Abstract: Forty plant species common in western Washington were investigated for the presence of allelopathic chemicals capable of influencing the distribution of subordinate species. Under laboratory conditions most species contained inhibitory volatile compounds and many contained effective water—soluble compounds. The results of up to 13 bioassays per species were compared with descriptions of the distribution of subordinate species under natural conditions. In several cases, notably Acer circinatum, Arbutus menziesii, and Rhododendron albiflorum, high inhibition values in laboratory experiments were associated with distinct changes in ground—cover composition beneath the canopy of these species. The ratio between inhibition values produced by volatile and water—soluble inhibitors was higher in species from arid regions than in those from humid regions. This finding supports Muller and Whittaker's hypothesis that allelopathic compounds are primarily waste products.

Abstract: Field observations of stands of Sporobolus pyramidatus indicated a reduction of growth of S. pyramidatus in the center of the stands. Associated species were either reduced in growth or eliminated from these stands. Rapid encroachment of S. pyramidatus on Cynodon dactylon was also observed. Experiments were designed to determine if the observed vegetational patterning was a result of allelopathy. Early studies eliminated shading and soil factors as causal agents. Experiments with field soil taken from around S. pyramidatus plants indicated that phytotoxins were present and were relatively stable in the soil. Decaying shoot material exerted the most phytotoxic effect on growth and germination of the test species; however, C. dactylon was adversely affected by all factors tested except leaf leachate. Buchloe dactyloides was inhibited by field soils and, by decaying shoots. The decaying shoots of S. pyramidatus inhibited the growth of its own seedlings. There was also an indication that the leaf leachate and root exudate of S. pyrami- datus may be responsible for its own decline. Para-coumaric acid and ferulic acid were extracted from S. pyrami- datus shoot residue in large quantities. Both compounds were very inhib- itory to germination of Amaranthus palmeri seed. The results of this study show that invasion and rapid encroachment of S. pyramidatus on C. dactylon and possibly on B. dactyloides are pri- marily due to an allelopathic effect, with competitive mechanisms and mechanical disturbance probably accentuating this effect. S. pyramidatus is more inhibitory to C. dactylon than to itself, thus allowing it to invade new areas before it eliminates itself.