I realize this is a relatively old question, but as a consulting arborist I have gained much experience with tree surveys and creating graphical representations of driplines and root protection zones, using QGIS specifically.
That said, I have to second CHenderson's approach, as it is the one I use for every inventory I conduct. I will qualify that endorsement by saying I have also tried the approach outlined by Underdark, and find that the size of the circles change as the map scale changes.
Creating additional shapefiles also has the advantage the graphical representation is retained should you want to export the shapefile to CAD (I do this practically 100% of the time). Since the tree measurements are used for the edification of engineers and the like, a CAD product is more valuable and easy to work with for them.
I would also discourage you from adding to the attribute table wherever possible. Depending on the size of your inventory, this can be very tedious and increases the likelihood of error. Instead, collect relevant data in the field, transcribe it to a digital format (if you're not collecting it digitally via PDA, tablet, GPS, etc.), and import it to QGIS as a .csv. Most of the other data can be added using the field calculator, including protection zone radii (which in Ontario are either based on the trunk diameter or dripline).