Responses Of Seedlings To Neighboring Plants Under Soil Acidity Stress

The direction and strength of plant-plant interactions, including competition or facilitation, is affected by the environment. According to the stress gradient hypothesis (SGH), under the conditions of harsh physical stress and intense consumer pressure, the inter-plant interactions often express in the form of facilitation; while in relatively low physical stress and consumer pressure, the interactions are more competitive. However, it is noteworthy that different species have different niche optima, and therefore, the intensity of stress is not an absolute concept with respect to different species. As a consequence, the SGH theory undergos defy, at least to some extend. We need to consider the tolerance of plants along the environmental stress gradient when we discuss the interaction between plants.Seedling is one of the most fragile stages in the plant growth. The survival and growth of seedlings often play an important role in structuring plant populations and communities. It was well documented that the plants have a profound impact on neighboring seedlings (competition or facilitation). However, whether and how the plants affect the germination and seedling survival of their neighboring seedlings along environmental gradients is not well investigated.In this study, I used the common garden experiments and field experiments to evaluate (1) the tolerance of different plant species towards acid soil and the niche optima; (2) the responses of seedlings to neighboring plants under soil acidity stress. The main results are as follows.1 Species tolerance to acid soilA soil acidity experiment by using legume species Lespedeza formosa Koehne, Indigofera pseudotinctoria Mats, and Medicago sativa L. showed that species differd in tolerance to soil acidity. According to germination rate and seedling survival rate, L formosa had widely tolerant to acidified soil (pH 2.5-6.1), while I. pseudotinctoria showed moderately tolerance (pH 3.1-6.1). Medicago. sativa had narrowly intolerant to soil acidity (pH 4.1-6.1). We therefore selected L. formosa as a neighboring plant to test plant-plant interactions in the follow up experiment.2 The responses of seedlings to neighbor seedling plants under soil acidity stressThe responses of seedlings to neighbor plants vary along soil acid gradients. The competition between conspecific seedlings of L. formosa decreased with the increasing pH values of the soil, while the relationship between I. pseudotinctoria and neighboring L. formosa shifted from competition to facilitation as soil pH values increased. For M. sativa, a U-shaped curve of relationship was observed. The relationship between M. sativa and L. formosa changed from facilitation to competition, and then to facilitation. The results by using Gaussian simulation showed that L. formosa, I. pseudotinctoria, and M. sativa had ecological optimal pH value at 4.67,4.66, and 5.42-5.56, respectively. Competition between the target plant and its neighbor occurred at the niche optimum of the target plant. However, under conditions deviated from the niche optimum, the inter-plant relationship became more facilitation. The relationship between facilitation and the deviation from the ecological optimum pH were linear. That is the further distance to ecological optimum, the more frequency of facilitation occurres.3 The responses of growth stage to adult neighbor plants under soil acidity stressPlant growth stage differed in responding to neighbor plants. In seedling stage, neighbor had no significant effect on the germination of I. pseudotinctoria compared to no neighbor. When soil pH was 4.1, however, the survival rate of I, pseudotinctoria were significantly higher with whole neighbor and canopy neighbor, indicating that both canopy and rhizosphere effect benefited survival of seedlings. For M. sativa, both germination and survival rates were significantly higher when neighbor plants were present.In the flowering stage, both effect (canopy effect and rhizosphere effect) benefited the growth of M. sativa at pH of both 4.1 and 6.1. In the case of I. pseudotinctoria, this beneficical effect only occurred at pH 6.1.4 The effect of arbuscular mycorrhizal fungi (AMF) on neighbor effectField experiment showed that the acid-tolerance neighbor plant (L. formosa) benefited the seedling growth and survival of I. pseudotinctoria and M. sativa under highly acidity soil, and AMF contributed to this effects. For the higher tolerant (L. formosa and I. pseudotinctoria), AMF enhanced competition between the neighbored plants. For the the lower acid-tolerant species (M. sativa), AMF enhanced facilitation between the neighbored plants. Experiment further showed that L. formosa increased organic matter and available phosphorus content of the soil.