| Caragana korshinskii is a commonly used desert shrub species in Northwest Region of China in vegetation restoration and desertification control. But it is difficult for C.korshinskii seedlings to live undergrowth, so the survival of seedlings is the limitation of natural regeneration of population. In order to provide more scientific basis for the natural or artificial regeneration and management of desert plant communities, this paper investigated the spatial and temporal structure of the natural C. korshinskii population, and surveyed seed yield and its composition and reproductive biomass allocation?RBA? of C.korshinskii shrubs and the fates of C. korshinskii seeds released in different habitats of C.korshinskii forest plantation in Longkeng forest region of Baijitan Nature Reserve in Ningxia, China. We investigated the seedling rate, emergency rate and characteristics of seedling growth of C. korshinskii at bottom, middle and top of sand dune?BSD, MSD and TSD respectively?, and 0.5 m, 1.5 m from C. korshinskii shrub base and shrub-interspace.Also, we studied the effects of light, sand burial, drought stress and nitrogen addition on strategy of seedling establishment through simulate natural environment of seedlings, then tested the responses of seedlings to artificially added water and nutrient. The main results are as follows:1.The age structure of natural C. korshinskii population showed a declining population, its survival curve close to curve I. There are at least three waves of death peak in the three age periods?the largest basal diameter of shrub branches: 0 mm?Young trees<10 mm, 10 mm?Adult trees<25 mm, 25 mm?Old trees<55 mm, the number of individuals respectively followed: 7, 113, 20?, the largest basal diameter of the three waves:<10 mm, 20 mm 25 mm, and 30 mm 35 mm. Seedlings regeneration is important in the succession process of C. korshinskii Population. The spatial pattern of different growth periods in C. korshinskii population: All trees, random distribution; Young trees, random distribution; Adult trees, random distribution; Old trees, cluster distribution?p<0.05?.Differentiation of survival strategy, what appears in old trees, may improve the fitness of survival and development of Caragana populations in adversity.2.The seedling regeneration of C. korshinskii population was slightly limited by seed sources. Significant difference were observed among different types of seeds sampled on June 10 and 23, 2013 in all there levels of individual, stem and pod?P<0.05?. With the delaying of development period, the proportion of intact seeds increased while that of insect-infested seeds decreased. It has been found that RBA increased with the extending of seed development and the RBA were all significantly lower than those of stem and leaf in three sampling dates?P<0.01?.3.The seedling regeneration of C. korshinskii population is easier under Artemisia desertorum shrubs. Both habitats of C. korshinskii shrubs?CKS?, A. desertorum shrubs?ADS? and bare sand?BS? and treatments of sand surface?SS? and sand burial?SB? have significant influence on seedling rate and survival rate of C. korshinskii seeds?P<0.05?. In ADS habitat, for example, the seedling rate of buried seeds was significantly higher than that of seed on sand surface during early survey?P<0.05? and significantly higher than those of buried seeds in CKS habitats and seeds treated by SS in ADS?P<0.05?. The final seedling rate maxsized in habitat of ADS, while no significant difference was detected among different habitats and between seeds treated by SB and SS. High volatility of irritant gas from A. desertorum shrub, and mechanical obstacles to animals under A. desertorum shrub, which is low and creeping, could be their reasons.4.The process of seed emergence is dominated by moisture and light conditions in habitats. BSD had the highest seedlings rate which was 20 times higher than that TSD had at the end of experiment. Emergency rate of seeds planted in BSD was significantly higher than that of seeds in TSD?P<0.01?. The maximum and minimum of seedling rate was observed at 1.5 m from C. korshinskii shrub base and shrub-interspace respectively on all survey date. All three habitats had the highest seedling rates on July 18 and there were no significant difference in the seedling rate and emergency rate among different habitats during the whole experiment period. Maximums of shoot height?SH?, basal stem diameter?BSD?, taproot length?TRL?, root stem ratio?RSR? and total dry mass?TDM? of seedlings were all observed at BSD, while the maximum of root shoot ratio?RSHR? was detected at TSD. Seedlings grown at 0.5 m from C. korshinskii shrub base had the highest SH whichwas higher than that of seedlings grown at 1.5 m from shrub base?P<0.05?. TRL, RSR and RSHR were all increased, but TDM declined, with the increasing of the distance from shrub base. The reason may be the redistribution of moisture and light and other ecological factors directed by slope position of sand dunes and nurse effect of C. Korshinskii shrubs.5.Sand burial can alleviate the suppression of strong light on the process of seed germination. Under 55.4% NS?natural sunlight? of light intensity, the germination of not buried seeds was inhibited?germination rate: 17.92%?, and with the increase of light intensity, the germination rate, germination index basically unchanged, but the germination value and vigor index decreased; When the burial depth at 0.5 cm 4.5 cm, with the increase of light intensity, germination rate, germination index, germination value and vigor index all increased. The main reasons are the water loss caused by strong light, which can be blocked by sand burial, and acceleration in the physiological and biochemical processes of seed germination under strong light and its accompanied high temperature.6.Strong light can alleviate the suppression of sand burial on the process of seedling growth. Under 55.4% NS, with increasing sand burial depth, the survival rate, growth indexes and specific leaf area of seedlings decreased, while root/shoot ratio, special shoot length, special root length of seedlings increase. Under 100% NS light intensity, after sand burial and water addition process, the seedling rate?35.40%?, emergence rate?0.98? of seeds and the survival rate?28.93%? of seedlings will be less than after water addition process but with no sand burial?72.57%, 1.54, 10.34%?, the branch number per plant,maximum of lateral branch, special shoot length, special root length decreased too. The reason is watering after sand burial can lead to soil compaction and permeability decreases.7.Nitrogen and phosphorus content of soil have an comprehensive effect on seed emergence and seedling growth of C. korshinskii. The germination percentage and seedling rate is higher, germination rate and emergence rate, germination index, germination value,vigor index of seeds, survival rate, seedling height, basal stem diameter, leaf area per plant,dry mass of young seedlings is the highest at 10 g N·m-2·a-1nitrogen addition, under which condition the seedling growth is limited by P element content in the soil. After supplementary fertilizing P(10 g N·m-2·a-1,2.5 g P·m-2·a-1), the emergence of seed andgrowth of seedling can be improved, but the survival rate of seedlings is reduced. the survival rate of seedlings is 0% at 80 g N·m-2·a-1. Though within endophytes like phosphate-solubilizing bacteria and nitrogen-fixing bacteria, seedlings of C. korshinskii,whose nutrient absorption capacity is limited. |
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