Robert Leverett
"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 metric units
OUR
LARGE OAK...
Circumference in breast height
CBH: 4,27 m
Diameter in breast height
DBH: 1,36 m
Height: 30,48 m
Dry biomass: 13 917 kg
Carbon stored: 6,96 tons
CO2 equivalent: 25,5 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, fastgrowing ones.
Numeric returns are expressed in metric units. A equivalent in imperial units can be displayed here.
Calculating the amount of carbon stored in a large northern red oak
Assume our oak measures 4,27 meters in circumference (1,36 meters DBH) and is 30,48 meters tall. Such an oak grows about 5 kilometres from where I sit in western Massachusetts, USA.
Given these dimensions and employing a U.S. Forest Servicederived analytical tool, which I have named the FIACOLE volumebiomass model, the oak has an estimated aboveground dry biomass of 12 101,41 kg. Another 15% (probably more) can be added for underground roots giving 13 916,78 kg. Approximately 50% of this dry weight is carbon, or 6 958,57kg (6,96 metric tons). This amount of elemental carbon has a CO2 equivalency of 25,46 metric tons.
How can we evaluate the 6 986 kg 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 30,48cm diameter at breast height (DBH), 15,24meter tall young northern red oak. This size oak is likely to be in a fast growth period.
Using FIACOLE and including roots, the oak’s dry biomass is an estimated 395,08 kg, of which 197,54 kg 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 15gallon containers or balled and burlapped. Such “standard” trees are usually approximately 2,5 cm in diameter at 1,4 m 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.
OFFSET OPTION #1
Circumference in breast height
CBH: 95,7 cm
Diameter in breast height
DBH: 30,5 cm
Height: 15,2 m
Dry biomass: 395 kg
Carbon stored: 197,5 kg
CO2 equivalent: 723,8 kg
Age: 30
COUNT:
35
OFFSET OPTION #2
Circumference in breast height
CBH: 47,9 cm
Diameter in breast height
DBH: 15,2 cm
Height: 12 m
Dry biomass: 92,1 kg
Carbon stored: 46,04 kg
CO2 equivalent: 168,7 kg
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 14,27meter CBH, 30,48meter tall northern red oak in mind, the next calculation may come as a surprise.
Dropping down to a 15,24cm DBH, 12,19meter tall oak, the number of young trees needed to match the one big tree increases dramatically. FIACOLE gives us 92,08 kg of dry biomass per tree. The carbon portion is 46,04 kg.
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 150,39 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 10,16cm DBH and 7,62meter height, on FIACOLE, each tree holds 14,74 kg 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.
OFFSET OPTION #3
Circumference in breast height
CBH: 32 cm
Diameter in breast height
DBH: 10,2 cm
Height: 7,6 m
Dry biomass: 29,48 kg
Carbon stored: 14,74 kg
CO2 equivalent: 54,01 kg
Age: 10
COUNT:
472
OFFSET OPTION #4
Circumference in breast height
CBH: 16 cm
Diameter in breast height
DBH: 5,08 cm
Height: 3 m
Dry biomass: 4,54 kg
Carbon stored: 2,27 kg
CO2 equivalent: 8,3 kg
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 nurserygrown stock planted in cities.
DBH is 5,08 cm and the height is 3,05 meters. Carbon stored is 2,27 kg.
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 2,54 cm and it is 1,37 meters tall, such as described by the DOE source above. The aboveground volume of the seedling can be approximated by:
where D = 2,54/100 = 0,0254 meters, H = 1,37 meters, and F (trunk form factor) = 0,5. This yields 0,00035 m3 of trunk volume. We increase this by a factor of 0,25 to add in limbs, branches, and twigs, giving 0,000434 m3. Finally, we add in belowground roots at an additional 15% to arrive at a total volume of 0,000499 m3.
With a dry weight of 579,75kg/m3 for Quercus rubra, we have 0,2896 kg of dry biomass in our small tree. The carbon part is 0,1448 kg.
Remembering that our large N. red oak holds 6 958,3 kg of carbon, it would take 48 061 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 eyeopening. So, there is no instantaneous solution, and there is an additional problem...
OFFSET OPTION #5
Circumference in breast height
CBH: 8 cm
Diameter in breast height
DBH: 2,54 cm
Height: 1,4 m
Dry biomass: 0,23 kg
Carbon stored: 0,12 kg
CO2 equivalent: 0,42 kg
Age: 3
COUNT:
48 061
SPACE NEEDED
Assuming each 2,54cm diameter seedling controls only 0,46 m2 of ground space, then the total area needed to hold the seedlings becomes 48 061 x 0,46 = 22 108 m2 or 2,2 hectares. 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: 31 cm
Height: 15,2 m
Age: 29
OAKS NEEDED
35
DBH: 15 cm
Height: 12,2 m
Age: 16
OAKS NEEDED
150
DBH: 10 cm
Height: 7,62 m
Age: 10
OAKS NEEDED
465
DBH: 5 cm
Height: 3 m
Age: 7
OAKS NEEDED
3 068
DBH: 2 cm
Height: 1,37 m
Age: 3
OAKS NEEDED
48 061
SUMMARY
Using the FIACOLE volumebiomass model, the large northern red oak with a DBH of 1,36 meters, a height of 30,48 meters holds 6 958,57 kg 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
CONCLUSION
The lesson is to plant new trees while preserving, and keeping healthy, the ones we already have. That is the winwin 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 winwin strategy.
for TREEIB.COM
Robert T. Leverett
June 2021
WHO
IS
ROBERT
LEVERETT
Recognized as
"Premier oldgrowth forest evangelist"

Environmental researcher and scientist

Environmental writer

Educator

Principal trainer and workshop leader for American Forests on treemeasuring methods for championship trees