Seed Life-history Strategies Of Plants And Restoration By Seed Addition In The Hill-gully Loess Plateau Region

The hill-gully Loess Plateau region has been subjected to long terms of severe soilerosion and drought stress, where the ecosystem is threatened with degradation. Regenerationand restoration of natural vegetation are identified as the essential and available approach tocontrol soil erosion and ecological degradation. Seed and seedling are regarded as the criticaland fragile stage during plant life history, which determines plant distribution and vegetationcomposition, as well as the natural revegetation. Thus, the characteristics of seed morphology,production, dormancy and germination, canopy seed bank and seed persistence of plants inthe study region were studied. Combining with characteristics of pecies distribution, the seedlife history strategies of plants adapted erosion environment were discussed. Additionally,seed addition was conducted to learn the factors of limiting seedling regeneration and toexplore the effective artificial approaches for revegetation. The main results are shown asfollows:（1）Species with seeds of small mass and spherical shape account for a large proportionin the study region, while seeds with appendages and secreting mucilage are advantagecharacteristics. These seed morphological characteristics are conductive to wide and densedistribution for species, which are defined as dominant morphological strategies to fit theenvironment stress and erosion disturbance. Species of different life forms and waterecotypes perform different strategies: seeds of annual herbs tend to be small mass, sphericalshape and having appendages; seeds of xeromorphytes and mesoxerophytes are more likelyto be small mass and having appendages. Seeds can resist water erosion with morphologicalcharacteristics, such as big mass, extreme elongated shape, appendages (except wing),mucilage secretion and special morhpology combination, which are not dominantmorphology determining species distribution and vegetation pattern. Nevertheless, resistlessmorphological characteristices can increase dispersal opportunities of seeds and make thempossible to reach suitable habitats.（2）Seed production of plants range from12to22774seeds/individual in the studyregion. Seed production of most species distributed densely or widely is medium. Annual/Biennial herbs prefer large seed production while perennial herbs are in balance ofseed propagation and vegetative reproduction. Shrubs and semi-shrubs are more likely toproduct extreme more seeds or extreme less seeds. There are interspecific difference in seedproduction responses on more severe erosion deterioration conditions, and three types ofseed production strategies: strategy of decreasing product components inputs and productingless small seeds, like Bothriochloa ischaemu and Stipa bungeana; strategy of increasingproduct components inputs and producting more big seeds, like Heteropappus altaicus andLespedeza davurica; strategy of increasing product components inputs and producting moresmall seeds, like Artemisia gmelinii, Artemisia giralaii and Potentilla tanacetifolia.（3）Species with high germination percentage and low dormancy percentage accountfor a large proportion in the study region, which is dominant germination strategy to adapterosion environment. Seeds of most perennial herbs, perennial grass in particular, have highgermination percentage and low dormancy percentage. There are great variations in seedgermination and seed dormancy of shrubs and semi-shrubs, as well as xeromorphytes andxerophytic-mesophytes. It shows seed strategies of keeping strong dormancy to avoid threator strong germinating to increase regeneration success. There are close relationshipsbetween seed germination/dormancy and seed mass. There is a decrease in seed germinationas seed mass increase, while seed dormancy is contrary. Seeds with the ability to secretemucilage have strong germination ability. The responses of seed germination/dormancy tosoil erosion are different. There are more dormant seeds of most main species in south slopethan in the other environments of hill and gully, which shows species respond toenvironmental stress by keeping seed dormancy.（4）At least64species have canopy seed bank in the study region, and species withoutappendage or species with late flower and fruit time tend to have canopy seed bank. For12studied species, seeds of Periploca sepium store on plant for less than four months, whileseeds of the Rose xanthina can store on plant over eight months. The other species store onplant for more than four months but less than eight months. Seed dispersals of most speciesrespond to the rainfall and temperature variety closely. The main mechanism of driving seedfall off is the alternation of wetting and drying or warm and cold temperature. Response ofseed germination and viability of nine species on storing on plant perform differently, withseed germination and seed viability of five species increase at different levels in the end ofFebruary in next year respectively. Studied species performed different canopy seed bankstrategies to increase the success of seed germination and seedling regeneration by persistinga large seed amounts, or regulating seed germination characteristics, or increasing percentageof live seeds. （5）Seed persistence in soil of main species in the study region is classified into fourtypes: transience(Periploca sepium and Hippophae rhamnoides), delayed transience(Syringaoblata and Ziziphus jujuba), short-term persistence(B. ischaemu and S. bungeana), long-termpersistence(Artemisia scoparia, A. gmelinii, A. giralaii, L. davurica, Sophora davidii,Cotoneaster multiflorus and R.xanthina). There are close relationships between seedpersistence and seed morphology and seed dormancy/germination. Small seeds, compactcoated and hard seeds tend to be more persistent in soil. Soil seed bank can fuction effectivelyby regulating seed germination. Burial can promote seeds of A. scoparia with highgermination percentage into dormancy, which is benefit to maintain seed persistence. Burialalso can promote seeds with high dormancy percentage to release dormancy, which is usefulfor seed germinating in right time and habitat. There are close relationships between seedpersistence and soil seed bank and species distribution. Species with transient and delayedtransient seeds have transient soil seed bank, which is useful for seeds to germinate timelywhen reach suitable habitat. Species with persistent seeds have regeneration potential, whichis useful for species (except C. multiflorus and R. xanthina) to distribute densely and widely.（6）In the study region, the key environmental limiting factor for seedling emergence,survival and growth is moisture condition. Seed germination and seedling emergence arelimited by the crust greatly, while seedling survival and establishment are limited by lowtemperature. Seedling regeneration is also limited by soil erosion, with seedling emergencelimited by water washing while seedling survival limited by sediment depositing. Seedadditions promote seedling regeneration and vegetation recruitment to some extent. Seeds ofdominant species with strong stress resistance or belonging to later succession stage can beused as effective additional seeds. Addition of seeds with more and quickly germinatedshould be conducted at early time with effective rainfall. Addition of seeds with slowly ornone geminating should be pre-processed to release dormancy. Seed addition withmicro-environments(pit, belt et al.) and shading is beneficial to ensure and promote theadditional plants to regenerate.