Manzanita (Arctostaphylos) Hybrids in the Pacific Northwest: Effects of Human and Natural Disturbance

Abstract

Three species of Arctostaphylos in cismontane northwestern North America hybridize where sympatric. The combination A. columbiana x A. uva-ursi, named A. media (or A. x media), is common on disturbed forest land in western Washington and southern British Columbia. It is transient due to forest succession. The combination A. columbiana X A. nevadensis is locally common on the lower western slopes of Mt. Hood, Oregon, and adjacent southern Washington. The disturbance that may have led to the latter instance of sympatry and hybridization is less due to man than to natural causes: a mudflow on Mt. Hood dated at 1690 years B.P. Morphology, floral biology, and ecological attributes of the hybrid populations are analyzed to document the interspecific gene flow. Three species of manzanita, Arctostaphylos (Ericaceae), are found in the cismontane and coastal areas of northwestern United States and adjacent Canada-A. columbiana Piper (Hairy Manzanita), A. uva-ursi (Linnaeus) Sprengel (Kinnikinnik), and A. nevadensis A. Gray (Pinemat Manzanita). Any two of them may occur sympatrically in particular habitats. Associ- ated with the coexistence of A. columbiana with A. uva-ursi are plants of intermediate character that have been named A. media (or A. x media) (Fig. 1-3); the intermediate associated with A. columbiana and A. neva- densis has not been named. The plants named A. media Greene are un- doubted hybrids as first pointed out by Piper (in Greene, 1891). The present paper describes the ecological, phytogeographic, and floral bio- logical attributes of the recurrent hybridization. In particular, it em- phasizes the effects of human disturbance on the occurrence of the pu- tative hybrid A. x media.