Blue wild-rye grass competition increases the effect of ozone on ponderosa pine seedlings
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Tree Physiology 21, 319–327
© 2001 Heron Publishing—Victoria, Canada
Blue wild-rye grass competition increases the effect of ozone on
ponderosa pine seedlings
CHRISTIAN P. ANDERSEN,1 WILLIAM E. HOGSETT,1 MILTON PLOCHER,2 KENT
RODECAP 2 and E. HENRY LEE1
1
US EPA, Western Ecology Division, National Health and Environmental Effects Research Laboratory, 200 SW 35th Street, Corvallis, OR 97333,
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USA
2
Dynamac Corporation, 200 SW 35th Street, Corvallis, OR 97333, USA
Received December 17, 1999
Summary Individual ponderosa pine (Pinus ponderosa al. 1995, US EPA 1996). However, it is difficult to identify the
Dougl. ex Laws.) seedlings were grown in mesocosms with role of ozone stress on plant growth in the field because many
three densities of blue wild-rye grass (Elymus glaucus Buckl.) other stresses are present in natural ecosystems. For example,
(equivalent to 0, 32 or 88 plants m –2 ) to determine if the pres- during periods of drought, plants may be partially protected
ence of a natural competitor alters the response of ponderosa from ozone as a result of stomatal closure (Temple et al.
pine seedlings to ozone. After 3 years of ozone exposure, grass 1992). Other stresses may interact with ozone to affect the
presence reduced total ponderosa pine dry mass by nearly magnitude, and in some cases the direction, of the response
50%, whereas ozone alone had no significant effect on ponder- (Heagle 1979, Tjoelker and Luxmoore 1991, Pääkkönen and
osa pine growth. The combination of ozone and grass further Holopainen 1995, Andersen and Scagel 1997). An under-
reduced needle, stem and branch dry mass significantly below standing of the role of combined stresses is important when
that induced by grass competition alone. Root:shoot ratios in-
determining which species are most at risk to damage from
creased in response to the combined grass and ozone treat-
tropospheric ozone and during which growth phases damage
ments. Grass competition significantly reduced soluble sugar
is most likely to occur.
concentrations in all ponderosa pine tissue components exam-
Because all possible combinations of stresses and condi-
ined. Starch concentrations were highly variable but did not
tions that exist in nature cannot be studied or fully character-
differ significantly between treatments. Ozone significantly
ized, it is important to understand the mechanisms of plant
reduced soluble sugar concentrations in fine roots and stems.
response to stress to extrapolate responses to greater spatial
In the absence of grass, ozone-treated seedlings tended to have
and temporal scales (Heck et al. 1998). One approach is to
higher tissue N concentrations than controls. In the presence of
study stress interactions under controlled conditions to iden-
grass, ozone-treated seedlings had lower N concentrations than
tify which stresses are likely to modify ozone response in the
controls, resulting in a significant interaction between these
field. By studying mechanisms of plant response under con-
two stresses in 1- and 2-year-old needles. Needle C:N ratios de-
trolled conditions, hypotheses can be formulated and used to
creased in response to grass competition, as a result of in-
identify key stresses in natural ecosystems.
creased N concentration and no change in C concentration. The
Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) is com-
opposite response was observed in ozone-treated seedlings as a
mon throughout western mid-elevation forests (Oliver and
result of decreased N concentrations, indicating that ozone-
Ryker 1994), and typically occurs on marginal sites character-
treated seedlings were unable to take up or retain as much ni-
ized by low nutrient availability and seasonal drought. Pon-
trogen when grown in the presence of grass. We conclude that
derosa pine is one of the conifers that is most sensitive to
ponderosa pine seedlings are more susceptible to ozone when
tropospheric ozone, and the long-term effects are well docu-
grown in competition with blue wild-rye grass.
mented (McBride et al. 1975, Taylor 1980, Miller et al. 1982,
Keywords: C:N ratio, Elymus glaucus, Pinus ponderosa, Miller et al. 1989, Pederson 1989). Blue wild-rye grass
root:shoot ratio. (Elymus glaucus Buckl.) is a perennial bunchgrass that grows
between 60 and 120 cm tall, and is found across much of the
same range as ponderosa pine (P.J. Burton and T.E. Duralia,
Symbios Research and Restoration, unpublished data). Al-
Introduction though blue wild-rye is not thought to be a strong competitor
Tropospheric ozone affects the physiology and growth of a among other grasses, it has been found to reduce performance
wide range of herbaceous and woody plant species in the USA of Quercus douglasii Hook. and Arn. (Koukoura and Menke
and Europe (Miller et al. 1982, Taylor et al. 1994, Ashmore et 1995) and is thought to compete with ponderosa pine seed-320 ANDERSEN, HOGSETT, PLOCHER, RODECAP AND LEE
lings during establishment (P. Miller, USFS, PSWS, unpub- ponderosa pine seedling within each mesocosm cell. A litter
lished data). layer was spread on the soil surface. Soil water content was
The majority of tropospheric ozone studies have been con- monitored by time domain reflectometry (TDR, Soilmoisture
ducted in the absence of other stresses to identify ozone-sensi- Equipment, Inc. Santa Barbara, CA) using probes buried at
tive species (US EPA 1996). Studies that have included more 25-cm depth in each cell and 15-cm probes inserted from the
than one stress have often revealed a range of complex interac- soil surface. To simulate normal summer dry-down during the
tions that are difficult to integrate into common process mod- 3-year study, seedlings were watered when the cells dried to
els of plant response (Fangmeier1989, Beyers et al. 1992, between 5 and 10% soil water by volume. Chambers were
Temple et al. 1992, Greitner et al. 1994, Pell et al. 1995, John- sprayed in the spring of 1996 and 1997 to control aphids and
son et al. 1996, Kasurinen et al. 1999). More information is mites.
needed about how combined stresses affect mechanisms of
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ozone response. This study was designed to examine the influ- Experimental design
ence of blue wild-rye grass on the response of ponderosa pine The experiment employed a split-plot design with two con-
seedlings to ozone. Although many interactions among spe- centrations of ozone (episodic 90 profile ozone, charcoal-fil-
cies occur within plant communities, and single factors cannot tered-air control) comprising the whole-plot treatments and
easily be ruled out (e.g., herbivory, allelopathy), the condi- three grass densities comprising the split-plot treatments.
tions and outcomes of this study strongly suggest that compe- Each grass treatment was replicated three times within each
tition was a major contributor to our results. open-top chamber, and four chamber replicates were used for
Grime (1979) states that competition is the tendency of each ozone treatment (eight total chambers, 72 sub-plots).
neighboring plants to utilize the same source of light, nutri- Main effects of and interaction between ozone and grass den-
ents, water or volume of space. Others have defined competi- sity were evaluated by analysis of variance. Data were log
tion as the induction of strain in one organism as a result of the transformed because the biomass measurements at final har-
use, defense or sequestering of resources by another organism vest showed significant heterogeneity of variance. Outliers
(Weldon and Slauson 1986). Goldberg and Fleetwood (1987) (i.e., residuals greater than three standard deviations from zero
separated competition into effect and response components, in absolute value) for total carbon and nitrogen, mineral nutri-
where effect is the ability of one plant to reduce the perfor- ent concentrations and glucose concentrations for current-, 1-
mance of a neighboring plant, and response is the ability to and 2-year-old needles on the main stem and branches of pon-
perform relatively well in the presence of competition. derosa pine seedlings were excluded from the analysis.
The goal of the present study was to focus on responses of
ponderosa pine seedlings grown with or without blue wild-rye Ozone exposures
grass. We examined physiological changes in ponderosa pine
to identify possible response mechanisms, and to obtain infor- Ozone was delivered to the open-top chambers for 112 days
mation on key processes that may be used to extend these re- during 1995 (June 16 through October 5) and 113 days during
sponses to the field. These results, in combination with field 1996 (June 17 through October 7) (Hogsett et al. 1985b). Ex-
examinations, are being used to develop process models that posure was ended after 97 days in 1997 (June 17 through Sep-
will allow us to examine possible responses of ponderosa pine tember 21) so that the seedlings could be harvested before
to ozone and climate scenarios that cannot be experimentally onset of dormancy. The ozone profile was developed based on
manipulated because of their spatial or temporal extent. Here regional air quality data from the midwest and consisted of an
we report the results of the 3-year study, with emphasis on episodic pattern of varying daily peak concentrations on
ponderosa pine seedling growth and nutrient status. 28-day cycles as described earlier (Lefohn et al. 1986, Clark et
al. 1995) (Table 1).
Methods Morphological measurements
Blue wild-rye grass plants were counted in the mesocosms in
Plant culture April, June and November 1996 and monthly from March
Ponderosa pine seeds from Shasta County, CA (California through September 1997. The grass was cut at 5-cm height
Seed Zone 522, elevation 2,000–2500 m), were grown for (above soil) on May 27 in both 1996 and 1997. Biomass from
1 year in a shade house at L.A. Moran Reforestation Center, each cutting was dried and weighed. The biomass was re-
Davis, CA. The 1-year-old seedlings were then transported to turned to the mesocosms in 1996 but not in 1997. After cut-
Corvallis, OR, and transplanted, one per mesocosm cell, to a ting, a tiller was marked in each mesocosm and measured for
pumiceous loamy sand collected under ponderosa pines in regrowth from June through October 1996 and June through
central Oregon. Nine mesocosm cells (53 × 53 × 50 cm deep) September 1997.
were nested within each modified open-top chamber and sur- At final harvest, aboveground ponderosa pine biomass was
rounded by a 15-cm border row of blue wild-rye grass. Blue separated into foliage and woody tissue by age class and grass
wild-rye grass seedlings were sown at one of three densities biomass was separated into tops (tillers and leaves) and stub-
(0 = no grass, 1 = 8 plants cell –1 or 32 plants m –2, or 2 = ble. Needle area and leaf area per unit weight for the ponder-
24 plants cell –1 or 88 plants m –2 ) along with the individual osa pines were determined by age class based on subsamples.
TREE PHYSIOLOGY VOLUME 21, 2001GRASS COMPETITION AFFECTS OZONE RESPONSE IN PINE 321
Table 1. Ozone concentrations for each of the three exposure seasons, dures (Bremner and Tabatabai 1971). In addition, subsamples
expressed using three different exposure statistics. Sum 00 is calcu- of 1996 and 1997 needle tissues were digested with 0.33 M
lated by adding the hourly mean ozone values, 24 h day –1, over the HClO4 and analyzed by ICP for P, K, S, Ca, Mg, Mn, Fe, Cu,
entire exposure season. The 12 h Sum 06 is calculated by summing
Zn, Mo and B (McQuaker et al. 1979). Accuracy of the test re-
the hourly mean ozone values only when they exceed 60 ppb for
each h, from 0800 to 2000 h each day. The 12 h AOT 40 is obtained by
sults for total nitrogen and carbon analysis as well as ICP anal-
summing only the portion of the peak that exceeds 40 ppb, from 0800 ysis were verified by analyzing reagent blanks and National
to 2000 h each day over the exposure season. Institute of Standards and Technology pine needles
(SRM-1575) along with the tissue samples. All assays were
Treatment Season Ozone concentration (ppm h) conducted in duplicate.
Sum 00 12 h 12 h
Sum 06 AOT 40
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Control 1995 19.5 ± 3.6 0.0 0.0 Results
1995 13.2 ± 3.6 0.0 0.0
1997 15.5 ± 2.7 0.0 0.0 Ozone concentrations during the 3-year exposure period are
shown in Table 1. Total exposure during the third growing
Ozone 1995 93.5 ± 1.1 30.7 ± 0.2 20.1 ± 1.1 season was less than total exposure during the first two grow-
1995 89.4 ± 0.8 26.7 ± 0.5 17.8 ± 0.3
ing seasons because the plants were harvested about 2 weeks
1997 75.7 ± 1.2 21.7 ± 0.4 14.6 ± 0.3
earlier.
Ponderosa pine seedling dry mass was significantly reduced
by the presence of blue wild-rye grass at both planting densi-
The litter layer was collected separately. Roots were separated ties (Table 2). In the absence of grass competition, ozone did
by species and ponderosa pine roots were sorted by diameter not significantly affect any of the seedling biomass parameters
(fine < 2 mm, mid 2–5 mm, and coarse > 5 mm). All tissues measured. Ozone reduced total seedling dry mass from
were dried at 60 °C to constant mass except for tissue subsam- 372.5 to 279.0 g at the highest grass density, indicating a weak
ples used to determine carbohydrate concentration. These tis- interaction (P = 0.07). When averaged across all grass treat-
sue samples were frozen at –70 °C and then lyophilized ments, ozone significantly increased root:shoot ratio (Ta-
(VirTis Genesis 12LE Lyophilizer, SP Industries, Inc., Gardi- ble 2). The dominant effect of the grass treatment compared
ner, NY) to constant mass. All tissue subsamples for chemical with the subtle effects of ozone precluded detection of signifi-
analyses were ground in a Wiley mill to pass a 40-mesh cant effects of ozone on fine root, total root and total shoot
screen. mass by ANOVA.
Ozone did not affect blue wild-rye grass dry mass at either
of the planting densities examined (Table 3). Total grass plus
Chemical analyses ponderosa pine dry mass was significantly reduced by ozone
Subsamples of 1996 and 1997 needle, stem and fine root tis- at the highest planting density (not shown). The response was
sues were analyzed for total nonstructural carbohydrate con- driven by a reduction in ponderosa pine dry mass as grass bio-
tent by an HPAEC-PAD method as described previously mass was unaffected.
(Wilson et al. 1995) and total nitrogen and carbon content Carbon concentrations of ponderosa pine needles were
were determined with a Carlo Erba EA1108 Elemental Ana- unaffected by the ozone or grass treatments (Table 4). Nitro-
lyzer (C.E. Elantech, Inc., Lakewood, NJ) by standard proce- gen concentration of 2-year-old main stem needles was signif-
Table 2. Tissue dry mass (g) and root:shoot ratios of ponderosa pine seedlings exposed to ozone in open-top chambers for three seasons with and
without blue wild-rye grass. Values are means with the standard error in parenthesis. Abbreviations: Grass 0 = no grass control, Grass 1 = 32 blue
wild-rye grass plants m –2, Grass 2 = 88 blue wild-rye grass plants m –2.
Ozone Grass Fine root Total root Total needle Total shoot Total plant Root:Shoot ratio
Control Grass 0 126.0 (8.3) 257.4 (15.6) 205.9 (19.0) 444.4 (40.6) 701.1 (52.7) 0.61 (0.04)
Grass 1 81.6 (5.3) 140.4 (8.5) 117.6 (10.8) 245.8 (22.5) 385.2 (29.0) 0.59 (0.04)
Grass 2 81.4 (5.3) 138.5 (8.4) 110.9 (10.2) 234.7 (21.4) 372.5 (28.0) 0.61 (0.04)
Ozone Grass 0 141.9 (9.3) 279.4 (16.9) 206.3 (19.0) 458.0 (41.8) 736.2 (55.4) 0.63 (0.04)
Grass 1 73.1 (4.8) 130.3 (7.9) 85.6 (7.9) 189.8 (17.3) 321.4 (24.2) 0.73 (0.04)
Grass 2 69.4 (4.6) 113.1 (6.8) 83.1 (7.7) 166.0 (15.2) 279.0 (21.0) 0.71 (0.04)
ANOVA (P value)
Ozone 0.412 0.461 0.087 0.135 0.202 0.039
Grass 0.000 0.000 0.000 0.000 0.000 0.446
Ozone × Grass 0.081 0.065 0.152 0.096 0.074 0.344
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com322 ANDERSEN, HOGSETT, PLOCHER, RODECAP AND LEE
Table 3. Blue wild-rye grass biomass (g) of shoots and roots after 3 nearly all needle ages examined, and statistically significant
years of ozone exposure. Values are means with the standard errors in interactions were found in 2- and 3-year old needles on the
parenthesis. Abbreviations: Grass 1 = 32 blue wild-rye grass plants main stem, and 2-year-old needles on branches. Carbon:nitro-
m –2, Grass 2 = 88 blue wild-rye grass plants m –2. gen ratios in needles were also significantly affected by ozone
and grass. The C:N ratios decreased in needles of control seed-
Ozone Grass Total shoot Total root lings grown in the presence of grass, and increased in
ozone-treated seedlings grown in the grass treatments.
Control Grass 1 46.7 (2.1) 55.1 (3.7) In current-year needles, ozone significantly increased Fe
Grass 2 56.3 (2.6) 83.1 (5.7) concentration (Table 5), but did not significantly affect any
Ozone Grass 1 53.0 (2.4) 79.7 (5.4) other nutrients. Grass competition increased S, Ca, Mn, B and
Grass 2 55.3 (2.5) 89.3 (6.1) Cu concentrations in either current-year or 2-year-old needles.
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ANOVA (P value) There appeared to be a small but statistically significant inter-
Ozone 0.756 0.302 action between ozone and grass competition for needle S and
Grass 0.015 0.001 Cu concentrations.
Ozone × Grass 0.122 0.032 Starch concentration in fine roots did not vary significantly
with ozone or with the presence of grass (Table 6). Starch con-
centrations were highest in fine roots and lowest in the oldest
icantly affected by ozone, and the pattern of response was age class of needles examined. Starch in aboveground tissues
influenced by the presence of grass. In control seedlings, need- generally decreased with grass competition; however, the
le nitrogen increased in the presence of grass, whereas in treatment effect was highly variable and only significant for
ozone-treated seedlings, needle nitrogen decreased in the pres- main stem needles. Grass competition significantly reduced
ence of grass. This general pattern of response was observed in glucose, fructose and sucrose concentrations of most of the
Table 4. Total carbon and nitrogen (%) concentrations, and mean C:N ratios of 2-year-old, 1-year-old and current-year needles of ponderosa pine
exposed to ozone with or without blue wild-rye grass for three growing seasons. Abbreviations: Grass 0 = no grass control, Grass 1 = 32 blue
wild-rye grass plants m –2, Grass 2 = 88 blue wild-rye grass plants m –2.
Main stem needles Branch needles
Two-year-old One-year-old Current-year Two-year-old One-year-old Current-year
Control Ozone Control Ozone Control Ozone Control Ozone Control Ozone Control Ozone
Carbon %
Grass 0 53.5 53.8 51.5 51.8 50.7 50.3 51.8 52.1 52.0 52.1 50.4 51.2
Grass 1 53.4 53.7 51.4 51.8 50.6 50.1 52.0 51.7 51.9 51.6 50.4 51.2
Grass 2 53.2 53.1 51.4 51.6 50.7 50.1 51.9 51.0 51.7 51.7 50.5 51.0
P value
Ozone 0.633 0.428 0.324 0.326 0.918 0.109
Grass 0.275 0.669 0.560 0.562 0.075 0.914
Ozone × Grass 0.737 0.718 0.903 0.430 0.435 0.607
Nitrogen %
Grass 0 0.85 0.99 1.02 1.14 1.24 1.27 0.85 0.91 1.07 1.19 1.16 1.27
Grass 1 0.89 0.88 1.16 1.05 1.39 1.43 0.98 0.89 1.18 1.13 1.31 1.40
Grass 2 0.95 0.80 1.22 1.03 1.42 1.35 0.97 0.858 1.26 1.13 1.36 1.34
P value
Ozone 0.804 0.059 0.933 0.621 0.695 0.115
Grass 0.560 0.306 0.000 0.517 0.047 0.000
Ozone × Grass 0.005 0.000 0.213 0.144 0.000 0.119
C:N ratio
Grass 0 63.6 54.9 50.7 45.9 41.4 39.9 62.1 57.7 49.0 44.3 44.3 40.5
Grass 1 60.5 63.2 44.4 49.5 36.7 35.3 53.2 61.2 44.3 45.8 38.7 37.0
Grass 2 56.3 66.2 42.3 51.3 35.9 37.4 53.9 61.0 41.3 46.3 37.1 38.3
P value
Ozone 0.719 0.028 0.277 0.543 0.707 0.286
Grass 0.709 0.481 0.0001 0.670 0.022 0.0003
Ozone × Grass 0.019 0.000 0.176 0.144 0.000 0.132
TREE PHYSIOLOGY VOLUME 21, 2001GRASS COMPETITION AFFECTS OZONE RESPONSE IN PINE 323
Table 5. Mineral nutrient concentrations in current-year and 1-year-old needles of ponderosa pine exposed to ozone with or without blue wild-rye
grass (88 plants m –2) competition for three growing seasons. Units are µg gdw –1.
One-year-old needles Current-year needles One-year-old needles Current year needles
Grass 0 Grass 2 Grass 0 Grass 2 P value P value
Control Ozone Control Ozone Control Ozone Control Ozone Ozone Grass Ozone Ozone Grass Ozone
× Grass × Grass
P 986.7 1071.5 1096.8 980.5 1199.8 1224.3 1166.3 1246.5 0.657 0.851 0.083 0.255 0.908 0.574
K 6827.5 7383.5 7748.5 7316.3 7081.8 7549.3 7145.3 7627.5 0.786 0.362 0.297 0.106 0.808 0.980
S 1764.5 2046.5 2354.3 2189.8 1376.8 1412.3 1657.5 1761.3 0.782 0.003 0.004 0.588 0.002 0.577
Ca 2914.5 3693.3 3715.3 3627.0 1447.0 1360.0 1827.8 1885.0 0.371 0.279 0.210 0.920 0.030 0.669
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Mg 1405.5 1621.5 1690.3 1496.0 1170.2 1075.5 1280.3 1398.0 0.952 0.683 0.311 0.863 0.060 0.298
Mn 165.4 180.8 208.5 227.2 118.7 104.5 141.3 128.4 0.476 0.021 0.912 0.149 0.028 0.937
Fe 49.2 61.6 80.0 57.9 32.0 37.3 33.7 54.5 0.688 0.441 0.209 0.021 0.166 0.240
Cu 3.0 2.8 4.4 3.3 3.5 3.4 3.9 4.6 0.108 0.006 0.078 0.508 0.002 0.074
Zn 64.6 102.8 98.0 68.6 33.4 41.6 42.8 44.2 0.938 0.684 0.343 0.292 0.155 0.390
Mo 0.27 0.57 0.46 0.33 0.23 0.27 0.17 0.14 0.688 0.253 0.726 0.972 0.684 0.740
B 28.3 27.6 35.0 36.7 23.6 29.5 33.7 33.5 0.849 0.001 0.419 0.322 0.028 0.249
Na 26.8 28.1 33.6 38.7 22.9 25.6 19.5 31.4 0.235 0.054 0.612 0.197 0.823 0.402
ponderosa pine tissues examined, whereas ozone only affected growth and physiology in the absence of other stresses (W.E.
glucose and fructose concentrations of fine roots and the glu- Hogsett, unpublished data; Andersen and Scagel 1997, Scagel
cose concentration of the main stem. and Andersen 1997). When ponderosa pine seedlings were
Over the course of the 3-year experiment, soil N and K con- grown in the presence of grass competition, ozone reduced to-
centrations decreased in all treatments, whereas soil P concen- tal dry mass of the ponderosa pine seedlings by about 25%, re-
tration and CEC increased (Table 7). There were no other sulting in a significant interaction (P < 0.07). Ozone interacts
apparent changes in the soil variables examined. After 3 years, with other natural and anthropogenic stresses affecting plant
the presence of grass plus ponderosa pine seedlings resulted in growth (Greitner et al. 1994, Andersen and Scagel 1997, Tem-
significant increases in soil NH 4 -N, NO 3 -N and K concentra-
ple et al. 1992); however, results vary with species and stresses
tions and soil pH compared with the soils where ponderosa
involved.
pine seedlings were grown alone. Phosphorus concentration
Competition is one of many potential interactions that oc-
also tended to decrease slightly (P < 0.003) in the presence of
curred within these reconstructed plant communities over the
grass. Ozone did not significantly affect any of the soil param-
3-year exposure period, and factors limiting growth may have
eters examined.
changed over time. The experiment was designed to enhance
To simulate the summer drought typical of ponderosa pine
the probability of competition between grass and ponderosa
forests, water was withheld during all 3 years of ozone expo-
pine for nutrients, water and light. Synchronous grass seed
sure. The lowest soil water content reached in August 1997
germination was encouraged so dominance and suppression
was about 7% (Figure 1). This value is typical of surface soils
resulting from early emergence were diminished. Density-de-
in the field (M. Johnson, US EPA, Corvallis, OR, unpublished
pendent mortality of the grass occurred, which is generally
data), and represents a soil water potential of approximately
considered a symptom of competition. The open-top cham-
–1.5 MPa (data not shown). There were no apparent treatment
bers excluded major herbivores, and insecticides were selec-
differences in degree of water stress experienced by the pon-
tively used to control insect outbreaks. Although it is difficult
derosa pine seedlings and grass during the experiment.
to demonstrate conclusively that competition occurred and
was responsible for altering ponderosa pine seedling response
to ozone, the conditions strongly suggest that competition
Discussion contributed to the structure and response of these communi-
The ponderosa pine seedlings were more susceptible to ozone ties. Therefore, we believe the results are best explained in
when grown with a grass competitor than when grown alone, terms of competition for resources, particularly below ground.
suggesting that single-factor exposure studies may underesti- The greatest impact of ozone on the ponderosa pine seed-
mate the impact of ozone on ponderosa pine seedling growth lings appeared to be on needle and shoot dry mass (Table 2).
in the field. In the absence of grass competition, total dry mass The ozone-induced decrease in shoot dry mass led to a signifi-
of the ponderosa pine seedlings was unaffected by a 3-year ex- cant increase in root:shoot ratio. An increase in root:shoot ra-
posure to ozone. The ozone concentration used in this experi- tio in response to ozone is opposite to the response generally
ment was relatively low, however, and has been found observed (Hogsett et al. 1985a, Cooley and Manning 1987,
previously to cause only subtle effects on ponderosa pine Andersen et al. 1997). However, Andersen and Scagel (1997)
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com324 ANDERSEN, HOGSETT, PLOCHER, RODECAP AND LEE
Table 6. Glucose, fructose, sucrose and starch concentrations in fine roots, stems and 1-year-old and current-year needles of ponderosa pine ex-
posed to ozone with or without blue wild-rye grass for three growing seasons. Units are µmol gdw–1. Abbreviations: Grass 0 = no grass control,
Grass 1 = 32 blue wild-rye grass plants m –2, Grass 2 = 88 blue wild-rye grass plants m –2.
Grass Fine root Stem One-year-old needles Current-year needles
Control Ozone Control Ozone Control Ozone Control Ozone
Glucose
Grass 0 86.60 63.48 159.16 155.82 157.04 166.30 150.96 161.52
Grass 1 77.77 50.62 155.66 99.40 134.72 146.10 134.86 140.03
Grass 2 72.72 46.94 136.48 100.51 139.25 136.62 131.10 128.96
P value
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O3 0.030 0.058 0.366 0.642
Grass 0.001 0.002 0.001 0.003
O3 × Grass 0.869 0.056 0.516 0.702
Fructose
Grass 0 113.84 87.21 68.78 70.32 125.78 136.37 132.85 145.02
Grass 1 102.61 62.88 81.94 61.74 107.86 121.16 120.47 128.36
Grass 2 94.18 59.86 71.51 56.57 112.19 116.99 116.97 118.55
P value
O3 0.009 0.328 0.130 0.346
Grass 0.000 0.256 0.050 0.008
O3 × Grass 0.326 0.070 0.852 0.728
Sucrose
Grass 0 49.39 52.73 38.43 23.96 17.87 20.38 17.13 18.30
Grass 1 52.37 59.19 39.49 20.54 34.93 23.61 33.05 29.09
Grass 2 52.81 61.93 29.22 19.82 25.50 25.19 35.54 28.65
P value
O3 0.477 0.181 0.554 0.658
Grass 0.086 0.004 0.020 0.003
O3 × Grass 0.608 0.074 0.128 0.664
Starch
Grass 0 179.91 138.84 152.66 129.49 153.54 154.31 114.78 100.85
Grass 1 149.73 130.73 144.44 112.42 144.58 121.42 88.74 80.04
Grass 2 155.26 130.58 140.11 93.36 123.95 173.64 80.94 85.68
P value
O3 0.486 0.324 0.405 0.528
Grass 0.457 0.303 0.411 0.049
O3 × Grass 0.785 0.781 0.077 0.693
also found an increase in root:shoot ratio after one season of experiment were N deficient, and no fertilizer was applied, it
ozone exposure in ponderosa pine growing at low nutrient is probable that growth in all treatments was N limited. Al-
availability. In our study, the ozone-induced increase in though ozone exposure did not significantly affect any of the
root:shoot ratio was most apparent in the two grass treatments, measured soil nutrient properties, there were several signifi-
although the interaction was not statistically significant. It is cant treatment interactions on tissue nutrient concentrations of
possible that, in the grass treatments, belowground competi- the ponderosa pine seedlings (Tables 4 and 5). In the presence
tion for nutrients and water kept the carbon sink strength high of blue wild-rye grass, ozone significantly decreased needle N
in ponderosa pine roots, at the expense of maintaining carbo- concentrations of ponderosa pine seedlings, whereas needle N
hydrates in the shoot for growth and repair processes. These concentrations increased in seedlings grown in the presence of
results show the importance of including other stresses when grass without ozone. This is the first report of a significant in-
evaluating the importance of ozone stress on vegetation re- teraction between ozone and grass competition on N and C:N
sponse, particularly allocation responses. ratios in ponderosa pines. The response cannot be explained
It appears that, in the presence of grass competition, ozone- by differences in soil N, because soil N increased with grass
exposed ponderosa pine seedlings were unable to obtain suffi- treatment (Table 7). One explanation is that, despite the rela-
cient nitrogen, despite slightly higher soil nitrogen concentra- tive increase in root biomass of ponderosa pine seedlings ex-
tions in the grass treatments. Because the soils used in the posed to ozone in the presence of grass competition, ozone re-
TREE PHYSIOLOGY VOLUME 21, 2001GRASS COMPETITION AFFECTS OZONE RESPONSE IN PINE 325
Table 7. Soil nutrient concentrations and soil pH at the beginning and after 3 years in plots containing ponderosa pine and three densities of blue
wild-rye grass, with or without ozone. Abbreviations: Grass 0 = no grass control, Grass 1 = 32 blue wild-rye grass plants m –2, Grass 2 = 88 blue
wild-rye grass plants m –2.
Initial Final P value
Grass 0 Grass 1 Grass 2 Ozone (O) Grass (G) O×G
Control Ozone Control Ozone Control Ozone
pH 7.16 7.13 7.18 7.25 7.23 7.33 7.18 0.274 0.081 0.111
P (ppm) 14.75 21.25 21.50 20.00 20.50 20.30 20.50 0.191 0.003 0.892
K (ppm) 494 422 438 437 438 443 449 0.141 0.060 0.498
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Ca (meq/100 g) 6.23 5.80 5.83 5.85 5.90 5.98 5.93 0.879 0.142 0.736
Mg (meq/100 g) 3.35 3.30 3.35 3.33 3.40 3.38 3.40 0.288 0.542 0.905
Na (meq/100 g) 0.05 0.13 0.13 0.13 0.13 0.13 0.13 0.885 0.861 0.861
NH4-N (ppm) 2.40 1.55 1.63 1.73 1.65 1.75 1.85 0.727 0.210 0.718
NO3-N (ppm) 3.11 0.30 0.45 0.63 0.60 0.68 0.60 0.757 0.001 0.220
NH4-NO3 (ppm) 5.54 1.85 2.08 2.35 2.25 2.43 2.45 0.688 0.015 0.560
OM(%) 2.61 2.59 2.57 2.60 2.53 2.56 2.63 0.851 0.765 0.292
CEC (meq/100 g) 10.98 15.23 16.18 17.83 14.60 15.53 14.43 0.323 0.663 0.328
duced the competitiveness of ponderosa pine or its ability to Although soil N decreased over the experiment, there was
obtain soil resources. An alternative possibility is that ozone- significantly more N in plots with grass than in plots without
exposed seedlings lost nitrogen through premature needle se- grass at the end of the 3-year study (Table 7). Increased grass
nescence and were unable to maintain an internal N reservoir. root mass and turnover may have stimulated microbial pro-
Litter mass was not significantly affected by ozone treatment, cesses in soil, leading to greater N mineralization and in-
but needle litter N may have been higher from ozone-treated creased N availability (Zak et al. 1993).
seedlings than from control seedlings. Fenn and Dunn (1989) Studies on interactions between nitrogen status and ozone
and Fenn (1991) found increased needle litter N in highly pol- have shown varying responses depending on the species ex-
luted plots compared with less polluted plots across the San amined and conditions used. In loblolly pine (Pinus taeda L.),
Bernardino Mountains in southern California. Studies are cur- ozone affected needle mass at high N availability but not at
rently underway to examine how ozone affects internal recy- low N availability (Tjoelker and Luxmoore 1991). Studies
cling of N and needle loss in ponderosa pine (J. Fletcher and with other species have shown an opposite response.
W. Hogsett, US EPA, unpublished data). Pääkkönen and Holopainen (1995) found increased sensitivity
Figure 1. Soil water content during
1997 measured with TDR probes in
each of the six treatment combinations.
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com326 ANDERSEN, HOGSETT, PLOCHER, RODECAP AND LEE
to ozone at low N availability compared with high N availabil- The research described in this article was funded by the US Envi-
ity in Betula pendula Roth. Andersen and Scagel (1997) exam- ronmental Protection Agency. It has been subjected to the Agency’s
ined CO2 flux from soils and found the greatest response to peer and administrative review and approved for publication. Men-
ozone at low N availability. No significant interactions be- tion of trade names or commercial products does not constitute en-
tween N status and ozone were observed in aspen (Populus dorsement or recommendation for use.
sp.) (Greitner et al. 1994). Thus, although it is clear that tissue
nutrient concentration and nutrient availability are important
factors influencing plant response to ozone, more detailed in- References
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