| Nitrogen (N) and phosphorus (P) are essential mineral nutrient elements in plant and the N:P ratios, as an important indication of soil nutritional conditions, plays vital roles in individual life history, carbon accumulation, metabolism and reproduction, and has important influences on interspecific relationship and community structure. Human activities have exerted a great impact on the original trophic conditions of ecosystems since the beginning of the Industrial Revolution. Due to disproportionately anthropogenic inputs of N and P in fertilization and the differences between element characteristics, the relative importance of nutrient elements (N:P ratios) has been changed in many ecosystems. As a result of those changes, patterns of plant competition would alter and dominated species in plant community may be changed. Biological invasion is also an important aespect of global change and a strong threat to native community biodiversity and ecosystem functioning. As a hot spot in ecology research, plant invasion is attracting more and more attention. Resource is a main environmental factor that determines invasion success of invasive species. Invasive plants with traits related to resource acquisition, resource conservation, or high resource-use efficiency could outperform the other native species, profiting from capability of responding quickly to nutrient fluctuation. Therefore, studying the effects of different N:P ratios on the interaction between exotic and native species will favor the understanding of occurrence of plant invasions and prediction of future invaders under the global change background.We conducted a greenhouse experiment to study the effects of different N:P ratios on the performance of alien R. typhina and native Q. acutissima, Vitex negundo var. heterophylla, and the interaction between them. All species are widely used in vegetation restoration in Northern China. The seedlings of Q. acutissima, V. negundo and R. typhina were grown in monoculture or mixture, with three different N:P ratios, to simulate different interspecific relationship. In this study, plant functional traits related to plant growth, physiology, leaf stoichiometry, biomass allocation, and nutrient absorption were determined, to evaluate impacts of R. typhina on native species.Results indicated that N:P supply ratio had significant effects on all species. The biomass of these species was highest under the N-limited condition (N:P=5) or the basic N and P supply condition (N:P=15). As a result of high N:P ratios, there was an unbalance between N and P, showing a P-limited condition, which inhibited the biomass production, they has lowest biomass under the high N:P ratio (N:P=45). Under N-limitation, plants had the highest RMR for obtaining nitrogen, while no significant adjustment of biomass allocation under high N:P ratio condition. With high performance, such as high plant growth, high crown area, high leaf mass ratio, high net photosynthesitic rate, low leaf mass per area, high net photosynthetic rate, R. typhina could product high biomass and root biomass, which help it in competition for nutrients and light, giving it strong capability of responding quickly to nutrient fluctuation. Compared with R. typhina, native species Q. acutissima and V. negundo invested more biomass in root and less biomass and nutrients in photosynthesis apparatus, with high root:shoot ratio and low leaf mass ratio, causing their lower competiveness for nutrients and light.In mixture, R. typhina was the superior competitor and had higher relative dominance index than native species Q. acutissima and V. negundo. RDI of R. typhina was not affected disadvantageously by N:P ratios and its dominance did not change. Compared with Q. acutissima, R. typhina has higher net photosynthetic rate, lower LMA and higher photosynthetic nitrogen-use efficiency and photosynthetic phosphorus-use efficiency, indicating its higher investment of nutrients in photosynthesis apparatus, to maintain higher biomass productivity. Similar to R. typhina, V. negundo had high net photosynthesitic rate and low LMA. However, photosynthesis of it was inhibited by shade effect, casuing by R. typhina with higher growth rate, height and crown area. In our study, there was no significant stress on physiology of V. negundo casued by R. typhina, but its biomass accumulation was significantly inhibited. In mixtures, under competitive pressure from R. typhina, Q. acutissima and V. negundo partitioned more biomass to roots and less to leaves than those in monocultures, to acquire more nutrients. Whereas, no significant difference in biomass allocation was observed for R. typhina, maintaining high shoot biomass ratio. This difference in biomass allocation strategies lead to an absolute advantage in light competition for R. typhina. And with high carbon accumulation capability, it can product much higher biomass than native Q. acutissima and V. negundo. Root mass raio of R. typhina was lower than native species, but owing to a high carbon assimilation capacity, R. typhina was able to maintain much higher root biomass in all treatments. Besides, with high performance root characteristics, high root surface area, R. typhina also has an absolute advantage in nutrients competition. Under high N:P ratio condition, Q. acutissima and V. negundo maintained similar biomass alloction to those under the medium N and P supply condition, whereas R. typhina significantly increased LMR and decreased RMR. This adjustment maintained its high biomass production and root mass, to keep its advantage in nutrients absorption, showing more adaptive to increasing N:P ratios. Biomass allocation of Q. acutissima and V. negundo indicated that they have greater tolerance to stressful environments. However, this strategy may be conservative for environmental changes that lead to fluctuating resource availability and cause the disadvantage in competition.Generally, compared to the native species Q. acutissima and V. negundo, the exotic species R. typhina, with higher growth performance in leaf and root traits, adopts more positive strategies for biomass and nutrient allocation to better acclimate to different nutrient conditions and maintain high biomass productivity. Given its higher nutrient absorption capacity and higher investment of biomass and nutrients in photosynthetic tissues, R. typhina shows greater biomass production under competition with Q. acutissima and V. negundo. The different trade-offs in biomass and nutrient allocation of the two species is the main reason for their distinct performances under competition and helps R. typhina to maintain absolute dominance under different N:P supply ratios. Furthermore, in low N:P ratio or high N:P ratio sites, exotic R. typhina might have higher biomass production and benefit more strongly from increasing anthropogenic nitrogen and phosphorus input. In our study, there was no significant difference between biomass of Q. acutissima in monocultures and mixtures, but a seriously reduce for V. negundo, caused by shade effect. Thus, the potential invasiveness of R. typhina and its effects on native plants needs further study in a long time scale. In future plant introduction and management programs, plant traits and local nutrient conditions should receive greater attention under the main factors of climate change, to evaluate their invisibility and avoid economic and ecological risk. |
PDF Full Text Request