If I understand well, you are talking about to use the function `predict()`.

So you have the model:

 `LM <- lm(PSR ~ Area+Forests, data = Wetlands)`

The function `predict()` provide this usage

`pred <- pred(your_model, your_data_test)`

but `your_data_test` have more variables than your model, right?

You can make a `new_data_test` using this,

`new_data_test <- data.frame(your_data_test$variable1,your_data_test$variable2)`


`pred <- pred(yourmodel, new_data_test)`

I suppose that this may be works well.