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| Through transpiration trees extract huge amounts of water from the soil and spew it into the atmosphere. |
Three Major Cycles in Forests
For a better understanding of the overall value of trees, we'll look at the three major cycles in forests. There are many other processes that also take place, but these three will provide a big-picture look at what's going on in forests, which can then be downsized to fit into our local activities in our yards and in our communities.1) The Water Cycle: Water moves through trees, evaporates into the air, forms clouds, rains or snows, both locally and globally. Water from the precipitation is soaked up by trees.
2) The Carbon Cycle: Carbon absorbed during photosynthesis and is released during respiration, fire & decomposition.
3) The Oxygen Cycle: O2 is produced through photosynthesis and absorbed through respiration.
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| Transpiration: the process of liquid water moving into the roots, up through the xylem, and evaporating out through the stomata (pores in the leaves) as a water vapor. Illustration by Marjorie Shropshire. |
Before we move to the water cycle, let’s look at how water moves through vascular plants: plants with xylem & phloem. Xylem cells form tubes that the water runs through unimpeded by cell walls. Phloem transports sugars and other nutrients up and down the plant as needed.
More than 90% of water runs right through a plant. The other 10% will be used for photosynthesis, used to keep the leaves and stems turgid, and used in other metabolic processes, but the vast majority evaporates through the stomata (pores) on the leaves.
What makes this work is the polarity of the water. Each water molecule is arranged like Mickey Mouse ears with a positive charge on the side with the H atoms and negative on the other side. So water adheres to itself as it is drawn through the plant’s xylem-the vessel cells or tubes.
The big driver is the differential between concentrations of the water in the plant and the water in the air. Nature wants everything to be equal, so this is what is sucking the water through the plants.
Also, as result of this evaporation the temperature is lowered. At 68 °F about 585 calories are required to vaporize one gram of water. This reduces the latent heat. In other words, the physical process of water shifting from a liquid to a vapor during transpiration causes the temperatures to be lowered. It’s like when you sweat, your body cools down. That’s why when you walk through a forest, it’s not just shady, but it's actually cooler.
A mature oak tree transpires more than 400 liters of water on one hot summers’ day and each liter is 1000 grams and each gram of water turned to water vapor removes heat, which varies depending upon temperature. So, when trees move a lot of water into the atmosphere, it cools the planet.
Water also evaporates from soil, so the total output of water vapor into the air from a forest or any ecosystem is called evapotranspiration. We need to know this term when we look at the overall water cycle.
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| The global water cycle. |
The evapotranspiration from rainforests, other forests, fields, and yards adds water to the atmosphere in the form of clouds and general humidity. Most of these clouds release the water right back onto the local areas.
But the rest ends up in the upper atmosphere and could be carried far away depending upon the global conditions.
When forests are cut down, less moisture goes into the atmosphere and rainfall declines. Severe droughts are becoming more common, which then causes the forests to suffer and some of the trees may be vulnerable to disease and infestation. These stressed trees are, in turn, more susceptible to fire, which many people in western North America are finding out right now. The effect is compounded globally.
See California has 149 million dead, dry trees ready to ignite like a matchbook for recent findings on this ecological disaster. For more details on water, read Water Science for Gardeners.
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| Photosynthesis and respiration |
Photosynthesis & Respiration
Before we move to the Carbon and the Oxygen cycles in forests, let's do quick review of life cycles here on Mother Earth, the vast majority of which, is based on that life-giving process of photosynthesis. Green plants combine carbon dioxide and water with energy from the sun to form sugar or glucose which can then combine to form starches and fats or whatever the plant needs.
Then most living organisms (plants, animals, fungi, bacteria) digest the sugars in one form or another to gain energy for life through a process the equal and opposite of photosynthesis—respiration.
This happens on a cellular level. When we breathe that’s just the final stages of our metabolizing process.
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| The Carbon cycle |
The Carbon Cycle
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| Trees sequester carbon when they are alive |
Carbon is absorbed through photosynthesis as carbon dioxide. Plants store carbon in their wood, leaves, flowers and fruit. When animals eat the plants the carbon is transferred to and cycled through their systems and some is stored while they are alive, but it’s released when they die.
Carbon is released during respiration, fires, & decomposition.
If you’re a fan of Star Trek, Spock, the science officer, would describe us as carbon-based life-forms.
Carbon is also stored in the soils as dead plant and animal matter. Actually, soil sequesters four times more carbon than all of the terrestrial plants. This number does not the ocean plants, but all those living in the soil worldwide—all the forests, all the fields, all of our yards, and more…
In our book Climate-Wise Landscaping, I summarize our section on soils with: “We have to stop treating our soil like dirt!
So, are forests carbon sinks?
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| Trees to lumber |
Carbon is sequestered in old trees until they rot or burn and carbon is also stored in harvested wood if it is used as lumber to build things, but not if it’s used for fuel.
In Baltimore and other urban areas, there are initiatives to reclaim wood before old buildings are razed so the wood can be used to make other stuff like composition board or paper, which keeps the carbon stored for longer and saves new forests from being needed.
Young forests are typically better carbon sinks than old forests, because growth is more vigorous and there are fewer old or dying trees. A typical growing oak tree will sequester up to 50 pounds of carbon a year.
Forests of all types temporarily store carbon, but definitely not as much as the oceans or the soil.
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| Why the asterisk? |
The Oxygen Cycle
Many people have said that forests are the lungs of the Earth, so why is there an asterisk?
Actually, the phytoplankton in the oceans produce up to 80% of world’s oxygen. Forests are important for many reasons, and yes, they produce oxygen.
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| Oxygen Cycle |
Oxygen is absorbed in fires & decomposition, but oxygen is stored in biomass & soils.
So in the end the forests provide about 20% of global oxygen, which is certainly important, but probably not enough to be labeled, "The Lungs of the world."
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| There is no Planet B! |
Plant more trees!
While we need to focus on deforestation as a large political problem, we can effect positive change in our yards and in our communities by replacing large portions of unneeded lawns with groupings of native trees and shrubs.
We can start in our own yards, which will provide shade and habitat for birds and butterflies and also cool the air. Then we can work within our neighborhoods, schools, churches, and towns to do the same.
These new trees that we all plant will join the global population of trees and all together it will make a big difference in the health of our only planet. There is no Planet B!
Green Gardening Matters,
Ginny Stibolt
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