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Rinnakkaistallenteet Luonnontieteiden ja metsätieteiden tiedekunta
2014
Particle capture efficiency of
different-aged needles of Norway
spruce under moderate and severe drought
Räsänen, Janne
NRC Research Press
info:eu-repo/semantics/article
© NRC Research Press All rights reserved
http://dx.doi.org/10.1139/cjfr-2014-0068
https://erepo.uef.fi/handle/123456789/2604
Downloaded from University of Eastern Finland's eRepository
1
Full title: Particle capture efficiency of different-aged needles of Norway spruce under 1
moderate and severe drought 2
3
Janne V. Räsänen*, Toini Holopainen, Jorma Joutsensaari, Pertti Pasanen and Minna 4
Kivimäenpää 5
6
*Corresponding author: Janne Räsänen, Department of Environmental Science, 7
University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland, tel: +358 8
40 355 3199, fax: +358 17 163 191, e-mail: janne.rasanen@uef.fi 9
10
Toini Holopainen, Department of Environmental Science, University of Eastern Finland, 11
P.O. Box 1627, FI-70211 Kuopio, Finland, e-mail: toini.holopainen@uef.fi 12
13
Jorma Joutsensaari, Department of Applied Physics, University of Eastern Finland, P.O.
14
Box 1627, FI-70211 Kuopio, Finland, e-mail: jorma.joutsensaari@uef.fi 15
16
Pertti Pasanen, Department of Environmental Science, University of Eastern Finland, 17
P.O. Box 1627, FI-70211 Kuopio, Finland, e-mail: pertti.pasanen@uef.fi 18
19
Minna Kivimäenpää, Department of Environmental Science, University of Eastern 20
Finland, P.O. Box 1627, FI-70211 Kuopio, Finland, e-mail: minna.kivimaenpaa@uef.fi 21
22 23 24
2 Abstract
25 26
Trees can remove particulate matter from the atmosphere, improving air quality and 27
providing ecosystem services. Particle removal capacity is known to differ between tree 28
species, but the influence of environmental factors on the removal capacity is still 29
unclear. In this study we measured particle capture efficiency (Cp) of Norway spruce 30
(Picea abies) in wind tunnel experiments under three watering treatments: well watered, 31
moderate drought and severe drought, and determined needle characteristics (stomatal 32
conductance and density, wax condition, needle area) that affect particle uptake. Trees 33
were exposed to 0.7µm (geometric mean diameter) NaCl particles with a mass 34
concentration of 1 mg m-3 in the wind tunnel and the Cp of the tree was determined for 35
the current year (C) and previous year (C+1) needles. Overall, the Cp was significantly 36
higher for C+1 needles than for C needles for all watering treatments. There was also a 37
trend for higher Cp of C+1 needles of less watered trees, but this was not observed for C 38
needles. We suggest that greater erosion of the wax layer of C+1 needles compared to C 39
needles increases hydrophilicity of the C+1 needle surface and this, in interaction with 40
low stomatal conductance, lead to the higher Cp.
41 42 43 44 45 46 47 48
3 Introduction
49 50
Particles smaller than 2.5 µm (PM2.5) are emitted from several anthropogenic and 51
natural sources and have adverse effects on human health (Pope III and Dockery 2006).
52
Considerable amounts of particulate and gaseous air pollutants are removed cost 53
efficiently by ecosystem services related to city forests (Nowak et al. 2006). The overall 54
improvement of air quality depends on the forest structure (Reinap et al. 2012) and 55
species diversity as both factors have major effects on total particle capture (Yang et al.
56
2005).
57 58
Particle deposition on trees has been studied in controlled wind tunnel experiments for 59
fine particles, PM2.5, (Beckett et al. 2000) and ultrafine particles, PM0.1, (Lin and 60
Khlystov 2012). Particle capture efficiency (Cp) and deposition velocity (Vg, usually m 61
s-1 or cm s-1) on trees have typically been used to describe particle deposition in the tree 62
canopy. Overall, the particle capturing of coniferous species is more efficient than that 63
of broadleaved trees in similar experimental conditions (Beckett et al. 2000). Cp 64
transformed for total leaf area varies between 0.15% and 0.21% for Douglas fir, 65
Corsican pine and Scots pine exposed to particles at a similar wind speed (3 m s-1), 66
whereas Norway spruce (Picea abies), had a far lower Cp of 0.06% at the same wind 67
speed (Summarized by Räsänen et al. 2013). Different experiments should be compared 68
with care because test conditions affect particle behavior (Belot and Gauthier 1975). For 69
example PM2.5 deposition in the tree canopy is strongly increased with increasing wind 70
velocity (Beckett et al. 2000).
71
4
A unitless variable of Cp has been calculated through several different methods, gaining 72
results that are not directly comparable. Belot and Gauthier (1975) used silhouette area 73
determined as the projected area of canopy that covers a cross sectional view. Beckett et 74
al. (2000) introduced a method that used the scanned area of one leaf side for 75
determining Cp. However, fine particles are captured by the whole leaf and needle 76
surface (total leaf area), which is two times the area of a single side for broad leaved 77
species (Freer-Smith et al. 2004) and can be over three times the scanned area of 78
coniferous species (Flower-Ellis and Ollson 1993). Other method than the total leaf area 79
leads to an overestimation of Cp values for coniferous species relative to broadleaved 80
species (Räsänen et al. 2013).
81 82
Plants can adapt to soil drought by controlling stomatal opening (Reynolds-Henne et al.
83
2010) and decreasing transpiration (Cornic 2000). Drought can reduce single leaf area, 84
which is one of the factors explaining increased particle deposition on tree foliage 85
(Räsänen et al. 2013). In addition, evaporative water molecules can act as particles thus 86
preventing fine particles to deposit on leaf surface (diffusiophoresis) or by cooling the 87
surface increasing the deposition (thermophoresis) (Hinds 1999). Cp of Norway spruce 88
was higher in drought treated than well watered saplings, which was linked to lower 89
stomatal conductance and transpiration under short-term drought probably lowering the 90
diffusiophoresis (Räsänen et al. 2012). Studies on broad bean (Vicia faba (L.)) showed 91
that stomata can also uptake water-soluble fine particles (Eichert et al. 2008) 92
93
The stoma area of coniferous needles is typically covered by a structural wax layer that 94
is sensitive to environmental stress and degrades with age (Turunen and Huttunen 95
5
1990). Burkhardt et al. (1995) showed that structural wax layers have an important role 96
in particle capture of coniferous species as almost no particles were detected on 97
dewaxed plastic model trees or to the adaxial side of silver fir (Abies alba) needles, 98
which are almost free of structural waxes. The amount of leaf surface wax also 99
correlated positively with the particle mass deposited to the leaf surfaces of the 13 100
examined coniferous and broadleaved tree species (Popek et al. 2013). Drought can 101
increase the amount of surface wax (Turunen and Huttunen 1990). On the contrary, leaf 102
aging and air pollution can cause wax degradation (Turunen and Huttunen 1990) which 103
may affect the particle capture efficiency. The canopy of coniferous species typically 104
contains more than one needle generation, but the age classification has rarely been used 105
in terms of particle deposition. Cape (1983), based on field studies, suggested that the 106
particles are accumulated more on older needles of Scots pine. However, our earlier 107
wind tunnel study with Norway spruce showed that particle deposition to current and 108
previous year needles was at the same level (Räsänen et al. 2012).
109 110
This study was performed to further explore the finding that increasing particle capture 111
efficiency of Norway spruce (Picea abies) occurs under lowered soil moisture (Räsänen 112
et al. 2012). To do this we measured needle (C and C+1 separately) characteristics that 113
are potentially affected by drought and determine particle pollution uptake. Different to 114
previous studies, two drought treatments were used: moderate drought arranged by 115
short-term exposure and long-term severe drought with very low soil moisture.
116 117
Materials and Methods 118
119
6
The effects of moderate and severe drought on particle capture efficiency of Norway 120
spruce (Picea abies (L.) Karsten) was examined in summer of 2011. Two year old 121
saplings were provided by Fin Forelia Ltd. at Tuusjärvi, Eastern Finland (62°51’N, 122
28°21’E). Saplings were repotted in two liter pots in a 2:1 peat sand mixture and an 123
additional 1g of N:P:K (9:3.5:5) slow release fertilizer. After repotting, the saplings 124
were randomly divided into three groups: a well watered control group, a moderate 125
drought group and a severe drought group. Well watered and moderate drought exposed 126
saplings were watered two times a week, but watering of moderate drought exposed 127
saplings was stopped 8 to 14 days before their use in experiments. Saplings in the 128
severe drought group were watered with 100 ml of water twice during the 45 day 129
drought period to avoid wilting. Soil moisture of the saplings was measured daily 130
(ThetaProbe, Delta-T Devices Ltd., Cambridge, UK) and the average ± SE soil moisture 131
(%) was in level of 40 ± 4; 9 ± 1 and 4 ± 0.4 in well watered, moderate drought and 132
severe drought group (n = 9), respectively. Saplings were maintained in a greenhouse at 133
the Kuopio campus of the University of Eastern Finland and transported to growth 134
chambers one week before the experiment started. Growth chambers (Weiss BIO 1300) 135
were adjusted to have a relative humidity (RH) of 52%, temperature ranging from night 136
time 12 °C to day time 19 °C and maximum photosynthetically active radiation (PAR) 137
of 375 μmol m-2 s-1. Temperature and light were simulated to reflect typical conditions 138
of June in Finland (see details in Räsänen et al. (2013)). Needles of current year 139
branches were mainly grown during greenhouse maintenance.
140 141
A wind tunnel, earlier described by Räsänen et al. (2012), was utilized to investigate 142
particle capture efficiency of the differently treated Norway spruce saplings at wind 143
7
speed of 3 m s-1. Six saplings of each treatment group were exposed for 2 hours to 144
particles with three saplings per group used as controls with no particle exposure.
145
Saplings were exposed in the experiment one by one in random order so that all groups 146
were tested at similar times of the day. The wind tunnel (6 m long and 50 cm by 147
diameter) was equipped with a particle generator (TSI 9306 Six-Jet Atomizer, T 160 SI 148
Inc., MN, USA) before the entrance to the tunnel. Particles were generated from 10 g L- 149
1 NaCl solution yielding an average NaCl particle mass concentration determined by 150
isokinetic filter collection of 1013 μg m-3 in the tunnel air. Particle mass size 151
distribution was measured with an impactor in a previous study and the geometric mean 152
diameter was 0.7 µm with a geometric standard deviation of 3.0 (Räsänen et al. 2012).
153
In total, 96% of the particles were in the size range of PM2.5. During the test run 154
saplings were illuminated with a greenhouse light (Philips Master Green Power 400 W) 155
providing PAR (measured with photometer, LICOR, model LI-185B, NE, USA) at the 156
level of 450 μmol m-2 s-1 to the mid-canopy. The mean temperature in the wind tunnel 157
during experiments was 24 ºC and the mean relative humidity (RH) 49%.
158 159
The effects of drought and the wind tunnel environment itself on Norway spruce gas 160
exchange was studied by measuring stomatal conductance (gs) and transpiration with a 161
porometer (LI-COR, model LI-1600 Steady state porometer, NE, USA) in the wind 162
tunnel. Two current year shoots (C), and one previous year shoot (C+1) per sapling 163
were measured at the start and end of each test run, with an average calculated for the C 164
needles. Stomatal conductance and transpiration measurements were expressed per total 165
needle area.
166 167
8
Particles deposited to an approximately 100 cm2 area of the needle surface of C and 168
C+1 shoots were dissolved into 40 ml of ion exchanged water and analyzed with ion 169
chromatography (Dionex DX-120 with AS40 autosampler, USA). The needles were 170
then scanned (HP Scanjet 3670) and their areas were determined with the ImageJ- 171
program (ImageJ, U.S. National Institutes of Health, Bethesda, Maryland, USA). The 172
scanned needle area was then transformed to total leaf area by multiplying it by 3.22 173
that was earlier shown to have a good correlation with the total needle area of Norway 174
spruce (Räsänen et al. 2012).
175 176
Particle capture efficiency (Cptot) of saplings was calculated by Eq. 1 (Räsänen et al.
177
2013):
178 179
A X Cp m
u
tot (1)
180
181
where m is mass (mg) of deposited NaCl particles on the needle surface; Xu is a product 182
of the NaCl mass concentration (c, mg m-3) in the air, the exposure duration (t, s) and 183
the average wind speed (u, m s-1) (i.e. Xu = ctu); and A is total needle area (m2). Thus, 184
the Cp indicates the share of the particles that were captured from the total amount of 185
particles that were available to be captured.
186 187
The same parameters were used to calculate deposition velocity (Vgtot, cm s-1) using the 188
Eq. 2 (Räsänen et al. 2013):
189 190
9 u
Cp
Vgtot tot (2)
191 192
Three needles from both C and C+1 shoots were collected for scanning electron 193
microscope (SEM, Philips XL30 ESEM-TMP, FEI Company, Holland) analysis.
194
Needles were attached to a double-sided tape at the needle base and dried and stored at 195
room temperature in desiccators. Tips of the dried needles were then removed and 196
needle pieces of ca. 1 cm were placed on copper tape on SEM stubs and coated with ca.
197
50 nm gold-palladium layer (Automatic Sputter Coater B7341, Agar Scientific Ltd., 198
Stansted, UK). Samples were digitally photographed with SEM and stomatal density (#
199
mm-1) was analysed with the ImageJ program by calculating the average number of 200
stomata in a stomatal row of approximately 1.3 mm long. In addition, 15 epistomatal 201
areas per sapling were photographed using a higher magnification of SEM (image area 202
ca. 3700 µm2). The coverage and condition of the waxes on the epistomatal areas was 203
then analysed from these pictures by applying the five stages (I-V) method introduced 204
by (Trimbacher and Eckmüllner 1997): Class I having single wax filament structures on 205
the stoma and maximum of 10% of the wax has fused. Class II having 10% to 25% of 206
the single wax filaments of epistomatal area fused. Class III having wax filaments fused 207
to plate-like structures and total fused wax area ranging from 25% to 50%. Class IV 208
having fused wax cover from 50% to 75% of the total epistomatal wax area. In class V 209
the wax microstructure has been completely destroyed.
210 211
Wax condition classes were assigned as a value representative of each class, e.g. class I 212
has a value of 1, class II has a value 2 etc., so that an average of the wax condition 213
10
classes could be calculated for each sapling. This quantitative parameter is referred to as 214
the wax index.
215 216
Statistical testing was done with IBM SPSS 19.0 program (SPSS Inc., Chicago, IL, 217
USA). Normality assumptions of the collected data were tested with the Shapiro-Wilk 218
test and homogeneity of the variances with Levene’s test. The main effects of watering 219
treatments, needle age and their interaction were tested with GLM repeated measures 220
ANOVA, since needles of different ages were sampled from the same tree. Interactions 221
with p < 0.2 were further studied using polynomial contrasts of Univariate ANOVA to 222
reveal effects of different watering treatments separately in C and C+1 needles.
223
Polynomial contrast gave p-values for linear and quadratic contrast of which the higher 224
significance level was selected. Main effects and contrasts were considered statistically 225
significant when p < 0.05 and marginally significant when p < 0.1. n = 6 for Cp and 226
Vgtot (only particle-exposed seedlings) and n = 9 for other parameters.
227 228
Results 229
230
Older C+1 needles captured particles more efficiently than C needles (Fig. 1A, Table 1).
231
C needles showed similar Cptot in all the treatments whereas C+1 needles showed a 232
trend for higher Cptot on less watered saplings (Fig. 1A, Table 1). Consequently, the 233
deposition velocity was higher on C+1 needles than on C needles (Fig. 1A, Table 1) and 234
there was a trend for a higher Vgtot of C+1 needles for less watered saplings (Fig. 1A, 235
Table 1). An effect of needle age was also observed for stomatal conductance, which 236
was higher for C+1 needles than for C needles (Fig. 1B, Table 1). Stomatal conductance 237
11
start values decreased with lower watering of saplings in C and C+1 needle age classes 238
(Fig. 1B, Table 1). Stomatal conductance of needles in both age classes (data combined) 239
decreased in the wind tunnel to constant levels of 0.027 ± 0.007 cm s-1, 0.009 ± 0.001 240
cm s-1 and 0.007 ± 0.001 cm s-1 in well watered, moderate drought and severe drought 241
groups, respectively (p = 0.002, linear contrast). Measurement time (07:00-18:00) did 242
not influence stomatal conductance, transpiration or Cptot (data not shown).
243 244
The areas of individual needles were larger in C shoots than in C+1 shoots (Fig. 1C, 245
Table 1). Contrast analysis showed that C needle unit area decreased with lower 246
watering of the saplings, but this was not seen in C+1 needles (Fig. 1C, Table 1).
247
Despite the observed change in needle area the stomatal density remained at the same 248
level in both needle age classes. Stomatal densities were also at similar levels in severe 249
drought (7.7 ± 0.3 # mm-1), moderate drought (7.2 ± 0.3 # mm-1) and well watered 250
groups (8.1 ± 0.2 # mm-1) (p = 0.164, univariate ANOVA). Waxes of the C needles 251
were generally in better condition than waxes of the C+1 needles (Fig. 1D, Table 1).
252
NaCl treatment did not cause changes in wax structures (data not shown).
253 254
Discussion 255
256
Our study highlight the importance of needle age related characteristics and soil 257
moisture as an environmental factors controlling particle capture on needle surfaces of 258
Norway spruce. Measured Cptot and Vgtot values for Norway spruce here are in the 259
lower range of those reported for conifers in the literature (Summarized in Räsänen et 260
12
al. 2013), but similar to our previous experiment with the same species (Räsänen et al.
261
2012).
262 263
The most noticeable age related modification was that C+1 needles were observed to 264
have more degraded waxes than C needles. This wax degradation leads to an increasing 265
hydrophilicity of C+1 needles (Neinhuis and Barthlott 1998) which may favor 266
deposition of hydrophilic particles. Other possible explanation is that part of the ‘wax 267
degradation’ could be optical bias caused by the deposited deliquescent particles which 268
are interpreted as plane wax structures (Burkhardt and Pariyar 2014). However, our 269
study do not support the latter explanation because NaCl did not influence wax 270
condition here or in Räsänen et al. (2012), fusion of waxes were minimal and C+1 and 271
C needles of Norway spruce had equally preserved waxes and similar Cptot in our earlier 272
experiment (Räsänen et al. 2012). In the present experiments the needles of C+1 shoots 273
developed in field nursery conditions in 2010 while the needles of C shoots were grown 274
in a greenhouse during the watering experiment. This explains the differences in size 275
between C+1 and C needles and between watering groups of C needles. In addition, 276
differences in the growing conditions of C (greenhouse) and C+1 (field) needle 277
generations most likely resulted in the better condition of epicuticular wax structures of 278
C needles (Grodzińska-Jurczak 1998). Current needle generations of conifers typically 279
have higher stomatal conductance than older needles (Zimmermann et al. 1988). Lower 280
stomatal conductance of C needles in this study may be explained by the developmental 281
stage of needles, since C needles were still maturing during the exposures.
282 283
13
Soil drought has been shown to have an effect on the Cptot of Norway spruce (Räsänen 284
et al. 2012), which was also seen in this study with C+1 needles whereby Cptot increased 285
linearly with soil drought severity but was not noted in the C needles which had 286
constant Cptot values in all watering treatments. Amount of tubular waxes cannot 287
explain the trend for higher Cptot on less watered saplings, because wax index was in a 288
same level between the treatments. Small single leaf or needle area has also been shown 289
to increase Cp (Beckett et al. 2000). Here we noticed that the C needle size decreased 290
with increasing drought, but this was not connected with higher values of Cptot. In our 291
earlier experiments a 20-40 times smaller needle area of Scots pine yielded only 2-5 292
times higher Cptot compared to broadleaved trees (Räsänen et al. 2013). Thus, 293
differences in single needle area of different watering groups or needle ages alone are 294
obviously too small to explain all the changes in Cptot in Norway spruce.
295 296
Stomatal conductance of the saplings decreased linearly with lower water availability in 297
soil, as was expected (Reynolds-Henne et al. 2010). Räsänen et al. (2012) discussed that 298
lower stomatal conductance leads to a higher Cptot via lowered diffusiophoresis, but the 299
results in the present study are controversial: Cptot was higher with lower stomatal 300
conductance in C+1 needles but with even lower stomatal conductance of C needles the 301
Cptot was lower than in C+1 needles. Increase in Cptot could be due to interaction of 302
increased wax erosion of C+1 needles (increasing hydrophilicity) (Neinhuis and 303
Barthlott 1998) and lower stomatal conductance (less resistance for particles to deposit) 304
(Räsänen et al. 2012). With a well-structured wax layer present on C needles lower 305
stomatal conductance does not have such a strong effect on particle deposition on 306
needle surfaces.
307
14 308
In conclusion, environmental and age related factors of conifer needles led to different 309
rates of Cptot and Vgtot. Lowered soil moisture level decreased the needle size of younger 310
needles, lowered stomatal conductance in both younger and older needles but increased 311
Cptot only in the older needle generation. The effect of smaller single needle area under 312
lowered soil moisture of Norway spruces was too small to explain changes on Cptot. Older 313
needles captured particles more efficiently in all watering treatments than the current year 314
needles. The majority of the differences in Cptot were suggested to be due to interaction 315
of the physiological state of the needles (stomatal conductance) and needle surface 316
characteristics (wax structure).
317 318
Acknowledgments 319
320
This work was funded by the Eemil Aaltonen Foundation, Finnish Cultural Foundation 321
and Finnish Doctoral Programme in Environmental Science and Technology (EnSTe).
322
We thank Timo Oksanen for technical assistance, Jukka Holopainen for soil moisture 323
surveillance, Jaana Rissanen for lab assistance, personnel of the Kuopio Campus 324
Research garden of the University of Eastern Finland (UEF) for maintaining and 325
transportation of saplings, personnel of Sib-labs (UEF) for use of SEM and Dr. James 326
Blande for language revision.
327 328
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Table 1. Statistical significance of parameters affected by age and watering treatments of Norway spruce needles.
394
Main effect Interaction Contrast
Parameter Age Watering Age x watering Watering in C Watering in C+1
n p F n p F n p F n p n p
Particle capture efficiency, Cptot (%)
6 < 0.001 32.259 6 0.256 1.506 6 0.195 1.842 6 0.798 6 0.076
Deposition velocity, Vgtot
(cm s-1)
6 < 0.001 32.259 6 0.256 1.506 6 0.195 1.842 6 0.798 6 0.076
Stomatal conductance start (cm s-1)
9 0.005 9.767 9 0.001 10.369 9 0.002 7.960 9 0.002 9 < 0.001
Stomatal conductance end (cm s-1)
9 0.606 0.273 9 0.004 6.882 9 0.965 0.035
Single needle area (mm2) 9 < 0.001 29.218 9 0.001 8.839 9 < 0.001 14.532 9 < 0.001 9 0.804 Wax index 9 < 0.001 43.788 9 0.939 0.063 9 0.463 0.794
Note: p and F for the main effects and interactions from GLM Repeated measures ANOVA. Linear contrasts for interactions p < 0.2 from 395
Univariate ANOVA.
396
19
Fig. I. A) Average particle capture efficiency (Cptot) and deposition velocity (Vgtot) in 397
current (C) and previous year needles (C+1) of Norway spruce seedlings exposed to 398
moderate drought, severe drought and in well watered controls (n = 6). B) Stomatal 399
conductance at the beginning of the test runs (n = 9). C) Single needle unit area (n = 9).
400
D) Wax index, giving wax class I value 1, class II value 2 etc., of the epistomatal wax 401
condition (n = 7). The lower wax index value indicates better wax condition. Error bars 402
indicate 1 SE.
403 404 405 406 407 408 409 410 411 412 413 414 415 416
20 417
Fig. I 418
Cptot (%)
0.00 0.01 0.02 0.03 0.04 0.05
Vgtot (cm s-1 )
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
Stomatal conductance (cm s-1 ) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Needle area (mm2 )
0 10 20 30 40 50
Well watered
Moderate drought
Severe drought
Wax index
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
A
B
C
D C
C+1
