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森林是陆地生态系统中重要的养分储存库之一,有关森林碳(C)、氮(N)、磷(P)储量和潜力的研究在全球和区域尺度上都十分广泛[1-4]。地处中国亚热带地区的森林生态系统具有极高的碳密度和养分含量,准确评估其碳、氮、磷储量具有重要现实和理论意义[5-6]。植被养分储量评估的准确程度与生物量和养分含量密切相关。以往研究所涉及的平均生物量密度法、平均生物量转换因子法、改善模型连续生物量转换因子法和异速生长方程模型等[7-9],都是通过提高生物量估算精度以提升养分储量评估准确性;针对树木养分含量计算误差和不确定性的研究相对较少[10-11]。虽有部分研究关注了不同树种、部位、树龄等因素间养分含量的差异[12-14],但受取样方法所限,对树干养分含量的研究仍较匮乏。此外,林学和生态学领域通常采用的小样本取样法(取样量n=3~5)难以客观代表总体养分含量,增加样本数可在何种程度上提高估算结果的精度值得开展进一步研究。因此,本研究以浙江省五洩国家森林公园中5种具有代表性的亚热带常绿阔叶树种为研究对象,采用有放回抽样方法(Bootstrap)模拟并比较小样本取样(n=3~5)和完整样本取样(n=18~32)的估算结果,评估小样本取样产生的养分质量分数估算误差。权衡取样量和估算精度,分别提出估算碳、氮、磷储量的最优取样量,为相关野外研究工作提供取样方法的建议。
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由图1和表1可知:不同树种树干碳、氮、磷质量分数之间具有极显著差异(P<0.001);杉木碳质量分数极显著高于其他4个树种(P<0.001),木荷和青冈最低;马尾松氮质量分数最高(P<0.001),青冈和毛竹次之,杉木和木荷最低,但青冈和毛竹、杉木和木荷之间无显著差异;马尾松和毛竹的磷质量分数极显著高于其他树种(P<0.001),杉木和木荷较低,两者之间差异不显著。
图 1 亚热带常绿阔叶林5个优势树种的树干木材碳、氮、磷质量分数差异
Figure 1. Carbon (C), Nitrogen (N), and Phosphorus (P) concentration variation in the tree boles of five tree species in subtropical evergreen broadleaf forest
由表1可知:马尾松树干碳变异系数为7%,其他树种为2%~3%;马尾松氮变异系数最低,为12%,杉木最高,为31%,其他树种为20%~25%;青冈的磷变异系数最低,仅为14%,木荷最高,为46%,其他树种为20%~33%。总体上,树种之间的碳、氮、磷变异系数差异较大。马尾松碳变异幅度高于其他树种,氮变异系数低于其他树种;青冈磷变异系数较小,木荷较高,其他树种处于中间水平。
表 1 5个优势树种的树干碳、氮、磷质量分数和变异系数
Table 1. Mean (standard deviation) and CV of C, N and P concentrations for 5 dominant tree species
树种 样本数/个 碳 氮 磷 质量分数/(g·kg−1) 变异系数/% 质量分数/(g·kg−1) 变异系数/% 质量分数/(g·kg−1) 变异系数/% 马尾松 32 449.1±29.4 b 7 3.4±0.4 a 12 1.10±0.29 a 26 杉木 21 487.6±10.7 a 2 1.3±0.4 c 31 5.80±0.19 c 33 木荷 30 431.1±12.5 c 3 1.2±0.3 c 25 0.52±0.24 c 46 青冈 30 435.9±11.5 c 3 2.0±0.4 b 20 0.98±0.14 b 14 毛竹 18 438.6±14.1 bc 3 2.2±0.5 b 23 1.28±0.26 a 20 说明:数值为平均值±标准差。同列不同小写字母表示树种间差异极显著(P<0.001) -
由图2可知:5个亚热带常绿阔叶林优势树种碳的建议取样量为5~10。其中马尾松建议取样量最大,为10,木荷为8,杉木和青冈为6,毛竹为5。亚热带常绿阔叶林优势树种氮的建议取样量为4~7。其中杉木建议取样数最大,为7,马尾松为6,木荷和青冈为5,毛竹最少,为4。亚热带常绿阔叶林优势树种磷的建议取样量为5~9。其中马尾松建议取样量最大,为9,青冈为8,杉木和木荷为7,毛竹最小,为5。
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将该领域常用的小取样量(n=3~5)与上述估算的取样量进行比较(表2)发现:对于碳估算变化,马尾松由于树干碳质量分数变异较大,n=5小样本条件下质量分数估算误差为6%~7%,采样样本增加至n=10后估算误差为4%~5%;而对于其他树种,n=3即可保证5%的估算误差。对于氮质量分数变化,马尾松n=5小样本条件下氮质量分数为14%~15%,采样样本增加至n=10不能显著提高估算精度;其他树种也表现出一致规律,增加取样量不能有效降低估算误差。磷质量分数变异较大,增加样本没有显著降低取样误差,n=5小样本可以保证马尾松、杉木、木荷估算误差为30%~50%,而青冈和毛竹估算误差为15%~20%。
表 2 5个优势树种建议取样量和小样本取样量(n=3~5)对总体的估算误差比较
Table 2. Comparison of the estimated extent of the population for different tree species suggested for ni and small sample size (n =3−5)
养分 树种 总样本/建议取样量 建议取样量估算误差/% 估算误差/% n=3 n=4 n=5 碳 马尾松 32/10 −3.99~5.12 −8.41~11.75 −7.47~9.46 −6.14~7.23 杉木 21/6 −1.90~2.00 −3.36~2.92 −2.81~2.58 −2.09~1.87 木荷 30/8 −2.95~2.22 −4.70~5.01 −3.98~4.40 −3.13~3.62 青冈 30/6 −2.89~2.78 −4.32~3.97 −3.20~3.02 −3.14~2.88 毛竹 18/5 −3.37~3.30 −5.15~4.48 −3.37~3.76 −3.37~3.30 氮 马尾松 32/6 −12.42~17.11 −19.30~23.01 −16.60~18.83 −14.97~13.96 杉木 21/7 −26.46~27.03 −41.27~51.44 −31.67~44.95 −29.82~37.00 木荷 30/5 −22.39~33.72 −31.16~43.34 −20.94~40.85 −22.39~33.72 青冈 30/5 −21.63~20.36 −31.89~20.93 −23.86~20.70 −21.63~20.36 毛竹 18/4 −26.04 ~25.65 −30.48~30.12 −26.04~25.65 −19.74~19.71 磷 马尾松 32/9 −21.94~22.49 −41.23~44.80 −31.65~30.50 −28.66~37.46 杉木 21/7 −30.70~26.10 −42.85~46.81 −40.42~49.80 −38.35~43.98 木荷 30/7 −41.40~42.89 −58.95~73.93 −54.89~66.40 −42.95~51.08 青冈 30/8 −11.79~13.29 −18.32~20.15 −18.76~17.18 −15.07~17.55 毛竹 18/5 −18.06~22.52 −24.53~26.72 −22.01~24.37 −18.06~22.52
Estimation and uncertainty analysis of carbon,nitrogen,and phosphorus concentrations in trunks of five dominant tree species in subtropical evergreen broadleaved forests
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摘要:
目的 相较于土壤和凋落物,树干取样破坏性强,样本获取难度大,因而通常以小样本取样估算其成分组成。本研究目的在于评估小样本采样引起的不同树种树干碳、氮、磷养分质量分数和储量估算的不确定性。 方法 基于亚热带常绿阔叶林马尾松Pinus massoniana、杉木Cunninghamia lanceolata、木荷Schima superba、青冈Quercus glauca、毛竹Phyllostachys edulis等5个优势树种的树干碳、氮、磷质量分数数据集(取样量n=18~32),采用有放回抽样方法(Bootstrap)对比一般小样本取样(n=3~5)与全样本(n=18~32)之间的估算差异,通过权衡取样量和变异之间的关系,给出对应指标的建议取样量以及不同取样量对应的估算误差范围。 结果 马尾松树干碳质量分数变异显著高于其他树种,n=3~5小样本引起估算误差约±10%,而其他树种仅±5%。对马尾松增加取样量n=10,估算误差为−4%~5%。 结论 建议在估算和评估马尾松林碳、氮、磷储量时,适当增加取样量(n=5~10)以降低估算误差,或在采用n=3~5小样本取样量的同时,将±10%估算误差范围考虑在内。其他树种碳、氮、磷质量分数估算时,取样量n=4~5时误差在可接受范围内。图2表2参27 Abstract:Objective Tree trunk sampling is destructive and difficult to obtain in comparison with soil and litter sampling. Therefore, small-size sampling is often used to estimate its composition. The purpose of this study is to evaluate the uncertainty in estimating mass fraction and reserve of carbon, nitrogen, and phosphorus (C, N, P) in trunks of different tree species caused by small-size sampling. Method Based on the data sets of C, N, and P concentrations of five dominant tree species in subtropical evergreen broad-leaved forests, including Pinus massoniana, Cunninghamia lanceolata, Schima superba, Quercus glauca, and Phyllostachys edulis (sampling size n=18−32), Bootstrap method was used to compare the estimated differences between small-size samples (n=3−5) and full samples (n=18−32). By weighing the relationship between sampling quantity and variation, the recommended sampling quantity of corresponding index and the estimation error range of different sampling quantities are given. Result The variation of C concentration of P. massoniana was significantly higher than that of other tree species. A small sample size of n =3−5 caused an estimation error of about ±10%, but only ± 5% for other tree species. For P. massoniana, the estimation error was −4% to 5% when the sample size was increased to 10. Conclusion It is suggested to appropriately increase the sampling quantity (n=5−10) to reduce the estimation error when estimating and evaluating C, N and P reserves of P. massoniana forest. If a small sample size of n=3−5 is used, an estimation error range of ±10% should be taken into account. When estimating C, N and P concentration of other tree species, the estimation error is within the acceptable range when n =4−5. [Ch, 2 fig. 2 tab. 27 ref.] -
表 1 5个优势树种的树干碳、氮、磷质量分数和变异系数
Table 1. Mean (standard deviation) and CV of C, N and P concentrations for 5 dominant tree species
树种 样本数/个 碳 氮 磷 质量分数/(g·kg−1) 变异系数/% 质量分数/(g·kg−1) 变异系数/% 质量分数/(g·kg−1) 变异系数/% 马尾松 32 449.1±29.4 b 7 3.4±0.4 a 12 1.10±0.29 a 26 杉木 21 487.6±10.7 a 2 1.3±0.4 c 31 5.80±0.19 c 33 木荷 30 431.1±12.5 c 3 1.2±0.3 c 25 0.52±0.24 c 46 青冈 30 435.9±11.5 c 3 2.0±0.4 b 20 0.98±0.14 b 14 毛竹 18 438.6±14.1 bc 3 2.2±0.5 b 23 1.28±0.26 a 20 说明:数值为平均值±标准差。同列不同小写字母表示树种间差异极显著(P<0.001) 表 2 5个优势树种建议取样量和小样本取样量(n=3~5)对总体的估算误差比较
Table 2. Comparison of the estimated extent of the population for different tree species suggested for ni and small sample size (n =3−5)
养分 树种 总样本/建议取样量 建议取样量估算误差/% 估算误差/% n=3 n=4 n=5 碳 马尾松 32/10 −3.99~5.12 −8.41~11.75 −7.47~9.46 −6.14~7.23 杉木 21/6 −1.90~2.00 −3.36~2.92 −2.81~2.58 −2.09~1.87 木荷 30/8 −2.95~2.22 −4.70~5.01 −3.98~4.40 −3.13~3.62 青冈 30/6 −2.89~2.78 −4.32~3.97 −3.20~3.02 −3.14~2.88 毛竹 18/5 −3.37~3.30 −5.15~4.48 −3.37~3.76 −3.37~3.30 氮 马尾松 32/6 −12.42~17.11 −19.30~23.01 −16.60~18.83 −14.97~13.96 杉木 21/7 −26.46~27.03 −41.27~51.44 −31.67~44.95 −29.82~37.00 木荷 30/5 −22.39~33.72 −31.16~43.34 −20.94~40.85 −22.39~33.72 青冈 30/5 −21.63~20.36 −31.89~20.93 −23.86~20.70 −21.63~20.36 毛竹 18/4 −26.04 ~25.65 −30.48~30.12 −26.04~25.65 −19.74~19.71 磷 马尾松 32/9 −21.94~22.49 −41.23~44.80 −31.65~30.50 −28.66~37.46 杉木 21/7 −30.70~26.10 −42.85~46.81 −40.42~49.80 −38.35~43.98 木荷 30/7 −41.40~42.89 −58.95~73.93 −54.89~66.40 −42.95~51.08 青冈 30/8 −11.79~13.29 −18.32~20.15 −18.76~17.18 −15.07~17.55 毛竹 18/5 −18.06~22.52 −24.53~26.72 −22.01~24.37 −18.06~22.52 -
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