| Pinus tabulaeformis is an important afforestation tree species in the mountainous areas of northern China,but there are a large number of natural regeneration barriers in Pinus tabulaeformis plantations in Beijing,hindering the restoration and growth of Pinus tabulaeformis forest resources.In the past,there have been many studies on the regeneration of Pinus tabulaeformis forest,but there is no study on the causes of natural regeneration barriers of Pinus tabulaeformis plantations in Beijing.There are a large number of naturally regenerated Pinus tabulaeformis plantations in Pingquan area,which is adjacent to Beijing.Therefore,this study sought to find out the key factor to natural regeneration barriers of Pinus tabulaeformis plantation in Beijing by comparing and analyzing the climatic conditions of Beijing and Hebei,the soil physicochemical properties of Pinus tabulaeformis,cone seed content,seed sowing quality,soil seed bank and soil germination tests,and has reached conclusions as below:(1)The results of the comparison of meteorological elements between the two areas(1981-2010)show that in the vigorous spring of seed germination,the precipitation in Pingquan and Beijing in the months of May and June have significant differences(p<0.05).In May and June,the average precipitation in Beijing was 37.6mm and 75.9mm respectively.In Hebei,it was 45.5mm and 96.4mm.The latter exceeded the former by 21%and 27%respectively.As the Chinese pine has a good natural regeneration in Hebei,the lower precipitation in May and June in Beijing may be a factor in the barriers to regeneration of Pinus tabulaeformis;monthly evaporation in Hebei Province is significantly lower than that in Beijing(p<0.05).Combined with the above-mentioned conclusions that the precipitation in Hebei Province in May and June was significantly higher than in Beijing,the lower soil moisture content in May and June may be a factor in the barriers to regeneration of Pinus tabulaeformis;due to differences in geographical location,temperatures in Pingquan,Hebei Province,are significantly lower than in Beijing(p<0.05),but the natural regeneration of Pinus tabulaeformis in Hebei Province is still good,so the atmospheric temperature should not be the cause of the impediments to regeneration of Pinus tabulaeformis in Beijing;The average relative humidity and the average length of daylight may have little effect on the natural regeneration of Pinus tabulaeformis in Beijing.(2)The results of the physical and chemical properties of the soils in the two areas show that the saturated soil water holding capacity,capillary water holding capacity,capillary porosity,and total porosity of the experimental soils in Hebei Province are the lowest among the four experimental plots.Therefore,these four indicators may be not a factor that affects natural regeneration;soil moisture content in the three test sites in Beijing is significantly lower than that in the Hebei area,and both are less than 20%.Therefore,soil moisture content may be a factor affecting the regeneration.The nitrogen content and organic matter content in Hebei experimental sites with well-regenerated Pinus tabulaeformis were significantly lower than those in the three experimental sites in Beijing;the difference in available potassium content was not significant;the available P content was the highest in the experimental area of Mujiayu,Beijing;There were no significant differences between the two experimental sites in Beijing;The soil pH at the experimental site in Hebei was significantly lower than that in the three experimental sites in Beijing,but both were less than 7.It can be seen that the above three soil nutrient elements,soil organic matter content and soil pH may have little effect on Pinus tabulaeformis regeneration.(3)The results of seed quality comparison between the two areas show that the average length of cone in Hebei is 4.2cm and that of Beijing cone is 3.6cm.The former is significantly higher than the latter;the average number of seeds in Hebei cones is 30,and that in Beijing cones is 22;the seed filling rate in Hebei is 98.6%,and the seed filling rate in Beijing is 83.1%.The 1000-grain weight of Pinus tabulaeformis irt Hebei area is 31.1g,which is 17.1g in Beijing;the germination rate of Pinus tabulaeformis seeds in Beijing is 3%,the germination potential is 1%;in Hebei,it is 73%and 15%respectively;The seed moisture content was 5.74%in Hebei,which was 5.15%in Beijing.The above indicators were significantly higher in Hebei than in Beijing(p<0.05).It can be seen that the cones in the experimental areas in Hebei Pingquan are better than the cones in the Beijing Xishan test site in terms of size,cone seed content,and seed plumpness;the seeds of Pinus tabulaeformis in 1000-grain weight,the germination rate and seed moisture content were significantly better than the Xishan area in Beijing.Low cone and seed quality in the Beijing area may lead to natural regeneration barriers in Pinus tabulaeformis.(4)The results of comparative analysis of soil seed banks in the two areas showed that the number of total seed,seed number of the first soil layer and the number of unbroken seeds in the soil bank in Hebei were the highest among the four experimental plots,and the difference was significant(p<0.05).It can be seen that the seeds in the soil seed bank in Hebei Province are more numerous and better quality than in Beijing,and the total seed amount in the soil,the first seed content in the soil,and the complete seed number may have an impact on the natural regeneration of Pinus tabulaeformis in Beijing.(5)The results of soil germination test of Pinus tabulaeformis showed that there was no seed germination in the combination of Pingquan soil and Beijing seed,Beijing soil and Beijing seed,but seed germination rates in combination of Pingquan soil and Pingquan seed,Beijing soil and Pingquan seed were 65%and 75%respectively.This germination test result is consistent with the results of the seed quality comparison test,verifying that the low seed quality in Beijing may be an important reason for the natural regeneration of Pinus tabulaeformis. |
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