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POTENTIAL INTERACTIONS
BETWEENGLOBALCHANGEAND
INTERMOUNTAIN ANNUAL
GRASSLANDS
Herman S. Mayeux
Hyrum B. Johnson
H. Wayne Polley
and others 1990). Changes in temperatures and precipitation patterns could influence species composition and
productivity of terrestrial plant communities (Gates 1990;
Joyce and others 1990; Perry and Borchers 1990; Peters
1990), but accurately predicting the consequences of a
changing climate is difficult when its theoretical cause
is also the primary raw material for plant growth (Long
1991; Long and Hutchin 1991).
ABSTRACT
Productivity of C3 plants apparently has risen with the
increase in atmospheric C02 since preindustrial times,
and continued increases are ezpected. Variation among
C3 species in their relative growth responses to increasing
C02 may cause changes in vegetation of the Intermountain
West, but the extent of such variation is poorly understood.
Even in the absence of variation among C3 species in response to C021 interactions between C02 effects and characteristics associated with plant lifeform may shift competitive relationships among annual grasses, perennial
grasses, and shrubs, or influence abiotic factors such as
the frequency and intensity of wildfires.
FUTURE EFFECTS OF INCREASING
C09 ON VEGETATION
A major source of uncertainty in predicting the response of vegetation to rising levels of atmospheric C02
is the lack of sufficient understanding of the direct effects
of increased C02 on plant performance and how this "C02
fertilization effect" will interact with possible changes in
abiotic factors such as temperatures and evaporation
rates (Long 1991). Recent research suggests that physiological and whole-plant processes like photosynthesis,
phenological development, leaf area and biomass accumulation, and reproductive output are often accelerated or
increased at the C02 levels expected in the next century,
while respiration and transpiration rates are suppressed
(Bazzaz and Fajer 1992; Bunce 1990; Idso 1989; Mooney
and others 1991; Strain 1987). Other responses, such as
changes in the way carbon is partitioned within the plant
and morphological and phenological acljustments, may
not represent immediate increases in biomass but can
strongly affect ecological success (Bazzaz 1990).
Future shifts in species composition have been proposed
because these effects are more strongly expressed in some
plant species than in others (Patterson and Flint 1980).
In general, plants with C3 metabolism (or the C3 photosynthetic pathway) are much more responsive to increased C02 than those with C4 metabolism at current
and future C02 levels. C4 photosynthesis is nearly C02saturated at current atmospheric concentrations, but C3
photosynthesis is not (Pearcy and Ehleringer 1984). This
difference may give a competitive advantage to C3 species
where the two functional groups occur together, as has
been shown under controlled conditions (Carter and
Peterson 1983; Johnson and others 1993; Marks and
Strain 1989). In the field, elevated C02 increased shoot
density and standing crop of a C3 grass but had no effect
on two associated C4 species (Curtis and others 1989).
INTRODUCTION
The C02 concentration of the atmosphere has increased
by almost 30 percent since the beginning of the Industrial
Age, from about 275 to 350 ppm (Keeling and others 1982).
It is expected to double again in the next century, primarily because of intensive combustion of fossil fuels
(Trabalka and others 1985). Ehleringer and others
(1991) reviewed evidence of even wider fluctuation in
atmospheric C02 over geologic time; C02 levels may have
been as high as 3,000 ppm in the mid Cretaceous, about
100 million years ago, decreased to about 300 ppm in the
Paleocene, and were as high as 1,000 ppm in the Eocene,
50 million years ago. For the last 160,000 years, C02 levels have been below today's concentration of 350 ppm, and
may have been as low as 160 ppm from 20,000 to 15,000
years ago (Barnola and others 1987; Delmas and others
1980). Atmospheric C02 rose to about 275 ppm at the
end of the last ice age, 10,000 years ago, and remained
fairly constant until the anthropogenic increase began
in the early 19th century (Neftel and others 1985; Stuiver
and others 1984).
Continued, rapid increases in atmospheric C02 and
other trace gases are viewed with alarm because of possible increases in temperatures of the lower atmosphere
and other climatic perturbations associated with intensification of the "greenhouse effect" (Schneider 1989; Watson
Paper t}resented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
HermanS. Mayeux is Range Scientist and Hyrum B. Johnson and
H. Wayne Polley are Ecologists, Agricultural Research Service, U.S. Department of Agriculture, 808 East Blackland Road, Temple, TX 76602.
95
EFFECTS OF INCREASING COs ON
CURRENT VEGETATION
among the dominant ca species. The extent to which
plants within each of the two major metabolic groupings,
C3 and C4, vary in their ability to respond to increasing
C02 is not yet known, but limited evidence suggests that
sufficient variability exists to influence species composition in natural vegetation (Johnson and others 1993).
For instance, Smith and others (1987) grew three 03
grasses and one C4 grass native to t4e Great Basin in atmospheres with near-current (340 ppm) and future (680
ppm) C02 concentrations and recorded a number of
physiological and whole-plant responses. All C3 species
responded favorably to higher C02, but wide variation
was observed among cheatgrass (Bromus tectorum), Indian ricegrass (Oryzopis hymenoides), and western wheatgrass (Agropyron smithii) in responses such as tillering,
aboveground biomass, and root:shoot ratios. They identified the annual cheatgrass as the C3 species most responsive to C02• For instance, cheatgrass was the only one of
these ca grasses in which leaf area increased in response
to a doubling of C02•
Woody and perennial herbaceous C3 plants may also be
highly responsive to additional C02 , but the possibility exists that inherent characteristics associated with lifeform
and life history may modify the role that C02 effects play
in vegetation change, especially in arid environments
where advantageous characteristics of preadapted annuals allow them to rapidly respond to change (Young and
others 1972).
We constructed a large growth chamber to test the hypothesis that increases in atmospheric C02 over the range
experienced since the last glaciation and since the beginning of the Industrial Age have influenced plant performance, especially that of ca annuals (Mayeux and others,
in press). Plants are grown in an elongated soil container
with a relatively large volume of about 12 m8, with topgrowth enclosed in an elongated, transparent polyethylene film chamber. The chamber is 38m long, has a
diameter of less than 1 m, and is enclosed in a vented
glasshouse. Movement of air through the chamber
creates a continuous daytime C02 gradient from nearcurrent ambient, a50 ppm, to below Ice Age levels, usually
150 or 200 ppm, by photosynthetic depletion. Dewpoint
and dry-bulb temperatures are automatically reset to conditions within the glasshouse at five equally spaced locations along the 38-m length to reduce temperature and
humidity gradients.
The annuals oats (Avena sativa) and wild mustard
(Bras sica kaber) were grown alone and in mixtures in the
chamber from 150 to about 340 ppm to compare responses
of a graminoid and a broadleafed C3 herbaceous plant to
C02 (Polley and others 1992). Stomatal conductance of
both species decreased with increasing C02, but decreased
conductance did not prevent a linear increase in net C02
assimilation of 300 percent in oats and an even greater
proportional increase in wild mustard. Leaf area/plant of
the two species increased linearly from about 0.6 m 2 at
150 ppm to 1.2 and 1.8 m 2 at 270 and 340 ppm, respectively. Increased assimilation per unit leaf area and comparable increases in total leaf area/plant combined to
cause aboveground biomass/plant of oats to increase from
about 5 gat C02 concentrations below 200 ppm to 12 gat
270 ppm and 20 gat a40 ppm at flowering. The biomass
response of mustard was almost as large.
Our current understanding of historic and prehistoric
changes in atmospheric C02 levels suggests that the
presettlement vegetation of the Intermountain West,
as elsewhere on Earth, developed in an atmosphere with
relatively low concentrations of C02; current levels are
twice that of the last glaciation and 30 percent greater
than they were about 200 years ago. The potential importance of this change in shaping the structure and species composition of contemporary plant communities becomes evident if the beneficial effects of additional C02
on plants, summarized earlier, occurred over the range
.
of C02 concentrations of the past.
The assumption that the increase in C02 since preindustrial times selectively favored C3 over C4 plants, as
has been predicted to occur with future C02 increases,
provided the basis for our hypothesis that the 30 percent
rise already experienced contributed to vegetation change,
particularly the concomitant increase in abundance and
density of C3 shrubs on the C4 grasslands of the American Southwest and the southern Great Plains (Johnson
and others 199a; Mayeux and others 1991).
EFFECTS OF COs ON VEGETATION
OF THE INTERMOUNTAIN WEST
In the Intermountain West, however, the vegetation
is not clearly divided into those two dominant functional
groups, C3 shrubs and C4 warm-season grasses. Most
Intermountain species are ca, including almost all of the
dominant grasses, native and introduced. Few important
C4 grasses occur, such as galletagrass (Hilariajamesii) in
the southern Great Basin. The genus Atriplex, including
the saltbushes and shadscale (A confertifolia), consists of
C4 plants; all other shrubs are ca. Russian thistle
(Salsola iberica) and halogeton (Halogeton glomeratus)
are important C4 invaders, but others are ca.
Little is known about the response of these species to
increasing C02 • A native lovegrass (Eragrostis orcutianna) responded to elevated C02 to a greater extent than
would be expected (Smith and others 1987). Productivity
of an introduced C3 annual grass, soft chess (Bromus
mollis), also increased at elevated C02 (Larigauderie and
others 1988). Big sagebrush (Artemisia tridentata var.
tridentata) may be the only Intermountain species that
has been studied at subambient, ambient, and superambient C02 levels (Johnson and Lincoln 1990). Increasing atmospheric C02 from the preindustrial level of 270
ppm to the current 350 ppm increased ovendry weight of
big sagebrush seedlings by 60 percent, while elevating
C02 to 650 ppm further increased plant biomass by only
10 percent. The larger response of big sagebrush to the
subambient increase supports the contention that historical increases in C02 may be more important than future
increases in terms of direct effects on vegetation.
If increased C02 has played a role in the changes in
species composition of vegetation observed in the Intermountain West, the positive effects of increased C02 on
plant performance must be expressed to varying degrees
96
conditions, from 220 to 660 g/m2 • Water stress during the
last half of the growing season decreased yields of both
cultivars across the range of C02 concentrations, and decreased the rate of biomass accumulation relative to wellwatered Seri M82, but both cultivars responded positively
to increasing C02 •
Like total aboveground biomass, grain yields of the two
varieties more than doubled over the entire C02 gradient,
and increased 50 and 54 percent as C02 rose from the
level of 200 years ago to that of today (data not shown).
Wheat used the same amount of soil water regardless of
C02 level or yield, so water use efficiency increased proportionally with herbage and seed yields. Similar improvements in performance of other wheats (Gifford 1977;
Neales and Nicholls 1978) and other CS crops (Allen and
others 1991; Baker and others 1990) have been demonstrated as C02 increased over subambient levels. The
relative magnitude of the improvements in performance
of C3 plants to increases in C02 representative of the
Holocene or just the last 200 years is greater than that
observed for CS plants when the current C02 concentration is doubled, which averages about SO percent (Kimball
1983).
Increases in the productivity of oats and wheat suggest
that other annual C3 grasses may also be more productive
and efficient than they were, if these annual C3 crops
are acceptable models for other C3 annual grasses. This
poses the possibility, relevant to recent vegetation
changes on intermountain rangelands, that productivity
of C3 plants like cheatgrass is increasing with C02 and
has been since its introduction.
A subsequent experiment with two cultivars of dayneutral spring wheat (Triticum aestivum) substantiated
the magnitude of physiological and growth responses of
CS annuals to subambient increases in C02• The dayneutral, spring wheat cultivars 'Yaqui 54' and 'Seri 82'
were grown to maturity under two watering regimes.
Soil water content was restored to field capacity weekly
through the duration of the 100-day experiment or watering was terminated at 50 days~ during early anthesis.
Under continuously well-watered conditions, total aboveground biomass of Seri M82 increased from about 300
glm2 at about 200 ppm C02 to 520 g/m2 at 275 ppm, and to
700 glm2 at near-current C02 concentrations at maturity
(fig. 1). Aboveground biomass of the older cultivar Yaqui
54 increased even more dramatically under well-watered
800
SERI M82
600
•
.
---·
......... -· 0. ....0-----0
0.
---·
0------0
·-
--,; 0
GLOBAL CHANGE, CHEATGRASS,
AND FIRE
YAQUI 54
•
400
0
•
..... o--·
..
__ .......0
0
_... -d
---·
0
______ .... .... ··'b
200
...... c; ..
... 0
•
Non Water Stressed (--)
Water Stressed (·-----)
0 ~--~----~-------~----~----~--~
0
200
225
250
275
300
325
350
C02 {ppm)
Figure 1-Total ovendry aboveground biomass
of two cultivars of day-neutraJ spring wheat
grown for 100 days along a continuous daytime
C02 gradient from about 200 to 350 ppm in the
subamblent C02 chamber at Temple, TX.
97
Ryan (1991) reviewed the relationships between climate, vegetation, and wildfire and considered in detail the
possible impacts of global warming on fuel characteristics,
ignition sources, weather, and other factors which influence the frequency and intensity of wildfires. He concluded that global warming may alter fire frequency and
intensity by modifying several factors that control wildfire, resulting in an even greater role of fire in shaping
vegetation in the future. Smith and others (1987), in considering the implications of the highly positive effects of
elevated C02 on cheatgrass, noted that cheatgrass invasion has been associated with a possible increase in fire
frequency (Young and Evans 1978) and thereby plays a
role in vegetation change (Billings 1990). They concluded
that "a substantial increase in productivity in Bromus under high C02 could exacerbate this condition and possibly
increase the number and severity of wildfires in the Great
Basin." These suggestions raise an important point. The
effects of global change on ecosystems may not be manifested straightforwardly as differences among plants in
their response to increasing C02 or higher temperatures,
but rather indirectly as C02 or climate effects on abiotic
factors like fire frequency and intensity.
Nonetheless, if the 30 percent increase in atmospheric
C02 over the last 200 years has dramatically increased
standing crop of C3 annuals like cheatgrass, as suggested
by the results of experiments conducted in the subambient C02 chamber, historical increases in C02 may have
contributed to the successful invasion by introduced annuals. Other C3 plant lifeforms also likely benefited from
increased C02 , but highly efficient dispersal mechanisms,
breeding system, and other characteristics of cheatgrass
relative to native perennials (Piemeisel1951; Young and
others 1972) may have interacted with C02 effects in
ways that selectively favor cheatgrass in the current
Great Basin environment. Furthermore, the suspected
occurrence of more frequent and severe wildfires over
recent decades (Billings 1990; Peimeisel1951; Pickford
1932) may be attributable to more uniform and dependable fuel loads associated with increased herbaceous productivity where cheatgrass occurs because of its response
to increased C02, in a manner similar to that described
for forests in the future by Ryan (1991).
It is tempting to assume that positive responses of
cheatgrass productivity to C02 contributed to increased
fire frequency by increasing fuel loads, but an extensive
comparison of fuel characteristics on Idaho's Snake River
Plains indicated that total amounts of fuel per unit area
decrease with increased abundance of cheatgrass, although fuel continuity was improved by cheatgrass
(Whisenant 1990). Higher flammability and other physical characteristics of cheatgrass relative to native perennials may also be a factor in wildfire behavior.
Similarly, historical increases in the abundance and
density of C3 woody species like big sagebrush (Amo and
Gruell1983, Branson 1985) andjunipers (Juniperus sp.)
(Blackburn and Tueller 1970; Burkhardt and Tisdale
1976; Christensen and Johnson 1964; Cottom and Stewart
1940) in the Great Basin could have been encouraged by
the favorable effects of increased C02 on growth and
water-use efficiency over the last two centuries (Mayeux
and others 1991). Woody plants (ldso and others 1991),
including conifers, are highly responsive to elevated C02
(Conroy and others 1990; Higginbotham and others 1985;
Kaushal and others 1989), and their growth appears
to have been favored by historical increases in C02
(Graumlich 1991; Kauppi and others 1992; Kienast and
Luxmoore 1988; LaMarche and others 1984). Elevated
C02 increased mycorrhizal density on roots of Virginia
pine (Pinus echinata), enhancing nutrient acquisition and
supporting additional biomass increases (O'Neill and
others 1987).
A wealth of information is accumulating concerning
C02 effects at the leaf and individual plant level, and
plant species appear to vary widely in the extent and nature of their reponses to additional C02 (Johnson and others 1993) and climate change (Graham and Grimm 1990).
Despite limitations of scale and the individualistic nature
of plants' responses, we consider future effects of global
change on natural vegetation by extrapolating to plant assemblages, landscapes, and even larger spatial scales
(Cohn 1989; Emanuel and others 1985; Joyce and others
1990; Mayeux and others 1991; Perry and Borchers 1990;
Ryan 1991; Verstraete and Schwartz 1991).
Even when the limitations are recognized, much of
what we propose in terms of vegetation response to global
change is speculation. The same complaint applies to our
efforts to reconstruct effects of changing climate and other
factors on rangeland vegetation over the recent past
(Branson 1985), a period in which interpretation should
be aided by a rich written and photographic record.
Nonetheless, recent research strongly suggests that the
direct effect of increased atmospheric C02 on plants is a
factor not sufficiently considered in seeking the causes
of historical and current changes in intermountain and
other vegetation, and changes in C02 levels will continue
to influence the structure and species composition of vegetation in the future as concentrations continue to rise.
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