Awakened trees are treated as animated objects, except that they gain the plant type and 3d6 Int, Wis and Cha. Animated objects have no Con score. Do awakened trees also have no Con score?
Frankly, they should have a Con score -- plant-creatures are supposed to have Constitution scores. For starters you can look at Treant statistics and modify up or down for different sizes. Furthermore, for the d20 campaign book "Garden of the Plantmaster", in the Appendix, I created a table of statistics entries for trees & shrubs of every possible size category.