Positive and negative biotic interactions and invasive Triadica sebifera tolerance to salinity: a cross-continent comparative study.

Exotic plant species may exhibit abiotic niche expansions that enable them to persist in a greater variety of habitat types in their introduced ranges than in their native ranges. Th is may refl ect variation in limitation by diff erent abiotic niche dimensions (realized niche shift) or phenotypic eff ects of biotic interactions that vary among ranges (realized niche expansion). Novel abiotic and biotic environments in the introduced range may also lead to genetic changes in exotic plant traits that enhance their abiotic stress tolerance (fundamental niche expansion). Here, we investigated how biotic interactions (aboveground herbivory and soil organisms) aff ect plant salinity tolerance using the invasive species Triadica sebifera from China (native range) and US (introduced range) populations grown in common gardens in both ranges. Simulated herbivory signifi cantly reduced survival in saline treatments with reductions especially large at low salinity. Soil sterilization had a negative eff ect on survival at low salinity in China but had a positive eff ect on survival at low salinity in the US. Triadica survival and biomass were higher for US populations than for China populations, particularly in China but salinity tolerance did not depend on population origin. On average, arbuscular mycorrhizal (AM) colonization was higher for US populations, US soils and low salinity. Th ese factors had a signifi cant, positive, non-additive interaction so that clipped seedlings from US populations in low saline US soils had high levels of AM colonization. Overall, our results show that phenotypic biotic interactions shape Triadica ’ s salinity tolerance. Positive and negative biotic interactions together aff ected plant performance at intermediate stress levels. However, only aboveground damage consistently aff ected salinity tolerance, suggesting an important role for enemy release in expanding stress tolerance.