Vico is used whenever a queue, or line, of individual objects is involved. I don't know why, but this has always made perfect sense to me. (despite Ĵetkubo's comment elsewhere on this page)
In Butler's Dictionary (Esperanto -> English; highly recommended) vico is defined as: Line, rank, turn; file, row, succession, suite (side by side, or one after another). Whereas linio is: Line (geometry); Long narrow mark, fold, furrow, seam, ktp.
Since a row of trees (probably a better translation than Duo's, IMHO) is not a mark or furrow that leaves us with vico, ĉeno (chain) or serio (series).
Now I'm trying to imagine a chain of trees…