Commentary: What do Trees and People have in Common? - Lots!

Lead Author: Dr. Jeff Howe

Publish date: 09.19.2011


Climate change is a hot environmental topic, and carbon sequestration, reducing carbon footprints, and carbon trading are among the solutions that are being discussed to mitigate man’s impact on global climate.  For the past two hundred years man has poured literally billions of tons of carbon based “greenhouse” gases such as carbon dioxide and methane into the atmosphere.  While there is still some debate (in some regions) over the degree to which man actually can influence the global environment, there appears to be general alignment behind the idea that behaviors that lead to increased long-term storage of carbon, particularly of atmospheric carbon, are a good thing.  Some have even been suggested that halting the harvest of trees might be an effective way to increase carbon storage.  However, this simplistic solution to the atmospheric carbon problem is misguided.


Carbon is a critical element and forms the basis for all life on earth.  The two most important chemical characteristics of carbon are that it has four chemical bonding sites and that the energy required to make or break a bond is just at the appropriate level for building molecules that are both stable and reactive.  Thus, carbon atoms bond readily to other carbon atoms to form long and complex molecules.


The chemical composition of wood is often referenced when discussing carbon storage in the forest.  Dry (moisture-free) wood is about 48-50% carbon, 38-42% oxygen, 6-7% hydrogen and a number of other elements, such as nitrogen and sulfur in very small percentages.  These percentages are based on the weight of the elements as a percentage of dry wood mass.


Living trees, however, are very wet.  In fact, although there can be great variation between tree species (and seasonally), a living tree may be made up of more than two thirds water by mass.  Thus, a living tree is made up of 15-18% carbon, 9-10% hydrogen, and 65-75% oxygen by mass.


Interestingly, human beings are also mostly water (two-thirds) and carbon-containing organic molecules.  In fact there is a striking similarity in the proportion of elements in the composition of humans and trees.  By mass humans are composed of about 18% carbon, 10% hydrogen, 65% oxygen, and 3% nitrogen, 1.5% calcium, 1.2% phosphorus and a number of elements in amounts less than 0.2 %.[1]    Obviously, humans are more than the elements they are made of, but with respect to carbon storage there is a marked similarity between humans and trees.


Though obviously plants are not warm-blooded creatures, trees are nonetheless actually very much like people in other ways as well.  As seedlings, for instance, they are very fragile, and when young grow rapidly in both height and in volume.  As they age, however, they gradually lose vigor, and over time become more susceptible to disease.    …and eventually they die.  Unlike humans, however, there is no “average” life span for trees as the age at death varies greatly by species.  Some tree species struggle to live past 60-70 years, while others commonly live into the hundreds of years.  Select trees of a few species live very long lives - into the late hundreds and even the thousands of years; but these are the exceptions rather than the rule.  In general, most tree species show measurable signs of aging after the age of 100. In addition, communities of trees – forests – are subject to periodic, unpredictable, sometimes catastrophic, disturbance events that impact the life spans of trees and forest stands.


In the same way scientists evaluate human populations rather than individuals, some key statistics are more appropriately measured by looking at forests rather than at individual trees.  As an example, the carbon dioxide exchange of individual trees may vary greatly by age.  While young established forests tend to capture carbon at a rapid rate, very mature forests tend to give off approximately as much carbon dioxide as they take in.  In fact, one way to define “old growth forest” would be to say that it is a forest in balance, or that it is giving off as many resources (such as carbon dioxide) as it is taking in.


Storing carbon in living trees is an attractive carbon mitigating strategy, and certainly one component of any solution to reducing man’s impact on the atmosphere.  Similarly, it could be argued that storing carbon in humans is an option; they are made up of similar elemental proportions.  Certainly, an increase in the human population by another few billion will increase the amount of carbon “sequestered” in humans.  In fact for every billion people added to the population we would store about 22.5 billion pounds of carbon.[2]    But it could be argued that humans are the problem that started this discussion in the first place.  Humans require so many other resources to live that adding more just to store carbon is probably not a good idea.


But trees require resources as well.  Every billion pounds of carbon stored in a living tree requires about 2 billion pounds, or almost 250 million gallons, of fresh water.  There are few regions of the world today where fresh ground water is in great excess and many regions where it is in critical short supply.  To offset the current 29 gigatonnes of carbon dioxide put into the air by humans each year by sequestering carbon in trees we would need to put into storage on the order of one point five trillion gallons of fresh water, annually.


Wood, on the other hand and as mentioned earlier, is approximately one-half carbon by weight, and for interior use contains generally only about 7% water by mass….or roughly that of the surrounding atmosphere.[3]   Thus, if the forests from which wood is harvested are growing at least as rapidly as the rate of wood removal[4] , utilizing wood in relatively permanent usage such as in wooden furniture and buildings represents an excellent means of storing carbon. All of the resources required in the production of wood can be compared directly for net carbon impacts with those of other raw materials and appropriate selection.


Storing carbon in living organisms such as trees is one part of the carbon solution, but it is not as simple a solution as it may seem.  Like humans, trees have finite life spans and require lots of fresh water to survive.     Thus, carbon in forests cannot be considered to be in permanent storage. Putting sustainably harvested wood and the carbon it contains into permanent and durable uses, complements carbon storage within forests   Climate change policies that recognize the importance of maintaining forests, but also the value of carbon storage within durable goods, quality engineering, and designs that enhance the long-term performance of wood houses, make more sense than policies aimed at simply letting forests grow.


Dr. Jeff Howe

September 2011


[1]  Chang, Raymond.  2007.  Chemistry, Ninth Edition.  McGraw-Hill, p. 52.

[2]  Assuming an average weight of 125 lbs. per human

[3]  In fact dry wood in most usages will acclimatize itself to its surrounding environment, meaning it will take on or give off moisture depending on the relative humidity – hence the terms shrinkage and swelling of wood.  

[4]  Forests of the United States and Canada have for many decades been growing far more rapidly than the rate of harvest, with the result that forest area, standing volume, average diameter, and net carbon storage are all steadily increasing.