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The Hot and the Classic
GLOBAL WARMING—
EFFECTS ON PLANTS
The burning of fossil fuels, the
large-scale clearing of forests, and
other human activities are altering
global climates at an alarming rate.
The continued consumption of fossil
fuels is expected to result in a dou-
bling of the current [CO
2
] by some-
time in this century. These increases in
CO
2
as well as other “greenhouse gas-
ses” are expected to raise world tem-
peratures by 0.03°C per year in the
21st century. Global warming and in-
creased atmospheric [CO
2
] are already
having a major impact on plant distri-
butions. Plants, in general, benefit
from slightly warmer temperatures
and higher [CO
2
], but not all plants
will benefit equally from these condi-
tions, and some may even be harmed:
There will be winners and losers in the
warmer world of the near future. If the
past is any indicator, the losers may
greatly outnumber the winners. Pala-
eobotanical evidence indicates that
there was a 4-fold increase in atmo-
spheric [CO
2
] across the Triassic-
Jurassic boundary and an associated
3°C to 4°C “greenhouse” warming
(McElwain et al., 1999). These environ-
mental conditions were calculated to
have raised leaf temperatures above a
highly conserved lethal limit, perhaps
contributing to the ⬎95% species-level
turnover of Triassic-Jurassic mega-
flora. Are we destined to witness a
floral mass extinction of similar pro-
portions in the coming few centuries?
The data and models discussed in this
month’s column suggest that the mass
extinction, or at least the mass ecolog-
ical upheaval, has already begun.
Effects on Carbon
Metabolism
The climate changes that we are cur-
rently undergoing include both in-
creases in temperature and increases
in atmospheric [CO
2
]. It is well known
that C
3
and C
4
plants respond quite
differently to temperature and atmo-
spheric [CO
2
]. Rising temperatures
will increase the ratio of photorespira-
tory loss of carbon to photosynthetic
gain, whereas rising [CO
2
] will have
an opposing effect. All else being
equal, C
4
plants tend to be favored
over C
3
plants in warm, humid cli-
mates; conversely, C
3
plants tend to be
favored over C
4
plants in cool cli-
mates. Empirical observations sup-
ported by a photosynthesis model pre-
dict the existence of a climatological
crossover temperature above which
C
4
species have a carbon gain advan-
tage and below which C
3
species are
favored. Model calculations and anal-
ysis of current plant distribution sug-
gest that this atmospheric [CO
2
]-
dependent crossover temperature is
approximated by a mean temperature
of 22°C for the warmest month at the
current [CO
2
] (Collatz et al., 1998).
In addition to favorable tempera-
tures, C
4
plants require sufficient pre-
cipitation during the warm growing
season. C
4
plants that are predomi-
nantly short stature grasses can be
competitively excluded by trees (near-
ly all C
3
plants)—regardless of the
photosynthetic superiority of the C
4
pathway—in regions otherwise favor-
able for C
4
. Collatz et al. (1998) exam-
ined changes in the global abundance
of C
4
grasses in the past using plausi-
ble estimates for the climates and at-
mospheric [CO
2
]. They predict that
global warming during this century
will favor C
3
vegetation because the
increase in C
3
photosynthetic effi-
ciency that occurs under higher atmo-
spheric [CO
2
] conditions will outweigh
the reduction of photosynthesis that is
attributable to higher temperatures.
Effects on Phenology
The growth and reproduction of
most plants is tightly regulated by the
time of season. The phenology or time
of flowering of a plant is one such
seasonal event that is critical for its
sexual reproduction. Although the ini-
tiation of flowering is typically medi-
ated by changes in daylength and, as
such, is independent of temperature,
the time required for flowers to de-
velop to maturity, like most growth
processes, is strongly dependent upon
temperature. Many recent reports in-
dicate the time of first flowering has
been affected by the warming trend of
the last half century. For example,
Abu-Asab et al. (2001) found that the
trend of average first-flowering times
per year for a group of 100 plant spe-
cies growing near Washington, DC,
have shown a significant advance of
2.4 d over the past 30 years. When 11
species that exhibit later first-
flowering times were excluded from
the data set, the remaining 89 show a
significant advance of 4.5 d on average
(ranging from ⫺3.2 to ⫺46 d). The
advances of first flowering in these 89
species were directly correlated with
local increases in minimum tempera-
ture. The average temperature during
the month or so preceding flower
opening appears to be largely respon-
sible for causing the advances of first-
flowering times.
A more recent report by Fitter and
Fitter (2002) has revealed just how rap-
idly these changes in flowering time
are occurring. These researchers found
that the average first flowering date of
385 British plant species has advanced
by 4.5 d during the past decade com-
pared with the previous four decades.
Sixteen percent of species flowered sig-
nificantly earlier in the 1990s than pre-
viously, with an average advancement
of 15 d in a decade. The authors also
found that different types of plants re-
sponded to varying degrees. For exam-
ple, annuals were more likely to flower
earlier than perennials, and insect-
pollinated species more than wind-
pollinated. Accelerated phenologies
may alter patterns of resource alloca-
tion, may affect interactions with polli-
nators, and could alter the size, species
richness, and intraspecific genetic di-
versity of the soil seed bank.
Massive Ecological
Upheavals
The distribution of many species
tends to be limited to a narrow range
of environmental conditions. One of
the consequences of the increased
growth seasons and earlier flowering
times afforded by global warming will
be that the natural ranges of many
plant species will shift polewards. For
www.plantphysiol.org/cgi/doi/
10.1104/pp.900042.
Plant Physiology, August 2002, Vol. 129, pp. 1421–1422, www.plantphysiol.org © 2002 American Society of Plant Biologists 1421
example, Iverson and Prasad (1998)
developed models to evaluate poten-
tial shifts for 80 individual tree species
in the eastern United States. They con-
cluded that roughly 30 species could
expand their range while an addi-
tional 30 species could decrease by at
least 10%, following equilibrium after
a changed climate. Depending on the
global change scenario used, four to
nine species would potentially move
out of the United States to the north.
Nearly half of the species assessed (36
out of 80) showed the potential for the
ecological optima to shift at least 100
km to the north, including seven that
could move ⬎250 km. Actual species
redistributions, however, may be con-
trolled by migration routes through
fragmented landscapes.
There is already ample empirical ev-
idence that many plant species are be-
ginning to invade formerly colder
climes as the world’s temperature has
begun to rise. For example, researchers
in Alaska combed through archives of
aerial photos, comparing those of the
same locations taken 50 years ago. Of
the 66 aerial photos included in the
study, growth increases were reported
in over half (Sturm et al., 2001). In the
Swedish Scandes since the early 1950s,
the range-margins of mountain birch
(Betula pubescens), Norway spruce (Pi-
cea abies), Scots pine (Pinus sylvestris),
rowan (Sorbus aucuparia), and willows
(Salix spp.) have advanced by 120 to
375 m to colonize moderate snow-bed
communities (Kullman, 2002). Ring
counting of a subsample of these sap-
lings revealed that, with one exception,
they were aged between 7 and 12
years, i.e. they germinated after 1987.
Another example is afforded by the re-
placement of macrolichens by invading
vascular plants in the climatically
milder parts of the Arctic (Cornelissen
et al., 2001). These macrolichens are
critical for the functioning and biodi-
versity of cold northern ecosystems
and their reindeer-based cultures.
In some cases, however, there may
not be enough intraspecific variation,
phenotypic plasticity, or continuity in
landscape to help certain plant species
to cope with the sudden climate
changes. For example, Etterson and
Shaw (2001) characterized the genetic
architecture of three populations of a
native North American prairie plant
in field conditions that simulate the
warmer and more CO
2
-rich climates
predicted by global climate models.
The predicted rates of evolutionary re-
sponse were much slower than the
predicted rate of climate change. The
local extinction of such species seems
a likely outcome.
Changes in Food Web
Structures
Of course, plants do not exist in iso-
lation but interact with other organ-
isms (e.g. pollinators, competitors,
mycorrhizae, pathogens, and herbi-
vores). These organisms, too, will be
affected by global warming in their
own ways. As such, it is virtually im-
possible to predict with any certainty
how any given species will succeed in
the face of global warming). One ap-
proach to this question is the use of
artificial microcosms (Petchey et al.,
1999). Microcosms permit experimen-
tal control over species composition
and rates of environmental change.
Petchey et al. (1999) concluded based
on such microcosm experiments that
extinction risk in warming environ-
ments depends on trophic position.
Warmed communities disproportion-
ately lost top predators and herbi-
vores and became increasingly domi-
nated by autotrophs and
bacteriovores. Changes in the relative
distribution of organisms among
trophically defined functional groups
led to differences in ecosystem func-
tion beyond those expected from
temperature-dependent physiological
rates. Diverse communities retain
more species than depauperate ones,
which suggests that high biodiversity
buffers against the effects of environ-
mental variation because tolerant spe-
cies are more likely to be found. Stud-
ies of single trophic levels clearly
show that warming can affect the dis-
tribution and abundance of species,
but complex responses generated in
entire food webs greatly complicate
predictions.
LITERATURE CITED
Abu-Asab MS, Peterson PM, Shetler
SG, Orli SS (2001) Earlier plant
flowering in spring as a response to
global warming in the Washington,
DC, area. Biodiversity Conserv 10:
597–612
Collatz GJ, Berry JA, Clark JS (1998)
Effects of climate and atmospheric
CO
2
partial pressure on the global
distribution of C
4
grasses: present,
past, and future. Oecologia 114:
441–454
Cornelissen JHC, Callaghan TV, Ala-
talo JM, Michelsen A, Graglia E,
Hartley AE, Hik DS, Hobbie SE,
Press MC, Robinson CH et al.
(2001) Global change and arctic eco-
systems: Is lichen decline a function
of increases in vascular plant bio-
mass? J Ecol 89: 984–994
Etterson JR, Shaw RG (2001) Con-
straint to adaptive evolution in re-
sponse to global warming Science
294: 151–154
Fitter AH, Fitter RSR (2002) Rapid
changes in flowering time in British
plants. Science 296: 1689–1691
Iverson LR, Prasad AM (1998) Pre-
dicting abundance of 80 tree species
following climate change in the
eastern United States. Ecol Monogr
68: 465–485
Kullman L (2002) Rapid recent range-
margin rise of tree and shrub spe-
cies in the Swedish Scandes. J Ecol
90: 68–77
McElwain JC, Beerling DJ, Wood-
ward FI (1999) Fossil plants and
global warming at the Triassic-
Jurassic boundary. Science 285:
1386–1390
Petchey OL, McPhearson PT, Casey
TM, Morin PJ (1999). Environmen-
tal warming alters food-web struc-
ture and ecosystem function. Na-
ture 402: 69–72
Sturm M, Racine C, Tape K (2001)
Increasing shrub abundance in the
Arctic. Nature 411: 546–547
Peter V. Minorsky
Department of Natural Sciences
Mercy College
Dobbs Ferry, NY 10522
1422 Plant Physiol. Vol. 129, 2002
