Natural Regeneration Of Platycladus Orientalis Plantation At The Weibei Loess Plateau

Platycladus orientalis is the major native tree species in Weibei Loess Plateau, and thevast majority has entered the solid phase. However, the prevalence problem of Platycladusorientalis in this region is the large density, growing recession, poor natural regeneration andother issues, there are obstacles in natural regeneration process is the core of its sustainablemanagement. Natural regeneration capacity research of Platycladus orientalis plantation inthis area is very important for further study of vegetation succession driving force and itssustainable management of the region.Three permanent plot（s50m×50m）were set in each population including tended (tendingrate was about30%), no-tended Platycladus orientalis plantations and Robinia pseudoacacia-Platycladus orientalis mixed (Platycladus orientalis rate was about75%) plantation inHuaiping forest farm of Yongshou county, Shaanxi province, seed rain, soil seed bank andseedling were investigated. Seed rain was collected every7days from August to October in2012and seeds in soil seed bank were collected in November2012and August2013,respectively. Viability of seeds was measured using seed germination method and density ofnatural regenerated seedlings in each plot was investigated, the results are as follows:1. The landing process of Platycladus orientalis seed rain can divide into three periods,the beginning period, the peak period and the ended period. The seed rain of three kindsPlatycladus orientalis all started from10thAugust; but the peak time of seed rain is different,the seed rain of no-tended forest reached the peak in later August and early September, theseed rain of mixed forest reached the peak during7thSeptember to14thSeptember; the endingtime of seed rain is different, the seed rain of tended forest ended at5thOctober, while theseed rain of no-tended forest and mixed forest ended at12thOctober.2. The seed rain of Platycladus orientalis was composed by intact seeds (full grain, noobvious infestation, grazing traces), incomplete seeds (poor, empty), insect damaged andanimal predated seeds. The composition of Platycladus orientalis seed rain is consistentduring the same period, at the beginning of seed rain, vacant and incomplete seeds were themajority in all the three kinds of plot; while intact seeds dominated during the peak time ofseed rain.3. The soil seed bank of Platycladus orientalis is short-term seed bank, the number of intact seeds reduced greatly with storage time, intact seeds reduced to0after nine months.90%of the seeds in soil seed bank distributed in the litter layer, seeds in the soil seed bankrarely distributed in the2~10cm soil layer.4. Due to the fact that forest density can affect the nutrition area of mother tree, so it hasa significant impact on the reserves of soil seed bank, showing that tended forest (130.55grain·m－2)>mixed forest(80.23grain·m－2)>no-tended forest (48.13grain·m－2).Totalreserves of soil seed bank decrease with storage time. The number of complete, incomplete,insect damaged and animal predated seeds showed a decreasing trend with storage time, butthe amount of rotten seeds was increase, the proportion of rotten seeds in no-tended forest wasthe highest.5. The horizontal distribution of soil seed bank of three kinds Platycladus orientalis wasno significant difference, mainly distributed in the range of2m from the mother tree, thenumber of seeds significantly reduced within the range of2~3m, the amount of seedsshowing a sharp downward trend from1.5m away of the mother tree to the periphery.6. The emergence time of seedlings of three kinds Platycladus orientalis was nosignificant difference, emergence from early April, reach the peak in June to July, no newseedlings appear after September. The density of seedling emergence of3forest wassignificantly difference (P<0.05). The density of seedling survival in tending forest was thebiggest, followed by mixed forest, no-tended forest was the minimum.