A study of population differentiation and variation in Abies procera

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Abstract
Abies procera Rehd. (noble fir) is a large-coned true fir of Washington and Oregon. Collections from areas north of any reported contact with its close relative, A. magnifica Murr., show population differentiation in response to the history of the site upon which the plants became established; immature trees that become established on sites denuded by clear-cut logging or mature (cone-bearing) trees that became established on sites denuded by fire are different from those that became established under mature forests. There is also a weak indication of geographic differentiation. Populations of mature trees from any one collecting site are generally more variable than immature trees from the same site. This unexpected result could be due to mature trees becoming established under many different climatic and hence selective regimes or to mature trees, which consist of more age-classes, including individuals that are made up of more generations resulting from sexual reproduction. The combination of history-related differentiation and higher variation in mature trees offers potential explanation for the high genetic variation in trees. History related population differentiation, in conjunction with the long-term fire history of forests, implies that some tree species are in fact mosaics of different populations depending on fire history and suggests that selection can cause nondirectional (= nonphyletic) changes in populations.