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The Response of Natural Ecosystems to the Rising Global CO2 Levels
Author(s): F. A. Bazzaz
Source: Annual Review of Ecology and Systematics, Vol. 21 (1990), pp. 167-196
Published by: Annual Reviews
Stable URL: http://www.jstor.org/stable/2097022 .
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Annu. Rev. Ecol. Syst. 1990. 21:167-96
Copyright ? 1990 bv Annual Reviews Inc. All rights reserved
THE RESPONSE OF NATURAL
ECOSYSTEMS TO THE RISING
GLOBAL C02 LEVELS
F. A. Bazzaz
Department of Organismic and Evolutionary Biology, Harvard University,
Cambridge, Massachusetts 02138
KEY WORDS: carbon dioxide, global change, ecosystems, growth, competition
INTRODUCTION
Evidence from many sources shows that the concentration of atmospheric
CO2 is steadily rising (61, 17). This rise is strongly correlated with the
increase in global consumption of fossil fuels (104). There are also significant
contributions from the clearing of forests, especially in the tropics (136, 55).
Controversy continues, however, as to whether the biosphere is presently a
source or a sink for carbon (see 52, 54, 56).
Despite this controversy, most scientists agree that rising CO2 levels will
have substantial direct and indirect effects on the biosphere (80). Because
CO2 is a greenhouse gas, its increase in the atmosphere may influence the
earth's energy budget. Several climatologists have used general circulation
models to predict changes in mean annual global temperature (58, 108).
While these models differ in detail, they all predict increased global warming
and substantial shifts in precipitation patterns. Recently, some scientists (60)
have questioned the predictions of these models. But regardless of changes in
global temperature and other climate variables, rising CO2 can influence
world ecosystems by direct effects on plant growth and development.
The large body of literature on the response of crops and intensively
managed forests to elevated CO2 is not treated in this review because there are
167
0066-4162/90/1120-0167$02.00
168 BAZZAZ
several excellent and recent reviews of it (e.g. 2, 28, 62, 127, 132 for crops,
and 37, 65, 111 for trees). Instead, this review concentrates on the response of
natural vegetation to elevated CO2 and some of the predicted climate change.
The review addresses the CO2 response of individuals at the physiological
level and the consequences of that response to population, community, and
ecosystem levels. It must, however, be emphasized that most of the findings
on the physiological and allocational response to CO2 were first discovered in
agricultural crops, and that much of the initial work on plants from natural
ecosystems (69) tests the variation among species in these responses.
PLANT RESPONSES AT THE PHYSIOLOGICAL LEVEL
TO ELEVATED C02
Plant biologists have long known some of the effects of high CO2 levels on
plants, and greenhouse growers have used CO2 fertilization to increase plant
yield. Work on plants from natural ecosystems has lagged behind that on
crops but, over the last few years, has produced a large body of information
(see 120 for extensive reviews). The major emphases have been on individual
physiological traits, but the consequences of these responses for the whole
plant, population, and ecosystem are less understood and, in some cases,
counter-intuitive. Many plant and ecosystem attributes will directly or indirectly
be influenced by elevated CO2 (118). Therefore, after briefly addressing
physiological responses at the leaf level, I concentrate on growth and
allocation, reproduction, plant-plant interactions, plant-herbivore interactions,
and some ecosystem level attributes.
CO2 and Photosynthesis
When other environmental resources and factors are present in adequate
levels, CO2 can enhance photosynthesis of C3 plants over a wide range of
concentrations. High CO2 reduces competition from 02 for Rubisco, increases
its activation (95), and reduces photorespiration. In contrast, in plants
with the C4 metabolism net photosynthetic rates rise steeply with increased
CO2 and level off at external CO2 concentrations slightly above ambient
(122).
Early studies on the response of plants to elevated CO2 examined shortterm
responses and used tissues that were grown in near-ambient but likely
quite variable CO2 levels of glasshouses and growth chambers. More recent
studies use plants grown under controlled CO2 levels. All these studies
showed an increase of photosynthetic rates with increased CO2 concentrations.
Measurements of photosynthetic rates of these plants grown
under ambient and elevated CO2 levels have shown that after a period of time
some species adjust their photosynthetic rates to the CO2 levels during growth
RESPONSE TO RISING CO2 169
(become acclimated) whereas other species show little or no adjustment (see
22, 87, 115, 121, 130, 141). The degree to which a species can adjust is
probably influenced by the levels of other environmental variables and the
timing of their availability (see later). Several investigators have also
observed that with time plants grown at elevated CO2 show a decline in
photosynthetic rates. Although the reasons for this decline are not fully
understood, several reasons for it have been proposed. They include: decline
in carboxylation efficiency which may be caused by a decrease in the amount
and activity of Rubisco (43, 105, 106); suppression of sucrose synthesis by an
accumulation of starch (51, 128); inhibition of the triose-P carrier; reduction
in the activity of sucrose-phosphate synthase; limitation of daytime photosynthate
export from sources to sinks (36) or insufficient sinks in the plant
(63). Because with acclimation there may be little overall increase in plant
photosynthesis and growth, understanding acclimation to a high CO2 environment
is critical in assessing the long-term response of plants to the high CO2
environments of the future.
From the extensive literature on the response of photosynthesis to elevated
C02, the following patterns emerged: (a) Elevated CO2 reduces or completely
eliminates photorespiration; (b) C3 plants are more responsive than C4 plants
to elevated CO2 levels, especially those above ambient concentrations; (c)
photosynthesis is enhanced by CO2 but this enhancement may decline with
time; (d) the response to CO2 is more pronounced under high levels of other
resources, especially water, nutrients, and light; (e) adjustment of photosynthesis
during growth occurs in some species but not in others, and this
adjustment may be influenced by resource availability; and (I) species even of
the same community may differ in their response to CO2 (Figure 1).
Dark Respiration
Little information is available on the effects of elevated CO2 concentration on
dark respiration rates. Enhancement of photosynthesis may lead to increased
respiration because of the increased availability of substrate for respiration.
Several arctic tundra species show a substantial increase in dark respiration
(88). However, there was no influence of elevated CO2 on dark respiration or
on light compensation in Desmodium paniculatum (141). There is evidence in
the agronomic literature that respiration may decline at high CO2 levels (4).
For species from natural communities, it is not known whether the change in
dark respiration is proportional to the rise in net daytime photosynthesis.
Furthermore, it is unknown if both growth and maintenance respiration
respond to the same degree to elevated CO2. These issues are important to the
understanding of the response of whole plants to the CO2 rise, especially in
regard to carbon gain and biomass accumulation, and they require much
attention.
170 BAZZAZ
C?2 Concentration Tlme~~~~~~~~~~~~HghCO
0~~~~~~~~~()0
00
4.1~~~~~~~~~~~~~~~~~~~~~~~~~9
01~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0
0~~~~~~~~~~~~~~~~~~~
AU00 nutrCents High
and water
'-4 ~ im High Cocetato
4.1 0 0 0
iqh ResourcHes r
O Amin Lo-wRsucs
j.1o 0~~~~~~~~~~~~~~~
C02 Concentration CO2 Concentration
Figure I General trends of response of plants to C02 concentrations.
Stomatal Conductance, Transpiration, and Water Use
There is now some evidence that growth in high C02 environments causes a
change in stomatal density in some species (e.g. 86). Woodward (134) has
shown that stomatal density and stomatal index increased markedly as the
C02 partial pressure is reduced below 340 0A I-'. 1A Above 340 l- l there is a
slight decrease in stomatal density in several species studied (135).
Most studies have shown a decline in stomatal conductance with an increase
C02 in concentrations (Figure 1). Stomatal response to C02 varies
greatly among species and may be influenced by other environmental factors
such as soil moisture and light levels (e.g. 125). Although strong evidence
suggests that stomata respond more to internal Ce2 concentration than to
RESPONSE TO RISING CO2 171
external concentrations (e.g. 78), the mechanism by which CO2 controls
stomatal activities is not known (95). Therefore, explanation of the differential
response of stomata to the CO2 rise is not possible at this time. Transpiration
rates decline as a result of decreased stomatal conductance. This decline
has been shown in several studies to lead to a favorable instantaneous water
use efficiency, improved plant water status, higher carbon gain and biomass
accumulation, and lower season-long water consumption rate (e.g. 100).
Enhancement of plant water use efficiency was observed also in plants grown
in the field (102). Drought stress in plants grown at elevated CO2 levels may
be also ameliorated by osmo-regulation and the maintenance of higher turgor
pressure (112). Lower transpiration rates should lead to higher leaf temperature
under high irradiance and low windspeed conditions (57). This increase,
coupled with the anticipated rise in air temperature, may have significant
effects on photosynthesis and plant growth.
Growth and Allocation
The critical issues that ought to be examined with regard to the effect of the
rising CO2 on plant growth are: (a) how long does the enhancement of growth
continue; (b) how do the allocational relationships in the plant change with
time under elevated CO2 levels; and (c) how will tissue quality change over
time and what are the consequences of this to herbivores, pathogens, and
symbionts?
Most studies on the effects of elevated CO2 show an initial enhancement in
growth, and like photosynthesis, this enhancement is especially large when
other resources are plentiful. In many species, however, this enhancement
may decline or completely disappear in time (1 1, 47, 114, 123-125). Most
studies have shown that there is generally an increase in allocation to roots,
especially when nutrients and water are limiting (68, 75, 79, 82-84, 114,
125). There is also strong evidence that specific leaf areas (SLA) decrease
with increasing CO2 levels (e.g. 47). Decreased SLA in high C02-grown
plants is often associated with increased starch levels in leaves and decreased
N concentration. Furthermore, the concentrations of C-based secondary
chemicals (e.g. phenolics) usually show no change in levels in leaves from
CO2-enriched plants even though the plants have greater carbon availability.
Several studies (e.g. 114), especially with woody seedlings, have shown that
branching increases with elevated CO2. Some evidence from tree ring analysis
suggests that growth in natural vegetation has been enhanced by the rising
global CO2 concentrations (66).
Phenology and Reproductive Biology
Despite its great importance to understanding the future impact of CO2 and
climate change as possible agents on natural selection, there is very limited
172 BAZZAZ
information on the effects of the rising CO2 on plant reproductive biology.
Most of the studies on the effects of CO2 on plants of natural communities
were terminated before reproduction. Because of the well-established effects
of elevated CO2 on plant growth, it is expected that aspects of reproduction
such as flowering phenology, allocation to reproduction and to various components
of reproduction, seed and flower abortion, and seed quality will also
be influenced (10). Studies have shown that depending on the species,
flowering time could be earlier or later under elevated CO2 (24, 46). In some
species these changes are only evident under unusually high levels of CO2
(e.g. 117). When plants are grown in competition, significant CO2 effects on
flowering among the species were found only under high nutrient conditions
(143), or the effects became less pronounced than when the plants were grown
separately (E. G. Reekie, F. A. Bazzaz, unpublished). Differences in flower
birth rate, flower longevity, and total floral display have been observed
among species in the same community as well as among populations of the
same species (e.g. Phlox drummondii) (46).
Reekie & Bazzaz (unpublished) examined the relation between CO2 level
and reproduction in four species from the annual community of disturbed
ground in Texas. Four insect pollinated forbs with showy flowers were used.
In Gallardia pulchela, doubling CO2 reduced the time required for flowering
by six days, though plant size at the time of flowering remained unchanged.
In Gaura brochycarpa, doubling CO2 also reduced time to flowering; however,
these reductions do not appear to be related to increased growth at elevated
CO2. The response of Lupinus texensis was the reverse: elevated CO2 increased
rather than decreased the time to flowering except when the plants
were given much underground space. No clear trends were found in
Oenothera laniculata. Shifts in flowering phenology caused by CO2 rise
could have marked effects on community structure and regeneration, especially
in communities where pollination is dependent upon animals or when
the growing season could be short, as is the case in this community where
drought can suddenly terminate the growing season. The combined effects of
elevated CO2 and other aspects of climate change, such as rising temperature,
may cause large shifts in phenology such that the activities of the plants and
their pollinators become decoupled.
Elevated CO2 can also affect other reproductive parameters, such as seed
number and size and seed nutrient content. In Datura stramonium, total fruit
weight was higher in plants grown in elevated CO2 than in plants grown in
ambient CO2. Plants grown in high CO2 produced thicker fruit walls, which
may prevent insects from laying eggs in these seeds, but, seed size was not
affected (46). In Abutilon theophrasti grown at elevated CO2 levels, total seed
production did not increase, but flower number, capsule number, and seed
number decreased (Figure 2). Individual seed weight was higher in plants
RESPONSE TO RISING C02 173
9 ~~~~~8
- ~~~~0-011-
8 - 7
7 30 665 9600
0~~~~~~~0c
7rw at hig CO 4) nsm pce eg Aboi reiEfl hr
0
zz 6-5 E 5-5 000
z
6 -5
0*008
53O4 6009001 -----000 ~~~~300 6w ( 00 9oo 00 300 600 900
Figure 2 The influence Of CO2 concentrations during growth on flower number, seed number,
and mean weight of individual seed in Abutilon theophrasti. From (46).
grown at high CO2 (46). In some species (e.g. Ambrosia artemisiifolia) there
was also much higher N concentration in seeds from plants grown at elevated
CO2 concentrations (47). Because of the well-established relationship between
individual seed size and nutrient content, and seedling success in
nature, the effects of rising CO2 and associated climate change may have
great impact on the demography and evolution of natural populations.
INTERACTION OF C02 WITH OTHER
ENVIRONMENTAL FACTORS
The interaction of CO2 with other plant resources has been amply demonstrated
(Figure 1). The response of plants to elevated CO2 is contingent upon
light levels (e.g. 95, 110, 113), soil moisture (7, 138), and nutrient availability
(2, 20, 50, 93, 103, 133). Several investigators have shown that the
enhancing effects of CO2 disappear under nitrogen and phosphorus limitation
(20, 50, 133, 143). Light saturation is usually higher under elevated than
under ambient CO2 (126), and high CO2 may compensate for low light (2).
Plant response to elevated CO2 is usually more strongly expressed under
higher levels of these resources, in a manner consistent with predictions about
the response of plants to multiple environmental resources (16, 25).
Elevated CO2 may modify the effects of stress factors on plant growth.
Such elevation has been shown to ameliorate effects of high salinity (19) by
174 BAZZAZ
supplying extra energy for maintenance respiration and by the reduction in the
entry of salt into the plant due to reduced transpirational pull (39, 44, 45).
High CO2 levels may influence the plant response to gaseous pollutants as
well. Coyne & Bingham (27) have shown that reduction in stomatal conductance
caused by high CO2 reduces both the amount of 03 entering the
leaves and the resulting damage. Similarly, decreased stomatal conductance
caused by high CO2 reduces entry of SO2 into leaves and lessens its damage in
C3 plants (23). SO2 reduced the growth of the C3 species at the ambient but
not at the elevated CO2 concentration. In contrast, in the C4 species, SO2
increased growth at the ambient CO2 concentration and reduced at a high
CO2. The results of this experiment support the notion that C3 species are
more sensitive to SO2 than are C4 species (131). This study shows that CO2
reversed the effect of SO2 on C3 but not C4 plants, results which correlated
with differences in sensitivity of stomatal conductance.
The interaction of CO2 with temperature is critical to the response of plants
to climate change. Acock (1) and Acock & Allen (2) present a model for the
response of photosynthesis to temperature and CO2. They show that at high
CO2 levels the optimal temperature for photosynthesis is higher than at
ambient CO2 and the range of optimal temperature for photosynthesis is
narrower. There are, however, only a few studies that consider their joint
effects. J. Coleman & F. A. Bazzaz (unpublished) examined growth and
resource acquisition and allocation in response to temperature and CO2 in a C3
and a C4 species that occur together in the field. The results show significant
interactive effects on these parameters, but the strength and direction differed
between the two species. In the C4 species (Amaranthus), final biomass was
increased by CO2 at 28?C but was depressed at 38?C. In the C3 species
(Abutilon), C02 enhanced initial biomass at both temperatures, but the final
biomass was not different in the two temperatures. These somewhat surprising
results were explained by the amount of standing leaf areas and changed
photosynthetic rates in the two species under these conditions. It was clear
from this model system that the interaction between factors may be complex
but could be understood by studying patterns of carbon gain and allocation.

Several of the climate models also predict that in addition to the global rise
in mean annual temperature there can be an increase in temperature extremes.
Furthermore, because of the generally reduced stomatal conductance under
elevated CO2 conditions, transpirational cooling of plant tissues will be
reduced. Few studies have addressed the joint effects on plant growth of
unusually high and unusually low temperature in conjunction with elevated
CO2. When the C4 weedy grasses Echinocloa crus-galli and Elusine indica
were grown in a range of temperatures and then subjected to one night of
chilling at 7? C, the decline in both conductance and photosynthesis was less
RESPONSE TO RISING CO2 175
in plants grown under elevated CO2 than in plants grown under ambient CO2
levels (97). Preliminary results with Abutilon suggest that individuals grown
at high CO2 concentrations are more sensitive to heat shock than are individuals
grown at ambient CO2 (F. A. Bazzaz, unpublished).
SUBSPECIFIC DIFFERENCES IN RESPONSE TO C02
Populations of the same species respond differently to C02, and these differences
may be related to the CO2 environment in which the plants grow (e.g.
140). However, differences among individuals of a population in response to
CO2 have rarely been investigated. Clearly, genetic differences among individuals
in response to atmospheric CO2 can affect the future of the genetic
structure of the population in a changing CO2 atmosphere. The studies that
have examined variation among individuals have detected differences among
them in response to CO2. For example, Wulff & Miller (142) found that
families of Plantago lanceolata differed in their response to CO2 enrichment
and to combinations of CO2 and temperature treatments. They suggested the
presence of genetic variability in this species in response to CO2 enrichment.
F. A. Bazzaz & G. Carlton (unpublished) found differences in CO2 response
in growth and architecture among several genotypes of Polygonum pensylvanicum
from a single population. Garbutt & Bazzaz (46) found differences in
the time of flowering, the number of flower births, and the maximum flower
display among four populations of the annual Phlox drummondii from central
Texas (Figure 3). Significant effects were also seen on plant final biomass and
in the number of flowers produced per unit of plant dry weight. These
responses may have significant implications for pollination success, dispersal,
and establishment.
60- e
55-
50-
45-
40-
35
30
25 888 8808 8 88
Popn 3 Popn 5 Plopn 6Popn 9
Figure 3 Deferences in floral display among natural populations of Phlox drummondii in
response to CO2 concentration. From (46).
176 BAZZAZ
PLANT RESPONSE TO C02 AT THE POPULATION
LEVEL
Almost no information is available on the response to elevated CO2 at the
population level. But because elevated CO2 affects growth, allocation, and
reproduction, undoubtedly there are some effects on populations. Using our
model system of the annuals Abutilon theophrasti and Amaranthus retroflexus,
we investigated how the simultaneous changes in CO2 and temperature
affect the recruitment of seeds into the population (S. Morse, F. A. Bazzaz,
unpublished). Although no differences appear in survivorship with respect to
ambient CO2 concentrations for either species, stand productivity was significantly
affected by both CO2 and temperature. In general, stand productivity
increased with both CO2 and temperature and was inversely proportional
to the number of survivors. CO2 magnified the intensity of plantplant
interactions and enhanced the growth of the remaining dominant
individuals.
INVESTIGATIONS AT THE COMMUNITY AND
ECOSYSTEM LEVELS
Productivity
Predictions about the changes in productivity of ecosystems are also based on
the generally observed increase in plant growth under high CO2 conditions. A
physiologically based graphical model (Beam 82) was proposed by a group of
scientists (see 119) to represent possible changes in productivity of ecosystems.
To address the relationship between elevated CO2 and productivity,
Gates (48) suggested a modification of the B factor, described by Bacastow &
Keeling (5), and proposed B' (the biotic growth factor), based on the
Michaelis-Menton equation. B' is the fractional increase in net primary
productivity (NPP) with a fractional increase in CO2 concentration. Using
data on single leaves, Gates (48) calculated B' factors for several deciduous
forest tree species and showed that they could be high, ranging from 0.33-
0.53. However, he also found that, depending on environmental limitations,
the B' values could be small (between 0.05-0.25). Using high B' values,
Gifford (49) estimated high carbon storage in the biosphere (1.65 Gt y-l for
B' = 0.60). Several other authors (e.g. 18, 48, 54, 64, 73) have pointed out
that because of the limits on plant growth already set by water and nutrient
deficiency, and temperatures at the northern limits of distribution, primary
productivity in natural ecosystems may not be enhanced much by the rising
global CO2. Furthermore, even in systems that have the potential for an
increase in production, Oechel & Strain (88) show that negative feedbacks
may soon lead to the elimination of any enhancement by the rising CO2. For
RESPONSE TO RISING C02 177
example in the chaparral, a water-limited system, increased water use efficiency
may lead to enhanced productivity. However, the chaparral is a
fire-prone system, and the increased accumulation of living and dead biomass
may increase the frequency of fire, which in turn would reduce biomass
accumulation. In contrast, Luxmoore (74) suggests a different scenario,
where increased photosynthesis in a high CO2 environment would increase
the amount of carbon allocated to roots, resulting in increased root exudation,
mycorrhizal proliferation, and increased N-fixation. Evidence also suggests
increased nitrogenase activity at high CO2 levels (81). These factors in turn
can lead to increased water and nutrient supply to the plants and increased
phytomass even in somewhat infertile habitats. The very limited evidence
from field studies shows both an increase in productivity with elevated CO2,
especially during the first year or two (e.g. 29, 90), and no change (121).
Thus, these responses to elevated CO2 remain very poorly understood despite
their great importance in predicting future productivity. Accurate predictions
about the response of natural ecosystems to global increase in CO2 levels still
require much additional data on the mechanistic bases of the responses of
several ecosystems (31).
RESPONSE OF SPECIFIC ECOSYSTEMS TO
ELEVATED C02
Graminoid-Dominated Ecosystems
CO2 AND ARCTIC TUNDRA Arctic ecosystems may be the ecosystems most
sensitive to climate change (see 119). All climate models show a greater
increase in mean annual temperature in these regions, compared to lower
latitudes. Arctic ecosystems possess several properties that make them of
particular interest to the study of CO2 response (12). Because of permafrost,
the active layer of the soil is shallow, and the top 10 cm of the soil contain
most of the root and rhizome systems, which constitute by far most of the
living biomass in this ecosystem. Up to 90% of the CO2 which evolves from
soil comes from root and rhizome respiration (12). Tundra soils also contain
large quantities of organic matter which, being mostly in the permafrost, is
normally unavailable to decomposers.
In a series of experiments with microcosms of intact cores of turf and soil of
coastal arctic tundra, W. D. Billings and associates (13-15, 96) examined the
effects on ecosystem carbon balance of doubling CO2, increasing temperature,
lowering the water table, and applying N-fertilizer. They concluded that
increasing summer temperature by 4?C would reduce net ecosystem CO2
uptake by half. Lowering the water table by only 5 cm and increasing
temperature greatly lowered ecosystem carbon storage. In contrast, doubling
CO2 concentrations per se had very little effect. They suggest that warmer
178 BAZZAZ
temperatures would extend the growing season into the short days of autumn,
expose much more peat to decomposers (which become more active in the
higher temperatures and the longer season), and lower the water table by high
transpiration under the warm conditions. Enhanced ecosystem carbon gain
caused by the release of nutrients would be more than offset by decreased
insulation and the resultant lowering of the permafrost table and increased soil
erosion. From these studies Billings reaches the dramatic conclusion that
doubling CO2 would convert the wet tundra ecosystem from a CO2 sink to a
CO2 source.
W. Oechel and coworkers have been studying the response of arctic
ecosystems to the increase in CO2 and temperature using environmentally
controlled greenhouses placed in situ in the tundra near Barrow, Alaska (99,
88, 121). Contrary to most results obtained on the response of single individuals,
Eriophorum plants in situ showed little response to high levels of
CO2. Plants grown at the high CO2 adjusted their photosynthetic rates within
three weeks so that their rates were similar to those grown under ambient CO2
when both were measured at CO2 levels of their growth. Although there was
no seasonal pattern of growth, a significant increase occurred in tillering
under the high CO2 conditions. When responses to elevated CO2 under
controlled conditions of six arctic tundra species of different growth forms
were compared (in 88), most of the species had increased their photosynthetic
rates on a leaf area basis, but they varied in the degree of response, and that
was influenced by nutrient level. All species except Eriophorum had increased
leaf dark respiration as well. Surprisingly, and contrary to the results
from the in situ measurements, the photosynthetic rate of Eriophorum vaginatum
was enhanced, especially under high nutrient conditions, and that
enhancement was still high after 2 months of exposure to the high CO2 level.
Oberbauer et al (87) found that Carex bigelowii, Betula nana, and Ledum
palustre responded to elevated CO2 and nutrient levels. They found that
nutrients enhanced growth much more than did CO2 and concluded that CO2
with or without nutrient limitation has little effect on the production of these
species. These results point out the importance of in situ measurements to
accurately assess plant response to elevated CO2 concentration. Analysis of
whole ecosystem response to elevated CO2 and temperature from the in situ
measurements shows that net CO2 uptake by tussock tundra was higher at
elevated CO2 than at ambient CO2. But, net CO2 uptake was reduced by
temperatures 4?C higher than ambient (53). Although the higher temperature
increased conductance and consequently gross photosynthesis, higher temperatures
also increased respiration to a degree that resulted in lower net CO2
uptake. These authors conclude that nutrient limitation in this system lowers
the ability of tundra plants to make full use of the elevated CO2 concentrations.

RESPONSE TO RISING C02 179
The following conclusions emerge from work on this ecosystem: (a) in
Eriophorum, the dominant species in this system, only tillering increases
dramatically with rise in C02; (b) photosynthetic acclimation to high CO2
occurs; (c) nutrients enhance the response to increasing C02; (d) species differ
in the degree to which growth is enhanced by C02; (e) different life forms do
not seem to respond differently to increase in C02; (I) conductance and
respiration increase; and (g) temperature rise lowers the CO2 enhancement
effects. Therefore, while some general responses are similar to those observed
in other ecosystems, the tundra ecosystem differs in some quite surprising
ways, particularly the increase in conductance.
From the available data, the following scenario emerges: As CO2 and
temperature rise, thaw of permafrost increases, the growing season lengthens,
decomposition of organic matter increases sharply, nutrient availability increases,
net CO2 uptake increases, and transpiration increases because of
higher temperature and increased conductance. After a while, however, the
water table recedes, photosynthesis and net ecosystem productivity decrease,
and the system becomes a CO2 source and a positive feedback loop would be
established.
THE ESTUARINE MARSH Another in situ study of the response of graminoid
ecosystem to elevated CO2 has been underway in the estuarine marsh of
Chesapeake Bay, Maryland, USA. Open top chambers were used by B.
Drake and his associates to enclose stands of Scirpus olneyi (C3), Spartina
patens (C4), and a combination of both species and to expose them to ambient
(350?22 ,u 1-1) and elevated (686?30 ul 1-1) C02 concentrations. Elevated
CO2 increased shoot density, delayed senescence, and increased biomass
in Scirpus, the C3 species, but there was no effect on Spartina, the
C4 species (30). Furthermore, Scirpus responded positively to elevated
CO2 both in pure and in mixed stands. Carbon-nitrogen relations were also
examined for these species (29). While carbon percentage did not change
with elevated CO2 in green leaves of Scirpus, nitrogen was reduced by
as much as 40%. Furthermore, aboveground tissue content of nitrogen on
a per leaf area basis was not influenced by C02, indicating that nitrogen
was allocated from storage pools. Surprisingly, litter C/N ratio was not
affected by CO2 level, and the authors suggested that CO2 rise will not influence
the rate of decomposition or N mineralization. Because of the
continued input of nutrients in water from the adjacent creek into this already
highly productive marsh, the authors conclude that continued exposure
to high CO2 levels may cause a continued increase in Scirpus productivity and
increased dominance in this system. Thus, this situation contrasts sharply
with that observed in the nutrient-limited tundra ecosystem discussed previously.

180 BAZZAZ
OTHER GRASSLANDS Information about the response of grasslands to elevated
CO2 is very limited. Smith et al (115) compared the response of four
grass species from the Great Basin. High CO2 resulted in increased growth,
especially basal stem production, in the C3 but not in the C4 species. This
enhancement was particularly strong for Bromus tectorum, an introduced
wee\d. Since Bromus predisposes rangelands to burning, the authors speculated
that this enhancement by high CO2 levels in the future may increase the
number and the severity of wildfires in this region, which could result in a
change in ecosystem function. Work with Blue grama (Bouteloua gracilis),
an important native perennial in the same region, showed that biomass and
leaf area were greatly enhanced at elevated CO2 levels, which is unusual for a
C4 plant (101).
When plants were grown individually, CO2 concentration differentially
influenced the growth of six species from the short annual grasslands found on
serpentine soil in California (129). In competition, however, these species did
not differ in their growth response to CO2. The species are of small stature
and presumably adapted to low nitrogen and calcium availability and to heavy
metals such as Ni and Mg. Apparently, the potential for these species to
respond to increased CO2 concentrations may be constrained by physiological
traits that enable these annuals to grow in their native, nutrient-limited
environment. Furthermore, in this low-stature community with a very short
growing season and nutrient limitation, competitive networks and adaptation
can develop and dampen the CO2 effects.
Regenerating Ecosystems
The speed of the rise in CO2 concentrations and the associated temperature
rise will far exceed the regeneration time of many woody species in the world
and their migration to new habitats (32). Thus, this rapid change would likely
result in the death of many individual plants and their replacement with early
successional species that, in general, are adapted to live in an environment
with initially high resource levels (6). Regenerating ecosystems may be the
dominant ones over much of the landscape in a high CO2 world. Thus, the
study of regenerating ecosystems is crucial to assessing the possible impact of
global change. Our extensive knowledge of their behavior at the physiological,
populational, and community levels under ambient and more recently
elevated CO2 may allow some predictions about their future.
The Early Successional Community: A Model System
NONCOMPETITIVE RESPONSE Work in our laboratory has focused mainly
on community level, using individual species responses to interpret communi-
RESPONSE TO RISING C02 181
ty level responses. A major premise of the research is that the response of
individuals is highly modified by the presence of other individuals in a
population or a community, and that these relationships themselves would be
modified by other factors in the natural environment.
Community-level investigations of CO2 effects on plant growth were reported
by Carlson & Bazzaz (22). The annual community of postagricultural
succession and the flood plain forest community in the midwestern United
States were studied. The experiments also included three crop species (corn,
soybean, and sunflower) in order to compare results with the published
agronomic literature. The results confirmed that species from natural communities
have physiological responses similar to those of the agronomic
species studied thus far. The degree of variation in response of different
species even of the same community was enormous. Based on these findings,
and without consideration of the associated climate change, three hypotheses
about the effects of elevated CO2 on plants were put forward: (a) Because of
increased water use efficiency, plant species will be able to expand their
ranges into drier habitats; (b) competitive interactions among species in a
community may change and will result in a change in community composition
and function; and (c) competitive interaction between crops and weeds may
change. The latter hypothesis was also proposed by Patterson & Flint (92) and
was later confirmed (94).
Further work with individually grown plants established the fundamental
physiological and morphological basis of the response of plants to CO2 and its
interactions with other environmental factors. Most of these studies involved
growing several species individually and studying differences among them in
their photosynthetic response, growth and allocation, or some other indicator
of their potential competitive success. These results were used to infer
competitive outcome among species (e.g. . The results have also been
useful in interpreting the response of communities to the rising CO2 levels.
We chose as a model system an early successional community of annual
plants to investigate in detail aspects of the CO2 response at the individual,
population, and community levels. Depending on the questions asked we
sometimes used all dominant species of the community and sometimes a
subset of these species. This community was chosen because annual plants
can be grown to maturity, so that the effects of CO2 on all phases of the life
cycle, including reproduction, could be studied, and also because we have
accumulated much background information on this community over two
decades. The community is dominated by a small number of species (five to
six) and has both C3 and C4 plants. Comparing the response of the major
species in this community to elevated CO2 when the plants were grown
individually (47), we found:
182 BAZZAZ
1. CO2 concentration had little effect on the timing of seedling emergence;
2. Photosynthetic rates increased and stomatal conductance decreased with
increased CO2;
3. The levels of CO2 during growth had no effect on photosynthetic rates;
4. Shoot water potential was less negative in plants grown at high CO2;
5. Relative growth rates were enhanced by CO2 early in the growth period
but declined later;
6. Specific leaf area (SLA) consistently decreased with increased CO2;
7. High CO2 caused one species to flower earlier and one to flower later,
while the rest showed no change;
8. There were significant species x CO2 interactions for leaf area, leaf
weight, weight of reproductive parts, and seed weight indicating
species-specific response to C02; and
9. Carbon/nitrogen ratios increased with increasing CO2.
The results of this experiment and others also suggest that the commonly
suggested C3/C4 dichotomy does not fully explain the responses of plants to
CO2. For example, Amaranthus (C4) often shows a greater increase in
biomass as a result of elevated CO2 than does the C3 species Abutilon
theophrasti (8, 47).
COMPETITIVE RESPONSE IN THE MODEL SYSTEM Under competitive conditions
the interaction between CO2 concentration and soil moisture showed
that total community biomass increased with increasing CO2 at both moist and
dry soil moisture conditions. The contribution of each species to total community
biomass was greatly influenced by CO2. For example, Polygonum pensylvanicum
contributed more at high CO2 and moisture levels. In contrast,
Amaranthus retroflexus declined under these conditions (7). These results are
commensurate with the response of these species individually to CO2 and
moisture separately. Work on the interaction of CO2 with light and nutrients
(143) using all six species from this community showed that total community
production reached its peak at 450 ,u 1- l CO2. While total community
biomass was higher under high light, relative to low light, and under high
nutrients, relative to low nutrients, the response of the community to elevated
CO2 was affected by light level but not by nutrient availability. The relative
success of some species, particularly in terms of seed biomass and reproductive
allocation, was significantly altered by CO2. The contribution of the C3
species in this community to total production increased with CO2 enrichment.
Competitive interactions and CO2 have been examined in more detail using
one C3 and one C4 plant from this community. Detailed growth analysis,
patterns of leaf display, and N allocation were used to understand the mechanisms
of interaction and to begin to model these interactions (11). The
species were grown both individually and in competition with each other. At
RESPONSE TO RISING C02 183
ambient CO2 levels Abutilon was competitively superior to Amaranthus
because the latter was unable to overcome the initial difference in starting
capital (larger seeds and seedling). But, at elevated CO2 that difference
disappeared, largely because of the enhanced relative growth rate (RGR) of
Amaranthus in high CO2 (especially earlier in the growth period) which
overcame the seed size advantage that Abutilon has over Amaranthus. High
CO2 caused an increase in root/shoot ratio in Abutilon and a decrease in
Amaranthus. But Amaranthus had a much higher rate of N uptake per unit of
root relative to Abutilon. Thus, the results of this experiment show that: (a)
the response to high CO2 is limited to early stages of growth; (b) elevated CO2
greatly increased RGR in Amaranthus; and (c) although, when compared with
C3 plants, C4 plants show a lesser enhancement of photosynthesis and net
assimilation rate (NAR) with increased CO2 levels, they did not "lose out" in
competition with C3 plants at elevated CO2 concentrations.
Bazzaz & Garbutt ( studied the influence of the identity of competing
species and that of neighborhood complexity on the interaction between CO2
and competition. Four species of the annual community were grown in
monoculture and in all possible combinations of two, three, or four species at
levels of CO2. Overall, the species responded differently to CO2 levels. In
mixtures the species interacted strongly, and in some cases these interactions
cancelled out the effects of CO2. For example, there were clear differences in
the responses of species in different competitive neighborhoods. All competitive
arrays that had C3 species in them depressed the growth of the C4 species
(Figure 4). The interactions between CO2 and the identity of the competing
species were particularly strong at the intermediate C02 level (500 ,u 1- 1).
These findings suggested that competitive outcome will be modified by CO2
and by the interaction of CO2 with other environmental factors. They show
that different species will behave differently in a high CO2 world and that
their response will depend on the identity of the competing species and
perhaps on community diversity.
Early Perennial Stage
The interaction between Aster pilosus (C3) and Andropogon virginicus (C4),
important species in old-field succession, was studied by Wray & Strain (137,
138, 139). They grew the two species both separately and in competition in
ambient and high CO2 levels, while half of them were subjected to a drought
cycle. In Aster, droughted plants grown at high CO2 had greater leaf water
potential and greater photosynthetic rates and total dry weight than did plants
grown at ambient CO2. In contrast, in Andropogon no differences appeared
among CO2 treatments in response to drought. In competition the differences
between the species in response to elevated CO2 were accentuated, and Aster
strongly dominated Andropogon. These authors suggested that CO2 enrich-
184 BAZZAZ
Ambrosia artemjsiifol' Abubilon ItMfphrasti
CO2 350 gL/L 7 C02 ? 350 L/L
Norm Al St Ar Al-St Al-Ar Ar-St Al-Ar-St Noo St Aa At Am-St Ar-St Aa-Ar Aa-Ar-St
108: CO2 500 gL/L 7 CO2 500 pi/L
Nom Al St Ar Al-St Al-Ar Ar-St Al-Ar-St Now St Aa Ar An-St Ar-St Aa-Ar Aa-Ar-St
l,
8 CO2 700 gL/L 007 co3700 iL
8= 5 2 * *
2Z - 0030 iL4 0250pl
Nom At St An Al-St Al-Ar An-St Al-Ar-St Nm St Aa At At-Sn At-SA An-Ar At-An-S
cn ~~~~Amaranthus retroflexus Seaa faberii
n 3.
CZs 3 C02 3500 UL 4 j0O2 350j? LJL
E O 0 U 4 C02 3500 gL/L
2 3 X
Nor At St An At-St At-Aa Aa-St At-Aa-St Nrm t Aa Ar At-Aa At-Ar Aa-Ar At-Aa-Ar
3
C02 700 gL/L 4 CO02 700 gL/L
2 . 3
Competitor
Figure 4 The relationship between plant growth, identity and diversity of competitors, and CO2
concentrations during growth in a community of annuals made up of two C3 species (Ambrosia
artemisEifolia (Aa),r and Abutilon theophrasti (At)) and two (C4) species (Amaranthus retroflexus
(Ar), and Setaria faberii (Sf)) From (.
ment may increase the competitive ability of Aster relative to Andropogon,
allowing Aster to persist for longer periods during old-field succession.
Early Successional Trees
Tolley & Strain (123-125), Sionit et al (114), and Fetcher et al (43) studied
the response of Sweetgum (Liquidambar siyraciflua) and loblolly pine (Pinus
taeda), two midsuccessional tree species, to elevated CO2. They found that
elevated CO2 increased components of growth more in sweetgum than in
loblolly pine, especially at high irradiance. Sweetgum developed more rapid-
RESPONSE TO RISING C02 185
ly, reached maximum size earlier, and maintained height dominance relative
to loblolly pine. Under drought stress high C02-grown sweetgum individuals
developed internal water deficits more slowly than did those grown under
ambient C02, and the seedlings maintained higher photosynthetic rates over
the drying cycle. In contrast, loblolly pine seedlings had a more severe
internal water deficit than did sweetgum, irrespective of CO2 level. The
authors concluded that sweetgum seedlings should tolerate longer exposure to
low moisture, especially under high CO2 conditions, and that these conditions
would result in greater seedling survival on drier sites in successional fields in
the piedmont. Furthermore, the height dominance and shading that sweetgum
presently exerts on pine may be intensified in a high CO2 environment. In the
climate of the future, with high C02, the authors suggest that sweetgum could
displace loblolly pine.
Forest Ecosystems
TEMPERATE FORESTS Only a few studies have examined the response of
tree species in a community context, and fewer still in competitive situations.
Seedlings of the dominants of a floodplain forest community and of an upland
deciduous forest community were grown as two groups in competition under
ambient and elevated CO2 concentrations (130). Photosynthetic capacity (rate
of photosynthesis at saturating CO2 and light) tended to decline as CO2
concentration increased. Stomatal conductance also declined with an increase
in CO2. Nitrogen and phosphorus concentrations generally decreased as CO2
increased. Overall growth of both communities was not enhanced by C02, but
the relative contribution of species to the total community biomass changed in
a complex way and was also influenced by light/CO2 interactions.
In four cooccurring species of Betula, elevated CO2 enhanced survivorship
in yellow birch only, but nearly doubled total weight and root/shoot ratio in all
species. However, differences among the species in growth response to
elevated CO2 were small despite the differences among the species in habitat
preferance (F. Bazzaz, unpublished). The response to CO2 of seven cooccurring
tree species from the Northern Hardwood forests in New England
was studied by F. A. Bazzaz, J. Coleman, & S. Morse, (unpublished).
Seedlings of Fagus grandifolia, Acer saccharum, Tsuga canadensis, Acer
rubrum, Betula papyrifera, Prunus serotina, and Pinus strobus were grown
under 400 ul I-l and 700 ,ul 1-1 CO2. The species differed greatly in their
responses; elevated CO2 significantly increased the biomass of Fagus, Prunus,
Acer saccharum, and Tsuga, but only marginally that of Betula, Acer
rubrum, and Pinus. Under the conditions of this experiment-relatively low
light (400-700 umole mole- 1) and high nutrients-the species that are
considered more shade tolerant and late successional (Fagus, Acer saccharum,
and Tsuga) showed the largest biomass increase, with high CO2 levels.
Furthermore, Betula and Acer rubrum grown from seed did not exhibit
186 BAZZAZ
different responses to elevated CO2 than did those individuals transplanted
from the field while dormant. These results suggest that seedlings of the late
successional trees in this system growing in the shade and with ample
nutrients will do relatively better in a high CO2 world than will early successional
trees in open environments. This may be particularly important
since young seedlings near the forest floor may experience a high CO2
environment caused by the efflux of CO2 from the soil (9). These findings, at
first glance, differ from those of other studies (e.g. 86) which found that
growth enhancement by elevated CO2 in Ochroma lagopus, a fast growing
pioneer species, was greater than that in Pentaclethra macroloba, a slower
growing climax species. Furthermore, Tolley & Strain (123) found a greater
enhancement of growth in the faster growing of two early successional tree
species Liquidambar styraciflua and Pinus taeda. The findings of these two
studies fit the general notion that early successional plants growing in open
environments are able to take opportunistic advantage of available resources
and that they have high growth rates (6). However, the results from the
seven-species study point once again to the importance of other environmental
resources in modifying the response of plants to elevated CO2.
TROPICAL RAINFORESTS Reekie & Bazzaz (100) studied competition and
patterns of resource use among seedlings of tropical trees under ambient and
elevated CO2 using five relatively fast growing early successional species
from the rainforest of Mexico (Cecropia obtusifolia, Myriocarpa longipes,
Piper auritum, Senna multijuga, and Trichospermum mexicanum). Elevated
CO2 only slightly affected photosynthesis and overall growth of the individually
grown plants but greatly affected mean canopy height. Though
stomatal conductance slightly declined with increased C02, leaf water potential
and plant water use were relatively unaffected. However, in the competitive
arrays there were marked effects of CO2 on species composition, with
some species decreasing and others increasing in importance. High CO2
increased the mean canopy height in Cecropia, Piper, and Trichospermum,
and decreased it in Senna (Figure 5). There were also some differences among
species in allocation to roots and in the timing of that allocation. Stepwise
regression analysis of several physiological and architectural measurements
showed that canopy height (leaf display in the canopy) was the single most
important variable determining competitive ability. Photosynthetic rates, especially
in low light, and allocation to root early in the growth period were
also significant. The results of this study suggest that competition for light
was the major factor influencing community composition, and that CO2
influenced competitive outcome largely through its effects on canopy architecture.
Early in the experiment competition for nutrients was intense. This
allowed Piper, with greater allocation to roots, to gain a competitive edge.
RESPONSE TO RISING C02 187
, ~~~~700
* ~~~~~M1.435 ga,, 350 sea 3
cS.S *A* 1.2
$2 0.6 01
0s 0.14. .0
Figure 5 Leaf area profiles, and mean canopy height of seedling of 5 fast-growing tropical
rainforest trees grown at 350 (left) and 700 ,ul 1 (right) CO2. Each unit on the horizontal axis
represent 1 dm-2. The species are Cecropia obtusifolia (C), Myriocarpa longipes (M), Piper
auritum (P), Senna multijuga (S), and Trichospermum mexicanum (T). From (100).
Very rapidly, however, the canopy closed and competition for light become
more intense. Therefore, Senna, Trichospermum, and Cecropia, with their
greater biomass allocation to shoots, were able to overtop the other species.
Senna was particularly successful because of its high photosynthetic rate and
tall shoot architecture. Thus, the major effect of elevated CO2 on competition
was through its modification of plant architecture.
CO2 AND EFFECTS ON SOIL
MICROORGANISMS/PLANT ROOT INTERACTIONS
It has been hypothesized that high CO2 and the resulting high availability of
photosynthate will enhance root growth and root exudation in the soil. These
will in turn influence plant nutrition by enlarging soil volume explored by
roots and by increasing mycorrhizal colonization (119), nodulation, and
nitrogen-fixing capacities (67, 74). There have been only a few tests of these
ideas with plants from natural ecosystems. In Quercus alba seedlings grown
in nutrient-poor forest soil, elevated CO2 increased growth, especially of the
root system (83). Much of the nitrogen was in fine roots and leaves, and the
plant's efficiency of N-use was enhanced. Furthermore, elevated CO2 increased
uptake of P which may have also been associated with a greater
proliferation of mycorrhizae and rhizosphere bacteria. The weight of new
buds of seedlings grown in elevated CO2 was greater than of those of
seedlings grown in ambient CO2, suggesting that shoot growth in the subsequent
year would be enhanced (84). Seedlings of Pinus echinata grown in
elevated CO2 allocated proportionally more photosynthate to fine roots,
produced larger fine root mass, and had higher mycorrhizal density than
plants grown in ambient CO2 (85).
Although there have been no experimental tests of the hypothesis, several
188 BAZZAZ
authors have predicted that the rate of litter decomposition may be slower in
high CO2 environments (119, 130). These predictions are based on the
finding in the majority of studies that the carbon-to-nitrogen ratio of tissues
grown under elevated CO2 levels declines and on experimental evidence that
tissue with high lignin and low nitrogen content decays slowly (77).
CO2 AND PLANT-HERBIVORE INTERACTIONS
Elevated CO2 concentrations within the range predicted by global change
scenarios are unlikely to influence herbivores directly (e.g. 42). However,
several investigators (see 119) have suggested that the tissue quality of plants
grown under high CO2 environments could be altered, thereby indirectly
affecting insect performance. Recent experimental evidence has supported
this notion. For example, most studies have demonstrated that foliar nitrogen
concentrations, a limiting nutrient for insect herbivores (76), decline with
increased CO2 (40-42, 59, 70, 71, 130, 133). Other important nutritional
factors, such as foliar carbon-based allelochemical and fiber concentrations,
do not seem to be affected by elevated CO2 conditions (40-42, 70, 59), and
foliar water content does not change in any consistent way in higher CO2
atmospheres (e.g. 71, 41). Too few systems have been examined to make any
general statements about these patterns.
Insect herbivore behavior and subsequent performance are affected when
they are reared on low nitrogen, high CO2 grown plants. To compensate for
the lower nitrogen concentrations, insect herbivores feeding on high CO2
grown foliage increase their consumption rate by 20%-80% compared to
those larvae feeding on low CO2 grown tissue (40, 59, 70, 72). Despite this
increased consumption, insect herbivore performance on high CO2 grown
plants is often poorer than on low CO2 grown plants. Lepidopteran larval
mortality increases (3, 41), and growth is often slower for larvae reared on
high C02-grown plants (40-42) (Figure 6). Slower growth might reduce
insect herbivore fitness in the wild due to an increased exposure to predators
and parasitoids (98) and a decrease in the likelihood of their completing
development in seasonal environments (e.g. 26). Reduced population numbers
have also been observed for foliage-feeding herbivores on plants in
enriched CO2 environments in open-top chambers (21). Interactions between
plants and other plant-eating organisms, such as mammals, have yet to be
investigated.
GLOBAL CHANGE AND PREDICTED CHANGES IN
SPECIES RANGES
Various modelling results, based on changes in temperature caused by the
increase in CO2 and other greenhouse gases, have suggested a significant
RESPONSE TO RISING CO2 189
600 -
500 - --o -- -high C02
Zcm 400 - low C02
300-
J 200-
aZ 100 :
0
0 10 20 30
Larval Age (days)
Figure 6 Growth of larvae of the Buckeye butterfly Junonia coenia on Plantago lanceolata
grown at ambient and elevated CO2 concentration. From (41).
change in patterns of regional plant productivity (109, 38, 116), in the
distribution ranges of some plant species (34, 33), and in species composition
on a regional scale (91). For example, the range of American beech (Fagus
grandifolia) could drastically change, and its distribution could be several
hundred miles north of its current position (33). Additionally, based on the
direct response to increased CO2 alone and the resultant decrease in water
consumption, it was also predicted that the ranges of species can expand
into drier habitats (22). Of course, neither of these approaches by itself
would yield definitive conclusions; the influence of both the direct and
indirect effects of the rising CO2 should be jointly considered. Using growth
and other physiological data on the response of the weedy vines Kudzu
(Pueraria lobata), and honeysuckle (Lonicera japonica) to elevated C02,
and considering the indirect effects of the C02-induced climate change,
Sasek & Strain (107) concluded that elevated CO2 levels and increased
winter minimum temperatures may allow northward and westward migration
of both species, but the decreased summer precipitation may minimize
the westward spread. It must be pointed out that these predictions concern
only the potential for range shifts in species and do not take into account
the new and potentially very effective barriers to and corridors for dispersal
of propagules, nor do they consider the important factors of the changed
plant/plant interactions, plant/animal interactions, and plant/microbial interactions.

190 BAZZAZ
CONCLUSIONS
It is clear from this review that some general patterns of response of plants,
especially at the physiological level, to the rising CO2 and the associated
climate change are beginning to emerge (Figure 1). Enhanced photosynthesis
and growth, increased allocation to underground parts, and particularly water
use efficiency have been strongly documented. However, photosynthesis and
growth enhancement in some species can be of limited duration, perhaps
because of shortages of sinks and the resulting simulation of photosynthates in
leaves. It is also clear that CO2 interacts strongly with other environmental
factors, especially nutrients and temperature, to generate the response at the
individual level.
Work at the community and ecosystem level has clearly shown that, in
most situations, the response at the individual level may become highly
modified and may not predict the response of communities. It is quite likely
that the impact on productivity of ecosystems may result mainly from changes
in species composition brought about by differential species response to
elevated CO2. The number and the identity of neighboring plants, the levels
of environmental resources, the activities of herbivores, pathogens, and
symbionts are crucial to the way plants respond to elevated CO2. Because
of the complexity of these interactions, and our limited knowledge of them,
our predictions about the future impact of the rising CO2 and associated
climate change are very tenuous. In fact, for some ecosystems we cannot
presently even predict the direction of the change that would result from
increasing CO2. Nevertheless, the work on a model system of annual plants,
and with other assemblages, is giving us some insights into the mechanisms of
the response to CO2 at the community level. We are beginning to identify
certain parameters that seem to explain significant amounts of the response
to elevated CO2. For example, initial relative plant growth rates and biomass
allocation seem very important determinants of plant response to
CO2. Responses at the population level are essentially unknown, but that
research in this area, particularly plant-animal interactions, will be of great
importance in understanding the future of biological systems in a high
CO2 world.
ACKNOWLEDGEMENTS
I thank J. Coleman, E. Fajer, S. Morse, K. Norweg, A. Quinn, and D.
Tremmel for much help in preparing this review. I also thank the U.S.
Department of Energy for their support.
RESPONSE TO RISING CO2 191
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The response of plants to elevated CO2.
II. Competitive interactions among annual
plants under varying light and nutrients.
Oecologia 62:412-17

Още информация, доста по-синтезирана:

An increase in atmospheric CO2 concentration upto 0.05% from the usual 0.03% can result in increased CO2 fixation rates but beyond this levels can be damaging for extended periods of time. In addition C3 and C4 plants respond differently to CO2 concentrations. C4 plants reach saturation at 360 μlL-1 while C3 plants do so at 450 μlL-1. So C3 plants respond better to increased CO2 concentrations in form increased productivity and this is used in the carbon dioxide enriched greenhouses with plants like tomatoes and bell peppers.

Повечето растения са тип C3.

http://www.asi.org/adb/04/03/05/co2-plant-growth.html With respect to CO2 utilization, plants are divided into two types: C3 plants and C4 plants. These names essentially distinguish two types of photosyntensis. C3 photosynthesis (so called because the photosynthetic process yields 3-carbon derivatives) has a problem in that sometimes O2 fills the role that CO2 is supposed to fill. When it does, much of the energy that goes into photosynthesis is wasted. C4 plants, on the other hand, starts with a gate, of sorts, that keeps much of the O2 out, so this waste happens less often. Most plants, including plants used in agriculture, are C3 plants. This includes lemon trees (virtually all trees, in fact), sugar beets, and potatoes. Corn and surgarcane are C4 plants. Each type of plant reacts to a change in CO2 concentrations differently. C4 plants already use CO2 efficiently. An increase in the concentration does not help them much. C3 plants, on the other hand, benefit greatly from increases in CO2 because less of the inefficient O2 photosynthesis occurs. Plants in a high CO2 environment increase their plant mass by 20 to 25%. Yields of some crops can be increased by up to 33%. This is the effect of doubling CO2 concentrations over Earth normal. Still higher concentrations can be expected to yield still better results. Note, however, that the effects vary even among different types of C3 plants. Some are better able to take advantage of higher CO2 concentrations than others, and a few actually suffer if CO2 concentrations are raised. But, there's a catch. These benefits occur only if the nutrient levels and the amount of water available also increase. CO2 alone does very little good. Consequently, to take advantage of a higher CO2 concentration, we must supply more water and bring in more nutrients (such as nitrogen). In fact, there is more than one catch. As a plant's production of starch from CO2 increases, it seems to reach some sort of saturation point. It reaches a point where it can no longer take advantage of the greater abundance of CO2. Scientists suspect that this is because there is a bottleneck in the plant's metabolic system. It can manufacture more starch, but it can't get it to where it is needed - or it can't use what it is getting. At this point, you might as well bring the CO2 concentration back down to normal levels for all the good you're doing. Or, if this point is close to the plant's maturation point, you can harvest it and plant the next crop. [Note: high conentrations of CO2 allows the plant to use water more efficiently. This is because the passageways that allow CO2 into the plant also let H2O out. Under higher CO2 concentrations, these passageways can be kept more tightly constricted, allowing less H2O to escape. But there is a tradeoff here between CO2 fertilization and efficient use of water. To the degree you have one, you must give up the other.] Reference: Fakhri A. Bazzaz and Eric D. Fajer, "Plant Life in a CO2-Rich World," SCIENTIFIC AMERICAN, January, 1992, pp 68-74.

Източник: https://www.physicsforums.com/threads/will-more-co2-increase-plant-growth.511645/

Още доказателства, че климатолозите прикриват климатичния оптимум (вж. първото мнение в темата), истинските данни и ни дават изопачени изводи.
Оказва се, че ледниците и вечният сняг дори в Пирин се запазват и дори се увеличават.
Площта на полярния лед се възстановява след отбелязания минимум през 2012-та.
Слънчевата активност все така намалява.
Линк към статия в Офнюз: http://nauka.offnews.bg/news/Skeptik_3/Mikrolednitcite-v-Pirin-rastat-vpechatleniia-na-Boian-Rashev_115665.html на Боян Рашев.

Всичко това хич не се връзва с предсказанията на климатичната наука отпреди 20 години. Да не говорим за драматичните прогнози, с които медиите ежедневно ни заливаха на фона на жегите в Западна Европа това лято – например за окончателното изчезване на летния арктически лед или „ООН: 2018 ще е най-горещата година в историята„.


Арктическият лед пак си остана с минимум, който е около 1.3 милиона кв. км. по-голям от абсолютния рекорд от 2012 г. А месечната амплитуда на средната глобална температура на долната атмосфера, измерена от сателитите, буквално лети надолу след рекорда от февруари, 2016 г. и годината ще остане много далеч от „най-горещата в историята“. Отклонението от средната температура за септември в периода 1981-2010 г. вече е само +0.14°С, което значи, че току-що преживяхме най-студения септември на планетата в последните 10 години.



Спадът на температурата не е за учудване на фона на новините от НАСА, че през 2018 г. Слънцето навлиза в най-студената си фаза, откакто мерим активността му. Тъй като много отдавна не се е случвало, има неприятно висока вероятност това да се окаже нов дълбок соларен минимум, известен още като Минимум на Маундер,

Айде и океаните няма да се покачват...
http://nauka.offnews.bg/news/Novini_1/Uchenite-namaliha-10-pati-prognozata-za-povishavane-na-morskoto-nivo-z_121741.html
Намалиха 10 пъти прогнозата за покачването на нивото на моретата до края на века.

Припомням, че същите типове не могат да обяснят защо Антарктида трупа повече лед отколкото губи...

Сега пък се оказа, че световният океан може да погълне много повече въглероден двуокис отколкото до сега се е знаело.
Толкова много колкото биха отделили 2.2 милиарда автомобила изминаващи по 15.000 км годишно. Това са всички автомобили и още малко кораби, влакове и самолети. Но всъщност едва 10% от отделяните от човечеството парникови  газове.

Линк: https://nauka.offnews.bg/news/a_1/a_156135.html?preview=ok

Ако превишаваме капацитета на планетата за усвояване на СО2 само 2-3 пъти и транспорта е само 10-20%, то остават енергетиката и отоплението, като второто може напълно да се поеме от първото.

Просто трябва с едно да изхвърлим въглищните, газовите и петролните електроцентрали. Ако петролни са останали.