Topography Significantly Influences The Growth Dynamics Of Above-ground Part And Rhizome Of Moso Bamboo Along A 10-year Chrono Sequence Following Afforestation

Moso bamboo(Phyllostachys heterocycla var.pubescens),as an important forest resource in southern China,has a great capability of carbon(C)sequestration and an important role of forestry in climate change.Therefore,the afforestation of Moso bamboo is getting more and more attentions.A lot of previous studies have revealed the influence of topography on the growth of mature aboveground parts of Moso bamboo forests,but few studies reveal the dynamic influence of topography on the growth of new Moso bamboo forests and the influence of topography on the growth of rhizome.Therefore,this article is divided into two experiments,revealing the dynamic effects of topography on the aboveground parts of Moso bamboo forests along a 10-year chrono sequence following afforestation and the influence on the growth of Moso bamboo rhizome.In the first experiment,thirty-one 20×20m long-term sample plots were established and the diameter at breast height(DBH),age,height and coordination of every newly sprouted culm were measured from 2009 to 2018.Four Moso bamboo expansion periods(rhizome expansion period,early period,middle period and stable period)were divided according to the patterns of change of newly sprouted culms,at the same time introducing new topographic factor(topographic complexity).Structural equation modeling and univariate analysis were combined to assess the dynamic effects of topographic factors(slope,aspect,slope position and topographic complexity)and the effect of single topographic factors and their interactions on the carbon stocks during the four expansion periods.In the second experiment,20 sample plots of 10×10m(5～15°,15～25°,25～35°,35～45°)were set up at 4 different slope grades,and the rhizomes of each sample plot were collected by the small window method,measuring stem length,diameter and rizhome biomass,and collect 0-20 and 20-40 cm soil water content,bulk density,alkalihydrolyzable nitrogen,available phosphorus,available potassium and the above-ground part of Moso bamboo(DBH,culms,biomass)through variance analysis and correlation analysis,revealing the influence of slope on rhizome growth,and the relationship among soil physical and chemical properties,above-ground part of Moso bamboo and rhizome growth.Finally,the ratio of rhizome biomass to above-ground Moso bamboo biomass and the distribution of different slopes were shown,the results are as follows:(1)the leading topographic factors influencing DBH,newly sprouted culms and carbon stocks were depended on the expansion period.Furthermore,the effective degree of leading topographic factor(aspect)decreased from early period to stable period and there was no significant difference in the effects of four topographic factors for carbon stocks and newly sprouted culms in stable period.(2)As for single aspect,east aspect was the optimal for the growth and development of Moso bamboo.As for the interaction,east aspect of lower and upper slope positions were the optimal interactive topography,and larger than those of a single aspect(east aspect),and significantly decreased in stable period.However,as for north and west aspects,which were not suitable for the growth of Moso bamboo,the carbon stocks of newly sprouted culms increased from rhizome expansion period to middle period,but kept stable in stable period.(3)Low topographic complexity was beneficial to the carbon accumulation under abrupt slopes but middle topographic complexity was needed under steeper slopes(steep and dangerous slope).(4)The length of rhizome stem decreases as slope,but due to the increase of the DBH of rhizome stem and the rhizome roots as slope,the rhizome biomass reaches the maximum at the 25-35° slope level but is not statistically significant.(5)It is worth noting that the moisture content of rhizome is larger in areas with stepper slopes,while that is lower in areas with flatter slopes(higher soil moisture content).(6)Rhizome biomass was significantly negatively correlated with water content and available phosphorus,and significantly positively correlated with culms.(7)The ratio of rhizome biomass to above-ground biomass in this study area decreased as slope,the value of this study is relatively small,comparing with previous studies.The main reason may be that our object is only 10-year Moso bamboo forests,so the rhizome system is relatively simple.