tree <- read.csv("trees91.csv", header=T, sep=",", dec=".", colClasses=c(C,N,REP,LFBCC,STBCC,RTBCC="integer", CHBR="character", "double"), nrows=70)

Error in read.table(file = file, header = header, sep = sep, quote = quote,  : object 'N' not found