Effect Of Nitrogen Availability And Defoliation Between Phragmites Australis And Spartina Alterniflora

Currently, Phragmites australis are invading North American ecosystems which are dominated by Spartina alterniflora, in the meantime, S. alterniflora and their descendants are threatening Europe and China East Coast which P. australis is the dominate species. From a global perspective, these two species have similar regional characteristics of mutual-invasion. Therefore, investigate how environmental factors influence mutual-invasion mechanisms of S. alterniflora and P. australis have significant ecological values. Two factors (defoliation and nitrogen concentration) were selected to explore these mechanisms, from the perspective of simulating animal grazing and soil nutrient to measure the two species'interactions and variations. The results show that:(1) Without animal grazing, P. australis have stronger competitiveness than S. alterniflora. Otherwise, their competitiveness will be weakened, furthermore, S. alterniflora would take this opportunity to gain the growth advantages by improve their competitive power.(2) In a high nitrogen level, P. australis maintain sustained photosynthetic potential while the photosynthetic capacity of S. alterniflora is nearly saturated. Consequently, in natural conditions, S. alterniflora own higher efficiency in exploiting solar energy, however, P. australis retain the potential photosynthetic by increased nitrogen. P. australis has the ability to enhance photosynthetic efficiency by super compensation effects, whereas in the condition of defoliation, the photosynthetic efficiency of S. alterniflora has reduced to some extent.(3) In the condition of increasing nitrogen concentration, S. alterniflora will reduce its plant height in order to allocate resources to the growth of leaves(photosynthetic organs). This may help S. alterniflora to achieve competitive advantages by modulating the proportion of input resources in photosynthetic organs in a high nitrogen levels.(4) With the increase of nutrients, P. australis will improve the investments of leaf (photosynthetic organs) and stem (support organ) resources, meanwhile reduce those of root (uptake organ) and rhizomes (foraging behavior), in that case, an adaptation to high nitrogen condition will be improved, thereby to obtain a competitive advantage.(5) In a medium nitrogen level, P. australis have the strongest asexual reproduction, whereas with increasing nitrogen concentration, the proportion of investment in asexual reproductive organs will be reduced. On the contrary, S. alterniflora will raise those investments by increase the number of tillers and rhizomes.(6) In the condition of defoliation, S. alterniflora will convert their resource investments from sexual to asexual reproduction. The same situation will occur in a sufficient nitrogen environment as well.