One of the more stupid, but more resilient, memories of climate change is based on the claim that CO2 is a plant food. Plants rely on it for photosynthesis, so increasing its level in the atmosphere should be beneficial for plants by increasing their growth. There are variants that claim that plants are currently starving for carbon dioxide, while others suggest that plants will grow so fast that they will protect carbon from warming the atmosphere.
Fortunately, if you look at the net "carbon dioxide is plant food," all the early blows you get are from people and organizations that break the idea. While CO2 can help the plants and may have contributed to the general landscaping of the Earth, this is just one of the many factors that affect the growth of plants. The new study shows that even when trees obviously benefit from rising atmospheric carbon levels, they do not seem to grow faster.
Ask the trees
Carbon dioxide is included in sugars by the action of a rather ineffective enzyme, so simply having more of it makes photosynthesis smooth. When there is more around, the pores that allow it in the leaves can also close a little more that retains the water. For these reasons, carbon dioxide has a toroidal effect on many plants.
But carbon dioxide almost never restricts plant growth. Nutrients such as nitrogen and phosphorus are more likely to be limiting and it is difficult for plants to use water more efficiently when there is not enough water. Increased temperatures driven by the entire CO2 placed in the atmosphere can also affect plant growth either directly through elevated temperatures or indirectly by increasing droughts.
Thus, the exact impact of elevated carbon dioxide depends on various local factors. Many of these have been tested in experiments that involve pumping additional carbon dioxide into the environment near plants. These experiments are relatively short-lived, often tracking effects over a single growing season. A Canadian research team decided to track the impact of carbon leakage in the long run. Thus, researchers have discovered a population of white cedars in eastern Canada that has allowed them to track over 700 years of tree history.
A core through the tree gives an indication of how well it has developed through the annual growth rings. But in this case, the kernel also provides data on the conditions of growth and efficiency of water use of the tree, calculated from isotopes of oxygen and carbon.
History has shown that things have remained stable until about 1850, after which atmospheric carbon levels have begun to grow. During this period, which is relatively humid and cool in the climate, trees respond to increased carbon levels by including more carbon. However, from 1965 onwards, when the area dried out and warmed, the use of water began to change in a way that would allow trees to cope more effectively with droughts.
At no point did the trees show an enhanced growth based on the trees of the trees. There are at least three periods in which trees grow at least as well as in recent years. While there may be higher growth that does not affect the boot, it is clear that the rise in CO2 did not have a general fertilizing effect on these trees.
What is happening with trees is not quite clear. The change can simply be a response to carbon integration, reaching the point where it is saturated, which allows an additional focus on water conservation. Alternatively, changes in the environment to a warmer and drier environment can lead to change. Regarding the reasons why none of the strategies lead to growth, researchers point to a study of trees elsewhere in Canada, indicating that phosphorus is a growth limiting factor.
Clearly, the situation is complex. Trees showed at least two different responses to elevated atmospheric carbon or a possible response to changes in water availability. There is also an additional possibility of limiting nutritional growth. All this is for one species in a single geographical area.
This complexity tells us that predicting how plants will react to our ongoing experiment with the atmosphere will require a lot of work to do. And with the rhythm that our carbon emissions have increased, it is possible that the reaction of plants will continue to change even when we study them.
PNASDOI: 10.1073 / pnas.1816686116 (for DOIs).