Herbivory Tolerance and Compensatory Differences in Native and Invasive Ecotypes of Chinese Tallow Tree (<Emphasis Type="Italic">Sapium sebiferum</Emphasis>)

We studied competitive interactions among three species (Corynephorus canescens, Hieracium pilosella and Carex arenaria) of different early successional stages on sand dunes. Our study focused on the influence of competition and water availability on biomass allocation patterns and the plasticity of root responses. Plants were grown for one growing season in a simple additive (target–neighbour) design under low or ambient water supply. Overall competition intensity (e.g., above–and below–ground), as well as root competition alone, were compared using control plants grown without competitors. Our results show high competition intensity leading to an average target plant biomass reduction of 56 relative to controls. Competition was mostly below–ground. With increasing water availability, the competitive effect of H. pilosella on both of the other species decreased significantly. All other tested species combinations were not influenced by water availability. Soil moisture seemed to be a key factor determining the plasticity of root responses. Under limited water availability, strong competitors caused a significant decrease of response ratio (lnRR) based on root: shoot ratios for H. pilosella and C. arenaria and a decrease in lnRR based on specific root length (SRL) for C. arenaria. Under sufficient water supply, however, there was no significant effect of competition on root: shoot ratios for any of the species and only C. arenaria in competition with C. canescens showed a lower lnRR based on SRL. These water–related, species–specific changes of root morphology and allocation patterns may point to an adaptive response to competition.