Science to Live By: Carbon Dioxide: The Essential Pollutant (Part Three)

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© J. Dirk Nies, Ph.D.

Last time, in Part Two of this series, we learned that edible yields of cassava, an important root crop, doubled when its leaves were exposed to elevated levels of CO2 in the atmosphere; levels we are likely to see by the middle of this century. This dramatic example of the ‘CO2 fertilizer effect’ arises as a direct consequence of plants having more carbon dioxide readily available for photosynthesis.

In this article, I wish to highlight a related but different direct impact that higher levels of CO2 currently are having on terrestrial plants and indirectly on hydrology as well. The subject I have in mind is water, more specifically, how efficiently trees use water. A recent communication in the journal Nature addresses this subject.

Researchers at Macquarie University in Australia “find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades.” They assert that the most credible explanation for this observation is the “strong CO2 fertilization effect” brought on by rising levels of CO2 in the air.

The boreal forests of North America and Eurasia (also referred to as the taiga) are the world’s largest terrestrial community of plants occupying a distinct region (biome). They are found just south of the polar tundra. Tree species found within these forests consist mainly of cold-hardy, cone-bearing evergreens, such as firs, pines, and spruces, along with some deciduous trees such as larches, birches, and aspens. Temperate forests are located further south and contain a higher abundance of deciduous trees such as oak, hickory, poplar, maple, beech and birch.

In both these forest types, the trees are not passive; they react dynamically to changes they perceive in the environment. In response to the enhanced availability of CO2 in the air, these forests and woodlands have adapted and are becoming ever more economical in their use of water. They now need less water to thrive than they did just 20 years ago.

But why is there any relationship between the amount of CO2 in the air and the amount of water trees need to grow?

Trees need both water and carbon dioxide to make food. The only source of CO2 available to trees is from the air. Trees absorb CO2 from the air through tiny, adjustable pores in their leaves called stomata. While doing so, however, they lose precious water through these very same pores. Under hot, dry conditions, upwards of 95 percent or more of the water absorbed by the roots of trees simply wafts away through the leaf canopy and is lost to the atmosphere (this phenomenon is called transpiration). Under ordinary conditions, roughly 400 water molecules are lost to the air for every single CO2molecule taken in. Four hundred-to-one are not good odds when water is scarce!

Clearly, the exchange of gaseous CO2 and H2O at the surface of leaves creates a biological conundrum. To maximize the amount of CO2 absorbed, trees open their stomata to make it easier for carbon dioxide to enter the leaf. To prevent becoming desiccated by water loss through transpiration, trees close their stomata as much as possible. Trees are tugged in opposite directions by these two opposing priorities.

As our human economy has pumped more and more carbon dioxide into the atmosphere, we have begun to tip the balance and ease this dilemma. Our inadvertent, unintended fertilization of the air with CO2 is making it easier for trees to get all the CO2 they need for photosynthesis. As a consequence, trees are reducing the size of the openings of their leaf stomata.  This in turn reduces water evaporation rates from their leaves. In the presence of higher levels of CO2, trees are drawing less water from the ground while still maintaining vigorous growth.

Nature’s biological economy is built around CO2. The vibrancy and diversity of plants, indeed all organisms populating earth, depend upon and are influenced by the presence of carbon dioxide in the air. This study has shown that in boreal and temperate forests today, photosynthesis is stronger, uptake of carbon from the atmosphere is increasing, water use is down, and these changes principally have arisen from increased concentrations of CO2 in the air over the past couple of decades.

This study also has reaffirmed that nature is surprising and complex. The adaptations occurring within these forests of the Northern Hemisphere have turned out to be greater than scientists expected. Our best theories and our finest, most sophisticated computer models did not predict the magnitude of the changes we see happening in these woodlands today. In the words of the Australian researchers: “The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models.”

And this is only the beginning of the story. Other effects, not measured in this study, may become apparent over time. For example, less transpiration, which naturally cools the tree canopy (like perspiration cools our bodies), may lead to higher temperatures near the ground. More efficient water use may lead to more surface runoff and flooding, or conversely, may result in more moisture ending up in groundwater. Who knows? What’s clear is that the carbon-based and water-based economies of large swaths of terrestrial vegetation are shifting in response to our changing the composition of the earth’s atmosphere.

Until recently over the course of human history, we could go about our lives without consequence, blissfully ignorant of the presence, properties, functions of CO2, and our alteration of its concentration in the air. The biological and environmental impact of our economic activities powered by burning fossil fuels has grown, however, from local and negligible to global and transformative. We are dramatically increasing levels of carbon dioxide in the atmosphere around the world and we are producing measurable impacts on the economy of life on earth. As more consequences emerge, blissful ignorance becomes less tenable.

Next time, I will report on recent and surprising research findings that describe how the world’s tropical rainforests are responding and adapting to higher levels of CO2 in our atmosphere.

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