Studies On Ecological Adaptation And Reproductive Strategy Of Caryopteris Mongolica

Caryopteris mongolica,a rare and endangered small shrub with high economic and ecological values,is mainly found in arid grasslands in northern China.Its distribution areas have rapidly shrunken in recent years.To explore the reasons for the plant's diminishing trend,the study combined the field investigation with laboratory tests to investigate the community structure,biodiversity status,population dynamics and spatial patterns in four C.mongolica communities of Daliuta,Hohhot,Saihantala and Wuhai in arid grasslands of Northern China.Reproductive features of the plant,including sexual reproduction and asexual propagation,was investigated in the aspects of seed production,seed germination and branch cloning behaviour.In the effort to reveal the sexual reproduction potencies,seed germination and seedling establishment were examined.To determine the population maintenance mechanism,seed soil bank dynamics was monitored.The main results are as follows:Stable native populations of C.mongolica were found in all the four sampling sites in the study.The population of C.mongolica distributed aggregately on large scale.The intensity of aggregation was negatively correlated with population density.Intraspecific relationship varied with population densities.There was no obvious competition relationship between the plants,mature plants and the young plants were mutually beneficial in symbiosis when the population density was lower than the environmental carrying capacity.As the population density increased close to the threshold of environmental carrying capacity,there was a competitive relationship between individuals.The mortality of young plants was high due to the intraspecific competition when the population density was high under a favorable environmental condition.Plant growth in height of C.mongolica mainly occured in the young stage.After entering the mature stage,the canopy of the plant enlarged quickly for grabbing resources more efficiently.C.mongolica produced false age-rings after 4 year-old.The frequency of false age-ring occurance increased with plant age.In the study of the age structure of the C.mongolica population,the linear regression model can be used to determine the age in accordance with the base diameter.C.mongolica was pseudo-dichotomously branched.Most of the primary branches was found in the most outer layer of the plant canopy.Newly born branches were thin and brown in colour.Most of the leaves were on the primary new branches.Branching architecture of C.mongolica was determined by height,crown and number of branches.Hydrothermal condition was the main factor influencing branching behaviour.It has complicated branching architecture in arid regions with high temperature.In the cold climate condition,the shrub accumulated biomass into the leaves and most of the energy was used for reproduction.The mature leaf of C.mongolica had a relatively small leaf area and thick blade.The upper surface of the leaf was covered by cuticle and epidermal hair.The leaf material was composed only of closely aligned palisade tissue,helpling the plant to adapt to arid environment.Temperature regime had a great influence on resource allocation in the leaves.Most of the energy was used for dry matter accumulation in leaves under low temperature.C.mongolica was mainly via sexual reproduction for regeneration,although it was capable of vegetative propagation by stolon.Large quantity of seeds was produced in September,sharply enriched the soil seed bank in 0-2cm depth.Seed germination followed with rainfall in the Spring quickly depleted the reserved seed bank.Seed germination in the field was mainly determined by temperature.The optimum burial depth for seed germination was 10–15 mm.Survival rate of seedlings was primarily determined by water regime.Seedling vigour was greatly influenced by temperature and light.Prolonged high temperature and frequent watering increased seedling death rate.Drought limited the growth of seedlings and make the plants smaller.Short period of high temperature was beneficial for seedling growth.Shoot development and secondary growth was undermined by weak light.In the natural field condition,seedlings survival were negatively affected by high temperature and frequent rainfalls,leading to a low renewal rate of plant population.Vegetative propagation occurred when the branches were buried in the field conditions.The number of ramet increased significantly with the increase of maternal plant base diameter.There was a very significant positive correlationship between the ramet and spacer.It was concluded that the mature plant of C.mongolica had high adaptability and competitive ability in arid regions.Through rational allocation of natural resources it has strong ability of spatial extension.Poor seedling establishment was considered the main reason for the fading of population.The discrepancy between germination and seedling growth requirements narrowed the seedling establishment window.In the natural habitats of C.mongolica,the conditions are often not able to meet the requirements for the entire process from seed germination to seedling establishment.This is the main reasons for the relatively poor dispersion of C.mongolica populations through sexual reproduction under field conditions.At the same time,the ability of vegetative propagation of C.mongolica was poor.In order to realize the maximum ecological and economic benefits,it was suggested that the artificial cultivation of C.mongolica should strengthen the management during seedling stage,especially the water control measures.