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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 doubling of the current
[CO2] by sometime in this century. These increases in
CO2 as well as other "greenhouse gasses" are expected
to raise world temperatures by 0.03°C per year in the 21st century.
Global warming and increased atmospheric [CO2] are
already having a major impact on plant distributions. Plants, in
general, benefit from slightly warmer temperatures and higher
[CO2], but not all plants will benefit equally from these
conditions, 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 atmospheric [CO2] across the
Triassic-Jurassic boundary and an associated 3°C to 4°C
"greenhouse" warming (McElwain et al., 1999
). These environmental
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 megaflora. Are we destined
to witness a floral mass extinction of similar proportions 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
ecological upheaval, has already begun.
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Effects on Carbon Metabolism |
The climate changes that we are currently undergoing include
both increases in temperature and increases in atmospheric
[CO2]. It is well known that C3 and
C4 plants respond quite differently to temperature and
atmospheric [CO2]. Rising temperatures will increase the
ratio of photorespiratory loss of carbon to photosynthetic gain,
whereas rising [CO2] will have an opposing effect. All
else being equal, C4 plants tend to be favored over
C3 plants in warm, humid climates; conversely,
C3 plants tend to be favored over C4 plants in
cool climates. Empirical observations supported by a photosynthesis
model predict the existence of a climatological crossover temperature
above which C4 species have a carbon gain advantage and
below which C3 species are favored. Model calculations and
analysis of current plant distribution suggest that this atmospheric [CO2]-dependent crossover temperature is approximated by
a mean temperature of 22°C for the warmest month at the current
[CO2] (Collatz et al., 1998).
In addition to favorable temperatures, C4 plants require
sufficient precipitation during the warm growing season. C4
plants that are predominantly short stature grasses can be
competitively excluded by trees (nearly all C3
plants)
regardless of the photosynthetic superiority of the
C4 pathwayin regions otherwise favorable for C4. Collatz et al. (1998) examined changes in the global
abundance of C4 grasses in the past using plausible
estimates for the climates and atmospheric [CO2]. They
predict that global warming during this century will favor
C3 vegetation because the increase in C3
photosynthetic efficiency that occurs under higher atmospheric [CO2] conditions will outweigh the reduction of
photosynthesis that is attributable to higher temperatures.
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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 initiation of flowering is typically mediated by changes in daylength and, as such, is independent of
temperature, the time required for flowers to develop to maturity, like
most growth processes, is strongly dependent upon temperature. Many
recent reports indicate 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 species 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 temperature. The average temperature during the month or so preceding flower opening appears to
be largely responsible for causing the advances of first-flowering times.
A more recent report by Fitter and Fitter (2002) has revealed
just how rapidly 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
compared with the previous four decades. Sixteen percent of species
flowered significantly earlier in the 1990s than previously, with an
average advancement of 15 d in a decade. The authors also found
that different types of plants responded to varying degrees. For
example, annuals were more likely to flower earlier than perennials, and insect-pollinated species more than wind-pollinated. Accelerated phenologies may alter patterns of resource allocation, may affect interactions with pollinators, and could alter the size, species richness, and intraspecific genetic diversity of the soil seed bank.
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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 example, Iverson and Prasad (1998) developed
models to evaluate potential shifts for 80 individual tree species in
the eastern United States. They concluded that roughly 30 species could
expand their range while an additional 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 controlled by
migration routes through fragmented landscapes.
There is already ample empirical evidence that many plant species
are beginning 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 (Picea 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 saplings 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 replacement 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 biodiversity 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
CO2-rich climates predicted by global climate
models. The predicted rates of evolutionary response were much slower
than the predicted rate of climate change. The local extinction of such
species seems a likely outcome.
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Changes in Food Web Structures |
Of course, plants do not exist in isolation but interact with
other organisms (e.g. pollinators, competitors, mycorrhizae, pathogens,
and herbivores). These organisms, too, will be affected by global
warming in their own ways. As such, it is virtually impossible to
predict with any certainty how any given species will succeed in the
face of global warming). One approach to this question is the use of
artificial microcosms (Petchey et al., 1999). Microcosms permit
experimental control over species composition and rates of
environmental change. Petchey et al. (1999) concluded based on such
microcosm experiments that extinction risk in warming environments
depends on trophic position. Warmed communities disproportionately lost
top predators and herbivores and became increasingly dominated by
autotrophs and bacteriovores. Changes in the relative distribution of
organisms among trophically defined functional groups led to differences in ecosystem function 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 environmental variation
because tolerant species are more likely to be found. Studies of single
trophic levels clearly show that warming can affect the distribution
and abundance of species, but complex responses generated in entire food webs greatly complicate predictions.
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GLOBAL WARMING
EFFECTS ON...
Effects on Carbon Metabolism