So here you go, only 3 easy steps to do it :

Step 1- You start doing the foliage wet in wet, applying a layer of yellow, then another layer of green, and why not small touches of orange here and there, for a feeling of fall. These pigments are all applied to wet paper and the results will vary from undefined lines to blurry shapes, depending on how wet is the paper. 

Step 2- Then mix a brown watercolor, let’s say Burnt Umber with some French Ultramarine and apply a thick and rich mixture in order to depict the branches of your tree, don’t wait until the foliage colors are dry… apply your dark color wet in wet in order for it to bleed into the initial foliage colours suggesting dark areas shaded by the foliage.

Step 3- Start the tree branches from up to down, from the smallest branches, and getting bigger as you go down, finishing with the tree trunk, applying only the dark colour from one side in order to suggest rounded form.

And the final result should be close to this:

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