Image courtesy of Wikimedia Commons.
By Helen Gloege ’23
Staff Writer
At the end of 2020, the U.K government approved planting trees in over 100 acres of a northern England peat bog. Peat bogs, areas where plants have been decaying over thousands of years into soil that traps their carbon, can store twice as much carbon dioxide as forests. When the trees were planted in northern England, they effectively dried out the soil, causing carbon to be released from the bogs and ending the project before it was ever finished.
Since plants are responsible for removing carbon dioxide from the atmosphere and releasing oxygen, it is expected that trees can help limit climate change. However, many tree planting programs like the trees planted in northern England face poor planning and lack of foresight that can cause more harm than good. Trees must be planted in locations where they match the existing biodiversity rather than disrupt it. Native trees are better equipped to handle a given area’s conditions, whereas non-native trees may increase the risk of fire, change the soil or decompose and release carbon faster.
Recent studies have found that terrestrial ecosystems, which often include trees, are becoming less reliable as temporary climate change mitigators.
A recent study conducted by an international team of scientists from December 2020 titled “Recent global decline of CO2 fertilization effects on vegetation photosynthesis” published in the research journal Science concluded that if the trend of rising CO2 levels continues, 86 percent of land ecosystems globally will become progressively less efficient at absorbing CO2. Carbon dioxide is necessary for plant growth, and elevated concentrations of CO2, in a process known as the CO2 fertilization effect, can cause an increase in photosynthesis.
The researchers analyzed satellite-derived and model-based datasets to understand the effect of increasing CO2 levels on CFE. The study found that, since 1982, the global average CFE has decreased from 21 percent to 12 percent per 100 parts per million of CO2 in the atmosphere. Without feedback between photosynthesis and elevated CO2, climate change will likely occur at a faster rate. Additional lack of water, nutrients and sunlight could also limit growth. In the tropics, there is not enough nitrogen or phosphorus to sustain photosynthesis. In the higher latitude temperate and boreal regions of the globe’s northernmost forests, soil moisture is more limited because of recent warming.
The decline in the CFE is more substantial than current land surface models have shown. The study also used remote-sensing observation instruments that allowed researchers to account for nutrient feedback and soil moisture limitations. This new information will force an adjustment of the remaining carbon budget to account for the weakening of plant CFE. These land ecosystems will not be as reliable for climate mitigation in the coming decades.
A separate study conducted by another international team of scientists in January of this year also supports these findings. Titled “How close are we to the temperature tipping point of the terrestrial biosphere,” the study pulls 20 years of data from 250 sites that measure the transfer of carbon dioxide between land, plants and the atmosphere. This study suggests that, due to rising global temperatures, forests will only be able to take up half as much CO2 from the atmosphere by 2040 as they do now. Forests and other land-based ecosystems take up 30 percent of human carbon emissions.
The data analysis provides a clear temperature limit above which the trees will begin releasing more CO2 than they can take in, and as a result, the land system will act to accelerate climate change. For many forest types, including rice, soy, pulses, grasses, oaks and pines, the optimum photosynthesis level peaks at 18 C (64.4 F). For plants like maize, sugar cane and various trees, the optimum photosynthesis level peaks at 28 C (82.4 F). The data does not show limits on the amount of CO2 that will be released.
Earth’s rising temperatures tend to pressure forests to emit more CO2 . At the current rate, these vegetated landscapes will turn from carbon sinks to carbon sources within the next 20 to 30 years. There are currently no signs that forests have been or will be able to acclimate to the rising temperatures. These results are likely conservative, as they do not take tree die-offs into account, such as the mass deaths of aspen trees in Colorado, insect outbreaks killing trees around the world and forest fires. Every type of land-based ecosystem will be affected. Even now, about 10 percent of the planet’s surface is crossing the threshold that limits photosynthesis during the warmest three-month periods of the year.
An article for Inside Climate News breaking down the study summarizes that it suggests there will be “about 50 percent of the terrestrial biosphere reaching temperatures above the threshold that limits photosynthesis by mid-century.” Additionally, the study indicates that “tropical rainforests in the Amazon and Southeast Asia, and Russia and Canada’s boreal forests, will be among the first to hit that tipping point,” according to Inside Climate News.
Currently, techniques to deal with hotter and longer droughts caused by climate change are not being integrated into forest management.