How green is your Christmas tree?
For many people, Christmas without a Christmas tree is unthinkable. In recent years, the sustainability of placing a living or slowly dying tree in your home has been questioned. Others have ‘defended’ this practice by comparing it to the habit of cutting off flowers and having them in a vase, where they generally last shorter than a Christmas tree.
We can state without any calculation that the CO2 balance of the tree can be neutral if a) it is grown in a responsible way (in particular: a new tree is planted for every tree that is cut), and b) it is used to generate energy after it leaves your home (in a biofuel power plant, for instance). The transport of the tree, however, causes additional emissions, so it is best to use a locally grown tree.
It can be argued that, if these conditions are met, buying a Christmas tree, is a way of subsidising the energy transition.
Reden makes sense is about numbers, so, although we already know the conclusion, we still calculate, out of curiosity, the amount of CO2 in the Christmas tree in your home or office.
How much carbon per tree?
Carbon comprises about 50% of the dry weight of a tree [1]. However, Christmas trees aren’t ‘air-dried, seasoned (20% moisture)’, so we must compensate for the moisture. According to engineering toolbox.com [2], fresh Douglas fir has a density of 609 kg/m3 in coastal regions, and dried (20% moisture content), this wood has a density of 529 kg/m3. Confusingly, the moisture content is defined as the mass of the moisture divided by the dry mass of the wood [3]. To complicate matters, the volume changes when the wood dries. Based on the same Wikipedia article, which is not very precise on this, we estimate 12% in volume shrinkage for fresh wood to 20% moisture content. Now, we can calculate the moisture content of a fresh tree. We assume it is the same in the needle-covered branches.
Which gives: k = 0.547 = 54.7%, and the fresh weight as 1.547 times the dry weight. Hence the dry weight is 64.6% of the fresh weight. 50% of this is carbon; 32% of the fresh weight.
The next question is: how much does a Christmas tree weigh? [4] According to one of many sites which Google finds for answering this question [5], a 2.4 m high tree weighs between 22 and 34 kg. Let’s take 28 kg. The figure below shows the simplified geometric shape: a cone. The volume of a cone with given top angle depends on its height to the power of 3, and so does its mass (for constant density): m (kg) = constant*H3. Using the average weight of the 2.4 m high tree, we can now calculate the weight for other heights. These are shown in the table below.
How much is that in C?
Now, we can calculate the Carbon content of the tree. Let’s assume we have a 2m high tree, then it weighs 16 kg, and 32% of this is carbon: 5.2 kg. Since CO2 has a molecular mass of 46 g/mol, and C has an atomic mass of 12 g/mol, 5.2 kg of carbon is equivalent to 5.2 * 46/12 kg CO2, or 20 kg of CO2. This CO2 was absorbed by the tree when it grew, and will return to the atmosphere when the tree is burnt or when it decomposes biologically. If the tree is used, after being discarded to generate sustainable electricity, for instance in a (former) coal fired station, then it serves a useful purpose after Christmas, too.
To get an idea of the amount of CO2: if you have a fairly ‘clean’ car with a CO2 emission of 125 g/km, then the carbon content of the Christmas tree is equivalent to 160 km worth of emission (still for the 2 m high tree).
The net emission effect of the tree itself is zero. The C released by burning the tree now was captured by the tree several years in the past. However, the tree may have been grown a long distance away. This could double the emission, without doubling the capture! [6]
This means that the greenest Christmas tree is the one which grew locally, perhaps in your own garden!
How many trees?
Wikipedia quotes a paper published in 2000 which gives the number of Christmas trees used each year as 30 - 36 million in the USA and 50 – 60 million in Europe [7]. Let’s use 100 million as a nice round figure (a small increase since 2000 seems likely). The CO2 content of all these trees, if their average height is 2m, is 20 kg * 100 million = 2 million tonnes of CO2, equivalent to 400 000 trips round the world [8].
Conclusion
The life cycle of the tree itself is largely CO2 neutral. Moving trees over large distances, however, is not (except the transport is fully sustainable).The amount of CO2 per tree is equivalent to the emission by an economical car covering a distance of roughly 160 km.
If you want to have the greenest possible Christmas tree, source it locally and make sure it is collected to generate sustainable energy after it has served as a decorative symbol.
[2] WoodSpecies - Moisture Content and Weight (engineeringtoolbox.com)
[4] Technically, we are talking about its mass (in kg). For non-technical readers, this is not confusing, science teachers may wince at this!
[5] How Much Does a Christmas Tree Weigh? The Surprising Truth! - My Heart Lives Here
[6] This point is often disregarded in the discussion about bio fuels: burning wood from a Canadian wood in a power plant in Europe is counted as CO2 neutral, but the energy used for transport leads to a negative balance. And if the trees are from an old forest which is not replanted carefully, then it is even worse.
[8] This is a measure of doubtful use, but it does convey the message that it is a lot.
