I'm planning a picture that may involve a tree so I've been practising trees in Illustrator and Photoshop. The tree and background were drawn in Illustrator and then bits were added in Photoshop (some texture, the light twinkles and the grass) I love to find really nice brushes such as the ones I used for the grasses (which you can find here http://qbrushes.net/plants/grasslands-brushes/) when I'm experimenting but I also like to make my own. Here I've made a brush to make the twiggy bits of the tree branches. So to share the love I've put the twiggy brush on my website as a free download so you can try it out if you like it. Go here and navigate to the 'links' page - the free download is at the bottom.
To use the brush open the zip file and open up the Illustrator file inside it. It will look like this:
Select this image and drag it over to your brushes palette. When prompted choose the option 'New Art Brush' and you will be shown an options window - I've left all the options at default but play around with.