I was reading this weekend about how Napoleon ordered all the roads in france to have trees planted along them, so his troops could march in the shade.
At present, trees are just eye-candy in simutrans.
Presumably, though, it would be possible to code something such that human or horse drawn vehicles get some benefit from moving along a route which has trees along it, at least in the months of say May-September. This would give some incentive to planting them, along roads and canals, perhaps just in certain climate zones (which the game has). ☺
Also, presumably, rail services could suffer some commensurate disadvantage in the months of October and November... ???
This is an interesting if somewhat eccentric idea; not a priority given the very long queue of balance critical tasks. Simulating the effect of poor adhesion on railways would be especially complex, especially given that one would also need to simulate the effect of lineside fires from steam trains keeping the tree growth in check automatically, but this ceasing to be the case with non-steam traction.