Effect of climate change on plant biodiversity
Environmental conditions play a key role in defining the function and distribution of plants, in combination with other factors. Changes in long term environmental conditions that can be collectively coined climate change are known to have had enormous impacts on plant diversity patterns in the past and are seen as having significant current impacts. It is predicted that climate change will remain one of the major drivers of biodiversity patterns in the future.
Palaeo context 
The Earth has experienced a constantly changing climate in the time since plants first evolved. In comparison to the present day, this history has seen Earth as cooler, warmer, drier and wetter, and CO2 (carbon dioxide) concentrations have been both higher and lower. These changes have been reflected by constantly shifting vegetation, for example forest communities dominating most areas in interglacial periods, and herbaceous communities dominating during glacial periods. It has been shown that past climatic change has been a major driver of the processes of speciation and extinction. The best known example of this is the Carboniferous Rainforest Collapse which occurred 350 million years ago. This event decimated amphibian populations and spurred on the evolution of reptiles.
Modern Context 
There is significant current interest and research focus on the phenomenon of recent anthropogenic climate changes, or global warming. Focus is on identifying the current impacts of climate change on biodiversity, and predicting these effects into the future.
Changing climatic variables relevant to the function and distribution of plants include increasing CO2 concentrations, increasing global temperatures, altered precipitation patterns, and changes in the pattern of ‘extreme’ weather events such as cyclones, fires or storms.
Because individual plants and therefore species can only function physiologically, and successfully complete their life cycles under specific environmental conditions (ideally within a subset of these), changes to climate are likely to have significant impacts on plants from the level of the individual right through to the level of the ecosystem or biome.
Effects of CO2 
Increases in atmospheric CO2 concentration for affect how plants photosynthesise, resulting in increases in plant water use efficiency, enhanced photosynthetic capacity and increased growth. Increased CO2 has been implicated in ‘vegetation thickening’ which affects plant community structure and function. Depending on environment, there are differential responses to elevated atmospheric CO2 between major ‘functional types’ of plant, such as C3 and C4 plants, or more or less woody species; which has the potential among other things to alter competition between these groups. Increased CO2 can also lead to increased Carbon : Nitrogen ratios in the leaves of plants or in other aspects of leaf chemistry, possibly changing herbivore nutrition.
Effects of temperature 
Increases in temperature raise the rate of many physiological processes such as photosynthesis in plants, to an upper limit. Extreme temperatures can be harmful when beyond the physiological limits of a plant.
Effects of water 
As water supply is critical for plant growth, it plays a key role in determining the distribution of plants. Changes in precipitation are predicted to be less consistent than for temperature and more variable between regions, with predictions for some areas to become much wetter, and some much drier.
General effects 
Environmental variables will not act in isolation, but also in combination with one other, and with other pressures such as habitat degradation and loss or the introduction of exotic species. It is suggested that that these other drivers of biodiversity change will act in synergy with climate change to increase the pressure on species to survive.
Direct impacts of climate change 
Changes in distributions 
If climatic factors such as temperature and precipitation change in a region beyond the tolerance of a species phenotypic plasticity, then distribution changes of the species may be inevitable. There is already strong evidence that plant species are shifting their ranges in altitude and latitude as a response to changing regional climates.
When compared to the reported past migration rates of plant species, the rapid pace of current change has the potential to not only alter species distributions, but also render many species as unable to follow the climate to which they are adapted. The environmental conditions required by some species, such as those in alpine regions may disappear altogether. The result of these changes is likely to be a rapid increase in extinction risk. Adaptation to new conditions may also be of great importance in the response of plants.
Predicting the extinction risk of plant species is not easy however. Estimations from particular periods of rapid climatic change in the past have shown relatively little species extinction in some regions, for example. Knowledge of how species may adapt or persist in the face of rapid change is still relatively limited.
Changes in the suitability of a habitat for a species drive distributional changes by not only changing the area that a species can physiologically tolerate, but how effectively it can compete with other plants within this area. Changes in community composition are therefore also an expected product of climate change.
Changes in life-cycles (phenology) 
The timing of phenological events such as flowering are often related to environmental variables such as temperature. Changing environments are therefore expected to lead to changes in life cycle events, and these have been recorded for many species of plants. These changes have the potential to lead to the asynchrony between species, or to change competition between plants. Flowering times in British plants for example have changed, leading to annual plants flowering earlier than perennials, and insect pollinated plants flowering earlier than wind pollinated plants; with potential ecological consequences. A recently published study has used data recorded by the writer and naturalist Henry David Thoreau to confirm effects of climate change on the phenology of some species in the area of Concord, Massachusetts.
Indirect impacts of climate change 
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All species are likely to be not only directly impacted by the changes in environmental conditions discussed above, but also indirectly through their interactions with other species. While direct impacts may be easier to predict and conceptualise, it is likely that indirect impacts are be equally important in determining the response of plants to climate change.
A species whose distribution changes as a direct result of climate change may ‘invade’ the range of another species for example, introducing a new competitive relationship.
The range of a symbiotic fungi associated with plant roots may directly change as a result of altered climate, resulting in a change in the plants distribution.
A new grass may spread into a region, altering the fire regime and greatly changing the species composition.
A pathogen or parasite may change its interactions with a plant, such as a pathogenic fungus becoming more common in an area where rainfall increases.
Increased temperatures may allow herbivores to expand further into alpine regions, significant impacting the composition of alpine herbfields.
There are innumerable examples of how climate change could indirectly affect plant species, most of which will be extremely difficult to predict.
Higher level changes 
Species respond in very different ways to climate change. Variation in the distribution, phenology and abundance of species will lead to inevitable changes in the relative abundance of species and their interactions. These changes will flow on to affect the structure and function of ecosystems.
Challenges of modelling future impacts 
Accurate predictions of the future impacts of climate change on plant diversity are critical to the development of conservation strategies. These predictions have come largely from bioinformatic strategies, involving modelling individual species, groups of species such as ‘functional types’, communities, ecosystems or biomes. They can also involve modelling species observed environmental niches, or observed physiological processes.
Although useful, modelling has many limitations. Firstly, there is uncertainty about the future levels of greenhouse gas emissions driving climate change  and considerable uncertainty in modelling how this will affect other aspects of climate such as local rainfall or temperatures. For most species the importance of specific climatic variables in defining distribution (e.g. minimum rainfall or maximum temperature) is unknown. It is also difficult to know which aspects of a particular climatic variable are most biologically relevant, such as average vs. maximum or minimum temperatures. Ecological processes such as interactions between species and dispersal rates and distances are also inherently complex, further complicating predictions.
Improvement of models is an active area of research, with new models attempting to take factors such as life-history traits of species or processes such as migration into account when predicting distribution changes; though possible trade-offs between regional accuracy and generality are recognised.
Climate change is also predicted to interact with other drivers of biodiversity change such as habitat destruction and fragmentation, or the introduction of foreign species. These threats may possibly act in synergy to increase extinction risk from that seen in periods of rapid climate change in the past.
See also 
- Global warming
- Extinction risk from climate change
- Effects of global warming
- Effects of climate change on wine production
- Effects of climate change on marine mammals
- Physical impacts of climate change
- Systems ecology
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Further reading 
- Thomas Lovejoy; Lee Hannah (2006). Climate Change and Biodiversity. TERI Press. ISBN 81-7993-084-X. Unknown parameter
- Tim Flannery (2006). The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. Grove/Atlantic Press. ISBN 0-8021-4292-3.
- (2008) Government report on the effects of climate change on agriculture, land resources, water resources, and biodiversity in the United States.
- (2003) Summary report from an international conference on Global Climate Change and Biodiversity, Joint Nature Conservation Committee
- (2008) Discussion on the future of modeling climate change impacts on plant species distributions. on wilfried thuiller's website
- (2005) The Millennium Ecosystem Assessment, including discussion of the effects of climate change on biodiversity
- Global Change Biology - a scientific journal with articles relating to the interaction between global changes such as climate, and biological systems
- (2011) After the birds vanish, the plants are next to go - New Scientist
- Loarie, S. R.; Duffy, P. B.; Hamilton, H.; Asner, G. P.; Field, C. B.; Ackerly, D. D. (2009). "The velocity of climate change". Nature 462 (7276): 1052–1055. doi:10.1038/nature08649. PMID 20033047.