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Effects of Elevated Concentrations
of Carbon Dioxide on Seedling Growth
of Mesquite and Huisache
Charles R. Tischler
H. Wayne Polley
Hyrum B. Johnson
Herman S. Mayeux
controls have reduced or eliminated stands of these plants,
rapid recolonization of the site by seedlings quickly returns
the area to its former vegetative composition (Gibbons and
others 1992).
We determined effects of elevated CO 2 concentrations on
shoot and root growth of mesquite and huisache seedlings,
effects that may contribute to alter the competitiveness of
these species on rangelands in the future.
Abstract-Plants such as mesquite (Prosopis glandulosa) and
huisache (Acacia smallii) generally show positive growth responses
to higher CO2 concentrations. Growth of the two legumes was
measured at five day intervals over 30 days from seedling emergence
at 380,700, and 1,000 ppm CO2 , Root elongation increased with CO 2
concentration in mesquite. Shoot mass of both species responded
positively to elevated CO2 • Positive responses to CO2 enrichment
were evident within one week after emergence. Predicted increases in
atmospheric CO2 could increase seedling establishment and survival
of these invasive shrubs, and ultimately contribute to a further
change in the species composition of Southwestern rangelands.
Materials and Methods --------------------Seedlings of both mesquite and huisache were grown in
split PVC tubes 145 cm (5 ft) long in glasshouses maintained
at atmospheric CO2 concentrations of 380, 700, and 1,000
ppm. No supplemental lighting was provided. The split PVC
tubes (held together with duct tape) facilitated easy removal
of the entire tap root. The soil medium was amended with
complete nutrient solution. Seedlings were harvested at
5 day intervals over 30 days following emergence. At harvest, shoot and root dry mass and length of taproots were
determined.
Atmospheric concentrations ofCO2 are predicted to double
(to about 700 ppm) by the middle of the next century. Plants
possessing the Ca photosynthetic pathway generally exhibit
positive growth responses to superambient concentrations,
while species with the C4 photosynthetic pathway usually
respond less to higher CO2 (Mayeux and others 1991; Johnson
and others 1993). This difference in response to CO 2 between
Ca and C4 plant types may contribute to modify the species
and growth form composition ofrangelands across the Southwest (Polley and others 1994). Because most brush species
are Ca plants, while warm-season grasses are of the C4 type,
the predicted changes in CO 2 concentration should be selectively advantageous to invasive shrubs.
Once established, woody species may persist for a century
or more unless controlled by humans. Gibbons and others
(1992) found that only a small percentage of the mesquite
(Prosopis glandulosa) that germinated survived into the
second year. However, this small percentage of surviving
seedlings was adequate to convert productive grassland into
a mesquite-dominated ecosystem. Mesquite and huisache
(Acacia smallii) have invaded vast areas of the Southwest in
recent years, and in many cases completely dominate the
landscape. Also, in cases where mechanical or chemical
Results
---------------------------------
Plots of log total biomass vs. aboveground biomass indicated that CO 2 concentration did not modify allometric
relationships between aboveground and total growth in
young seedlings of either species (data not shown). However,
the ratio of aboveground to total biomass decreased with age
regardless of CO2 concentration (fig. 1).
Rooting depth ofhuisache did not respond to CO2 , and was
consistently lower than that of mesquite (fig. 2). Mesquite
rooted more deeply at elevated CO2 levels than at the
current concentration. Rates of root depth extension for
mesquite averaged 3.67 em darl at the 380 ppm CO2 level
and 4.52 cm day-l at elevated concentrations.
In mesquite, total biomass at the elevated CO2 concentrations was greater throughout the experiment (fig. 3), with
differences being statistically significant at day 30. For
huisache, differences between atmospheres were not as
pronounced (fig. 4), but trends were for greater biomass at
elevated CO2 concentrations.
In: Barrow, Jerry R.; McArthur, E. Durant; Sosebee, Ronald E.; Tausch,
Robin J., comps. 1996. Proceedings: shrubland ecosystem dynamics in a
changing environment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep.
INT-GTR-338. Ogden, UT: U.S. Department of Agriculture, Forest Service
Intermountain Research Station.
'
Charles R. Tischler is a Plant Physiologist, H. Wayne Polley and Hyrum B.
Jo~son are Plant Ecologists, and Herman S. Mayeux is a Range Scientist,
Agricultural Research Service, U.S. Department of Agriculture, 808 East
Blackland Road, Temple, TX 76502.
246
0.75
0.5
Mesquite
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.........
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a::
1
380 Jlmol mor
1
700 Jlmol mor
1
1000 Jlmol mor
A
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0
•
>
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(0
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Huisache
Mesquite
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-l
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10
20
30
o
0.2
0.1
0.0
L-._ _.l...-_ _L - - - - L - - - - L . . . - - - L . . . . - - - - - '
o
HARVEST DATE (DAYS)
10
20
30
HARVEST DATE (DAYS)
Figure 1-Means of the ratio of aboveground
biomass to total biomass of mesquite and huisache
seedlings after 10 to 30 days growth averaged across
three CO2 levels (Huisache: r2= 0.63, P < 0.0001,
n = 75; Mesquite: r2= 0.58, P < 0.0001, n =7 5).
Figure 3-Means of total biomass of mesquite
seedlings at zero to 30 days post-emergence
grown at three CO 2 levels (at 380 ppm, r2= 0.95,
P < 0.0001, n = 35; at 700 and 1,000 ppm, r2= 0.95,
P < 0.0001, n = 70).
0.4 . - - - - - - - - - - - - - - - - - - - - - - - ,
1.00
r:::::=J Huisache
~
E
.........
Huisache
Mesquite
ab
0.75
a
.-Ia..
o
CI)
CI)
(!)
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o
o
oa::
~
o
w
o
o
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.........
0.50
z
i=
6.
1
380 Jlmol mor
1
700 Jlmol mor
1
1000 Jlmol mor
0.2
OJ
-l
0.25
0.00
.- 0.1
L.-...I._ _
0.0 '---_ _
...1.-_ _- - 1 -_ _----'-_ _- - - - ' ' - -_ _- ' - - - - - '
380
700
1000
CO 2 CONCENTRATION (Jlmol mor
1
o
10
20
HARVEST DATE (DAYS)
)
Figure 2-Rooting depth of mesquite and huisache
seedlings (averaged over sampling days 5-30) at
three CO2 levels. Bars headed with the same letter
are not significantly different (P < 0.05) by StudentNewman-Kuel's Test (n = 35).
Figure 4-Means of total biomass of huisache seedlings at zero to 30 days post-emergence grown at
three CO 2 levels (Single regression for all atmospheres, r2= 0.84, P < 0.0001,
247
Discussion
-------------------------------
The response of mesquite seedlings to CO2 enrichment
was quite dramatic. Root and total biomass were increased
as much as 38% at elevated CO 2, Positive responses to
elevated CO2 were evident in about a week. Positive responses were noted for huisache, but were not as impressive
as those measured on older plants (H. B. Johnson, unpublished results). Differences in shoot mass present on days 15
and 20 between huisache plants grown at elevated CO2 and
the current level were not present at later harvests. We
believe that this may reflect a temporary lag in growth at the
elevated CO 2 levels, where the earlier abscission of cotyledons and lower leaflets precedes a burst of leaf initiation
and growth. We have observed that leaf and cotyledon
longevity may be shorter at elevated CO2 , Senescence of the
cotyledons and lower leaves represents a significant shift in
relative masses of seedling structures that further complicated growth analyses of these young plants.
Obviously, the root lengths we observed in the tube system
represent maximum potential elongation, but are indicative
248
of the significant amount of resources both species invest in
deep rooting.
Further experimentation is required to determine responses of mesquite and huisache to elevated CO2 where
competition, water and heat stress, herbivory, and other
complicating factors are also present.
References ------------------------------Gibbens, R. P.; Beck, R. F.; McNeely, R. P.; Herbel, C. H. 1992.
Recent rates of mesquite establishment in the northern
Chihuahuan Desert. Journal of Range Management. 45:585-588.
Johnson, H. B.; Polley, H. W.; Mayeux, H. S. 1993. Increasing CO2
and plant-plant interactions: effects on natural vegetation.
Vegetatio. 104/105:157-170.
Mayeux, H. S.; Johnson, H. B.; Polley, H. W. 1991. Global change
and vegetation dynamics. In: James, L. F.; Evans, J. 0.; Child,
R. D., ed. Noxious range weeds. Westview Press, Boulder CO: 62-64.
Polley, H. W.; Johnson, H. B.; Mayeux, H. S. 1994. Increasing CO2 :
comparative responses of the C4 grass Schizachyrium and grassland invader Prosopis. Ecology. 75:976-988.