Plant-soil Interactions And Triadica Sebifera Invasion: A Cross-continent Comparative Study

Biotic interactions involving exotic plants in their introduced ranges may differ from those of co-occurring plant species and from interactions in their native ranges. When interactions are less negative (Enemy release hypothesis), or more positive (Enhanced mutualist hypothesis) compared to native plant species, this may increase invasion success, and differences among ranges may cause changes in exotic plant traits. Biotic interactions of exotic plant species in their introduced ranges, such as those with mutualists or enemies, may differ from those of co-occurring plant species and from interactions in their native range. Biotic and abiotic conditions of exotic plant species in their introduced range may differ from those in their native range. Novel biotic and abiotic environments in the introduced ranges may also lead to genetic changes in exotic plant traits associated to new selection pressures, particularly enhanced ability of tolerance to abiotic stress.1. We investigated arbuscular mycorrhizae (AM) associated with Triadica sebifera seedlings from populations in native (China) and introduced ranges (US) and with seedlings from US and China species within three co-occurring genera(Liquidambar, Ulmus, Celtis) grown in multiple common gardens in both ranges. No general pattern of higher or lower AM colonization was found in the introduced range for China and US Celtis, Liquidambar, or Ulmus species. However, AM colonization was significantly higher for Triadica than for other genera, particularly in the introduced range, suggesting AM may improve Triadica’s invasion success. Triadica AM colonization was higher in US than China gardens, decreased with increasing soil nitrogen in China, but was independent of soil nitrogen in the US. This might reflect a different effect of soil fertility on this mutualism among ranges. Introduced Triadica populations had higher AM colonization than native populations, particularly in US gardens, implying a possible advantage from greater AM association in the introduced range. This is the first study supporting post-introduction changes in mycorrhizal dependence of an invasive species. It indicates that there are both phenotypic and genetic components to the effect of positive interactions on plant invasions.2. We investigated plant-soil interactions of Chinese tallow (Triadica sebifera) from China (native range) and US (introduced range) populations and with US and China species within four co-occurring genera (Liquidambar, Ulmus, Celtis, and Platanus) grown in both ranges. Seedling survival was higher for Triadica than for other genera, particularly in the US where Triadica is introduced. In China, Triadica had higher survival and biomass in "away" (heterospecific) soils than in "home" (conspecific) soil, but in the US, only biomass was higher. Biomass in sterilized soil was lower than in either home or away soils in the US but biomass in sterilized soil was intermediate in China. In China, Triadica biomass was higher in sterilized soil than in a mix of active soils of all genera but the opposite pattern occurred in the US. Triadica mycorrhizal colonization was higher in the US than in China but did not differ among home, away, and active mix soils within either range. This suggests mycorrhizae enhanced Triadica performance in away, home, and active mix soils compared to fungicide-treated or sterilized soil, especially in the US (Enhanced mutualist hypothesis). The pattern of performance in home and away soils suggests a stronger negative effect of specific pathogens in China (Enemy release hypothesis) whereas greater performance of Triadica in active mix than in fungicide-treated soil in the US suggests pathogenic soil fungi impacted Triadica. On average, Triadica performance was similar for native and invasive populations. However, invasive populations had greater biomass in active mix than in sterilized or fungicide-treated soil and native populations had higher mycorrhizal colonization in home than in away soils or active mix. This suggests genetic differences in strength or specificity of mycorrhizal interactions. Overall, our results support both ecological and evolutionary variation in interactions with the soil community that may influence the invasion success of exotic plants.3. We investigated how biotic interactions (aboveground herbivory and soil organisms) affect 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 had no effect on seedling survival or growth in no salinity (no stress) or high salinity (high stress) treatments. However, clipping significantly reduced survival and mass in low salinity (low stress). Soil sterilization had a negative effect on survival in low salinity in China but had a positive effect on survival in low salinity in the US. Triadica survival and biomass were higher for US populations than for China populations, particularly in China. On average, arbuscular mycorrhizal (AM) colonization was higher for US populations, US soils, and low salinity. These factors had a significant, positive, non-additive interaction so that clipped seedlings from US populations in low saline US soils had extremely high levels of AM colonization. Overall, our results show that biotic interactions shape salinity tolerance. Positive and negative biotic interactions together affected plant performance at intermediate stress levels. However, plant performance was insensitive to biotic treatments in benign conditions and it was already severely reduced at high salinity independent of biotic treatments. The strength of abiotic*biotic effects at intermediate stress varied among ranges and population origins. We conclude that both ecological and evolutionary variation in interactions with herbivores and the soil community could influence abiotic stress tolerance for invasive plants.4. We investigated how fertilization effects on plant-soil interactions and plant tolerance to soil salinity using invasive tree species Triadica sebifera from China (native range) and US (introduced range) populations grown in Triadica’s native range. Our results showed that fertilization could increase negative net soil effects on Triadica and plant tolerance to salinity. Salinity treatment could increase positive net soil effects on Triadica. Invasive populations of Triadica could have greater total biomass and quicker growth rate than those for native populations. Overall, our results suggest that fertilization and salinity stress together could affect plant-soil interactions for invasive species. Together our results reflect that both ecological and evolutionary variation in interactions with the soil that may influence successful plant invasions.