Keep in mind that the lessons focus on different topics not to specifically address how to draw everything under that specific umbrella, but rather as different lenses through which to look at the same core problems of developing our spatial reasoning skills through constructional drawing exercises.

I have generally found that trees do tend to be more complicated and don't necessarily contribute in any unique fashion to that core goal that other plants don't already. While I don't have anything against students trying their hand at a tree or two, it's not an area I feel there's any benefit in exploring further in the lesson material itself.

So- no harm either way in trying them, but it is totally normal that they'd be quite a bit more challenging.