Plant productivity is expected to be strongly affected by climate change within the next 50 years. Lobell et al. (2011) showed that climate trends since 1980 were large enough in many countries to offset a significant proportion of the potential increases in average crop yields due to technological advances, CO2 fertilization and other factors. By 2100, potentially, atmospheric [CO2] will rise to 1000 mmol mol21, temperature will rise by 2–4 8C or more, precipitation will become more variable, and episodes of extreme weather will become more frequent, intense and last longer. It is important to assess whether there will be sufficient food and energy production under future climate conditions. Such an assessment can also assist in developing adaptation strategies that improve the resilience of crop systems to stresses induced by climate change. Hundreds of studies conducted over the last 30 years have confirmed that plant biomass and yield tend to increase significantly as CO2 concentrations increase above current levels. Such results are found to be robust across a variety of experimental settings, such as controlled environment closed chambers, greenhouses, open and closed field top chambers, and free-air carbon dioxide enrichment experiments. Elevated CO2 concentrations stimulate photosynthesis, leading to increased plant productivity and modified water and nutrient cycles. Experiments under optimal conditions show that doubling the atmospheric CO2 concentration increases leaf photosynthesis by 30%–50% in C3 plant species and 10%–25% in C4 species, despite some down-regulation of leaf photosynthesis by elevated atmospheric CO2 concentrations. Crop yield increase is lower than the photosynthetic response. On average across several species and under unstressed conditions, compared with current atmospheric CO2 concentrations of ≈380 ppm, crop yields increase at 550 ppm CO2 in the range of 10–20% for C3 crops and 0–10% for C4 crops (17–19). Increases in above-ground biomass at 550 ppm CO2 for trees are in the range 0–30%, with the higher values observed in young trees and little to no response observed in the few experiments conducted to date in mature natural forests. Observed increases of above-ground production in C3 pasture grasses and legumes are ≈+10 and +20%, respectively. Reference: http://aob.oxfordjournals.org/content/112/3/465.full.pdf+html
The potential for rapid warming to induce a selective sweep in natural populations is likely to be greatest in, although not restricted to, isolated populations, where new variation is unlikely to be supplied by gene flow from neighbouring populations. However, if throughout the species range, climatic warming causes the displacement of a population’s climatic optimum to occur at a rate that exceeds the maximum rate of gene flow between populations the effect will be felt range-wide. This would involve not only a potential range-wide reduction in population fitness, growth and survival but also a reduction in genetic variation both determining and linked to the species climate response. The action of rapid climate change in decoupling locally adapted populations from their typical climate may significantly increase population extinction risk throughout the species range. Using a species-based climate envelope approach for predicting extinction risk may therefore lead to underestimation, as populations throughout the species range will be left outside their typical climate, not just those populations at the range margins. Maintenance of genetic diversity within populations is a key conservation aim, as it will enhance their ability to adapt to future environmental changes. Different genotypes may show different responses to competitive interactions with other genotypes and variation in their susceptibility to attack by pests and Diseases. Reduction of genetic diversity within populations may significantly reduce the ability of the population to resist and recover from perturbations such as pest and disease outbreaks or extreme weather events and may increase their risk of extinction. Studies reported by Burdon & Thrall (2001) show that populations with reduced genetic diversity may be more susceptible to pest and disease outbreaks because of lower occurrence of resistant individuals within the population. Although the consequences of reduced genetic diversity will vary between species and populations, decreased climate-related diversity is likely to reduce a population’s ability to withstand and recover from future climatic perturbations. Species-specific reductions in fitness and diversity will change community dynamics by altering species competitive abilities. This will contribute to the expected changes in both the occurrence and relative abundance of individual species in plant communities.
In species with widespread distributions and well-connected populations, a reduction of genetic diversity within populations is likely to contribute to population extinctions, but is less likely to threaten the existence of the species. The consequences for rare species and those occurring in isolated habitats (e.g. high alpine species) are likely to be more severe, because their populations are likely to be less numerous and may be less well connected or occur over narrow geographical regions. For many such species, even where efforts are made to manage population sizes, a reduction in fitness may result from the effects of changing climate alone. The combination of reduced fitness interacting with a potential reduction in diversity may be catastrophic and lead to local or widespread population extinctions.