Abstract

Climate has a defining role in growth, composition and distribution of plant populations and communities of natural as well as manmade ecosystems of the world. Changing climatic variables such as -increasing carbon dioxide (CO₂) concentration, rising global temperature, altered precipitation patterns and significant increase in ‘extreme’ weather events, such as drought, floods, cyclones, fires or storms variably affects different ecosystems plant populations and communities. The effects of climate change on plant populations and communities are assessed through various approaches - controlled field experiments, modeling and correlation between long term climate data and associated changes etc. The manuscript is an attempt to report the direct or indirect effects of the increase or shift in temperature, alteration in precipitation in different regions on plants population and community ecology attributes such as growth, phenology, composition, distribution range, incidence of pest, productivity etc. Therefore, the manuscript emphasizes that ecological understanding of plant populations and communities responses to climate change from a holistic and pragmatic perspective is critically important to address the issues pertaining to plant biodiversity, global food security, poverty elevation and sustainable development.

1. Introduction

“The fate of humanity in the light of climate change, and of all known species, is inseparable from the fate of plants” (Gran Canaria Group).

There are explicit evidences that the Earth’s climate is warming at an alarming rate. Human-induced climate change is now widely perceived and affecting many plant communities. Climate model projections indicate that global surface temperature will rise by 1.1 to 6.4°C during the 21^st century. 5^th Intergovernmental Panel on Climate Change (IPCC) assessment report (AR5) ¹ states “continued GHG emissions at or above the current rates would cause further warming and induce changes in the global climate system.” Long term environmental changes due to anthropogenic activities - called climate change - are going to exert enormous impact on ecological and economic system ^{2, 3}.

Climate is determining factor for growth, composition and distribution of plant populations and communities. Changing climatic variables which effects plant population and communities based ecosystem are - increasing Carbon dioxide (CO₂) concentration, rising global temperature, altered precipitation patterns, and significant increase in ‘extreme’ weather events, such as drought, floods cyclones, fires or storms. The manuscript attempts to address some of the important issues related to plant population and communities response to climate change.

2. Rising level of Atmospheric CO₂

CO₂ is a trace gas in the atmosphere. Its pre-industrial concentration was 280ppm, since industrialization its concentration has been increasing more rapidly due to accelerated use of fossil fuels. Its concentration has reached 400ppm and continues to rise further. Elevated level of atmospheric CO₂ contributes in global warming due to its capturing potential of reradiates long wave radiations from earth surface.

CO₂ is the main gaseous raw material required by green plants for manufacturing food through the process of photosynthesis according to the following general equation:-

(1)

Some plants species would grow better at higher CO₂ concentration as compared to other and this is called as carbon dioxide fertilization effect. Elevated levels of CO₂ caused enhanced vegetative growth in selected plant species and altered the structure and function of plant community. While rising CO₂ levels can push plant productivity but this may not be realized because of other environmental constraints, such as rising temperature, changes in precipitation pattern, soil moisture, invasion of invasive species etc.

3. Rising Temperature

Temperature is a crucial factor to determine the pace physiological activity of all living organisms including plants. It has been reported that global surface temperature has increased ≈ 0.2 °C per decade in the past 30 years and global warming of more than ≈ 1°C, relative to 2000, will constitute “dangerous” climate change impacts (Figure 1) ⁴. Each plant species has its own specific temperature tolerance range beyond which they are unable to survive. Many physiological and reproductive processes such as photosynthesis, transpiration, stomatal conductance, phenology, anthesis, pollen viability and pollination are highly temperature sensitive. Various studies have shown that every 1°C rise in temperature can reduce crop production by 10%.

4. Approaches for Assessing Impacts of Climate Change on Plants Population and Community

Apart from field observations various experimental approaches have been developed to study climate change variables effect on plant population and communities. CO₂ enrichment studies have been carried out to observe plants population and communities’ response to elevated CO₂ in greenhouses, growth chambers, open top chambers (OTCs), Free Air CO₂ Enrichment (FACE) and WEB-FACE infrastructures_. Whereas for studying plant population and communities response to elevated temperature - Temperature Gradient Tunnels (TGTs) or glasshouses were also used by various workers (Figure 2 and Figure 3). Thus far, these effects of climate change variables have been reported mainly through experiments conducted in controlled environments such as growth chambers, greenhouses, and plastic enclosures. Long term experimental studies on the effect of CO₂ under more realistic field settings have not attempted in a comprehensive manner. The results of short term CO₂ enrichment studies in greenhouses, growth chambers and open top chambers suggests that growth of many plants could increase about 30% on an average with a doubling of the atmospheric CO₂ concentration under optimum growth conditions with respect to water, nutrients, light and temperature. However, plants responses to steep increase in CO₂ level over short-term in enrichment experiments may not be same for long-term exposure to elevated CO_2. It is important to understand that apart from the projected change in temperature and CO_2, pattern of rainfall, snowfall, soil moisture, erosion, nutrient loss, competitive ability and pest and disease environment may also change to varying extent. Response of plants to the combined effect of all these climate dependent variables may be totally different from the results of such experiments.

5. Elevated Temperature: Plants Population and Communities Response

Elevation or shift in temperature in different regions will affect plants growth cycle as well as distribution range. Warming induced growth stimulation will be more at higher altitudes. Some crops yield will increase due to global warming, if warming is not accompanied with other abiotic stresses. Higher temperatures affect Himalayan plant species productivity, as many of them require certain period of exposure to chilling temperature for their seeds to germinate and for dormant vegetative buds of trees to resume growth in the spring. It has been estimated that for plant communities of regions located on middle and higher latitudes, climate change will be beneficial, as it increase the growing season duration, prepone sowing of crops in the spring, crops become mature in short duration and > 2 cropping cycles can be implemented during single season. Global warming will reduce and enhance leaf longevity of plant communities of north-eastern and western Himalayas respectively, indicates that global warming may induce varied response in plant communities of different ecosystems.

Many plant species will shift slowly from lower to higher latitudes due to rise in temperature; however their rates of upward shifting are difficult to predict because of the complexities in interaction of abiotic and biotic factors. Vegetation above thirty degrees north latitude is changing very fast (Figure 4). Vegetation in this region has been growing more vigorously as compared to other regions over the past two decades. The major changes recorded above forty degrees in the northern latitude, where growing season length has increased. Countries at higher latitudes such as Canada and Russia, crop cultivated area will increase, although crop productivity may not be optimum due to less availability of fertile soils. In warmer, lower latitudes, increased temperatures may accelerate the rate of plant respiration releasing much larger amounts of CO₂ resulting sub-optimal condition for plant growth.

Climate change will cause generation of new altitudinal belts of vegetation in which the proportion of grasses species will be more than that of shrubs. Himalayan vegetation communities at slopes are likely to be most susceptible to climate change adverse impacts. High attitudinal vegetation zones are the most threatened because here replacement of oak forests by pine forest (due to warming driven upward progression of biomes) will reduce quality as well as quantity of forest products needed to sustain livelihood.

Results of a large number of modeling studies showed that effect of climate change on world food production may be small, as productivity gains of some regions will be balanced by loss of other regions. Vulnerability of plant communities to climate change impact is high for the developing countries of lower and warmer latitudes.

When temperatures exceed the optimum range required for plants species and communities of different latitudes, their growth and productivity will decline. It has been estimated that due to climate change night time temperature minima rise more than day time maxima, which will enhance the energy expenditure of plants on night time respiration at the cost of productivity. Global warming will shorten the lifecycle of some crops through enhanced physiological development, early crop maturation, reducing the grain filling period and decline in productivity.

6. Altered Pattern of Precipitation: Plants Population and Communities Response

Water plays a key role for plant species and communities growth, development distribution and abundance. It is predicted that the rainfall patterns are going to change as a result of climate change. Some parts of the world will grow wetter, but others will grow drier, and by far the drying is going to have upper hand. The areas of the earth’s land surface classified as very dry, have doubled since 1970 and according to IPCC this trend will worsen by 2050. IPCC has projected that India will become hotter and wetter by the end of this century. However, any benefit arising from increased rainfall may not be realized, because of higher day and night temperature leads to reduce water use efficiency and nutrient loss from already stressed soils of cultivated fields.

Climate change will alter precipitation pattern at different latitudes of Earth which will affect plants growth processes. The occurrence of moisture stress due to variability in precipitation pattern during vegetative growth, flowering, pollination, and grain-filling period is harmful for most of the plant species including crop plants. In warmer climatic zones (semiarid regions) rise in temperature and reduced/altered pattern of precipitation increase the demand of water for irrigation in regions where there are competing demand for water among agriculture (agriculture being already a major consumer of water), urban, domestic and industrial uses. Enhanced irrigation and evapo-transpiration promote salt accumulation and adversely affects soil fertility. Areas predicted to have increased rainfall may suffer from floods and prolonged inundation affecting plants and crops adversely.

Altered precipitation pattern will reduce plant communities based ecosystem net primary productivity and cause changes in community composition ⁵. Alterations in precipitation pattern are responsible for spatial and temporal variation in soil moisture dynamics, which is critical for plants growth ⁶. Deficit in soil moisture induces plant water potential that, in turn, may cause dehydration, turgor loss, closure, and reduction of photosynthesis ⁷.

Plant species which appear in late succession stages and have greater capacity of storing resources in underground root systems appear to withstand climate change impacts as compared to other species. Increased precipitation and temperature will enhance leaves age.

7. Climate Change and Phenology of Plants

Phenology refers to the study of plants life-cycle events along with associated climate. Plant populations and communities phenological patterns are very diverse and unexplored, but important for plants survival and reproduction. Therefore, it is important to keep track of phenological events namely- leaf fall, appearance of new leaves, flower buds, anthesis, pollination, fertilization and seed dispersal from year to year, and determine how they relate to the prevailing climate patterns. Phenology offers evidence of climate change happening now and helps in assessment of significant effects on plants in the future ⁸.

The flowering and fruiting phonological stages of plants communities lifecycle were susceptible to abiotic stresses such as temperature and moisture. Global warming altered flowering and fruiting phenology in plant communities from the normal ⁹. Selected plant species began to flower before the peak of summer heat but delayed reproduction in species that start flowering after the peak temperature in a tall grass prairie in North America. The warming-induced divergence of flowering and fruiting toward the two ends of the growing season resulted in a gap in flowering and fruiting in the community during the middle of the season ¹⁰. Due to climate change there is gap between the flowering periods of males and females resulting in ineffective pollination, appearance of pest which feed on these plants and pollinators. Such alterations may disturb the linkages between different trophic levels in the community and disturb the ecological balance.

Phenological changes have been studied by many plant scientists in Europe. Abu-Asab et al. ⁹ have studied changes in first-flowering times of over 100 plant species, representing 44 families of angiosperms for 29 years (1970–99) in Washington DC. They observed that most of the trees now flower 3–5 days earlier than they did some years ago.

Climate change will alter the pace and pattern of seasonal community dynamics which in turn will disrupt the coupling between phenological events and seasonal changes evolved through long years of evolution. Changing climate is altering the timing of phenological events. Flowering and fruiting time for many plant species have changed. It now realized that historical records (of the year-by-year) of plants phenology can also provide valuable information on plants’ response to climate change. For example, a comparison of flowering records of plants growing in The Royal Botanic Garden, Edinburgh (RBGE) back in 1850 with phenological recordings, made on daily basis after 2002, revealed that around two thirds of RBGE plants are sensitive to temperature. Flowering time of Rhododendron species, which grows at high altitudes in Himalayas, has been reported to have shifted from March/April to Feb-March due to changing climate ¹¹. Changes in phenology have the potential to create asynchrony between species and may seriously upset the intra-specific competitive relations among species of a plant community.

8. Climate Change and Plant Physiological Response

Enhanced levels of atmospheric CO₂ will increase the difference in partial pressure between the air outside and inside the plant leaves, and as a result more CO₂ is absorbed and converted to carbohydrates. C₃ plants respond favorably to elevated CO₂ levels as compared to C₄ plants which are otherwise photosynthetically more efficient than C₃ plants. and plant response to elevated CO₂ is different on account of their carbon fixation pathway and it will be further affected by other abiotic stresses such as drought. Enhanced CO₂ assimilation by plants may increase their C:N ratio and change nutrient status to varying degrees under different habitat conditions. Under field situation plants may not be able to "store" excess carbon from rising atmospheric CO₂ levels

In a long term field study (of 10 years) carried out with perennial rye grass (Lolium perenne L. cv. Bastion) treated by elevated (CO₂) of 600 µmol mol⁻¹ (in FACE infrastructure), two levels of nitrogen and induced alteration in the source:sink ratio by manipulated destructive harvests ^{12, 13}. In this study > 3000 observations of photosynthesis and stomatal conductance of the grass canopy to elevated CO₂ across each growing season were recorded. No significant change in photosynthetic stimulation, rate of carboxylation and electron transport across a 10-year period. The study of Wand et al. ¹⁴ found that photosynthetic rates in C₃ and C₄ grasses increased on an average of 25 and 33% respectively, with doubling of CO₂ concentration. The increased CO₂ will also be advantageous for CO₂ uptake and productivity of CAM plants.

Plants exposure to elevated levels of CO₂ induces closure of stomata, which regulates leaves CO₂ and water vapour exchange with surrounding air. Thus, under CO₂ enrichment crops may transpire less and fix more carbon dioxide as carbohydrates and improve water-use efficiency (the ratio between crop biomass and the amount of water consumed). Lee et al. ¹⁵ in a FACE study on grassland ecosystem found that stomata conductance of 13 perennial species in response to elevated CO₂, causing about 40% increase in instantaneous water use efficiency. At the same time, associated climatic effects, such as higher temperatures, changes in rainfall and soil moisture, and increased frequencies of extreme meteorological events, could either enhance or negate potentially beneficial effects of enhanced atmospheric CO₂ on crop physiology.

Plants can and do adapt to changes in their environment, their adaptive responses to climate change are likely to be much slower as compared to exposure to single abiotic stress. Since adaptation to a single stress factor may involves only one or two selected character whereas adaptation to climate change may involve many characters.

9. Climate Change and Plant Population and Communities Composition

The rapid pace of climate change has the potential of not only altering species distributions, but may also render many species vulnerable that are unable to keep pace with the shifting of their favorable climate zone. The environmental conditions required by some species, such as those in alpine regions, may disappear altogether ¹⁶. As a consequence, extinction risk of many plant species has increased by many folds. Recent predictions suggest that almost half of today's known plant species could be threatened with extinction ^{17, 18}.

Changes in species composition and abundance, as a consequence of climate change, are inevitable in almost all plant communities. However, our understanding about the response of different plant species to the escalating climate change, in different habitats, is woefully limited. Successful establishment of a species in the newly migrated area not only be depends on the favorable climatic features, but will depend on its ability to compete with other plant species growing in the newly migrated area.

Climatic variables do not act in isolation, but in combination with one another along with other environmental constraints such as air pollution, buildup of ground level ozone, atmospheric brown cloud, status of soil fertility, and extent of habitat degradation or introduction of alien invasive species. More than often these drivers of change will act in synergy with climate change, affecting growth and survival of plant species.

Plants response to climate change at community level will involve changes in species composition and migration of plants northwards and to higher elevations. Isolation of plant communities due to fragmentation of habitats, liquidation of co-evolution; between plants and their pollinators or dispersal agents may lead to the breakdown of community organization and function. Plant communities in general act as 'carbon sink' (store carbon), by sequestering carbon dioxide from the atmosphere ¹⁹.

Plant species inhabiting different ecosystems (agro-ecosystem, mountains, grasslands, deserts, wetlands etc) would respond differentially to the upcoming events of climate change and thus some will succeed and others may lose. For example, recently tropical ecologist Bill Laurence and his colleagues at Smithsonian Tropical Research Institute, Balbo, Panama, examined the record of 18 undisturbed plots in heart of Brazilian Amazon to determine how the composition of trees in these plots has changed over time. Of the 150 tree genera, they found that two become significantly more common and fourteen become significantly rare over the past 15 years ²⁰.

Plants respond to climatic changes through dispersal, death, invasion and displacement events. Some plant species have effective means of dispersal, allowing them rapidly to reach newly available habitat. Species with long life cycles, endemic species, isolated are likely to be more vulnerable. Species occurring in wetlands, salt marshes and dune ecosystems in the coastal areas are likely to get 'squeezed' between human settlements and the rising sea level. In the coastal marine ecosystem, the algae species present in corals are highly susceptible to rise in temperature. Increasing incidences of coral bleaching reported from various part of the world had been attributed to the rising temperature of sea water. Algal bleaching of coral reefs seriously threatens marine fisheries resources and marine productivity. Loss of plant communities or species associations, increase in invasions of species will be observed due to climate change.

10. Climate Change and Plants Pest

Incidence of pest and diseases is most severe in tropical regions due to favorable climate/weather conditions. Global warming may affect growth and development of all organisms including insect pest. Among all the abiotic factors, temperature and humidity are the most important parameters affecting pest distribution and abundance in time and space since these are cold blooded. Exposure to temperature on either side of the optimal range exerts an adverse impact on the growth and development of pest ²¹. Availability of long optimum growing period will enable insects such as grasshoppers to complete more reproductive cycles. Warmer winter temperatures may also allow larvae to extend their range (where they are restricted due to extreme cold condition) and causing greater vegetation infestation. Altered wind patterns may change the spread of pest dispersed or dependent on wind for their dispersal. Dynamics of crop-pest interactions will be altered due to altered timing of hosts and pest growth stages. Livestock diseases may be similarly affected. The possible increases in pest infestations may bring about large scale changes in pest control and management strategies.

Climate change attributes will directly and indirectly affect parasitic plants through alteration in their physiology and host plants, respectively. While limited number of studies have addressed the responses of few parasitic plants such Striga and Rhinanthus species to climate change attributes. The mechanism of action of elevated CO₂ as well as warming will be through alteration in physiological processes i.e., rate of photosynthesis and stomatal conductance. Enhanced photosynthesis of the hemi-parasite and host will increase parasite carbon gains but may also increase the host demand for mineral nutrients ¹². Availability of soil nutrients to host plants may, therefore, mediate the impacts of climate change on host-parasite interactions. Climate change impacts on host-parasite interactions at the individual level will ultimately affect hemiparasite impacts at the community level. Impacts of parasitic plants at community will be high or minimum if climate change attributes support or inhibit growth of hemiparasite populations and other natural enemies of hosts respectively. Hemiparasitic plants with wide host range may migrate under a changing climate ²².

Plant species will not only be impacted directly by changes in climatic parameters, but may also change indirectly through their interactions with other species. Increased temperatures may alter the composition of the alpine plant communities due to herbivores territory expansion.

A species, whose distribution has changed, as a direct result of climate change, may invade the range of another species; encounter new competitive or symbiotic challenges. For example, a pathogen or parasite may change its interaction with plant species, for example, a pathogenic fungus may become more common and virulent in an area where rainfall has increased can threaten some new plant species which were earlier outside its host range. Similarly, if the distribution range of symbiotic fungi associated with roots of a particular plant species may drastically change due to altered climate, will eventually undermine the competitive ability of it traditional host plant species. Similarly, a new grass species may spread into a new region may alerter fire regime and frequent fires over time may change the species composition of the vegetation.

11. Climate Change and Plants Productivity

Green Plants productivity is very vulnerable to current inter annual climatic variability. Global climate change could further enhance this vulnerability and frequent occurrence of extreme climatic events adds up on this. In the coming years, more food, fiber and other agricultural products would be required to feed the increasing population under diminishing availability of arable land, water and degrading soil resources and expanding both biotic and abiotic stresses. In different field studies it had been found that crop species of different agro-ecological regions of world respond differently to global climate change factors. The response of some of some crop species in terms of biomass and yield towards the predicted CO₂ and temperature increase are summarized in Table 1. Till now it has been reported that elevated CO₂ will act as fertilizer and have CO₂ fertilization effect and increasing crop yield. These studies have greatly enhanced our understanding of plant response to the rising CO₂ and temperature, but remain much short of elucidating plant response under field condition largely due to inherent limitations in experimental protocol. Most of the studies were conducted under controlled conditions using single crop species by exposing to individual factors at a time. Although in some studies a combination to two factors, mostly temperature and CO₂ were incorporated keeping the other environmental variables in optimal range.

In real life field situation many factors, other than temperature, CO₂ and rainfall, will have interactive effect and plants will respond to their integrated impact. Long term experimental field studies incorporating all the potential major factors are lacking but needed urgently.

Results of some modeling studies show that the impacts of climate change on crop production vary geographically. North America, and for the former Soviet Union may experience a gain of 3–10% in cereal production for Western Europe suffers losses up to 6% under most projections. Impacts on developing regions are mostly negative for South Asia (–8% to 17% for scenario A2) and Africa (– 3.5% to + 0.5%), mixed for Centrally Planned Asia (–6% to +1%), and overall positive for Latin America. It is predicted that by 2050, if climate change continues on its current trajectory, world supplies of rice, corn, and wheat - will decline ^{23, 24, 32}. Countries near the equator or at low elevations may feel the pinch sooner and disproportionately more. In addition, global warming could diminish the nutritional value of food, oil and fodder crops creating serious public health problems.

Changes in climatic variables such as temperature, rainfall, humidity when exceed the ecological tolerance of a species, then survival of the species depend on its successful migration to a new area having favorable climate. Reports about plant species moving their ranges northward or to higher altitudes as a response to changing climate are progressively increasing.

Plants responses to climate change are species specific and it is obvious that different species will respond differently according to their tolerance range and phenological behavior (such as onset of flowering, pollen sterility, reproductive success). Some plant species may exhibit genetic in response to the selection pressure exerted by changing climate. Plants with restricted range of distribution or which occur in highly specialized habitats such as the insectivorous plant Nepenthes found in Khasi Hills (Plate 1), and most of the endemic species are relatively at a much higher risk of extinction.

12. Mitigation Strategies and Policy Interventions

As per the IPCC report, it is now unequivocal that there is warming of climate system. A holistic approach is needed to deal with the problem of climate change and global warming. In long term, both phenomenons are going to have irreversible impact on whole earth. We have to take mitigation and adaptation steps related to our growth and development activities so that we achieve sustainable development and stable earth. Climate change requires two fold approaches: first by reducing and stabilizing the concentration of greenhouse gases (GHG) in the environment i.e., mitigation and/or adapting to the climate change already in the pipeline i.e., adaptation ^{33, 34}. The few suggestions are as follows: - development of technology in every field so as to reduce the release of the GHGs. There is need to give emphasis on the use of alternative energy resources as far as possible in energy generation. There is need of behavioural and attitudinal change of the public towards their environment by making them aware about the impact of climate change and global warming in long term. There is need for development of crop varieties which are able to survive the impact of climate change ³⁵. The developed countries should transfer technologies to the developing countries so that they can fight the impact of climate change. Since the climate change and global warming does not follow the political boundaries made by the countries, the whole world has to fight against it and take steps to reduce the impact and achieve the status of sustainable earth.

13. Conclusions

Historically climate change has had a profound effect on plant biogeography, thus it is reasonable to expect that the ongoing climate change would have significant impact on plants and vegetation. Climate change will induce instability in food and fiber production that will adversely affect the socio- economic environment at local, regional and global levels. Climate change would alter timing of plant activities, and may have a domino effect, impacting feeding, breeding or migration patterns of animals that rely on specific plant species.

Our understanding of plant responses to climate change for different ecological zones is highly inadequate. Global climate change and consequent habitat changes will put many plant species outside their survival range leading to species extinction. It is unfortunate that issues related to the impact of climate change on plants and vegetation has not received the attention they deserve in national policies and international debate. A proper understanding of plant interaction with the changing climate is of critical importance, for developing appropriate adaptation strategies for agricrops and for conserving plant biodiversity. Systematic studies for assessing response of crop plants, trees, forests and other vegetation types are urgently needed, particularly in biodiversity hot spot areas which include India and other tropical regions of the world. It is imperative that international bodies such as UNEP, WMO, FAO and international NGOs act proactively for promoting systematic scientific studies in different climatic zones for studying climate change implications for plants and vegetation. Ecological understanding of plant responses to climate change from a holistic and pragmatic perspective is critically important to address the issues pertaining to plant biodiversity, global food security, poverty elevation and sustainable development.

Conflict of Interest Statement

All authors hereby declare that there is no conflict of interest.

References