Modelling Categories and Data

Hi, any help would be greatly appreciated.

I am trying to create growth models for different tree species which correlate tree weight with height. This in itself is not a problem.

I am then trying to see if there is a significant difference between species, or, if it is possible to use a single model/equation for all species.

I have been using R and SAS but have reverted to Sigmaplot as my coding was not up to scratch.

Would something as simple as a Multiple Regression between all species work?

Thanks in advance.