Competition mechanisms of native and exotic tree species

The amount of exotic plant species introduced to new regions by humans has considerably increased in the last two centuries. Worldwide, the invasion of exotic species represents an important threat to native biodiversity and ecosystem functions. Several biological traits (e.g., high growth rate and rapid propagation) result in a superior competitiveness of invasive species and often cause changes in natural species composition. Specifying the attributes that turn exotic species into strong competitors may improve the ability to understand and effectively manage plant invasions in the future. Conducting a pot experiment ensures the investigation of plant interactions under relatively controlled conditions without distracting effects of heterogeneous environmental factors. However, pot experiments with tree species raise more problems in comparison with herbaceous plants due to their longevity and bigger dimensions. This is shown by a comprehensive literature review giving an overview on the practical implementation of pot experiments studying exclusively tree species. It is evident that the advantage of pot experiments is also a disadvantage at the same time: Due to the controlled conditions, pot experiments are always restricted in their ability to imitate natural situations. Thus, the reliability of pot studies for predicting the growth and performance of trees in the field can be problematic. One option to improve the transferability of pot experiments could be to implement additional measurements under natural conditions. In a pot experiment, I investigated the competition mechanisms due to differences in growth rate, biomass production, and biomass allocation of two native (Quercus robur L., Carpinus betulus L.) and two exotic tree species (Prunus serotina Ehrh., Robinia pseudoacacia L.). One-year-old tree seedlings were planted in different intra- and interspecific, competitive situations with or without the influence of root competition. To determine the competition mechanisms in more detail, I distinguished between root and shoot competition by installing either above- or belowground plastic partitions in the pots. I hypothesized that total biomass production of the exotic tree species is significantly higher compared to the native species resulting in a biomass reduction of Q. robur and C. betulus. Furthermore, I analyzed the effect of belowground competition on native plant performance and biomass allocation patterns according to the ‘balanced-growth hypothesis’. The results supported the assumptions that biomass production of exotic P. serotina and R. pseudoacacia is significantly higher, which leads to a strong competitive advantage and to a biomass decrease of the admixed less competitive native species. The competitive pressure of exotic tree seedlings on Q. robur and C. betulus was largely driven by root competition. The exclusion of belowground interactions by partitions led to an increasing biomass production of both native species. Thus, even a limited rooting volume seemed to provide better growing conditions than direct root interactions by invasive competitors. In accordance with the ‘balanced-growth hypothesis’, Q. robur and C. betulus allocated more biomass towards the roots due to the strong effect of belowground competition by exotic species. The higher proportion of the root fraction was mainly achieved at the expense of leaf and branch biomass. Furthermore, the results showed a higher biomass production in mixtures of native and exotic tree species than their growth in monocultures would have predicted. Competition was lower for exotic species in mixtures with the less productive Q. robur and C. betulus compared to the competition in monocultures or in mixture with the other exotic species. Regarding both exotic species, P. serotina produced a significantly higher biomass. Nevertheless, R. pseudoacacia negatively affected the biomass production of P. serotina due to its strong root competition. Accordingly, both highly competitive exotic species inhibited each other’s growth and produced less biomass in mixture with each other compared to the respective monocultures. There is evidence that the strong competitiveness of invasive exotic species is often achieved at the expense of a tolerance to environmental stress. Accordingly, both exotic species had a higher mortality rate in the pot experiment and especially P. serotina seemed to be sensitive to shade, drought, and flooding. Possibly, this weakness could be used to prevent a further spreading of invasive species.