Facilitation Among Common Halophytes In Coastal Wetlands Of The Yellow River Delta

The interaction between species is one of the most important research contents in ecology.Among them,facilitation has attracted much attention in recent years.However,our understanding of the ecological consequences of facilitation is limited by the imbalance in the study of the performance of beneficiaries and benefactors along the stress gradient.At present,the stress tolerance theory is mainly relied on to identify the beneficiaries and benefactors in the facilitation.However,when the facilitation mechanisms was closely related to individual size and stress-sensitive species had obvious body size advantage,the positive effect of beneficiary feedback effect might outweigh that of benefactor facilitative effect,leading to species roles reversal.We tested this hypothesis using a greenhouse experiment with six halophytic species from the Yellow River Delta,China.They are Apocynum venetum,Limonium bicolor,Salicornia europaea,Suaeda glauca,Phragmites australis and Suaeda salsa.A controlled greenhouse experiment was conducted to compare the responses of these six species to salt stress and to explore how the results of intra-and interspecific interactions changed along the salt stress gradient.Partial least squares path models were used to analyze the pairwise interaction data and to explore that our inference about roles reversal is correct or not.A total of 6 intraspecific interactions,15 interspecific interactions as well as 6 single-species controls and 5 salinities were evaluated.After five weeks of salt treatments,plant height was measured,plant biomass was harvested,root to shoot ratio was calculated and ion content was measured.The main results of this study are presented below:Six species showed significantly different coping strategies for salt stress.The root to shoot ratio of S.glauca and L.bicolor decreased significantly under salt stress,and the salt ions mainly concentrated in the aboveground part.The root to shoot ratio of S.europaea and S.salsa was always low,and they were not affected by salt stress,and the ions in the body mainly concentrated in the aboveground part.The root to shoot ratio of P.australis was not significantly affected by salt stress,but the ions were mainly concentrated in the root.The root to shoot ratio of A.venetum decreased significantly under salt stress,and the salt ions in the body showed an equal distribution between ground and ground.The changes in the results of intraspecific and interspecific interactions of the six species with the salt stress gradient were consistent with the prediction of the stress gradient hypothesis.For the beneficiaries,the facilitation became more important with the increase of salt stress level.There was no obvious regularity in the performance of the benefactors as a function of the salt stress gradient.We found that this is because the main stress faced by the benefactor is the competition from the beneficiary rather than the external environment.The results confirm our hypothesis was correct.When the stress-sensitive species had obvious body size advantage,the predictions of stress tolerance theory were moderated by initial plant size.Here,BFEs outweighed the facilitative effects,leading to the reversal of species’ roles.The results of intraspecific path analysis were confounded by priority effects caused by small differences in germination time.However,in the future,studying the density dependence of populations can help us to deepen our understanding of intraspecific facilitative effects and beneficiary feedback effects.In this study,we found that stress tolerance difference and relative size together determine the role of species in facilitation.Species with greater capacity for coercive amelioration are more likely to be benefactors in facilitation.This study provided new ideas for understanding the ecological consequences of facilitation from a holistic perspective,further expanding the general framework of the stress gradient hypothesis,and providing sample for species selection and matching in coastal wetland ecological restoration.