Climate change is any significant long term change in the expected pattern. Any change in climate overtime, whether due to natural variability or as a result of human activity. 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 current plant diversity patterns; further impacts are expected in the future. It is predicted that climate change will remain one of the major drivers of biodiversity patterns in the future. Human actions are currently triggering the sixth major mass extinction our Earth has seen, changing the distribution and abundance of many plants.
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 CO
2 (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.
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 CO
2 concentrations, increasing global temperatures, altered precipitation patterns, and changes in the pattern of ‘extreme’ weather events such as cyclones, fires or storms. Highly variable species distribution has resulted from different models with variable bioclimatic changes.
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.
CO2 concentrations have been steadily rising for more than two centuries. Increases in atmospheric CO2 concentration 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. Studies show that doubled concentrations of CO2 will show an increase in photosynthesis in C3 plants but not in C4 plants. However, it is also shown that C4 plants are able to persist in drought better than the C3 plants.
Increases in temperature raise the rate of many physiological processes such as photosynthesis in plants, to an upper limit, depending on the type of plant. These increases in photosynthesis and other physiological processes are driven by increased rates of chemical reactions and roughly a doubling of enzymatic product conversion rates for every 10 °C increase in temperature. Extreme temperatures can be harmful when beyond the physiological limits of a plant which will eventually lead to higher desiccation rates.
One common hypothesis among scientists is that the warmer an area is, the higher the plant diversity. This hypothesis can be observed in nature, where higher plant biodiversity is often located at certain latitudes (which often correlates with a specific climate/temperature).
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. A change in water availability would show a direct correlation to the growth rates and persistences of plant species in that region.
With less consistent, more intense rainfall events the water availability will have a direct impact on the soil moisture in an area. A decrease in soil moisture will have negative impacts on plant’s growth, changing the dynamics of the ecosystem as a whole. Plants rely not only on the total rainfall during the growing season, but also the intensity and magnitude of each rainfall event.
Environmental variables act not in isolation, but in combination with other pressures such as habitat degradation, habitat loss, and the introduction of exotic species that can potentially be invasive. It is suggested that these other drivers of biodiversity change will act in synergy with climate change to increase the pressure on species to survive. As these changes add up, our overall ecosystems are predicted to look much different than they do today.
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 evidence that plant species are shifting their ranges in altitude and latitude as a response to changing regional climates. Yet it is difficult to predict how species ranges will change in response to climate and separate these changes from all the other man-made environmental changes such as eutrophication, acid rain and habitat destruction.
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.
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.
Species richness and species evenness play a key role in how quickly and productively an ecosystem can adapt to change . By increasing the possibly of a population bottleneck through more extreme weather events, genetic diversity in the population would drastically decrease. Since genetic diversity is a main contributor of how an ecosystem can evolve, the ecosystem would be much more susceptible to getting wiped out since each individual would be similar to the next. An absence of genetic mutations and decrease in species richness greatly enhances the possibility of extinction.
Altering the environment puts stress on a plant to increase its phenotypic plasticity, causing species to change faster than predicted. These plastic responses will help the plants respond to a fast changing environment. Understanding how native species change in response to the environment will help gather conclusions of how mutualistic relationships will react.
All species are likely to be directly impacted by the changes in environmental conditions discussed above, and 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 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 or 'be invaded' for example, introducing a new competitive relationship or altering other processes such as carbon sequestration.
In Europe, the temperature and precipitation effects due to climate change can indirectly affect certain populations of people. The rise of temperatures and lack of precipitation results in different river floodplains, which reduce the populations of people sensitive to flood risk.
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 plant's 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.
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. Bird migration patterns are already showing a change in flying south sooner, and returning sooner, this could overtime affect the over all ecosystem. If birds are leaving sooner this would decrease the pollination rates of some plants over time. The observation of bird migrations is more evidence of the climate changing, which would result in plants flowering at different times.
With certain species of plants having a disadvantage with a warmer climate, their insect herbivores may also be taking a hit. Temperature will directly affect diversity, persistence and survival in both the plants and their insect herbivores. As these insect herbivores decrease, so will the higher levels of species that eat those insects. This cascading event would be detrimental to our earth and how we view nature today.
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 modeling individual species, groups of species such as ‘functional types’, communities, ecosystems or biomes. They can also involve modeling species observed environmental niches, or observed physiological processes.
Although useful, modeling has many limitations. Firstly, there is uncertainty about the future levels of greenhouse gas emissions driving climate change  and considerable uncertainty in modeling 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.
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