| Understory regeneration is an important ecological process in the growth and development of forest stands and an important element in the study of forest ecosystem dynamics.The selfsuccession and restoration of forest ecosystems are mainly achieved through natural forest regeneration.The Daxing’anling region is one of the main distribution areas of natural forests in China,with abundant forest resources,but over-harvesting in the past has destroyed most of the forest structure,and most of the original forest stands have been transformed into secondary forests.The understory regeneration makes it difficult to maintain the natural succession of forests,so it is especially important to explore and study the regeneration influence factors in this region.This study is based on the survey data of standard sample plots in the Cuigang,Xinlin,and Zhuangzhi forestry fields of the Xinlin Forestry Bureau in the Daxing’anling area.The study was conducted from three research perspectives: spatial distribution pattern of regeneration;regeneration impact factors;and regeneration quantity prediction,with a view to providing a theoretical basis for sustainable forest management planning in the Daxing’anling area.The main findings are as follows:(1)Based on the data from 100m×100m sample plots of Larix gmelinii,Betula platyphylla,and mixed Larix gmelinii + Betula platyphylla forests in Cuigang Forestry Station,Xinlin Forestry Bureau,Daxing’anling,the spatial distribution patterns and scale effects of trees under different forest types and different forest classes were analyzed.The results showed that the good category of regeneration qualities was observed only for LF forest,while the other two forest types both belonged to the poor category.The number of regenerations for each forest type was significantly different from that of tree layer;the distribution of diameter [except for L.gmelinii forest and B.platyphylla forest,where the q values were 1.58 and 1.30,respectively] and height were all not reasonable,indicating that the communities of the studied forest types were all relatively unstable.For different tree species,the spatial distributions of the main species in each plot were mainly clumped.However,the relations between the five used indicators and the spatial scales varied significantly among different forest types,in which the linear increases(40%),the power increases(22%),and the negative quadratic polynomials accounted for approximately 40%,22%,and 20%,respectively.For different size classes,significant clumped distributions were observed for the regeneration levels(I ～ III),while the spatial distribution of tree layers(IV ～ V)usually fluctuated among various distribution patterns.The scale effects of different size classes also mainly focused on the linear increases(44%),the power increases(15%),and the negative quadratic polynomial(12%).For each forest type and sampling scale,the cluster degrees of trees decreased significantly with increases in the sizes of trees.Within each specific forest type,the pattern sizes of non-dominant species were significantly larger than those of dominant tree species,while the pattern sizes of regeneration layers were significantly larger than those of tree layers.(2)Based on the investigation data of 49 sample plots for natural L.gmelinii forests in the Cuigang forest farm of Xinlin Forestry Bureau,Daxing’anling mountains,37 commonly used measurable variables that mainly focused on stand non-spatial structure,stand spatial structure,species diversity,soil conditions,and site conditions were employed to construct the structural equation model of natural regeneration densities and size diversities(i.e.,height and grounddiameter).Then,the direct,indirect,and total influence coefficients of each path were quantified to extract the critical and controllable factors that influence the regeneration density and regeneration diversity of natural L.gmelinii forests,which may provide some insights for implementing sustainable forest management in the region.The results indicated that the effects of various latent variables on regeneration density were ranked as: stand non-spatial structure(-0.410)> species diversity(0.380)> soil condition(0.250)> site condition(0.249)> stand spatial structure(0.197),while the order was changed to: soil condition(0.778)> site condition(0.748)>stand spatial structure(0.684)> stand non-spatial structure(0.287)> forest diversity(0.105),when evaluated on the stand regeneration size diversities.On the whole,the critical and controllable factors that affect the regeneration quantity and diversity included: p H,total potassium,species diversity,tree height diversity,uniform angle index,and stand volume per hectare.(3)Based on the survey data of 96 standard sample plots in Cuigang,Xinlin,and Zhuangzhi Forest Farm of Xinlin forestry bureau,twenty-nine basic indicators were selected from four aspects: stand,soil,diversity,and stand structure.The Poisson model,negative binomial model,and random forest regression model were constructed for L.gmelinii and B.platyphylla regeneration quantities,and the OOB replacement method was used to determine the contribution of candidate variables to the number of regenerations of L.gmelinii and B.platyphylla after selection of the best model.The accuracy of the random forest regeneration number prediction model was considered higher than that of the Poisson model and the negative binomial model by the ten-fold crossover test.The relative importance of each variable was obtained by the OOB replacement method,where the variables important for the number of L.gmelinii regeneration were,in order of importance,the Shannon index of diameter at breast height(17.57%),the Pielou evenness index of diameter at breast height(16.88%),the unit volume(13.29%),the Simpson index of diameter at breast height(12.92%),and the average diameter at breast height of the stand(12.91%);the variables that were important for the number of regeneration of B.platyphylla in order of importance were: diameter at breast height Shannon index(18.53%),diameter at breast height,Pielou evenness index(16.13%),herbaceous cover(12.62%),average diameter at breast height(12.34%)and shrub coverage(11.31%).Tree size diversity and stand density are important factors influencing the amount of regeneration and can be harvested or replanted to ensure a scientific and reasonable stand density,thus promoting natural regeneration and the natural succession of forest ecosystems.This study analyzes the spatial distribution patterns of major forest species at different sampling scales and constructs a model for predicting regeneration numbers based on data from sample plots in the Daxing’anling region,aiming to describe the distribution types of forest trees and reflect the dynamic trends of forest numbers from different perspectives,which is important for revealing the mechanisms of population recovery,guiding forest regeneration work and sustainable management planning of forest stands. |
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