Is it possible (preferable using fairly common tools) to extract nutrition from grass or trees? Say for instance in a time of hunger. Can you make a grass-soup – and could you live off it?
Rune
No - grass and trees contain lots of celluose, which the human body can’t break down, thus you would get little nutrition from these types of plants.
Depending on the type of tree, yes - quite apart from the very nutritious seeds(nuts) and fruits produced by trees, many of them (such as birch and maple) can be tapped for their sap.
The bark stripped from young birch twigs has (in years gone by) been pounded into a pulp and baked into a sort of breadlike cake.
Grass is a little more difficult to extract nutrition from, but some grasses have thickened root systems that can be crushed to extract carbohydrates. - the seeds of some wild grasses are edible and nutritious if ground up or boiled.
(not forgetting, of course, that wheat, maize, bamboo and sugar cane are grasses)