There is an ongoing decline in plant biodiversity, just like there is ongoing biodiversity loss for many other life forms. Islands have a tremendous quantity of plant biodiversity.^[1] That being said, island ecosystems hosting endemic plant species are subject to an increased risk of extinction.^[2] One of the causes for this decline is climate change.^[3][4][5] Environmental conditions play a key role in defining the function and geographic distributions of plants, in combination with other factors, thereby modifying patterns of biodiversity.^[6]

Extended fire weather seasons may result in more severe burn conditions and shorter burn intervals, which can threaten the biodiversity of native vegetation.^[7] Besides, species habitat changes or migrations under changing weather conditions can cause non-native plants^[8] and pests to impact native vegetation diversity, making the latter less structurally functional and more vulnerable to external damage,^[9] leading to biodiversity loss.

Predicting the effects that climate change will have on plant biodiversity can be achieved using various models. Bioclimatic models are most commonly used.^[10][11]

Approximately one-third of all terrestrial plant species face the threat of extinction, while the conservation status of 94% of these species remains to be assessed.^[12]

Direct impacts

Changing climatic variables relevant to the function and distribution of plants include increasing CO₂ concentrations (see CO2 fertilization effect), 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 temperature

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).^[13] Plant species in montane and snowy ecosystems are at greater risk for habitat loss due to climate change.^[14] The effects of climate change are predicted to be more severe in mountains of northern latitude.^[14]

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.^[15] There is already evidence that plant species are shifting their ranges in altitude and latitude as a response to changing regional climates.^[16][17] 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.^[18][19][20]

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.^[21] 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.^[22] Adaptation to new conditions may also be of great importance in the response of plants.^[23]

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.^[24] Knowledge of how species may adapt or persist in the face of rapid change is still relatively limited.

It is clear now that the loss of some species will be very dangerous for humans because they will stop providing services. Some of them have unique characteristics that cannot be replaced by any other.^[25]

Distributions of species and plant species will narrow following the effects of climate change.^[14] Climate change can affect areas such as wintering and breeding grounds to birds. Migratory birds use wintering and breeding grounds as a place to feed and recharge after migrating for long hours. If these areas are damaged due to climate change, it will eventually affect them as well.^[26]

Lowland forest have gotten smaller during the last glacial period and those small areas became island which are made up of drought resisting plants. In those small refugee areas there are also a lot of shade dependent plants.^[25] As an example, the dynamics of the calcareous grassland were significantly impacted due to the climate factors.^[27]

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

Plants typically reside in locations that are beneficial to their life histories.^[28] The timing of phenological events such as flowering and leaf production, are often related to environmental variables, including temperature, which can be altered by climate change.^[29] Changing environments are, therefore, expected to lead to changes in life cycle events, and these have been recorded for many species of plants, therefore, many plant species are considered to be adequate indicators of climate change.^[16][30] These changes have the potential to lead to the asynchrony between species, or to change competition between plants. Both the insect pollinators and plant populations will eventually become extinct due to the uneven and confusing connection that is caused by the change of climate.^[31] 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.^[32] Other observed effects also include the lengthening in growing seasons of certain agricultural crops such as wheat and maize.^[33] 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.^[34] Another life-cycle change is a warmer winter which can lead to summer rainfall or summer drought.^[27]

Ultimately, climate change can affect the phenology and interactions of many plant species, and depending on its effect, can make it difficult for a plant to be productive.^[35]

Extinction risks

Data from 2018 found that at 1.5 °C (2.7 °F), 2 °C (3.6 °F) and 3.2 °C (5.8 °F) of global warming, over half of climatically determined geographic range would be lost by 8%, 16%, and 44% of plant species. This corresponds to more than 20% likelihood of extinction over the next 10–100 years under the IUCN criteria.^[36][37]

The 2022 IPCC Sixth Assessment Report estimates that while at 2 °C (3.6 °F) of global warming, fewer than 3% of flowering plants would be at a very high risk of extinction, this increases to 10% at 3.2 °C (5.8 °F).^[37]

A 2020 meta-analysis found that while 39% of vascular plant species were likely threatened with extinction, only 4.1% of this figure could be attributed to climate change, with land use change activities predominating. However, the researchers suggested that this may be more representative of the slower pace of research on effects of climate change on plants. For fungi, it estimated that 9.4% are threatened due to climate change, while 62% are threatened by other forms of habitat loss.^[38]

Alpine and mountain plant species are known to be some of the most vulnerable to climate change. In 2010, a study looking at 2,632 species located in and around European mountain ranges found that depending on the climate scenario, 36–55% of alpine species, 31–51% of subalpine species and 19–46% of montane species would lose more than 80% of their suitable habitat by 2070–2100.^[39] In 2012, it was estimated that for the 150 plant species in the European Alps, their range would, on average, decline by 44%-50% by the end of the century - moreover, lags in their shifts would mean that around 40% of their remaining range would soon become unsuitable as well, often leading to an extinction debt.^[40] In 2022, it was found that those earlier studies simulated abrupt, "stepwise" climate shifts, while more realistic gradual warming would see a rebound in alpine plant diversity after mid-century under the "intermediate" and most intense global warming scenarios RCP4.5 and RCP8.5. However, for RCP8.5, that rebound would be deceptive, followed by the same collapse in biodiversity at the end of the century as simulated in the earlier papers.^[41] This is because on average, every degree of warming reduces total species population growth by 7%,^[42] and the rebound was driven by colonization of niches left behind by most vulnerable species like Androsace chamaejasme and Viola calcarata going extinct by mid-century or earlier.^[41]

It's been estimated that by 2050, climate change alone could reduce species richness of trees in the Amazon Rainforest by 31–37%, while deforestation alone could be responsible for 19–36%, and the combined effect might reach 58%. The paper's worst-case scenario for both stressors had only 53% of the original rainforest area surviving as a continuous ecosystem by 2050, with the rest reduced to a severely fragmented block.^[43] Another study estimated that the rainforest would lose 69% of its plant species under the warming of 4.5 °C (8.1 °F).^[44]

Another estimate suggests that two prominent species of seagrasses in the Mediterranean Sea would be substantially affected under the worst-case greenhouse gas emission scenario, with Posidonia oceanica losing 75% of its habitat by 2050 and potentially becoming functionally extinct by 2100, while Cymodocea nodosa would lose ~46% of its habitat and then stabilize due to expansion into previously unsuitable areas.^[45]

Indirect impacts

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.^[46][47] 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.^[48]

The range of a symbiotic fungi associated with plant roots (i.e., mycorrhizae)^[49] may directly change as a result of altered climate, resulting in a change in the plant's distribution.^[50]

Challenges of modeling future impacts

Predicting the effects that climate change will have on plant biodiversity can be achieved using various models, however bioclimatic models are most commonly used.^[10][11]

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. The velocity of climate change can also be involved in modelling future impacts as well.^[51]

Although useful, modeling has many limitations. Firstly, there is uncertainty about the future levels of greenhouse gas emissions driving climate change ^[52] 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.^[53]

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.^[54]

Singh et al. (2023) highlighted the urgent need for comprehensive understanding and management of plant diseases in the face of climate change. The paper emphasized the importance of integrating ecological and evolutionary theories, along with advanced technologies like genomics and machine learning, to predict and mitigate disease outbreaks. The establishment of a dedicated knowledge hub, in collaboration with existing intergovernmental bodies under the One Health framework, was proposed to address these challenges through coordinated research and policy actions. Also, increased investment and commitment from stakeholders worldwide were deemed essential to achieve effective detection, monitoring, and management of plant pathogens.^[55]