| In order to explain ecological processes of seedlings settlement of Quercus variabilis Blpopulation and their drought tolerance, different distribution regions(including Loess Plateau,Qinling north slope and Qinling south slope), and different slope aspect were taken as theobject of our study in Shaanxi province. By field investigation and controlled experiment inthe laboratory, methods of population and physiological ecology were applied to exploresystematically process from seed and to seedling establishment; meanwhile avoidance or/andtolerance mechanisms of Q. variabilis Bl was clarified in drought environment, and we foundsuitable soil moisture conditions for seed germination and seedling establishment.The resultsof the present study provide information for vegetation restoration with this species in aridand semi-arid regions of northwestern China. The main results were as follows:1.The sunny slope showed greater fruiting number, seed morphology and quality(including1000-seed mass and seed vitality) in parent tree individual of Q. variabilis Bl thanshady slope in Loess Plateau, Qinling north slope and Qinling south slope. The fruitingnumber from high to low was Qinling South Slope, Qinling North Slope and Loess Plateau,and the sunny side and upper canopy of parent tree produced relatively more fruits. Fruitingnumber maintained significant difference among different distribution, aspect and canopyposition. Stepwise multiple regression analysis for fruiting number, seed morphology andquality related17environmental factors, which showed that crown width area and volume ofparent tree individual, the temperature, light conditions, soil available nitrogen and potassiumin Q. variabilis Bl. forest were key factors, whereas the temperature and light conditionsalways play the greatest role in enhancement of fruiting number, seed morphology andquality.2. There were differences in the dissemination process, occurrence time, and composition ofseed rain among the three distribution regions. The lastest landing time and earliest endingtime were observed in Loess Plateau; seed rain lasted about76days, while peak time lastedabout14days. In contrast, the earliest landing time and lastest ending time was found inQinling South Slope. The whole seed rain lasted91days and the peak time lasted about33days. Seed rain in sunny slope started earlier than that in the shady slope in same distribution region, while apparent peak time was presented in shady slope. Total intensities of seed rainand soil seed bank was Qinling South Slope＞Qinling North Slope＞Loess Plateau, which insunny slope were greater than in shady slope in same distribution region. The process ofseed rain was dominated by mature seeds, and mature seeds mainly concentrated in the peakof the landing. The largest proportion of mature seeds was found in Loess Plateau, accountingfor56.60%. Immature, seeds eaten by insect pests and animals mainly landed in the startingand final phase of seed rain, the largest proportions of seeds eaten by insect pests and animalswere found in Qinling South Slope, while the largest proportion of immature seeds reaching33.34%, was observed in Qinling North Slope. The number of seed development changeddynamically in soil seed bank by time, and the speed of the disappearance of mature seeds insunny slope was faster than that in shady slope. The seed descreased by seeds eaten byanimals and moldy resulted in soil seed bank.3. The densities of seedlings decreased with age increasing in three distribution regions.In order to adapt to the Loess Plateau habitat, the seedlings increase root growth and decreasethe growth of leaves and stem. Northern slope of seedlings enhanced ability to adapt in theQinling North Slope with age increasing, more dry mass was gradually accumulated in leavesand stem. Seedling growth in sunny slope of Qinling South Slope was better than that inshady slope, and improvement of leaf area index and light competitiveness was adopted topromote seedling regeneration. Path analysis showed that improving lighting conditions, soilmoisture, soil available nitrogen content and the thickness of the litter layer, and reducing theshrub layer cover and forest stand density are conducive to the seedlings settement. Moreover,the best seedling growth was found in the sunny slope of Qinling South Slope, but the worstone was displayed in sunny slope of Loess Plateau.4. With decreasing water potential caused by increasing concentrations of PEG tosimulate different drought stress intensities, germination time delayed and germination vitalityindex(VI) decreased gradually. When compared to controlled treatment(0MPa), seedgermination percentage (GP) and daily average germination rate under light drought stress(-0.1MPa) both increased by9.81%, germination vigor (GRI) increased by4.58%, as well asthe growth of seed radicle promoted. The moderate and severe drought stress treatmentsinhibited seed germination, as well as slowed growth of the seed radicle and plumule.Germchit adapted to the decreasing environmental water potential (from-0.1to-1.2MPa) byincreasing the radicle to plumule ratio (R/P), whereas seed could not produce plumule undersevere drought treatment (-1.5MPa). It must take more time to finish50%seeds radicalemergence, plumule emergence and seedling establishment than that of controls underdrought stress. Critical water potential of50%seeds radical emergence, plumule emergence and seedling establishment was-0.12,-0.08and0.06MPa, respectively, which indicated thatseedling establishment was most sensitive to external water potential. When soilenvironmental water potential was below-0.6MPa, the contents of three osmoregulationsubstances (free proline, soluble sugar and protein), the activities of four protective enzymes[(superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbateperoxidase(APX)], and antioxidant content [ascorbic acid (ASA)] were significantlydecreased, and malondialdehyde (MDA) significantly increased, resulting in lipidperoxidation. Therefore seed germination was inhibited.5. The seedlings had optimal height, basal diameter, and healthy number of leaves, leafarea under controlled treatment. Afterwards, these parameters significantly decreased asdrought intensity increased. The number, length, surface area, volume of different diameterclass root under light drought treatment significantly increased compared with those atcontrolled treatment, whereas these parameters decreased at moderate and severe droughtstress. Leaf area ratio (LAR) and specific leaf area (SLA) increased to withstand progressivedrought stress, but their effects were not significant. The long-term water-use efficiency(WUEL) started to decline at moderate drought stress, resulting seedling slow growth.Increasing accumulation of osmoregulation substances, decreasing osmotic potential anddeveloping antioxidant defense system of leaf, thin fine root (0<D≤0.5mm) and thick fineroots (0.5<D≤2mm) withstood light drought stress. Part of the protective antioxidant enzymeactivity and antioxidant content have declined, leading to decreasing ability of droughtresistance under moderate drought stress. However, prolonged severe drought stresssuppressed to increase in soluble protein, activities of CAT, POD and APX, as well as ASAcontent, leading to a rapid increase in MDA and decline in fine root vitality. Therefore,seedling growth was almost stagnant at severe drought stress.6. Q. variabilis Bl seedlings still were able to maintain an adequate leaf water status(RWC>60%) at light and moderate drought treatment. The deficient leaf water status (RWC<42%) was observed at severe drought resulting in serious leaf wilting. Exposure to lightdrought resulted in quickly significant reductions in the saturated osmotic potential (Ψ100s) andinitial plasmolysis osmotic potential (Ψ0s), meanwhile seedling maintained a relatively highernon-permeate water content (RWCa) and lower bulk modulus of elasticity (). However, Ψ100sand Ψ0swere improved at prolonged severe drought stress, leading to an inhibitedosmoregulation. The photosynthesis diurnal courses of Q. variabilis Bl seedlings werebimodal curve during clear days of the growing season. Peak value appeared at10:00to11:00and15:00to16:00, having a significant photosynthetic midday depression phenomenon at13:00. Drought stress induced significant decline in net photosynthetic rate (Pn), and the main reason for this depression of seedling included stomatal factor and non-stomatal factor.Leaves instantaneous water use efficiency (WUEI) was improved at light drought, butsignificantly decrease at severe drought. Drought stress led to a reduced photosyntheticpigment content, while Chla/Chlband Car/Chla+benhanced.7. Seedlings should be protected in Q. variabilis Bl secondary forests to adapt adverseenvironmental. Cultivation of high-yield Q. variabilis Bl forest should collectes in peak timeof seed rain, and the location should be selected at Qinling forest area. In addtion, thinningwas suitable for cultivation and management to provide the favorable light conditions and soilwater contents keep being between21.1±0.6%to14.6±1.2%for Q. variabilis Bl seedgermination and seedlings growth. |
PDF Full Text Request