| In recent years, due to its profound ecological impacts, global environmental change has aroused widespread concern. Owing to the rapid development of industry and agriculture, fossil fuel burning and nitrogen fertilizer use has risen to a large extent, which has led to a sharp increase in reactive nitrogen (N) emission and atmospheric nitrogen deposition since the Industrial Revolution. The continuous increase in nitrogen deposition may influence plant growth and biochemical cycling and biodiversity of terrestrial ecosystems. Frequent anthropogenic activities, including deforestation, forest harvesting and tending, have made the light environment of forest ecosystems under fluctuation. In the context of global change, exotic plants may have a series of adaptive responses; the interspecific interactions may also change between exotic and native plants. Functional traits determine the adaptive strategies of plants in the face of complex environments, and may play an important role in the adaptation and expansion of exotic plants. Therefore, in the context of global change, it is very important to study the interspecific interactions of exotic and native plants and compare them from the aspect of functional traits for revealing the mechanisms of adaptation and expansion of exotic plants and their impacts on native communities.Robinia pseudoacacia is the most common exotic species used for revegetation in the warm temperate region in China. In order to study the impacts of R. pseudoacacia on native species when nitrogen deposition increases and reveal its competitive strategy, we conducted an experiment with the seedlings of R. pseudoacacia and Quercus acutissima, a typical constructive species of native forests. According to the present nitrogen deposition rate in China, we set four simulated nitrogen deposition levels (0,3,6,12 g N m-2 year-1) in the experiment. Under each nitrogen deposition level, the seedlings were arranged into three cultivation types: mixture with one seedling of each species to simulate interspecific interaction and monocultures with two seedlings of the same species as control. At the end of the experiment, we measured plant total biomass and functional traits, such as biomass allocation, seedling height and leaf physiological, morphological and stoichiometrical characteristics and calculated the relative competition indexes between R. pseudoacacia and Q. acutissima. The results showed that there existed an obvious competition between the two species and R. pseudoacacia inhibited the growth of Q. acutissima. Due to its active strategies in resource acquisition and utilization, R. pseudoacacia dominated in competition. Compared with Q. acutissima, the huge root system, lower main root to lateral root ratio and higher root biomass conferred R. pseudoacacia an advantage in soil nutrient competition, while the higher height and lower root to shoot ratio ensured that R. pseudoacacia also dominated in the competition for light. The maximum net photosynthetic rate, specific leaf area and leaf nitrogen/phosphorus content of R. pseudoacacia were all much higher than those of Q. acutissima, which indicated that R. pseudoacacia was positioned further along the leaf economic spectrum towards an active strategy. R. pseudoacacia was able to allocate more resources to photosynthesis and growth, and less to leaf structural defence, which helped to obtain a larger and quicker carbon return on the investment in resources in leaves, accumulate more biomass and achieve a large competitive advantage. In addition, the photosynthetic nitrogen and phosphorus use efficiencies of R. pseudoacacia were also much higher than those of Q. acutissima, which also conferred R. pseudoacacia an advantage in growth and competition.Simulated nitrogen deposition showed little impacts on the growth of Q. acutissima in monoculture and R. pseudoacacia, but significantly promoted the growth of Q. acutissima in mixture. Therefore, nitrogen addition alleviated the competitive effects of R. pseudoacacia on Q. acutissima. At the end of the experiment, soil available nitrogen content in mixture was not significantly different from that in monoculture of Q. acutissima, but soil available phosphorus content was significantly lower, indicating that R. pseudoacacia had a large demand and competitiveness for soil phosphorus and Q. acutissima may confront phosphorus deficiency. Nitrogen addition may mitigate the phosphorus limitation of Q. acutissima and thus alleviated the competitive effects of R. pseudoacacia.In order to study the adaptive mechanisms of R. pseudoacacia to different global change factors, we compared the seedlings of R. pseudoacacia and a phylogenetically related native species Sophora japonica under different light and nitrogen deposition levels. Both species belong to the Fabaceae family. We set two light regimes:the light intensity of the high light regime equals to 66% of the full radiation, while the light intensity of the low light regime equals to 10% of the full radiation. Under each light regime, we set three simulated nitrogen deposition levels (0,3,9 g N m-2 year-1) according to the present nitrogen deposition rate in China. At the end of the experiment, we measured plant total biomass and functional traits, such as biomass allocation and leaf physiological, morphological and stoichiometrical characteristics. The results showed that light showed much greater impacts on the growth of R. pseudoacacia and S. japonica than nitrogen deposition, which only had very limited effects. In the low light regime, no significant difference was detected on the total biomass of R. pseudoacacia and S. japonica; functional traits were also similar between the two species. However, when light intensity increased, R. pseudoacacia exhibited significantly different plastic responses on many functional traits relative to S. japonica, and therefore, in the high light regime, R. pseudoacacia had significantly higher maximum net photosynthetic rate, leaf nitrogen content, lower leaf carbon to nitrogen ratio, leaf thickness and root to shoot ratio than S. japonica. This suggested that in the high light regime, R. pseudoacacia adopted a more active strategy than S. japonica in resource acquisition and utilization, which enabled it to capture light more efficiently, allocate more resources to photosynthesis and growth and obtain a larger carbon gain on the investment in dry mass and nutrients in leaves. Therefore, although R. pseudoacacia did not show obvious advantage in the low light regime, it could better capitalize on light increase and promoted its growth to a larger extent and thus obtain a significant growth advantage in the high light regime.Rhus typhina is also a common exotic species used for reforestation in the warm temperate region in China, but its invasiveness has long been a controversial issue. In order to study the adaptive mechanism and strategy of R. typhina in the context of global change, we compared the seedlings of R. typhina with Q. acutissima and Vitex negundo var. heterophylla, a common shrub in native forests, under different light and nitrogen deposition levels. We set two light regimes:the light intensity of the high light regime equals to 66% of the full radiation, while the light intensity of the low light regime equals to 10% of the full radiation. Under each light regime, we set three simulated nitrogen deposition levels (0,3,9 g N m-2 year-1) according to the present nitrogen deposition rate in China. At the end of the experiment, we measured plant total biomass and functional traits, such as biomass allocation, seedling height, crown area and leaf physiological, morphological and stoichiometrical characteristics. The results indicated that nitrogen deposition significantly increased the total biomass of R. typhina and V. negundo var. heterophylla, and enhanced the photosynthetic capacity of R. typhina, but showed little impacts on Q. acutissima. Therefore, nitrogen deposition promoted the growth of R. typhina to a larger extent than the two native species. Compared with Q. acutissima, R. typhina adopted a more active strategy in resource acquisition and utilization. R. typhina had a much larger crown area and a much higher photosynthetic nitrogen use efficiency than Q. acutissima in all the treatment combinations, which conferred it an obvious advantage in the gain and utilization of light and nutrients together with its huge root system and developed fine roots. In the low light regime, R. typhina also exhibited a much higher specific leaf area and a much lower root to shoot ratio than Q. acutissima, which further promoted its acquisition for light. Therefore, R. typhina showed much higher biomass accumulation than Q. acutissima in all the treatment combinations. Compared with V. negundo var. heterophylla, in the high light regime, R. typhina did not show obvious growth advantage and functional traits were similar between the two species. However, as light intensity decreased, R. typhina significantly increased its crown area, while the crown area of V. negundo var. heterophylla decreased significantly. Therefore, in the low light regime, R. typhina had a much lager crown area and could still obtain much light resource and thus exhibited an obvious growth advantage relative to V. negundo var. heterophylla. In general, R. typhina could not only take better advantage of increased light intensity and promote its growth, but also show strong adaptation and tolerance to low light, and thus could maintain stable growth in different environmental conditions.In summary, compared with different native species, the exotic species R. pseudoacacia and R. typhina did not always exhibit absolute growth advantage in different environmental conditions, but both were able to adopt active strategies in resource acquisition and utilization through altering functional traits. Therefore, they could better capitalize on increased resource availability and promote their growth to a larger extent, or maintain stable growth in unfavorable conditions. Since R. typhina was able to maintain stable and vigorous growth in both favorable and unfavorable environmental conditions, and nitrogen deposition may promote its growth to a larger degree than native species, in the future, we should monitor the populations of R. typhina and use it in reforestation reasonably. |
PDF Full Text Request