Experimental approaches to test allelopathy: A case study using the invader Sapium sebiferum

Allelopathy occurs when plants release chemicals that inhibit neighboring plants. Invaders can have particularly effective allelochemicals which facilitate invasions. Allelopathy tests often compare the leaf extracts to water controls or employ activated carbon to neutralize allelochemicals. Each has limited power to detect the relative allelopathic effects of native versus exotic species. Here we use these approaches, combined with a new approach using interspecific leaf extract mixtures with all but one leaf extract treated with activated carbon, to test the allelopathic role of Chinese tallow tree (Sapium sebiferum) in invasions. We used foliar leaf extracts of Sapium and three native tree species and seeds of Sapium, two other exotic species and two native species. We measured seed germination and seedling growth. Results indicated that allelopathy does not contribute to Sapium’s invasive success. Effects of Sapium leaf extracts were within the range of native species of activated carbon on Sapium leaf extracts. The effect of non-carbon treated Sapium leaf extract was within the range for native species and statistically indistinguishable in the mixtures. Together these results demonstrate the variety of experimental designs that can be used to investigate allelopathy and invasions.