Brush pen, copic marker, and white colored pencil.
When my sister and I were young, we would spend summers at our grandparent’s cabin. I remember loading up my pockets with candy bars and cans of soda, and climbing the large pine trees on the edge of their property to see how high I could get, and would stay up there as long as I could. Getting scratched and covered in sap was totally worth it.

So sweet. And a very neat, simple style. Did you use the white colored pencil over the marker? That little hint of white is a great effect.
Neat and simple… because I was running out of time to do something for this week! XD But I really do like how it came out. Sometimes simple can be better. Recently listened to a podcast (http://www.makingcomics.com/2012/04/22/jake-parker-on-making-comics-12/) where an artist was saying that he noticed as he got better, his drawings became simpler, and effectively told a better story because there was less to distract from it.
And yes! Colored pencil for highlights over the marker. Figured I’d try it out after seeing this post from Katie Cook http://katiecandraw.typepad.com/katiecandrawcom_internet_/2012/03/wonder-woman-marker-demo-piece.html
This is lovely, and I spent many hours of my childhood in trees! Also just attempted an illustration with marker and prisma pencils and was really pleased.
I never got to have a tree house, but I keep telling my husband that someday I’d love to have one for my art studio. I peeked at your site and didn’t see it? But yes, the combination makes a lovely effect.