozone, and gext is the external leaf
or cuticular conductance (s m-1). gext was set
to 0.0004 s m-1 (Emberson et al. 2007).
Leaf boundary layer resistance (rb) was cal­culated from the wind speed at canopy
height, u (z1) in m s-1, and the mean leaf
width, Ld (m, UNECE 2004 - eqn. 7):
0.5

( )

r b=1.3⋅150⋅

Ldu(z1 )

where the factor 1.3 accounts for differencesin diffusivity between heat and ozone. Ld foroak and beech/birch were determined as 0.1
and 0.05 m, respectively, from leaf shapedata in Japan (Endo 1940).
In Europe, the accumulative stomatalozone uptake (AFst Y) was recommended toassess the ozone risk for forest species(Karlsson et al. 2007). It is given by the fol­lowing equation (eqn. 8):

Tab. 2 - Observed time (day of year) of leaf onset and leaf fall for deciduous forest trees inJapan.

RegionWestern Japan(35°10’ N 132°40’E)
Central-eastern Japan(34°55’ N 137°45’E)
Central-eastern Japan(36°30’ N 138°20’E)
Northern Japan(39°90’ N 141°00’E)
Hokkaido(42°59’ N 141°23’E)

Observation (day of year)
Leaf
Leaf
onset
fall
107
339

References
Ozaki et al. (2000)

109

319

Fujimoto (2008)

121

297

Kato & Hayashi (2008)

131

298

Aoki & Hashimoto (1995)

135

304

Nakai et al. 2003

Tab. 3 - Mean values (± SD) of estimated AOT40 and AF st0 for deciduous tree species ineach region of Japan. The values were calculated for each region including more than 5
grids. Within the same parameter, values with different letters are significantly different(Kruskal-Wallis test, p < 0.05 - number of 40 × 40 km grids, p < 0.05).

RegionAOT40(ppm·h)Hokkaido(42°-44°N 140°-145°E)
Eastern Japan(35°-42°N 139°-142°E)
Central-eastern Japan(34°-38°N 136°-139°E)
Central Japan(33°-36°N 133°-136°E)
Western Japan(31°-35°N 130°-133°E)

5.1 ± 1.6 a(41)
16.7 ± 5.0 b(53)
28.0 ± 5.6 c(41)
30.8 ± 3.5 d(31)
26.4 ± 4.3 c(40)

Quercusserrata
35.9 ± 3.4 a(44)
41.2 ± 3.7 b(41)36.0 ± 3.5 a(30)
40.6 ± 2.3 b(30)

AFst0 (mmol m-2)
Fagus
Betulacrenataermanii
26.7 ± 1.6 a
31.9 ± 2.3 a(6)(35)
35.1 ± 3.2 b
40.8 ± 4.4 b
(43)
(14)
41.1 ± 2.7 c
48.3 ± 4.0 c
(32)
(21)
40.1 ± 2.1 c(17)41.8 ± 2.2 c(19)

Significance of difference
Kruskal-Wallis test

< 0.001< 0.001< 0.001< 0.001

AF st Y =∫ max (F st −Y , 0)⋅dt
where Y is a threshold of stomatal ozone up­take (nmol O3 m-2 s-1). In Europe, a threshold
Y is currently used (Emberson et al. 2007,
Karlsson et al. 2007). However, it is notclear whether the same threshold of Fst canbe applied in Japan. Therefore, we did not
set a threshold for the Fst value (AFst0) in thepresent study.

Estimation of leaf duration
Leaf duration was simply assumed basedon phenological data in Japan (Tab. 2). We
chose the data by including several species
in Japan. Because we could not collectenough phenological data for each species,
we assumed no difference in leaf duration
among species. From these data, a leaf onset
time was set depending on the latitude in Ja­pan (leaf onset time = 3.4 · latitude - 9.1;
r2 = 0.89). In contrast, time of leaf fall did
not show a linear relationship with latitude.

iForest (2011) 4: 128-135

Fig. 3 - A map of estimated AOT40 values from April to September in Japan along the 40 ×
40km grid of the CHASER and WRF/Chem models used to estimate climate and ozone input
data.
131

© SISEF http://www.sisef.it/iforest/

Comparison between ozone uptake and AOT40 of deciduous trees in Japan

Fig. 4 - Maps of estimated AFst0 (accumulated stomatal ozone uptake) values of the deci­
duous tree species in Japan. (A): Quercus serrata; (B): Fagus crenata; (C): Betula ermanii.
Therefore, the averaged value of the time
(DOY = 311, s.d. = 17) was adopted as timeof leaf fall in this study.

Statistics and data analysis
To compare AOT40 and AFst0, maps ofboth indices were provided. In addition, ave­raged AOT40 and AFst0 were calculated foreach region. The linear correlations between
both indices, and between f VPD in summer© SISEF http://www.sisef.it/iforest/

and AFst0 were tested. Moreover, differenceof the indices among regions was tested witha Kruskal-Wallis test. Results were con­sidered significant at p < 0.05. Statisticalanalysis was performed with SPSS software
version 11.5 (SPSS, Chicago, USA).

Results

Fig. 3 shows a map of the AOT40 values
from April to September in Japan, and Tab.

132

3 shows an averaged value of AOT40 and
AFst0 of each region in Japan. There weredifferences in the AOT40 values among re­
gions. The AOT40 values showed thehighest value more than 30 ppm·h from
central-eastern Japan (34°-38°N 136°139°E) to western Japan (31°-35°N 130°133°E - Fig. 3). In contrast, the AOT40 va­
lues were relatively low (0-10 ppm·h) in
Hokkaido (42°- 44°N 140°- 145°E).
Fig. 4 (A-C) shows maps of the AFst0 va­
lues of the deciduous tree species in Japan.
For Quercus serrata, AFst0 showed thehighest value in central-eastern Japan (Fig.
4A). The averaged AFst0 values in this re­gion were 41.2 mmol m-2 s-1 (Tab. 3). In thisarea, the AFst0 values reached about 50
mmol m-2 for Quercus serrata and Fagus
crenata, and 55 mmol m-2 for Betula er­
manii. In Hokkaido or eastern Japan (35°42°N 139°-142°E), the AFst0 values werelower than in central-eastern Japan (Fig. 4,
Tab. 3). In Hokkaido, the averaged AF st0
values were 26.7 mmol m-2 and 31.9 mmol
m-2 for Fagus crenata and Betula ermanii,
respectively (Tab. 3). The averaged AFst0
values for Quercus serrata were 35.9 mmol
m-2 in eastern Japan (Tab. 3). In central Japan(33°- 36°N 133°-136°E), the AFst0 values
were similar to the levels in eastern Japan for
Quercus serrata (Fig. 4A), Tab. 3). The
AFst0 values showed below 30mmol m-2 for
Quercus serrata in this region (Fig. 4A).
A comparison between the AOT40 and AF­
st0 maps (Fig. 3, Fig. 4) suggested a diffe­
rence in the spatial pattern. Although the
AOT40 values were different between
eastern Japan, and central Japan, the AFst0
values were not significantly different for
Quercus serrata (Tab. 3). These areas are thewarmer parts of Japan. Fig. 5 shows the ave­raged fVPD values for Quercus serrata insummer (July-August). In central Japan, theaveraged fVPD values were about 0.8 in thesummer (Fig. 5) because of high tempera­
ture. The lower fVPD values correspondedwith the lower AFst0 areas for Quercus ser­rata.
For further analysis, we conducted a corre­
lation analysis between both indices in Japan(Tab. 4). The result shows that the correla­tion between AOT40 and AFst0 was lower
for Quercus serrata compared to other two
species. Quercus serrata is mainly distri­buted in warm-temperate climate region(Fig. 1). Especially, the correlation was not
significant in warmer part of Japan such as
central and western Japan (Tab. 4). This re­sult was influenced by effects of VPD on
stomatal conductance (Fig. 5, Tab. 5).
For the other two species distributed in hu­
mid and cool region and high mountainous
area, the correlation between AOT40 and AF­
st0 was very high (Tab. 4). Fagus crenata isalso distributed in the warmer part of Japansuch as central and western Japan (Fig. 1). In

iForest (2011) 4: 128-135

Hoshika Y et al. - iForest 4: 128-135

Fig. 5 - Spatial distribution of averaged fVPD values for Quercus serrata from July to Augustin Japan. fVPD shows the response of stomatal conductance to vapor pressure deficit, scaled
from 0 to 1.
Tab. 4 - Correlation between AOT40 and AF st0 (accumulative stomatal ozone uptake) for
each tree species in Japan. The correlation coefficient was calculated for each area including
more than 5 grids. (*) and (**) denotes the significance at 5% and 1 % level, respectively.
(NS) indicates no significant correlation.
Quercus serrata
RegionAll regions
0.33**
Hokkaido - Central-eastern Japan
0.53**(34°-44°N 136°-145°E)
Central Japan - Western Japan-0.06 NS(31°-36°N 130°-136°E)

Fagus crenata
0.89**
0.93**

Betula ermanii
0.91**
0.93**

0.34*-

Tab. 5 - Correlation between fVPD (response of stomatal conductance to vapor pressure defi­cit) in summer and AFst0 (accumulative stomatal ozone uptake) for Quercus serrata in Ja­
pan. The correlation coefficient was calculated for each area including more than 5 grids. (*)
and (**) denotes the significance at 5% and 1 % level, respectively. (NS) indicates no signi­
ficant correlation.
RegionAll regions
Hokkaido - Central-eastern Japan(34°-44°N 136°-145°E)
Central Japan - Western Japan(31°-36°N 130°-136°E)
the warmer part of Japan, the correlation
between AOT40 and AFst0 decreased for
Fagus crenata (r = 0.37).

Discussion

The purpose of this study was to compare
the spatial map of the AFst0 and AOT40 va­
lues for deciduous forest trees in Japan. Em­
berson et al. (2000) and Simpson et al.
(2007) similarly compared the spatial distri­bution of the stomatal ozone uptake and
AOT40 values in Europe. The AOT40 value
was relatively high in southern Europe com­pared to northern Europe. The spatial gra­dient of the AOT40 values was remarkablefrom south to north, but the spatial gradientiForest (2011) 4: 128-135

Value
0.43**
0.38**
0.9**
of the stomatal ozone uptake was much less.
Stomatal ozone uptake was limited due todrought stress during the summer in Medi­terranean regions where ozone concentra­
tions were high (Paoletti 2006).
In this study, AFst0 showed similarly muchless regional differences than AOT40. How­ever, there is a difference of the result com­pared to Europe. In Europe, the expo­
sure-based critical level for forest species issuggested. Values of the critical levels aredefined as 5 ppm·h of AOT40. AOT40 va­
lues exceeding the European critical levels
were shown in 89% of Japan. In Japan,
Kohno et al. (2005) suggested that 15-30
ppm·h AOT40 for moderately sensitive spe­

133

cies (e.g., Quercus serrata) from April to
September as critical level for forest species.
AOT40 values exceeding 30 ppm·h wereshown in 21% of Japan. Some regions where
the AOT40 values reached 30 ppm·h corres­
ponded to cool and humid climate such as
central-eastern Japan. Averaged stomatalozone uptake was estimated to be higher
than 40 mmol m-2 for the three species incentral-eastern Japan (Tab. 3). In contrast,
stomatal ozone uptake for Quercus serrata
did not show any difference between easternand central Japan, although AOT40 in cen­tral Japan was twice that in eastern Japan.
Emberson et al. (2000) showed that VPD
played a major role in limiting the stomatalozone uptake. Also in this study, VPD is alimiting factor of the stomatal ozone uptake
especially in warmer part of Japan (Tab. 5).
For this reason, this region showed a dis­
crepancy between the AFst0 values of Quer­cus serrata and AOT40. These results sug­gest that not only ozone concentration but
also stomatal closure induced by VPD af­
fected the AFst0 in the warmer part of Japan.
The results of correlation analysis for
Fagus crenata and Betula ermanii suggested
that AFst0 and AOT40 values are similar forozone risk assessment in humid and cool cli­mate region, because the correlation between
AOT40 and AFst0 was very high (Tab. 4),
i.e., humid and cool climate was favorable
for stomatal opening. However, in the war­mer part of Japan, the correlation between
AOT40 and AFst0 decreased for Fagus cre­nata (r = 0.37), because VPD induced a sto­matal closure.
In warmer part of Japan, we also could findthe difference of spatial pattern of the AF st0
values between Fagus crenata and Quercus
serrata (Fig. 4). Stomatal conductance for
Fagus crenata was less sensitive to high
VPD (>2 kPa) than that for Quercus serrata(Tab. 1; Fig. 2). Therefore, the result is dueto the species-specific characteristics of thestomatal conductance parameters. The
assessment approach using AOT40 does notconsider the difference of species-specificstomatal response to the climate factors. The
result of this study illustrated the limitationsof performing ozone risk assessment using
AOT40 in Japan.
In this study, fSMD could not be applied in
the model because of insufficient publisheddata about soil moisture deficit. However,Sirisampan et al. (2003) reported that thesoil water content had no effect on the sto­matal conductance of six tree species in cen­tral Japan. However, soil water deficit may
be a limiting factor in estimating AF st0 evenfor short periods during a growing season.
The parameter of fSMD would bring about afurther refinement of the calculations for Ja­
pan. In addition, we assumed that plants
absorbed ozone continuously when climate
condition was favorable for stomatal ope­

© SISEF http://www.sisef.it/iforest/

Comparison between ozone uptake and AOT40 of deciduous trees in Japanning. However, when leaves experience a
longer period of ozone exposure, stomatal
response would be changed (Paoletti &Grulke 2005). Pleijel et al. (2002) and
Danielsson et al. (2003) reported that ozone
exposure over a long time caused a decline
in gmax for crops in their modelling studies.
This effect has not included in the model ofthis study yet. Long-term ozone exposure
would also be limiting factor of stomatalozone uptake. Including the effects of ozone
on stomatal conductance would be expected
to improve the ozone uptake modelling for
deciduous forests in Japan.

Conclusions

The maps between AFst0 and AOT40 for
dominant temperate deciduous tree species
in Japan were compared. Estimations of sto­matal ozone uptake were accomplished using
estimated ozone concentration and climate
data, and vegetation data. As a result, sto­matal closure induced by vapour pressure
deficit affected the AFst0 values in warmerparts of Japan.
Therefore, the areas with the highest
AOT40 did not correspond to the areas withthe highest AFst0, suggesting that the use of
AOT40 for protecting Japanese forests may
be misleading.
Stomatal ozone uptake was suggested forassessing ozone damage more than three
decades ago in Japan (Omasa et al. 1979).
Many studies (e.g., Paoletti & Manning2007) suggested that the stomatal flux-based
approach is scientifically-sound and would
be a useful tool for ozone risk assessment.
This paper is a contribution to develop thestomatal flux-based approach in Japan. Fu­
ture work should validate and improve ourmodel with field measurements. Parameteri­zations for more species would also contri­bute further developments because ozone
sensitivity of plants and stomatal response toenvironmental condition are species-specific(e.g., Omasa et al. 2000).

Acknowledgements

This work was supported by JSPS Fellow­
ships for Young Scientists and was partly
supported by the Global Environment Re­
search Fund (C-062) of the Ministry of theEnvironment, Japan.

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