If you ever visited a forest in real life, the ground is mostly old leaves and little twigs. More like splinters but there are also many bigger twigs.
(from: File:Forest-floor076.jpg - Wikipedia)
Currently it is actually hard to find firewood without an axe in the forest. As someone who actually gathered firewood for an actual fire in the forest, it is actually pretty easy to find a bit of firewood for a small camp fire you use to cook food.
This is why I want to potentially add a new tile: Forest soil. You get some leaves, splinters and twigs from it. If you are lucky you might even find a stick. The reason why I do not want to spread all that as items on the forest floor is performance, so adding a new tile and letting the player either examine it or use some sort of construction action to gather stuff seems sensible.
I think a construction action might be better here because if you can examine literally almost any tile in the forest it gets spammy.
Why leaves? There is the pile of leaves and a few constructions in my PR which need an insane amount of withered plants or straw because we do not have leaves yet. Both twigs and leaves are mostly intended to be burned, but can be used for some crude constructions if you have to live in the forest. I could imagine they can also be used for composting, but this is out of scope.
Leaves and twigs should also be dropped from trees when you cut them down. This should be the only thing requiring C++. I would also buff shrubs because they only give you 0-2 withered plants. They would drop leaves and a slightly higher amount of withered plants.
I am aware that in real life you have to fight against wet firewood, but this is also out of scope here, until we have support for wet terrain.