Excerpt from Vegetation Development on Disturbed Grand Fir Sites, Swan Valley, Northwestern Montana Grand fir (abies grandis [doug1.] western redcedar (thuja plicata western white pine (pinus monticola and western hemlock (tsuga heterophylla [raf.] Sarg.) are major forest trees of northern Idaho and areas farther west, but' decrease in abundance and become more localized eastward across northwestern Montana. They all reach their range limits near the Continental Divide. This distribution pattern is related to the influx of moist Pacific air along a prominent storm track roughly paralleling the Canadian border (daubenmire Upon crossing each mountain range the moisture supply is diminished. The last major pockets of heavy, low elevation precipitation occur where these Pacific air masses intercept the main ranges of the Rocky Mountains east of the Flathead Valley. These areas contain luxuriant forests, in which grand fir, western redcedar, or western hemlock are potential climax species.