What Killed the Last Tree?
I was tossing around some theories with @ctrl-shift-alt-9 and @water-activated-light about what may have killed the Last Tree on Eden and ended up doing some bullshit math based on Molly Brown's concept art for the Tree, an hour's worth of research into fertilizer and soil nutrient levels, and some wild speculation.
This post has a much more unsteady foothold in canon than most of my other posts, so consider everything you read here as a headcanon. It will be tagged as #wild conjecture accordingly.
I am also, notably, not an arborist nor am I too familiar with agriculture beyond basic plant care, so feel free to rip me apart peer review!
so without any further ado:
This is working off the (basically confirmed at this point) theory that Eden was feeding burying its dead within the roots of the Last Tree to sustain it. The Last Tree is (semi-reliably) confirmed to be dead by Simon's flashback of speaking with Ava at timestamp: 01:40:34
"The Last Tree's dead... It's Eden's grave."
But how did it die? There are a lot of possibilities (lack of space, lack of essential nutrients, disease, sabotage, etc) but one that intrigues me the most is the possibility that Eden overfed over-fertilized it. Trees require a balance of various nutrients to stay healthy, and in enclosed, sterile environments those nutrients where those nutrients aren't being naturally replenished with plant/animal matter, they need to be added back in via fertilization. There's a wide variety of nutrients that trees require, but the main one I focused on in my theory is nitrogen. Nitrogen is one of the major components that fuel a tree's ability to grow, and too little can lead to stunted growth and dying leaves. Too much nitrogen can cause trees to grow excessively, expend a shit ton of energy doing so, imbalance their other nutrient levels, and leave them susceptible to disease. Nitrogen is one of the three major components of most fertilizer, and without a ready supply of fertilizer, Eden would have been left scrambling to find sources of nitrogen and other nutrients to keep the Last Tree alive.
The good news: decomposing organic matter is a great source of nitrogen
The bad news: not a lot of organic matter available in space
the horrifying news: a decomposing human body releases 32 grams of nitrogen for every kilogram of dry body mass (Source, be warned, article contains images of decomposing pig carcasses photographed for scientific and education purposes).
So how many bodies much nitrogen is too much?
I unfortunately was only able to hunt down ideal nutrient levels for crops, at least in an easily usable form. I wasn't committed enough to try and calculate the ideal nitrogen ppm for deciduous trees based on the lbs/ft² data that i could find, so we're sticking with crop data for now. Again, we're in wild conjecture territory.
The ideal range for nitrogen in the soil is between 10-15 parts per million (ppm). Basically, for every unit of soil mass, 10-15 millionths of that mass should be nitrogen. For 1 kilogram of soil, that's 10-15 milligrams.
Anything above 20 ppm puts the tree at risk of the effects mentioned above and the eventual death of the tree if not rectified. So, if we roll with my theory of the tree being over-fertilized, how many bodies would Eden have to bury for the soil to hit critical levels of nitrogen saturation and the tree to start suffering?
First up we have to calculate the volume of the soil in the Grove. Made some rough guestimates based on DeadlyMelodic's concept art, seen below.
Using the very small person seen at the base of the tree, I measured out a diameter of 26.5 units. Multiply that by the average height of a human being (1.65~ m) and we get a 43.725 meter diameter (143.45 ft for the imperial folk). The depth was a little trickier; I ended up just extending the slope of that outside ring to see where it would hit the interior wall, then called upon Pythagoras to get a depth of 4.95 meters (16.24~ ft). Assuming that the Last Tree is some species of maple (just going based on looks here), its roots could extend anywhere from 1.8-3.0 meters below ground, maybe deeper based on its size, so it feels fair to say that they would have designed the arboretum to accommodate that. Plug those numbers into the formula for the volume of a cylinder, and we get: 7432.418 cubic meters
Not done there though! Still need to get the mass of all that dirt since ppm is based on mass, not volume. The 1 cubic meter of loosely packed soil has a mass of 1200 kg, so if we take that number multiplied by the cubic volume of the arboretum we get a whopping 8.919x10⁶ kg (19 million lbs jesus fucking christ).
Using that total mass of the soil, we can calculate how many kilograms of nitrogen are needed to get to 20 ppm by making 20 = (nitrogen/8.919x10⁶) * 10⁶. Solving for nitrogen gives us 178.38, so the total mass of nitrogen in the soil would have to be 178.38 kilograms or greater to get the soil to harmful levels.
The nitrogen levels wouldn't have been at 0 ppm when the first body batch of fertilizer was added though. I ended up subtracting the equivalent of 8 ppm to simulate a low nitrogen level, so now we just need to add 107.03 kg of nitrogen to the soil to reach that harmful 20 ppm.
Last batch of scribbled equations, I swear.
The human body releases 32 grams of nitrogen per 1 kilogram of dry body mass as it decomposes, dry mass meaning everything besides the 60% water the body is composed of. Using the average body mass of a human being (67.7 kg) and multiplying that by 0.4 to get the dry mass only (27.08 kg), then multiplying that by 32 grams (0.032 kg), we get the average amount of nitrogen released by a decomposing human body: 0.867 kilograms. Divide the total amount of nitrogen by the amount released by a body and we get an estimate of how many bodies would have to have been buried in the roots of the Last Tree for it to suffer from nitrogen toxicity:
Which is... a lot. I'm not even sure if that amount of mass would fit beneath the roots of the tree. All of this bullshit, however, has been looking at nitrogen toxicity alone; there are a lot of other factors to consider when looking at the logistics of fertilizing the Last Tree with corpses. Repeated root disruption, pH imbalances, hell even the sheer amount of mass they're trying to compress into the soil could result in an anaerobic bacterial infection and root rot.
I am still of the opinion that the Last Tree died because it was overfed. Using human bodies as fertilizer comes at a great risk to the Tree they're supposed to be keeping alive. It may have sustained it at first, but the practice absolutely contributed to its death in the end. Hypothetically. Again, wild conjecture land.
