Having an understanding of chemistry and knowing how poisonous copper and arsenic can be i would steer well clear of this treated wood. In the short term it may have little effect on your trees but over extended periods (years) it may slowly build up and poison your tree. I found some info on it
here which mentions up to 13% of the chemical leaching into the soil and binding with it. In the small amount of soil held in your box, this could be a worry.
Al