[R] rowMean, specify subset of columns within Dataframe?

[aaront]

I would like to calculate the mean of tree leader increment growth over 5
years (I1 through I5) where each tree is a row and each row has 5 columns.
So far I have achieved this using rowMeans when all columns are numeric type
and used in the calculation:

Data1 <- data.frame(cbind(I1 = 3, I2 = c(0,3:1, 2:5,NA), I3
=c(1:4,NA,5:2),I4=2,I5=3))
Data1
Data1$mean_5 <- rowMeans(Data1, na.rm =T)
Data1

My real dataset has many columns including several Factor type. Is it
possible to specify a range of columns within a data frame  using rowMeans
dims= (say where there is a one 
Factor  column called Species leading I1 to I5, and one Factor column called
Moisture following, so 7 columns total) , or do I either need to extract
those columns to a new data frame, calculate means, and reattach to the
original data frame, or use a different function such as apply?

Unfortunately I am fairly new to R and have a difficult time with some
terminology and concepts in R Help.

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