Is Bowen ratio an absolute indicator of land surface type based on the amount of heat transfer from the surface?
A very simple answer is that it is related to the demands of the vegetation types and their ability to maximize the net radiation. A "perfect" plant that covered the entire surface would theoretically result in a BR of 0 as 100% of Rn is used for photosynthesis and released as ET, such that H=0 and LE=RN and B=H/L. It obviously doesn't work that way though. 

Since $B=y\frac{\partial T}{\partial e}$ where y is the psych constant (~0.4 and really not a constant) it can be seen that the gradients of temperature and moisture, not the actual values, are the only two factors that matter in determining the Bowen ratio. Each plant essentially has a "specific Bowen Ratio" range where they are able to maximize their use of RN for the environment they are in. If the climate changes in that area the plant would not be able to survive. 

That is a very very basic and highly simplified explanation, but hopefully it helps