Abstract: Investigating how nutrient conditions influence morphological plasticity induced by plant-plant interactions can help reveal the adaptation strategies of karst-adaptive plant species in coping with variations of environmental factors. In this study, we explored the effects of nutrient availability on the plasticity in biomass and morphological traits induced by intraspecific or interspecific interactions by treating Bidens pilosa and Buddleja davidii with and without intraspecific and interspecific interactions under two conditions of nutrient addition and no addition (control). In the no-nutrient condition, the total biomass of B. pilosa and B. davidii under the intraspecific interaction decreased by 9.2% and 14.4%, respectively, which mainly resulted in competition effect, with little interspecific competition. When nutrients were added, intraspecific facilitative effect became dominant, increasing by 35.1% and 41.3% respectively. B. davidii showed interspecific facilitative effect, increasing by 34.6%, but not in B. pilosa. Compared to control, intraspecific interaction decreased root biomass of B. davidii and leaf biomass of B. pilosa in low nutrient conditions. When nutrients were added, it increased stem biomass of B. davidii and decreased specific leaf area of B. pilosa. By contrast, interspecific interaction increased root∶shoot ratio of B. davidii, decreased root biomass, and increased stem biomass for B. pilosa under low nutrient conditions. When nutrients were added, it increased root∶shoot ratio and root biomass for B. davidii, and increased ground diameter of B. pilosa. These results suggested that abiotic factors can affect the plastic response of plants through mediating the strength of intraspecific and interspecific interactions. The distinct contrast between trait plasticity induced by intraspecific and interspecific interactions reflected striking differences in growth and adaptation strategies between the two species, with belowground growth and aboveground growth being dominant for B. davidii and B. pilosa, respectively. Intraspecific interactions can lead to more intense belowground and aboveground competition for the two species, respectively. Under interspecific interactions, however, they may be able to avoid growth advantages, complementing each other in resource utilization, thus competition weakened and the facilitation pronounced. Such a strategy would be beneficial for the better coexistence of karst-adaptive plant species in coping with the abiotic challenges.

Key words: aboveground competition, belowground competition, facilitative effect, phenotypic plasticity, plant-plant interaction, adaptive strategy