| Ambrosia artemisiifolia and Ambrosia trivia are the world’s most aggressive invasive weeds,which have an important impact on agricultural production,ecological environment and human health.This paper aimed at a phenomenon that the seed yield per plant of A.artemisiifolia and A.trivia is up to 2 000-62 000,the number of plants per unit area is 300-750 and are high-density cluster distributed.Combined with field observation with laboratory experiments.Studied the seed germination tendency and population density dynamics at different growth stages,the fate of underground seed banks and the effects of allelopathy and autotoxicity on their own seed germination of A.artemisiifolia and A.trivia.Analyzed the self-regulation mechanism of two species in order to avoid sibling competition.At the same time,the Allopathy of the plant extracts to the crops--Corn(Zea mays)and Wheat(Triticum aestivum)and companion species--Alfalfa(Medicago sativa)and Clover(Trifolium repens)was verified.It is expected to lay a foundation for insight into the population maintenance and growth of the two species and analyze their invasion mechanisms.The main conclusions are as follows:First,the population density of A.artemisiifolia and A.trivia decreased with the growth period lengthens and increased with the increase of establishment years(1)The results of population density comparison of A.artemisiifolia and A.trivia showed that different growth stages was:Seedling stage>Vegetative stage>Breeding stage>Mature stage.According to the population density changes from seedling stage to maturity stage,the population density of A.artemisiifolia in different habitats decreased by 99,119 and 406 plant/m~2 respectively,while that of A.trivia decreased by 93,480 and 698 plant/m~2 respectively.It showed that there are a massive die-off in the number of plants per unit area of two species in the whole growth period.(2)The results of population density comparison of different habitats in the same growth period are:The invasion of six years>four years>two years.The longer the invasion time,the higher the population density in the same growth period.It indicated that the longer the settlement time is,the greater the cumulative input of seed bank is,and the greater the seed germination is.Second,the number of seeds in the soil seed bank of A.artemisiifolia and A.trivia is huge,but the number of active seeds decreased with the increase of invasion ages and soil depthThe results of seed bank size comparison are:The invasion of six years>four years>two years.Among them,the proportion of withered,insect eaten and moldy seeds in the six-year habitat of A.artemisiifolia invasion reached 23.81%,and in the two,four and six-year habitat of A.trivia invasion reached 22.81%,33.51%and 15.15%,respectively.The seed bank size in different soil depth was:0-10 cm>10-20 cm>20-30 cm,with the increase of soil depth,the seed quantity and seed type decreased.After the seeds of A.artemisiifolia and A.trivia enter the soil seed bank,the loss of different types of seeds,such as withered seed,insect feeding and mildew,constitutes various elements of the dynamic change of the soil seed bank,which will affect the seed germination pattern and become one of the factors determining the population density.Third,A.artemisiifolia and A.trivia had autotoxicity on seed germination,which is characterized by"low promotion and high inhibition",gradually weakened with the extension of growth period,and autotoxicity does not make seed vigor lose(1)The effect of low concentration(0.1,1,2,5 g/100m L)extracts of A.artemisiifolia and A.trivia on the germination of their own seeds was the highest in the vegetative stage.Among them,the highest germination promotion rate of A.artemisiifolia was12.74%,A.trivia is about 12.74%.Seedling stage is next,the germination rate of the two species was 4.80%and 6.25%,respectively.And there was no effect of promoting germination in breeding and mature stage.(2)The germination inhibition of A.artemisiifolia and A.trivia in high concentration(10,15 g/100m L)was the highest in the vegetative stage,with the highest germination inhibition rates were 32.86%and 92.03%,respectively.At the seedling stage were27.21%and 77.94%respectively.At the breeding and mature stage,the germination rate of autotoxic effect inhibition decreased,the highest were 12.02%and 46.96%,16.42%and 48.48%,respectively.The results showed that the autotoxic effect was important in the seed germination and seedling rapid growth stage of A.artemisiifolia and A.trivia.(3)There was no significant difference(P>0.05)in the germination of the large and small seeds of A.artemisiifolia and A.trivia,which indicated that the autotoxicity of the two species had no specificity for the germination of different seed sizes.(4)The seed vigor of A.artemisiifolia and A.trivia was not lost after self-poisoning.After washing,the germination rate increased by 6%and 28%,respectively.The results showed that the two species could inhibit the germination of partial seeds by secreting autotoxic substances.Keeping the stability of the populations can better maintain the competitive advantage among species,and it is conducive to the re-establishment and long-term continuation of its populations.Forth,the inhibitory effects of A.artemisiifolia and A.trivia on crops and associated forages were more strong in seedling and vegetative stageExtracts from A.artemisiifolia and A.trivia at seedling and vegetative stages had more strong inhibition on seed germination of corn,wheat,alfalfa and clover.Among them,the highest germination inhibition rates of corn and wheat seeds were 31.46%,27.71%and 41.44%,45.11%respectively.And the highest germination inhibition rates of alfalfa and clover were 100%.Seedling and vegetative stage are also important stages of autotoxicity,and the effect intensity gradually weakens after vegetative stage.To sum up,this paper studied the relationship between population density changes with the growth period and seed types in soil seed bank of A.artemisiifolia and A.trivia,and the effect of plant extracts at different growth stages on the germination of associated species and their own seeds.It was founded that the seed types of the soil seed bank and the autotoxicity and allelopathy of plants jointly affect the population density change.This may be an important reason for A.artemisiifolia and A.trivia to invaded,spread and stabilized the population successfully. |
