As you can see, the first image is a very simple outline sketch. Don't go into detail trying to draw loads of twigs, just the main trunk and the main boughs and the rest we will do with a paintbrush. 

Now the left-hand side, yellow ochre, trying to capture light. 

In the middle section of the tree, raw umber, then at the right-hand side a mixture of ultramarine blue and burnt sienna. 

At this point I start to invent a few twigs.  I used my rigger brush for all of the tree - a rigger brush is a lovely bouncy flicky brush, so don't try to hold it stiff and tight, hold it further back and let the brush bounce around and make the twigs for you.

Another use for a rigger brush is to lay the full length of the hairs on the side of the tree and just flick it out quickly using a mixture of ultramarine blue and burnt sienna and there we have rough growth on the side of the tree. 

For the foliage I changed to my 3/4" wash brush and dabbed the brush flat, filling all of the hair with yellow ochre followed by burnt sienna.

Then a mixture of ultramarine blue and burnt sienna. Although the paint is fairly well watered down, don't have surplus water in the brush as these are fairly dry brush strokes.  I have put a little bit of grass underneath the tree to finish it off and flicked up using my finger nails to make a clump of rough grass.  

And there we have a lovely autumnal tree.