Plants are essential to our survival as humans. Everything we eat is made up of plants or animals that rely on plants at some point in the food chain. Plants are also the foundation of natural ecosystems, absorbing roughly 30% of all carbon dioxide emitted by humans each year. But, as the effects of climate change worsen, how rising CO2 levels in the atmosphere and rising temperatures affecting the plant world?
CO2 Increases Plant Productivity
Photosynthesis is the process by which plants use sunlight, carbon dioxide from the atmosphere, and water to produce oxygen and carbohydrates that they can use for energy and growth.
The carbon fertilization effect occurs when CO2 levels in the atmosphere rise, causing an increase in plant photosynthesis. As per new research, global plant photosynthesis increased by 12 percent while CO2 levels in the atmosphere increased by 17 percent (between 1982 and 2020). Carbon dioxide fertilization was responsible for the vast majority of this increase in photosynthesis.
Some plants grow faster when photosynthesis is increased. Scientists discovered that in response to increased CO2 levels, above-ground plant growth increased by an average of 21%, while below-ground growth increased by 28%. As a result, some crops, such as wheat, rice, and soybeans, are expected to benefit from increased CO2 levels, with yield increases ranging from 12 to 14 percent. However, increased CO2 has no effect on the growth of some tropical and subtropical grasses, as well as several important crops such as corn, sugar cane, sorghum, and millet.
Plants use less water during photosynthesis when CO2 levels are high. Plants have stomata, which are openings that allow CO2 to be absorbed and moisture to be released into the atmosphere. When CO2 levels rise, plants can maintain a high rate of photosynthesis while partially closing their stomata, reducing water loss by 5 to 20%. Scientists speculate that this could result in plants releasing less water into the atmosphere, allowing more to remain on land, in soil, and in streams.
Climate change-induced CO2 increases may allow plants to benefit from the carbon fertilization effect and use less water to grow, but it's not all good news for plants. It's a little more complicated than that, because climate change affects other factors important to plant growth, such as nutrients, temperature, and water.
Nitrogen Limitations
Between 1980 and 2017, researchers studied hundreds of plant species and discovered that most unfertilized terrestrial ecosystems are becoming deficient in nutrients, particularly nitrogen. This decrease in nutrients was attributed to global changes such as rising temperatures and CO2 levels.
Nitrogen is the most abundant element on Earth, accounting for roughly 80% of the atmosphere. It is a necessary component of DNA and RNA and is required by plants to produce carbohydrates and proteins for growth. Plants, on the other hand, are unable to use nitrogen gas found in the atmosphere because it contains two atoms of nitrogen that are triply bonded together in such a way that they are difficult to break apart into a form that plants can use. Lightning has enough energy to break the triple bond, which is referred to as nitrogen fixation. Nitrogen is also fixed in the fertilizer manufacturing process.
However, the majority of nitrogen fixation occurs in the soil, where certain bacteria attach to the roots of plants such as legumes. The bacteria take carbon from the plant and fix nitrogen in a symbiotic exchange, combining it with oxygen or hydrogen to form compounds that plants can use.
Trees currently absorb about one-third of human-caused CO2 emissions, but their ability to do so in the future is dependent on the amount of nitrogen available to them. The benefit of increased CO2 will be limited if nitrogen is limited.
Earlier nitrogen fixation research based on measurements of free-living bacteria predicted that the fixation process works best at 25°C and that as temperatures rose above 25°C, the rate of fixation would decrease. This would have resulted in a runaway scenario in which nitrogen-fixing decreased as temperatures rose, resulting in lower plant productivity in a warming world. Plants would then remove less CO2 from the atmosphere, resulting in more warming and less nitrogen fixation, and so on. Griffin and his colleagues created an instrument that allowed them to measure the temperature response of nitrogen on bacteria that formed associations with plant roots rather than free-living bacteria.
Rising Temperatures
Griffin's research also discovered that the temperature response of nitrogen fixation is distinct from the temperature response of photosynthesis, which involves nitrogen-containing enzymes. Higher temperatures can reduce the efficiency of these enzymes. Rubisco is a key enzyme in photosynthesis that helps convert carbon dioxide into carbohydrates, but as temperatures rise, it "relaxes" and the shape of its pocket that holds the CO2 becomes less precise. As a result, one-fifth of the time, the enzyme fixes oxygen rather than carbon dioxide, lowering photosynthesis efficiency and wasting the plant's resources.
Rising temperatures are also lengthening and warming the growing seasons. Plants will use more water because they will grow more and for a longer period of time, canceling out the benefits of partially closing their stomata. Contrary to what scientists previously believed, the result will be drier soils and less runoff required for streams and rivers.
This could also lead to more local warming because evapotranspiration- the process by which plants release moisture into the air- keeps the air cooler. Furthermore, when soils become dry, plants becomes stressed and don’t absorb as much CO2, potentially limiting photosynthesis. Researchers discovered that even if plants absorbed excess carbon for photosynthesis during a wet year, the amount could not compensate for the reduced amount of CO2 absorbed during the previous dry year.
Crops are also more vulnerable as temperatures rise and moisture levels rise. Weeds, many of which thrive in heat and elevated CO2, already account for approximately 34% of crop losses; insects account for 18% of crop losses, and the disease accounts for 16%. Climate change will almost certainly amplify these losses.
Temperatures above 32° to 35° C cause stress in many crops, though this varies depending on crop type and water availability. According to models, each degree of additional warmth causes a 3 to 7 percent loss in yields of some important crops, such as corn and soybeans.
Furthermore, an increase in temperature accelerates the plant lifecycle, so that as the plant matures faster, it has less time for photosynthesis, resulting in fewer grains and lower yields.
Extreme Weather:
Climate change will increase the frequency and severity of extreme weather events such as heavy rain, windstorms, heat waves, and drought. Extreme precipitation events can disrupt plant growth, especially in recently burned forests, and make plants and soils more vulnerable to flooding and erosion. Increased frequency of high winds can put a strain on tree stands.
Climate change is also expected to bring more heatwaves and droughts, which would likely cancel out any benefits from carbon fertilization. While crop yields typically decline during hot growing seasons, the combination of heat and dryness could reduce maize yields by 20% in some parts of the US and 40% in Eastern Europe & Southeast Africa.
Plants Face an Uncertain Future
Many studies on plant life's response to climate change appear to indicate that most plants will be more stressed and less productive in the future. However, many unknowns remain regarding how the complex interactions between plant physiology and behaviour, resource availability and use, shifting plant communities, and other factors will affect overall plant life in the face of climate change.
(Source: Columbia Climate School)
