[R] rpart and randomforest results

[Schillo, Sonja]

Hi all,

I have a question on rpart and randomforest results:

We calculated a single regression tree using rpart and got a pseudo-r2 of roundabout 10% (which is not too bad compared to a linear regression on this data). Encouraged by this we grew a whole regression forest on the same data set using randomforest. But we got  pretty bad pseudo-r2 values for the randomforest (even sometimes negative values for some option settings).
We then thought that if we built only one single tree with the randomforest routine we should get a result similar to that of rpart. So we set the options for randomforest to only one single tree but the resulting pseudo-r2 value was negative aswell.

Does anyone have a clue as to why the randomforest results are so bad whereas the rpart result is quite ok?
Is our assumption that a single tree grown by randomforest should give similar results as a tree grown by rpart wrong?
What am I missing here? 

Thanks a lot for your help!
Sonja