基于结构方程模型的杉木公益林林分空间结构评价

doi:10.11707/j.1001-7488.20220808

摘要/Abstract

摘要：

目的: 综合评价杉木公益林林分空间结构，确定不同龄组林分空间结构现状，为制定精准可行的林分空间结构优化和经营措施提供理论依据。方法: 以湖南省平江县芦头实验林场杉木公益林为研究对象，基于30块样地实测数据，采用4株木法确定林分空间结构单元，选取全混交度、角尺度和空间密度指数作为内生潜变量水平空间结构的观测变量，林层指数、开敞度和交角竞争指数作为内生潜变量垂直空间结构的观测变量，以林分空间结构为外生潜变量，构建林分空间结构评价结构方程模型，模型输出的路径系数通过归一化处理确定各指标权重，在此基础上对林分空间结构进行综合评价。结果: 1) 林分空间结构评价结构方程模型的卡方自由度比值(χ²/df)为1.708，介于1~3之间，比较拟合指数(CFI)、非正规化拟合指数(NNFI)和递增拟合指数(IFI)分别为0.944、0.924和0.946，均大于0.90，各拟合指数均符合适配参考值，模型整体适配度良好；2) 内生潜变量水平空间结构、垂直空间结构的权重分别为0.5229、0.4771，对外生潜变量林分空间结构的影响大致相同，空间密度指数所占权重最高，是影响林分水平空间结构的关键因素，开敞度和交角竞争指数所占权重相等，是影响林分垂直空间结构的关键因素；3) 杉木幼龄林、中龄林和近熟林的综合得分分别为0.3781、0.4526和0.3775，3个龄组的林分空间结构等级处于Ⅱ、Ⅲ等级。总体来看，研究区杉木公益林林分空间结构未达到理想状态，林分空间结构较差，树种混交程度低，接近零度混交至弱度混交状态，林分拥挤程度中等，林木透光条件一般，林层结构单一，但林木间竞争强度较弱，且空间分布格局较好，处于均匀至近似随机分布状态。结论: 林分水平空间结构和垂直空间结构均是影响林分空间结构的关键因素，空间密度指数是影响林分水平空间结构的最重要因素，开敞度和交角竞争指数是影响林分垂直空间结构的最重要因素，为改善研究区整体林分空间结构，应采取多树种补植和单株抚育间伐的综合经营措施。研究区3个龄组杉木生态公益林林分空结构评价结果较客观地反映了林分空间结构现状，采用结构方程模型对林分空间结构进行评价是科学合理的，可为林分空间结构评价提供一个全新的思路。

关键词: 杉木公益林, 林分空间结构, 结构方程模型, 指标权重

Abstract:

Objective: The spatial structure of Cunninghamia lanceolata public welfare forest was comprehensively evaluated, and the current situation of spatial structure of different age groups was determined, in order to provide theoretical bases for formulating accurate and feasible stand spatial structure optimization and management measures. Method: Taking the Cunninghamia lanceolata public welfare forest in Lutou experimental forest farm of Pingjiang county, Hunan Province as the research object, based on the measured data of 30 sample plots, the stand spatial structure unit was determined by four adjacent trees.Taking the totalmingling degree, uniform angle index and spatial density index as the observed variables of the horizontal spatial structure of endogenous latent variables, taking the storey index, open degree and angle competition index as the observed variables of the vertical spatial structure of endogenous latent variables, and taking stand spatial structure as the exogenous latent variables, the stand spatial structure evaluation structural equation model was constructed. The path coefficient output from the structural equation model was normalized to determine the weight of each index. On these bases, the stand spatial structure was comprehensively evaluated. Result: 1) For the structural equation model of the evaluation of stand spatial structure, the normed Chi-square (χ²/df) was 1.708, which was between 1-3. The values of comparative fit index(CFI), non-normed fit index(NNFI) and incremental fit index(IFI) were 0.944, 0.924 and 0.946, respectively, which were all greater than 0.90. All the fitting indexes conformed to the appropriate reference value, and the overall fitness of the model was good. 2) The weights of horizontal spatial structure and vertical spatial structure of endogenous latent variables were 0.5229 and 0.4771 respectively, and their effects on the stand spatial structure of exogenous latent variables were roughly the same. The spatial density index of the observed variables was the key factor affecting the horizontal spatial structure. The open degree and competition index were equally weighted, and the both were key factors affecting the vertical spatial structure. 3) The comprehensive scores of young, middle and near-mature Cunninghamia lanceolata forests were 0.3781, 0.4526 and 0.3775 respectively. The stand spatial structure grades of the three age groups were in grade Ⅱ and Ⅲ. Overall, the spatial structure of Cunninghamia lanceolata public welfare forests in the study area did not reach the ideal state. The spatial structure of the forest was poor, and the mingling degree of tree species was low, being close from zero degree mixing to weak mixing state. The degree of crowding was medium, and the light transmission conditions of trees were general. The forest layer structure was single, but the competition intensity among trees was weak, and the spatial distribution pattern was good, which was in the state of uniform to approximate random distribution. Conclusion: Both the horizontal spatial structure and vertical spatial structure of the stand were the key factors affecting the stand spatial structure. The results of the structural equation model further found that the spatial density index was the most important factor affecting the stand horizontal spatial structure. The open degree and competition index were the important factors affecting the stand vertical spatial structure. Therefore, in order to improve the overall stand spatial structure in the study area, comprehensive management measures such as multispecies replanting and singleplant tending-thinning should be adopted. From the evaluation results of the stand spatial structure of the three age groups of Cunninghamia lanceolata ecological public welfare forests in the study area, it objectively reflected the current status of the stand spatial structure, indicating that the idea of using the structural equation model to evaluate the stand spatial structure was scientific and reasonable, and it was expected to provide a new idea for stand spatial structure evaluation.

Key words: Cunninghamia lanceolata public welfare forest, stand spatial structure, structural equation model, index weight

中图分类号:

S757

赵文菲,曹小玉,谢政锠,庞一凡,孙亚萍,李际平,莫永俊,袁达. 基于结构方程模型的杉木公益林林分空间结构评价[J]. 林业科学, 2022, 58(8): 76-88.

Wenfei Zhao,Xiaoyu Cao,Zhengchang Xie,Yifan Pang,Yaping Sun,Jiping Li,Yongjun Mo,Da Yuan. Evaluation of Stand Spatial Structure of Cunninghamia lanceolata Public Welfare Forest by Using Structural Equation Model[J]. Scientia Silvae Sinicae, 2022, 58(8): 76-88.

图/表 8

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表2

林分空间结构评价等级划分标准"

林分空间结构综合得分 Comprehensive score value of stand spatial structure	林分特征描述 The description of stand feature	评价等级 Evaluation grade
[0, 0.2)	单木所在结构单元内空间结构差。树种混交程度低，林层单一，透光条件差，单木生长处于劣势状态，林木聚集分布 The spatial structure in the structural unit where the single tree is located is poor. Themingling degree of tree species is low, the structure of forest layer is simple, the light transmission condition is poor, the growth of single tree is in an inferior position, and the trees are in contagious distribution	Ⅰ
[0.2, 0.4)	单木所在结构单元内空间结构较差。树种混交程度较低，林层多样性一般，透光条件较差，单木生长处于被压状态，林木聚集分布 The spatial structure in the structural unit where the single tree is located is slightly poor. Themingling degree of tree species is slightly low, the diversity of forest layer is general, the light transmission condition is slightly poor, the growth of single tree is in the pressed state, and the trees are in contagious distribution	Ⅱ
[0.4, 0.6)	单木所在结构单元内空间结构一般。树种混交程度中等，林层较为多样，透光条件一般，单木生长处于中庸状态，林木均匀分布 The spatial structure in the structural unit where the single tree is located is general. Themingling degree of tree species is medium, the structure of forest layer is comparatively diverse, the light transmission condition is general, the growth of single tree is in a moderate state, and the trees are in uniform distribution	Ⅲ
[0.6, 0.8)	单木所在结构单元内空间结构较好。混交树种较多，林层结构较为复杂，透光条件较好，单木生长处于优势状态，林木接近随机分布 The spatial structure in the structural unit where the single tree is located is slightly good. There are a number of mixed tree species, the structure of forest layer structure is complex, the light transmission condition is good, the growth of single tree is in a dominant state, and the trees are close to random distribution	Ⅳ
[0.8, 1.0]	单木所在结构单元内空间结构好。林木处于理想的生长状态：树种丰富，林层多为3层以上的复层林，透光条件好，单木生长处于优势状态，林木随机分布 The spatial structure in the structural unit where the single tree is located is good. The trees are in an ideal growth state: the tree species is abundant, the structure of forest layer is mostly multi-layer forest with more than 3 layers, the light transmission condition is good, the growth of single tree is in a dominant state, and the trees are in random distribution	Ⅴ

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龄组 Age group	样地号 Plot code	树种组成 Species composition	平均胸径 Mean DBH/cm	平均树高 Mean height/m	平均冠幅 Mean crown/m	坡位 Slope position	坡度 Gradient/(°)
幼龄林 Young forest	1	10杉	4.6	3.2	1.8	中Mesoslope	28
	2	10杉	5.8	3.6	1.9	上Upslope	30
	3	10杉-柳	4.1	2.3	1.5	中Mesoslope	28
	4	10杉	4.9	2.6	1.8	上Upslope	29
	5	10杉	5.1	3.0	2.1	中Mesoslope	32
	6	10杉	4.7	2.8	1.9	下Downslope	29
	7	10杉+马	6.3	4.0	2.1	下Downslope	31
	8	10杉	8.5	4.8	2.0	中Mesoslope	24
	9	10杉	6.1	3.6	2.3	上Upslope	20
	10	10杉+毛	6.8	4.1	2.2	上Upslope	25
中龄林 Middle aged forest	11	10杉-山	11.2	6.1	2.9	中Mesoslope	28
	12	10杉+黄	10.9	5.4	2.2	下Downslope	30
	13	10杉+山	10.6	6.0	2.1	下Downslope	25
	14	10杉	12.4	6.1	3.1	中Mesoslope	23
	15	10杉+柳	11.9	5.9	3.0	中Mesoslope	30
	16	10杉-黄	10.3	5.8	2.8	下Downslope	24
	17	10杉	12.0	8.1	2.5	中Mesoslope	32
	18	10杉	12.4	8.8	3.1	中Mesoslope	20
	19	10杉-青	10.5	7.9	2.8	中Mesoslope	18
	20	10杉	11.9	7.6	2.7	中Mesoslope	25
近熟林 Near-mature forest	21	10杉	13.5	8.8	3.4	中Mesoslope	28
	22	10杉-毛	13.0	9.2	3.4	中Mesoslope	31
	23	10杉	12.3	8.9	3.0	下Downslope	29
	24	10杉	16.1	10.9	4.3	中Mesoslope	25
	25	10杉-毛	14.9	11.2	3.2	下Downslope	26
	26	10杉	15.6	9.9	3.9	中Mesoslope	27
	27	10杉	13.9	9.7	3.2	中Mesoslope	31
	28	10杉+苦	13.6	9.4	2.9	中Mesoslope	30
	29	9杉1毛	16.1	10.7	3.1	中Mesoslope	28
	30	10杉	14.8	10.3	3.9	下Downslope	22

模型拟合指数Model fitting index	统计值Statistical value	参考值Reference value	是否适配Degree of adaptation
χ²/df	1.708	1~3	是Fit
CFI	0.944	>0.90	是Fit
NNFI	0.924	>0.90	是Fit
IFI	0.946	>0.90	是Fit

目标层Target layer	准则层Criterion layer	权重Weight	指标层Index layer	权重Weight
林分空间结构 Stand spatial structure	水平空间结构 Horizontal spatial structure	0.522 9	全混交度Totalmingling degree	0.338 6
			角尺度Uniform angle index	0.291 3
			空间密度指数Spatial density index	0.370 1
	垂直空间结构 Vertical spatial structure	0.477 1	林层指数Storey index	0.306 8
			开敞度Open degree	0.346 6
			交角竞争指数Angle competition index	0.346 6

龄组 Age group	全混交度 Total mingling degree	角尺度 Uniform angle index	空间密度指数 Spatial density index	林层指数 Storey index	开敞度 Open degree	交角竞争指数 Angle competition index	综合得分 Comprehensive score	等级 Grade
幼龄林 Young forest	0.029 8±0.04a	0.424 6±0.03b	0.552 7±0.06a	0.150 5±0.04b	0.402 2±0.09a	0.254 3±0.02b	0.378 1±0.11a	Ⅱ
中龄林 Middle aged forest	0.056 9±0.04a	0.451 3±0.02a	0.539 7±0.15a	0.248 8±0.07a	0.270 2±0.07b	0.279 6±0.03b	0.452 6±0.09a	Ⅲ
近熟林 Near-mature forest	0.043 9±0.03a	0.466 9±0.02a	0.548 3±0.11a	0.291 8±0.05a	0.140 0±0.05c	0.381 9±0.08a	0.377 5±0.08a	Ⅱ

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