Natural Regeneration Strategies Of Calligonum Mongolicum Under Different Aeolian Environments

Calligonum mongolicum,a great windbreak and sand-fixation pioneer species occurred in mobile and semifixed dunes,is regarded as one of the most important building groups of sandy vegetation,and most suitable afforestation species for sand-fixation in arid areas,which plays an extremely important role in protecting the ecological security in Western China.C.mongolicum shows a strong ability of natural regeneration in desert mobile sand dune,but this ability is sharp decline when the sand dune is getting fixed.The ecological and physiological mechanisms which behinds this phenomenon is still poorly known.In this thesis,we investigated reproductive features and natural regeneration strategies of C.mongolicum in three different aeolian environments from various of aspects including the number of seeds,the pattern of seed dispersal,the density of seeds in soil seed banks,seed germination rate,seedling emergence rate and clonal growth to reveal the factors resulted in weak natural regeneration after sand dune were fixed.The results are shown as following:1)C.mongolicum had several reproductive strategies to adapt to various aeolian environments in deserts.First,it flowered and produced seeds 2-3 times a year.The phenology played an important role in seed production.The seed mass of matured seeds is: September <August<June.Second.The matured seeds were dispersed by wind,and the dispersal direction was positively correlated to wind direction.The seeds dispersed in long-distance and short-distance strategies,with the former to provide a chance for seeds to settle down in a new habitat,and the later to contribute to the soil seed band in situ.The third-year seeds in seed bank retained most of the seed vigor and germination rates of the freshly harvested seeds,constituting the persistent soil seed bank.Third,the germination rate of seeds was low(<45%)and it took ~ 14 days to observe the first seedlings.C.mongolicum could be delayed within-season germination timing acts as a bet hedging strategy to avoid the unpredictable environmental changes.Moreover,the seed germination and seedlings showed a strong tolerate to drought(PEG –6000,up to 20.3%)and salty(NaCl,up to 0.8%)enviroments.2)C.mongolicum took different strategies to power population natural regeneration in the three aeolian environments.First,the regeneration in the wind erosion plot and the sand accumulation plot is mainly relied on clonal reproduction with plants flowered and produced seeds twice a year while there is no clonal reproduction in the intersand dune plot with plants flowered and produced seeds three times a year.Second,the vertical distribution of soil seed bank was related with the soil texture and the aeolian environment.The high soil compaction in intersand dune resulted in the seeds hardly deep into the soil seed bank,leading that seeds only just settled in the shallow soil layer.However,the strong sand movement in wind erosion plot resulted in a much lower seed bank density at shallow than deep soil layer,and a relative higher soil seed bank at 0-10 cm depth in sand accumulation plot.There was significant difference for the seedling emergence rate and the seedling survival rate among the three different aeolian environments,and the soil seed bank at 2-5cm depth was significantly and positively correlated with seedling emergence rate.Third,the sand accumulation plot showed much higher density of seedlings in both spring and autumn(2300/ha and 1300/ha,respectively)than the wind erosion plot(2/ha and 30/ha,respectively).The height,the basal stem diameter and leaf area of seedling were similar in all the three aeolian environments and seedlings were resistant to mobilesand by increasing the number and the length of assimilation branches to maintain a higher water holding capacity.Fourth,clonal seedlings with connections with their maternal plants were growing faster than seedlings generated from soil seed banks,which was beneficial to the survival and growth under wind stress to complete the whole life history,suggesting a better ability to adapt to the wind erosion environment.3)After the dune fixation,the natural regeneration strategy of C.mongolicum have changed significantly.When the afforestation area formaed the fixed dune,the seedlings were only generated by seeds,and the seedling density was only 67 plants/ha,which was significantly lower than that of the mobilesand area(310 plants/ha).After the dune fixation,the surface sandstorm activity decreased and the aeration porosity of soil increased,which limited the development of the root of seedlings,and resulted in the fixed dune with no clonal reproduction.The density and distribution pattern of seeds in the soil seed bank of fixed dune did not differ from those in mobilesand,indicating that soil seed bank was not a limiting factor of natural regeneration in afforestation.However,the increase of soil aeration porosity after dune fixation resulted in a low seedling unearthed rate and massive decease of the survival rates of seedlings.Therefore,the change of soil physical features is the main reason for the change of the natural regeneration strategy after sand dune fixation.In conclusion,C.mongolicum was benefited from multiple life cycles per year,the longdistance dispersal,the formation of a persistent soil seed bank,and the seed germination in both spring and autumn,for its local adaptation to the harsh environments in the desert area.The natural regeneration strategies changed at the three different aeolian environments.This study not only systematically revealed the natural regeneration strategies for the sand-fixation pioneer species C.mongolicum,but also has important guiding significance for the management and plantation of Calligonum acerata in the desert area.