A series of laboratory and field experiments were conducted to determine if mature ponderosa pines produce a substance (phytotoxin) that inhibits the germination and growth of seedlings directly under the tree crown. Neither live nor dead materials collected from ponderosa pines produced either volatile or water-soluble phytotoxins that drastically inhibited germination of seeds or growth of seedlings. Seed overwintering beneath the canopy of mature pine, or planted in soils collected there, showed reduced germination. Exact cause of the reduction was not determined. If weak phytotoxins were responsible, they did not inhibit growth of seedlings that germinated.
United States Department of Agriculture, Forest Service, "A Search for Phytotoxins Influencing Germination and Early Growth of Ponderosa Pine" (1979). Forestry. Paper 25.