Investigating Allelopathy and Soil Moisture as Factors Determining Community Composition of an Invaded Southern California Black Walnut Woodland

Allelopathy is the chemical inhibition of one plant by another. Southern California black walnut (Juglans californica) occurs in a pure walnut woodland at the Lyle Center for Regenerative Studies on the campus of Cal Poly Pomona and produces the allelopathic compound juglone. Juglans californica is suspected to influence community composition in the woodland by allelopathy as well as by influencing microclimate conditions such as radiation exposure and soil moisture through its canopy. Black mustard (Brassica nigra) is an invasive annual forb that occurs in the walnut woodland and produces its own allelochemical allyl-isothiocyanate (AITC). A field experiment was designed to investigate the effects of walnut canopy cover on eight understory species. A greenhouse experiment was designed to investigate whether a subset of these species was tolerant of allelopathy from J. californica and B. nigra by recording percent germination, growth, and physiological measurements. Brassica nigra, Marrubium vulgare, Phacelia distans, and Silybum marianum seeds were treated with six allelopathy treatments: walnut mulch, mustard mulch, coconut coir, 0.5 mM juglone solution, 0.5 mM AITC solution, and a control. Percent germination was recorded over 28 days. Seedlings of the four species mentioned plus Sambucus nigra ssp. caerulea and J. californica were grown in the greenhouse and treated with the same six allelopathy treatments. In addition to an allelopathy treatment, each seedling was also given a dry or wet soil treatment. Growth, chlorophyll fluorescence, and leaf water potential were measured. Seedlings of the six species mentioned plus Rhus ovata and Diplacus longiflorus were planted in a field experiment to determine whether being in the open or under the canopy of a walnut tree affected growth, chlorophyll fluorescence, and leaf water potential. Most species were negatively affected by AITC and juglone solutions in the greenhouse, exhibited by decreased germination, lower biomass, and increased water stress. The germination stage was when plants were most influenced by allelopathy since species germinate during times of abundant soil moisture. Only J. californica was tolerant of allelopathy from both chemicals. Brassica nigra was tolerant of its own allelochemical AITC, which could be another competitive advantage it has over other species. However, it was clear that mulches do not contain inhibitory amounts of allelochemicals because none of them provided a negative effect similar to AITC and juglone. The lack of observable inhibition in the Lyle Center walnut woodland is likely due to low concentrations of allelochemicals in the soil most of the year. AITC and juglone do not persist long in the soil, especially in arid conditions such as those in the walnut woodland. Thus, allelopathy is likely only a factor during infrequent times of abundant rain, making soil moisture a greater factor in determining community composition in the walnut woodland.