Robert Leverett

Cofounder of the Native Tree Society

www.nativetreesociety.org

for TREEIB.COM

"WE CAN’T PLANT OUR WAY OUT OF THE CLIMATE CRISIS."

HOW MANY TREES must be planted and HOW LONG must they grow to match the carbon volume stored in one big tree?

This is the version in imperial units

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OUR

LARGE OAK...

Circumference in breast height

CBH: 14 feet

Diameter in breast height

DBH: 4.46 feet

Height: 100 feet

Dry biomass: 30,681 lbs

Carbon stored: 15,340 lbs

CO2 equivalent: 28.1 tons

Age: n.a.

...is
felled.

COUNT:

1

Introduction

Trees are the repositories of carbon originally absorbed from the atmosphere in the form of CO2.

There is no debate that the carbon is stored in the wood and bark, nor should there be any debate that the larger the tree, the more the carbon.

 

However, we are losing sight of the importance of retaining the bigger trees as they continue to sequester carbon at a critical time in our Earth’s history.

 

What follows is a look at the value of a hypothetical tree, a northern red oak (Quercus rubra), for storing carbon, comparing the carbon sequestered by replacing the one big oak with young, fast-growing ones.

 

Numeric returns are expressed in imperial units. A equivalent in metric units can be displayed here.

Calculating the amount of carbon stored in a large northern red oak

Assume our oak measures 14 feet in circumference (4.46 DBH) and is 100 feet tall. Such an oak grows about three miles from where I sit in western Massachusetts, USA.

Given these dimensions and employing a U.S. Forest Service-derived analytical tool, which I have named the FIA-COLE volume-biomass model, the oak has an estimated above-ground dry biomass of 26,679 lbs. Another 15% (probably more) can be added for underground roots giving 30,681 lbs. Approximately 50% of this dry weight is carbon, or 15,340 lbs (7.7 tons). This amount of elemental carbon has a CO2 equivalency of 28.1 tons.

How can we evaluate the 15,340 lbs of carbon in our northern red oak in terms of what it would take to replace that specific amount? A common approach is to attempt replacing the lost carbon with new tree plantings, recognizing the rapid growth of young trees.

HOW MANY TREES

must be planted

and

HOW LONG

must they grow to match

the carbon volume stored in one big tree?

 

Let's assume that we have a 12-inch diameter at breast height (DBH), 50-foot tall young northern red oak. This size oak is likely to be in a fast growth period.

Using FIA-COLE and including roots, the oak’s dry biomass is an estimated 871 lbs, of which 435.5 lbs are carbon. It would take 35.22 young trees of this size to match the carbon in the large oak. These 35.22 younger trees would need to grow for roughly 26 years after planting. This last figure comes from the tables in Methods for Calculating Carbon Sequestration by Trees in Urban and Suburban Settings: April 1,1998, U.S. Department of Energy, Energy Information Administration.

The 26 years does not include the time spent in the nursery, which would be several years. The full time would likely be 29 to 30 years. This age follows from the above government source.

Quoting: The tables included for estimating sequestration were designed for reporters who have planted ordinary, nursery- raised trees, typically sold in 15-gallon containers or balled and burlapped. Such “standard” trees are usually approximately one inch in diameter at 4.5 feet above the ground when planted.

The source does not say how long it takes to reach this size in the nursery, but for the sake of the following analysis, as seen above, I am assuming three years.

autumn-landscape-bottom-view-tree-tops-t

OFFSET OPTION #1

Circumference in breast height

CBH: 37.7 inch

Diameter in breast height

DBH: 12 inch

Height: 50 feet

Dry biomass: 871 lbs

Carbon stored: 435.5 lbs

CO2 equivalent: 1,596 lbs

Age: 30

COUNT:

35

autumn-landscape-bottom-view-tree-tops-t

OFFSET OPTION #2

Circumference in breast height

CBH: 18.85 inch

Diameter in breast height

DBH: 6 inch

Height: 40 feet

Dry biomass: 202.6 lb

Carbon stored: 101.5 lbs

CO2 equivalent: 371.9 lbs

Age: 16

COUNT:

151

HOW MANY TREES

must be planted

and

HOW LONG

must they grow to match

the carbon volume stored in one big tree?

 

Keeping the 35.22 trees needed to replace the 14-foot CBH, 100-foot tall northern red oak in mind, the next calculation may come as a surprise.

Dropping down to a 6-inch DBH, 40-foot tall oak, the number of young trees needed to match the one big tree increases dramatically. FIA-COLE gives us 202.6 lbs of dry biomass per tree. The carbon portion is 101.5 lbs.

The number of small trees needed to equal the big one becomes 151.14.

 

Using the US Department of Energy (DOE) tables again, it would take approximately 13 years from outdoor planting to reach this size plus three in the nursery for 16. In other words, if we can achieve the plan of having the 151.14 oaks alive in 16 years, we would have the same amount of carbon as was held in the big oak. Thought of another way, it would take 151.14 plantings 16 years to get us back to where we were. In addition, it is highly unlikely that all the seedlings would survive. The actual number of plantings would need to be more.

HOW MANY TREES

must be planted

and

HOW LONG

must they grow to match

the carbon volume stored in one big tree?

 

Dropping to a 4-inch and 25-foot height, on FIA-COLE, each tree holds 32.5 lbs of carbon. 

The number of oaks required to replace carbon stored in our big tree skyrockets to 472.01.

On the DOE tables, it takes about 7 years to get a young red oak up to this size, or 10 with nursery time.

autumn-landscape-bottom-view-tree-tops-t

OFFSET OPTION #3

Circumference in breast height

CBH: 12.57 inch

Diameter in breast height

DBH: 4 inch

Height: 25 feet

Dry biomass: 65 lbs

Carbon stored: 32.5 lbs

CO2 equivalent: 119.1 lbs

Age: 10

COUNT:

472

autumn-landscape-bottom-view-tree-tops-t

OFFSET OPTION #4

Circumference in breast height

CBH: 6.28 inch

Diameter in breast height

DBH: 2 inch

Height: 10 feet

Dry biomass: 10 lbs

Carbon stored: 5 lbs

CO2 equivalent: 18.3 lbs

Age: 7

COUNT:

3,068

HOW MANY TREES

must be planted

and

HOW LONG

must they grow to match

the carbon volume stored in one big tree?

 

The next comparison takes a typical nursery-grown stock planted in cities.

DBH is 2 inch and the height is 10 feet. Carbon stored is 5 lbs.

The number of saplings needed to equal the large oak skyrockets to 3,068.

 

HOW MANY TREES

must be planted

and

HOW LONG

must they grow to match

the carbon volume stored in one big tree?

 

As a final comparison, let’s take a young, newly planted tree from nursery stock. Let’s say that its diameter is one inch and it is 4.5 feet tall, such as described by the DOE source above. The above-ground volume of the seedling can be approximated by:

Formula.png

where D = 1/12 = 0.0833, H = 4.5, and F (trunk form factor) = 0.5. This yields 0.01227 ft3 of trunk volume. We increase this by a factor of 0.25 to add in limbs, branches, and twigs, giving 0.01153 ft3. Finally, we add in below-ground roots at an additional 15% to arrive at a total volume of 0.01764 ft3.

With a dry weight of 36.19 lbs/ft3 for Quercus rubra, we have 0.63837 lbs of dry biomass in our small tree. The carbon part is 0.31918 lbs.

Remembering that our large N. red oak holds 15,340,425 lbs of carbon, it would take 48,061.33 newly planted trees to match the carbon in our one large oak, and they would be three years old! This is a staggering number and the physical space they would require is equally eye-opening. So, there is no instantaneous solution, and there is an additional problem...

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OFFSET OPTION #5

Circumference in breast height

CBH: 3.14 inch

Diameter in breast height

DBH: 1 inch

Height: 4.5 feet

Dry biomass: 0.64 lbs

Carbon stored: 0.319

CO2 equivalent: 1.169 lbs

Age: 3

COUNT:

48,061

SPACE NEEDED

Assuming each 1-inch diameter seedling controls 5 square feet of ground space, then the total area needed to hold the seedlings becomes 48,061 x 5 = 240,305 square feet or 5.5 acres. It hardly seems a fair exchange. While we shouldn’t expect to get instantaneous results from seedlings, planting our way out of the climate problem proves not so easy.

DBH: 12″ 

Height: 50 ft

Age: 30

OAKS  NEEDED

35

DBH: 6″

Height:  40′

Age: 16

OAKS  NEEDED

151

DBH: 4

Height: 25′

Age: 10

OAKS  NEEDED

472

DBH: 2

Height: 10′

Age: 7

OAKS  NEEDED

3,068

DBH: 1

Height: 4.5′

Age: 3

OAKS  NEEDED

48,061

SUMMARY

Using the FIA-COLE volume-biomass model, the large northern red oak with a DBH of 4.46 feet, a height of 100 feet holds 15,340 lbs of carbon.

Number of younger trees needed to supply equivalent amount of carbon:

The inescapable truth is that we need continuing help from our existing mature trees. There is a storage efficiency gained through their size. We need them for the carbon they are presently sequestering and for the amount they can continue adding if we keep them healthy. To be sure, planting is important, but keeping large trees standing and healthy takes on extra importance during this climate crisis.

We need continuing help from our existing mature trees.

IMPORTANT NOTE:

The numbers given in these offset examples represent only the carbon stored in the above defined large northern red oak and their offset by planting new trees.

In case the tree is burned, which means the carbon stored in the tree is emitted into the atmosphere as CO2, the number of planted trees has to be doubled. We have to replace the carbon absorbed and stored in the big tree and to absorb the emitted CO2 from burning back from the atmosphere.

The numbers given in this article do not include the following carbon footprints:

  • Seeds collection

  • Planting in nursery

  • Planting to the final stand including transport

  • Aftercare including watering

  • Logging, transport and processing of the large tree.

  • Carbon footprint of all necessary items needed to rise, plant and care of the new trees.

Cutting large urban trees under the assumption that we can quickly plant enough smaller ones to replace the carbon sequestered in the big trees is not a sensible strategy.

 

A far more climate friendly path is to plant new trees while keeping the existing big ones alive and healthy.

NOT A SENSIBLE SOLUTION

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CONCLUSION

The lesson is to plant new trees while preserving, and keeping healthy, the ones we already have. That is the win-win strategy.

If we plant enough young trees, we can offset the loss of the big tree, but the number of young trees needed, their time of growth, and the ground space they require should encourage us to retain our big tree for as long as we can.

 

As time progresses, and the young trees grow, the number needed to offset the one big tree drops.

 

However, urban trees planted today have a surprisingly short lifespan. So, while it is true that in 40 years, we can capture a lot of CO2 through plantings, if the extra sequestration simply offsets what we previously held in existing, mature trees, we have gained nothing, and we have lost 40 years!

The lesson is to plant new trees while preserving, and keeping healthy, the ones we already have. That is the win-win strategy.

for TREEIB.COM

The Native Tree Society

out.png

-Robert T. Leverett-

June 2021

WHO
IS

 

ROBERT

LEVERETT
 

Recognized as 
"Premier old-growth forest evangelist"
 

  • Environmental researcher and scientist

  • Environmental writer

  • Educator

  • Principal trainer and workshop leader for American Forests on tree-measuring methods for championship trees


A close colleague to William R. Moomaw, the Professor Emeritus of International Environmental Policy at the Fletcher SchoolTufts University, a lead author of the Nobel Prize-winning Intergovernmental Panel on Climate Change