Allelopathic interactions have been well documented in a variety of deciduous tree 

 species. Eucalyptus globulus Labill. and E. occmaldulensis Dehn. influence herbaceous 

 vegetation through chemicals that escape into the environment (del Moral and Muller 

 1969, 1970). Sycamore, Platanus Ocoidentalis L. (Al-Naib and Rice 1971), and hackberry, 

 Celtis laevigata Willd. (Lodhi and Rice 1971, Lodhi 1975) are characterized by sparse 

 vegetation beneath their canopies. Experiments with decaying leaves, leaf leachates, 

 and soils collected beneath the trees lead to the conclusion that phytotoxins were 

 a significant factor. Several oak species including Quevaus stellata Wang., Q- 

 mavilandica Muench. (McPherson and Thompson 1972) and Q- falcata var. pagodaefolia 

 Ell. (DeBell 1971) produce compounds that inhibit the growth in understory plants. 



Another tree species, Grevillea robusta Cunn., in the subtropical rain forest of 

 Australia cannot regenerate in pure stands (Webb and others 1967) . Experiments were 

 conducted to show that a water-transferable factor in the rhizosphere from the older 

 trees was responsible for this phenomenon. The commercial production of these trees 

 may not be possible in pure stands, and a polyculture will be required unless special 

 care and treatment is used to remove toxins from the soil. 



Since ponderosa pine is climax only on the driest of sites, the production of an 

 autotoxic chemical could provide survival value to the species. When moisture is 

 limiting, adequate growth and vigor can only be maintained at lower tree densities. 

 Therefore, to insure lower densities, chemicals from the mature trees could, perhaps, 

 reduce the germination and growth of seedlings, which could then be eliminated by 

 drought or frequent fires. This would prevent overcrowding, stagnation, and competition 

 between members of the same species. Hall (1976) stated that he suspected "... a 

 selective inhibitory substance in ponderosa pine litter that is destroyed with periodic 

 underburning . Without fire, this substance is free to build up in the soil and reduce 

 pine growth." If this is the case then fire played a dual role in the ecology of 

 ponderosa pine, it eliminated unhealthy seedlings from within mature pine stands, and 

 it prepared seedbeds for pine seeds in open areas by destroying phytotoxins, releasing 

 nutrients and reducing competition from other species. 



There is some experimental evidence that suggests ponderosa pine produces phyto- 

 toxins. Jameson (1961) reported that water extracts of ponderosa pine needles in- 

 hibited wheat radicle growth by 86 percent. Needle extracts also inhibited the 

 radicle growth of two native grass species, Sitanion hystrix (Nutt.) J. G. Smith and 

 Bouteloua gvacllis (H.B.K.) Lag., and Vinus ponderosa (Jameson 1968). Del Moral and 

 Gates (1971) observed inhibition of barley by ponderosa pine volatiles and litter water 

 extracts, although field analysis suggested only weak allelopathic influence with 

 associated native species. Eckert (1975) has revealed results which indicate that 

 ponderosa pine needles produce chemicals that adversely affect some associated under- 

 story species, creating obvious ecotones near ponderosa pine trees. 



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