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The carbon dioxide boost for trees can not offset the climate impacts



  Close-up picture of cedar buds.
Enlarge / Buds on a cedar tree.

One of the more stupid but persistent memes for climate change is based on the claim that CO 2 is plant food. Plants rely on it during photosynthesis. Therefore, increasing the level of the atmosphere should benefit the plants by promoting their growth. There are variants that claim that plants are currently starving for carbon dioxide, while others suspect that plants will grow so fast that carbon does not warm the atmosphere at all. Fortunately, if you search the web for carbon dioxide plant food, "all the early hits you get come from people and organizations that expose the idea." While CO 2 can help and plants contributing to a general greening of the earth, this is only one of several factors that affect plant growth, and a new study shows that even if trees benefit significantly from the rising carbon content in the atmosphere, they do not seem to grow faster.

Ask the Trees

Carbon dioxide is incorporated into sugar through the action of a rather inefficient enzyme: if you just have more of them in the vicinity, photosynthesis runs smoothly, and if there are more, the pores that leave it in leaves can also close a little more, which saves water.For these reasons, carbon dioxide on many plants fertilizing.

But carbon dioxide is d Plant growth almost never limits. Nutrients such as nitrogen and phosphorus are more restrictive, and it is difficult for the plants to use water more efficiently if there is not enough water. The elevated temperatures introduced into the atmosphere by all CO 2 can affect plant growth, either directly by elevated temperatures or indirectly by increased periods of drought.

The exact effect of elevated carbon dioxide depends on a variety of local factors. Many of them have been tested in trials that pump additional carbon dioxide into the environment of the equipment. These experiments are usually relatively short-term and often track the effects of a single growth period. A Canadian research team decided to track the effects of a change in CO2 levels for much longer. For example, researchers in eastern Canada found a population of white cedars that enabled them to follow the more than 700-year history of trees.

A kernel through the tree indicates how well it grew through the annual growth rings. In this case, however, the core also provided a record of the growth conditions and water use efficiency of the tree, calculated from isotopes of oxygen and carbon.

History showed that things stabilized until around 1

850, at that point atmospheric. The carbon content began to rise. During this time, which is relatively humid and cool in climate change, the trees responded to the increased carbon content with more carbon. From 1965, however, the consumption of water began to change as the area dried out and heated up so that the trees were better able to cope with droughts.

No Growth

At no time did the trees show improved growth based on the tree rings. There are at least three periods in the record in which the trees have grown at least as well as in recent years. While it may be possible that there was higher growth that is not reflected in the stem, it is evident that the rising CO levels 2 did not have general fertilization on these trees.

What's going on? with the trees is not exactly clear. The change could simply be a response to the carbon input reaching a point at which it is saturated, allowing an additional focus on saving water. Alternatively, switching the environment to a warmer, drier environment may have fueled the shift. Why neither of these strategies leads to growth, the researchers point to a study on other trees in Canada, suggesting that phosphorus limits growth.

It is only clear that the situation is complex. The trees showed at least two different responses to elevated atmospheric carbon or a possible response to changes in water availability. And there is the additional possibility that growth is limited by nutrients. All of this applies to a single species in a single geographic 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 get organized. And with the pace of our CO2 emissions, it is possible that the response of plants will continue to change, even if we examine them.

PNAS 2019. DOI: 10.1073 / pnas.1816686116 (via DOIs).


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