| Since the introduction of Black locust(Robinia pseudoacacia L.)into China from Europe at the end of the 19th century,it has been extensively used and cultivated in different areas for several decades due to its characteristics of drought and infertility resistance,and fast growth.At present,it has become one of the top ten tree species for afforestation in China,however,with the increase of afforestation cost,it is particularly important to improve the cultivation and management measures of multi-rotation Black locust coppice forests.This study took multi-rotation Black locust coppice forests in Luoning County,Henan Province as an example,we analyzed the stand growth,seed banks,plant diversity,spatial patterns,and spatial point processes of different multi-rotation coppice stands in fifteen 50 m×50 m plots under the existing management measures;discussed the effect of retention density on growth of second-rotation stands of different ages through the experiment of thinning.About 150 stumps of different sizes were selected from the second-and third-rotation stands in the first year after clear-cutting,the investigation of regeneration was done for four consecutive years to examine the regeneration dynamics of stumps in multi-rotation stands.The main findings of this study are summarized here below:(1)After clear-cutting,the growth of the second-rotation stands after thinning was better than that of the third-rotation stands,since the average annual DBH of III-17 stand was 0.402cm,while that of II-30stand was 0.461cm,what’s more,the mean DBH of stump sprouts was larger than that of root sprouts except for II-18 and III-17 stands;with the increase of age,the proportion of root sprouts in the second-rotation stands increased continuously(59.57%～79.84%),while the proportion of root sprouts increased first and then decreased(42.93%～64.26%～51.53%)in the third-rotation stand;the fitting degree of different distribution models to DBH and height was improved after thinning,but there were still fewer trees with larger diameter classes,which indicated unreasonable thinning measures.Thinning experiment in the second-rotation young stand demonstrated that the increase of stock volume per hectare in the stand with a retention density of 2800 stems/hm~2 from 4a to 6a was 25%,while the increase of stock volume per hectare in the stand with a retention density of 3500 stems/hm~2 from 8a to 10a was 49.77%,and the effect of thinning on distribution of DBH and height of stand was limited within two years.Therefore,it is necessary to retain a higher stand density in the younger stand age and increase the proportion of root sprouts and larger diameter trees with the growth of the stand during the management of multi-rotation coppice forests of Black locust.(2)The size of the stump remarkably promoted the growth of sprouting branches.The average number of sprouts in the second-rotation stand was 18.56,15.32,9.74,and 7.82,respectively,with the average base diameters of 12.74,11.91,11.86,and 9.88 mm for stumps of four diameter classes(>25 cm,20–25 cm,15–20 cm,<15 cm),respectively in the first year of regeneration.With an increase in age,the diameter of the stump was positively associated with the number of sprouts and base diameters of sprouts.In the third-rotation stand,the average number of sprouts was 10.07,8.36,and 7.06,respectively,with the average base diameters of 8.92,8.39,and 8.07 mm in the stumps of three diameter classes(12-16cm,8-12cm,<8cm)in the first year of regeneration,at 4a,as compared with the stumps of second-rotation stand,the average base diameter of stump sprouts in third-rotation stand increased more and the number of sprouts decreased more.The sprouts were primarily distributed in the southeast and northeast directions of stumps at the age of four,and the competition index of maximum sprout showed a decrease with an increase in age of stumps.In the process of tending felling,different number of well-shaped spouts should be reserved in the direction of the southwest or northwest which were conducive to sprouts growth according to the size of the stumps.(3)An analysis of characteristics of the seed bank in second-and third-rotation stands revealed abundant seed bank resources;the density of different types of seed banks ranged from 1762 seeds/m~2 to4722 seeds/m~2,the seed bank density of the third-rotation stand was greater than that of the second-rotation stand,the seed bank density of the litter layer was higher than that of the soil layer and the density of seed banks showed significant variation in different locations.In addition,the seed vigor of the litter layer was higher than that of the soil layer.There is a material basis for seed regeneration in multi-rotation coppice forests,and it should be considered to promote seed regeneration in the larger gaps formed by thinning in future.(4)Sixty-three herbaceous plants from thirty families were observed in fifteen stand plots of different rotations.Non-metric multidimensional scaling(NMDS)showed that herbaceous plant diversity in the second-and third-rotation stands had an obvious separation point;herbaceous plant richness decreased with an increase in rotation,but the difference was not significant;a total of twelve woody plants of ten families were observed,which had low species and density,and Vitex negundo L.was the common woody species found in all the plots.Thinning measures could improve herbaceous plant diversity and maintain herbaceous community stability but significantly reduce the diversity of other tree species.(5)Aggregation,random,and repulsion distribution patterns were detected in the second and third-rotation stands.Aggregation decreased with the age of stand and scale.We also observed that the thinning measures could reduce the degree of aggregation;the aggregation degree of the spatial pattern detected by the L(r)function was higher than that of g(r);in the first year after clear-cutting(III-1 stand),the spatial correlation between the stumps and the root sprouts was primarily positive at different scales,however,in other stands,the spatial correlation between stumps and root sprouts was primarily negative and independent;the spatial correlation between stumps and root sprouts was mainly negative on the smaller scale;the positive spatial autocorrelation of DBH in the second-and third-rotation stands decreased with increasing stand age,and the decrease degree in the second-rotation stands was higher than that in the third-rotation stands.(6)The Poisson point process,Cox Cluster point process,Cluster point process,and Gibbs point process models were employed to fit the point patterns of the total stand,root sprouts,and stump sprouts.The analysis showed that the Poisson point process had no good fitting effect on all stands.Gibbs point process model displayed better fitting effect on I-50,II-12,II-25,and II-30 total stands,because of thinning measures that resulted in repulsion distribution in small scale and random distribution in large scale.The spatial pattern of the stand was controlled by the combination of stumps and clonal integration at the initial regeneration stage,with the increase of stand age,thinning,and stand ages being the main mechanisms controlling stand point patterns.In this study,multi-rotation Black locust coppice forests in Luoning Country,Henan province was taken as the research object,we explored the growth and spatial pattern characteristics of multi-rotation Black locust coppice forests under the current management system,the results indicated that the unreasonable thinning measures lead to cluster distribution of trees,thus seriously affect the stand growth,what’s more,the diameter and height distribution of stand lacked large diameter trees,meanwhile,point process fitting method was used to explore the formation reasons of the spatial pattern of the regeneration stand.Based on the results of thinning experiment and stand investigation and analysis,the cultivation and management methods of multi-rotation coppice forests of Black locust were put forward,which provided effective technical and theoretical support for the cultivation measures,future regeneration and development of multi-rotation Black locust coppice forests. |
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