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Let us know how access to this document benefits you.The allelopathic potential of Rhododendron macrophyllum in a western Cascades clearcut" (1979). Dissertations and Theses. Paper 2766. Rhododendron macrophyllum is a dominant species in.the shrub stage of secondary succession on burned and logged sites in the Tsuga heterophylla zone of the western Cascades of Oregon. A study was undertaken to determine if R. macrophyllum has the potential to inhibit the germination and growth of surrounding ~egetation through water-solub le toxins which. . are produced in its leaves and leached out of its litter by rainfall. Aqueous extracts of R. macrophyllum leaf litter significantly reduced germination and radicle growth in Btomus tectorum, ·Epilobitim ansustifolium, Picea sitChensis, ·pseudOtStiga·menzieSii, and·Tsu8a heterophylla. Osmotic pressure of the leaf extract solution was determined not to be a significant causative factor in the inhibition observed. Field studies showed four plant species to increase in frequency of presence with increasing distan~e from _1!. macrophyllum drip lines. Two species, both ericaceous, decreased in frequency of presence. Five species increased in density with increasing distance from R. macrophyllum drip lines, while only one species, which was ericaceous, decreased, It is postulated that these patte~s result fr~ the allelopathic influence of _1!. macrophyllum. Members of the Ericaceae are thought to either have a common response to environmental conditions or to be able to tolerate the toxins produced by members of thei~ family. iii ACKNOWLEDGEMENTS The author would like to extend his appr.eciation to his major advisor, Dr. Robert O. Tinnin, for his help and advice during the course of this investigation and for his helpful suggestions and criticisms in the preparation of this manuscript. Appreciation is likewise extended to the other members of my committee, Dr. Richard D. ~ocher and Dr. Richard B. Forbes, for their advice ~nd criti~isms and for the use of their laboratories and equipment during portions of this study. Appreciation is given also to Dr. Mary L. Taylor for use of her laboratory and equipment during a P?rtion of this s~udy. concerning the statistical analyses of my data. Thanks are given to Jim Sweet for his helpful laboratory measure-men ts. Gratitude is also expressed to the Crown Zellerbach Nursery in Aurora, Oregon for the donation of conifer seeds for use in this study. Finally, sincere appreciation is.extended to Kathryn A. Farr, who had to put up with me throughout the entire process.