?mvpart
 
#Prepare the data: remove “distance from source”
env <- subset(env, select = -das)

# Create the regression tree
doubs.mrt <- mvpart(as.matrix(spe.hel) ~. ,env,
            legend=FALSE, margin=0.01, cp=0, xv="pick",
            xval=nrow(spe.hel), xvmult=100, which=4)