Nitrogen availability is a major determinant of successional patterns in many ecosystems. Increased levels of soil nitrogen, caused by atmospheric nitrogen deposition, continuously fertilize a large (and growing) portion of the terrestrial biosphere. Increased nitrogen deposition onto natural ecosystems is disadvantageous to slow-growing native plants that have adapted to nutrient-poor habitats by creating environments favorable for faster-growing plants, such as grasses. In this paper, two invasive plant species, Ageratina adenophora and Chromoleana odorata, were studied. Both of them were planted under five soil nitrogen levels for more than four months. By investigating their traits related to morphology, biomass allocation, growth and photosynthesis, we compared their phenotypic responses to nitrogen. Our main objectives were to 1) explore how the two species acclimate to soil nitrogen availability, 2) evaluate which plant traits were associated with the invasiveness of the two species, and 3) determine whether the increased levels of soil nitrogen could facilitate their invasion.
The two species were very plastic in their response to nitrogen availability. They exhibited considerable (nitrogen-acclimation abilities. With an increase in nitrogen levels, their root mass ratio and root mass/crown mass decreased, but their leaf mass ratio (LMR), leaf area ratio and leaf area to root mass ratio increased. At lower nitrogen levels, more biomass was invested into the root system, a nutrient absorbing organ, which could enhance nutrient-capture ability. At higher nitrogen levels, more biomass was invested into the leaves, an assimilative organ, which could increase their carbon accumulation and improve their competitive abilities. A. adenophorum could acclimate better to nitrogen environments than C. odorata.
The two invasive plant species could benefit from high nitrogen levels, which were usually excessive and/or harmful for most native species. Under a wide range of nitrogen levels, relative growth rates (RGR), total biomass, branch numbers, leaf area index, maximum net photosynthetic rate, chlorophyll and carotenoid content increased significantly with increasing nitrogen levels, and did not decrease significantly at over-optimal nitrogen levels. The two species could maintain relatively higher RGR in the dry season when native plant species almost stopped growing. Having the ability to use resources at times when native plants could not, their competitive abilities and invasiveness were promoted.
Mean leaf area ratio (equal to LMR/SLA (specific leaf area)) and net assimilation rate were coequally important in determining the response of RGR to nitrogen levels in A. adenophora and C. odorata. LMR was a very important determinant of RGR, which played the most important role in determining differences in RGR among nitrogen treatments and between species. With an increase in nitrogen levels, the SLA decreased in A. adenophora whereas it increased in C. odorata. But under all nitrogen levels, SLA was higher in A. adenophora than in C. odorata. The higher SLA of A. adenophora compensated this species for its lower LMR and was favorable to its growth. The response trend of SLA to nitrogen levels in A. adenophora was more profitable than in C. odorata.
In conclusion, our results indicated that the two invasive plant species were able to acclimate to a wide range of nitrogen environments and could grow better in higher nitrogen environments, suggesting that enhanced soil nitrogen levels might promote their invasion.