Bottom-up, Interspecific And Top-down Determinants Of Plant Communities In Salt Marshes In The Yellow River Estuary

Understanding the determinants of species abundance, distribution and community organization in nature is one of the most fundamental issues in ecology. Coastal salt marshes widely distributed at middle and high latitudes of the world are ecologically and economically important for storm buffering, pollutant filtering and providing habitats for rare and endangered species. With striking plant zonation patterns across steep environmental gradients, coastal salt marshes have been considered as ideal systems to investigate mechanisms of species distribution in natural habitats. In this research, I used a series of field surveys, lab analysis of environmental factors, field manipulative experiments, common garden experiments and multivariate analyses to examine the determinants of plant distribution in coastal salt marshes in the Yellow River estuary, China. More specially, I examined the roles of bottom-up physical factors including salinity and flooding, interspecific competition and facilitation, and crab herbivory on the growth and distribution patterns of marsh plants in the estuary. Each of my specific studies and their findings are given below:(1) Bottom-up factors and distribution of marsh plant communities: variation along elevation gradientsI used detrended correspondence analysis(DCA), detrended canonical correspondence analysis(DCCA), correlation analysis and generalized linear models to analyze the composition, diversity of plant communities, and their correlation with abiotic factors along an elevation gradient in the estuary. DCCA showed that plant communities and abiotic factors differed in their relation between in high zones and in low zones. In high zones, flooding and soil salinity were both positively correlated with the primary axis of the DCCA plot. In low zones, however, flooding and soil salinity were positively and negatively related to the primary axis of the DCCA chart along which plant communities were mainly distributed, respectively. Analysis of species diversity showed that in high zones, species richness and Shannon diversity had significant relationships with all examined abiotic factors(P < 0.05), with negative relations to soil salinity, flooding and soil bulk density and positive relations to soil water content and p H. In contrast, in low zones, species richness and Shannon diversity had no relationship with any of the five abiotic factors, except that there was a significant negative relation to flooding.(2) Plant interspecific interactions: the roles in determining the zonation of Tamarix chinensis along a salinity gradientI surveyed the zonation of T. chinensis along a salinity gradient and quantified its salt tolerance using a pot experiment. In two field experiments, I transplanted T. chinensis seedlings into salt marsh, transitional zone and upland habitats, manipulated neighbours and quantified survivorship and biomass to examine neighbour effects. I also quantified vegetation effects on abiotic conditions in each zone. T. chinensis dominated the transitional zone, but was absent in upland and salt marsh habitats. In the pot experiment, T. chinensis grew well in freshwater treatments, but was inhibited by increasing salinity. Field experiments revealed that competition from neighbours limited T. chinensis growth in the uplands, while T. chinensis transplants were limited, with or without neighbours, in the salt marsh by high soil salinity. In the transitional zone, however, T. chinensis transplants performed better with than without neighbours. Vegetation removal significantly elevated soil salinity in the transitional zone, but not in other zones. These results suggest that Competition, facilitation and abiotic stress are all important in mediating the zonation of T. chinensis. Within its physiological stress tolerance range, or fundamental niche, it is limited by plant competition in low salinity habitats, and facilitated by neighbours in high salt stress habitats, but cannot survive in salt marshes having salinities above its salt stress tolerance limit.(3) Plant interspecific interactions: variation in the effects of a nurse shrub(T. chinensis) on herbaceous communities along a salinity gradientIn each of two different salt stress habitats, I quantified canopy sizes of T. chinensis, percentage covers of herbaceous species and soil salinities beneath T. chinensis canopies and in open areas at 6 different sites. Effects of different T. chinensis canopy sizes on soil salinity, species compositions and diversity of herbaceous communities were examined in each habitat. The results showed that in the low salt stress habitats, soil salinity was significantly higher in canopy plots than in open area plots, while in the high salt stress habitat, soil salinity was lower in canopy plots than in open area plots. The species composition of herbaceous communities significantly differed between in canopy plots and in open area plots in both salt stress habitats, while only in the low salt stress habitats, species diversity of herbaceous communities significantly differed, being lower in canopy plots than in open area plots. In the low salt stress habiats, the effects of T. chinensis canopies on hebaceous communities appeared to result from neither its elevating soil salinity by secreting salts nor decreasing soil salinity by shading. In contrast, in the high salt stress habitats, the effects of T. chinensis canopies should strongly be caused by decreasing soil salinity through shading. Furthermore, the results showed that the nature and strength of the effects varied with size of the nurse plants.(4) Plant interspecific interactions: the importance of plant strategies on the effects of the nurse shrub T. chinensis on herbaceous communitiesI examined interactions between the shrub T. chinensis and two congeneric forbs(Suaeda salsa and S. glauca) on the Chinese coast. Although S. salsa and S. glauca are both annuals, morphologically similar, and have synchronous phenologies, they have contrasting adaptive strategies. S. glauca is salt intolerant but competitively superior, and S. salsa is salt tolerant but competitively inferior. Field surveys showed that S. glauca was associated with T. chinensis canopies while S. salsa was more abundant in open areas. A T. chinensis removal experiment showed that S. glauca cover was lower and soil salinity higher after two years in removal than in control plots. Transplant experiments showed that S. salsa performance under T. chinensis canopies was reduced by competition from S. glauca and T. chinensis, while in open areas S. glauca was not affected by S. salsa competition. Thus, contrasting competitive abilities and stress tolerances of S. glauca and S. salsa underlie their facilitative and competitive interactions with T. chinensis, suggesting that plant strategies are critical to the outcome of species interactions.(5) Top-down factors and growth/distrubition of marsh plants: variation in the effects of crab herbivory in the Yellow River estuaryI conducted a series of field experiments and common garden experiments to examine whether crab herbivory plays a role of importance in mediating population dynamics and vegetation patterns in the salt marshes in the Yellow River estuary and whether the importance of crab herbivory vary with environmental variation. Field experiments showed that the herbivorous crab H. tientsinensis readily ate and strongly limited the survival and growth of the dominant marsh plant Suaeda salsa(the survivorship of transplanted S. salsa can be reduced by crab herbivory by 100%), while the strength and importance of crab herbivory varied with changing biotic and abiotic factors among different marsh habitats that include flooding and vegetation abundance. Common garden experiment showed that flooding and grazing by H. tientsinensis synergistically limited the survival and growth of S. salsa. Under every other day flooding treatments, the strength of crab herbivory was significantly higher than under weekly flooding treatments(P < 0.05). These results suggest that(1) crab herbivory is an important factor in mediating S. salsa population dynamics and vegetation patterns in the salt marshes in the Yellow River estuary,(2) the importance of crab herbivory varies as a function of different factors including flooding, vegetation abundance and crab abundance.My findings presented in this research systematically explain the mechanisms underlying the spatial variations in the determinants of plant communities in coastal marshes, and have implications for hypotheses currently tested and heavily debated in ecology, such as the stress-gradient hypothesis(Bertness & Callaway 1994) and the relationship between predation and environmental stress as hypothesized by Menge & Sutherland(1976). My findings also have implications for management and restoration of coastal salt marshes.