A comparison of direct and indirect methods for measuring leaf and surface areas of individual bushes

Indirect estimates of leaf area from measurements with three commercially available instruments (DEMON, LAI-2000 and Sunfleck Ceptometer) were compared with directly measured areas of individual Retama sphaerocarpa bushes. The three indirect methods gave good estimates of the total surface area of individual bushes. For the DEMON, the method of log-linear averaging of transmitted radiation gave estimates closer to directly measured surface area than the method of averaging transmission linearly. For the LAI-2000, estimated surface area index multiplied by canopy projected area gave the best agreement with directly measured values. For measurements with the Sunfleck Ceptometer, values of surface area estimated from the transmission of photosynthetic quantum flux density, without correcting for diffuse radiation, gave the best agreement with directly measured values. Surface areas estimated by the three instruments were not significantly different from directly measured total (leaf + branch + stem) surface areas. Leaf surface area could be calculated from estimated total surface area minus directly measured branch surface area. Measured branch surface area was linearly related to canopy projected area.     

小麦LAI-2000观测值对辐亮度变化的响应

利用LAI-2000植物冠层分析仪和ASD光谱仪,通过固定点拔节期冬小麦叶面积指数(LAI)观测实验和同步光谱辐亮度实验,测量了晴天条件下,固定点冬小麦从中午至傍晚24个不同时刻的LAI值及对应的辐亮度。继之分析了此段时间内测得的定点冬小麦LAI值分别与对应时刻可见光和近红外谱段的天空光辐亮度、总辐亮度和太阳直射辐亮度值之间的相关性。结果表明,无论波长小于490nm的谱段,还是波长大于490 nm的谱段,LAI与天空光辐亮度、总辐亮度和太阳直射辐亮度都呈负相关,相关系数(R2)高达0.8左右;尤其LAI与天空光辐亮度的负相关性最高,这种相关性随着波长的增大而减小。LAI与各谱段天空光辐亮度的相关性特征可为LAI-2000晴天观测的LAI值归一化修正处理提供一种新思路和技术途径,以消除太阳直射光的影响,从而解决LAI-2000只能在晴天观测的局限性,拓展LAI-2000在晴天观测条件下的适用性。     

小兴安岭白桦次生林叶面积指数的估测

叶面积指数(LAI)是量化冠层结构最常用的参数之一,准确估测LAI对森林生态系统结构特性的研究具有重要意义.利用半球摄影图像法和LAI-2000法及半球摄影图像法结合凋落物法估测了小兴安岭白桦次生林LAI及其动态变化.首先对该林型叶凋落末期(11月初)的半球摄影图像进行合理校正(包括木质部分所占比例α,冠层水平集聚指数ΩE,校正值作为该时期常绿树种的真实LAI(LAIt),结合各调查期的凋落物数据,得到落叶季节(7-11月)的LAIt,并以该值为参考值,对比分析了两种光学仪器法估测值.结果表明:两种光学仪器法在LAI最大时期低估(分别低估2.83％、6.20％),其他时期显著高估(平均高估118.13％、89.34％),但两种光学仪器法与探讨方法估测值存在很好的相关性:LAIt=-1.1393+1.0934·LAIHP,R2=0.80; LAIt=-0.1712+0.6259·LAILAI-2000,R2=0.83.研究结果可为将来方便、快捷、准确的估测白桦次生林的LAI提供参考.     

基于光学仪器法测定谷地云冷杉林叶面积指数的季节变化

利用半球摄影法(DHP)和LAI-2000植物冠层分析仪两种光学仪器法(间接法)以及凋落物法(直接法),研究了小兴安岭谷地云冷杉林叶面积指数(LAI)的季节变化,并构建了不同季节直接法与间接法测定的LAI间的相关关系.结果表明: 在整个试验期间,DHP测定的LAI比直接法测定值低估40%～48%, LAI-2000植物冠层分析仪的低估范围为15%～26%;不同时期直接法与DHP和LAI-2000植物冠层分析仪测定的LAI均显著相关, 且均可合并为A、B、C 3类预测模型, 可以分别预测5和11月, 6、9和10月, 7和8月的LAI.本研究结果可为高效、准确地测定针叶林LAI的季节变化提供参考.     

利用3种校正方案提高间接法测定兴安落叶松人工林叶面积指数的精度

木质部和集聚效应是影响间接法测定叶面积指数(LAI)精度的主要因素, 尤其是木质部的校正一直存在争议。针对这一问题, 该研究首先利用半球摄影法(DHP)和LAI-2000植物冠层分析仪法(LAI-2000法) 2种间接法测定了小兴安岭兴安落叶松(Larix gmelinii)人工林叶面积最大时期的有效LAI (L_e), 然后提出了A、B、C 3种校正方案来提高间接法的测定精度。同时, 利用凋落物法和异速生长方程法2种直接法测定LAI, 以凋落物法测定值为标准来评估3种校正方案的校正效果, 并检验天顶角范围对校正结果是否存在显著影响。结果表明: 在0-45° (1-3环)、0-60° (1-4环)、45°-60° (4环)及0-75° (1-5环) 4种不同天顶角范围内, DHP测定的L_e比凋落物法、异速生长方程法测定值分别低估19%-32%和8%-29%; 而LAI-2000法也得到相似的结论, 分别低估9%-30%和8%-28%。虽然校正方案A高估了木质部对LAI的贡献, 但在45º-60º天顶角范围内, 能有效地校正DHP测定的L_e, 在1-3环和1-4环天顶角范围内, 能有效地校正LAI-2000法测定的L_e。4种天顶角范围内, 校正方案B均能有效地校正DHP测定的L_e。整体来看, 4种天顶角范围内, 校正方案C对DHP和LAI-2000法测定值的校正效果均优于其他2种方案。研究结果表明除木质部和集聚效应外, 天顶角范围的选择也是决定间接法测定LAI精度的重要因素。     

利用直接法和间接法测定针阔混交林叶面积指数的季节动态

利用光学仪器法能够快速、高效地测定森林生态系统的叶面积指数(leaf area index, LAI)。然而, 评估该方法测定针阔混交林LAI季节动态准确性的研究较少。该研究基于凋落物法测定了小兴安岭地区阔叶红松(Pinus koraiensis)林LAI的季节动态, 其结果可代表真实的LAI。参考真实的LAI, 对半球摄影法(digital hemispherical photography, DHP)和LAI-2000植物冠层分析仪测定的有效叶面积指数(effective LAI, L_e)进行了评估。首先对DHP测定LAI过程中采用的不合理曝光模式(自动曝光)进行了系统校正。同时, 测定了光学仪器法估测LAI的主要影响因素(包括木质比例(woody-to-total area ratio, α)、集聚指数(clumping index, Ω_E)和针簇比(needle-to-shoot area ratio, γ_E))的季节变化。结果表明: 3种不同方法测定的LAI均表现为单峰型的季节变化, 8月初达到峰值。从5月至11月, DHP测定的L_e比真实的LAI低估50%-59%, 平均低估55%; 而LAI-2000植物冠层分析仪测定的L_e比真实的LAI低估19%-35%, 平均低估27%。DHP测定的L_e 经过自动曝光, α、Ω_E和γ_E校正后, 精度明显提高, 但仍比真实的LAI低估6%-15%, 平均低估9%; 相对而言, LAI-2000植物冠层分析仪测定的L_e经过α、Ω_E和γ_E校正后, 精度明显提高, 各时期与真实的LAI的差异均小于9%。研究结果表明, 考虑木质部和集聚效应对光学仪器法的影响后, DHP和LAI-2000植物冠层分析仪均能相对准确地测定针阔混交林LAI的季节动态, 其中, DHP的测定精度高于85%, 而LAI-2000植物冠层分析仪的测定精度高于91%。     

自动曝光对半球摄影法测量叶面积指数及其季节变化的影响

自动曝光是影响半球摄影法(DHP)测量叶面积指数(LAI)精度的重要误差源之一.本研究基于小兴安岭地区的阔叶红松林、白桦次生林、红松人工林和兴安落叶松人工林,利用DHP和LAI-2200植物冠层分析仪分别测量6—9月每个月中旬的LAI,首先比较两种方法测量LAI的差异性,再检验森林类型和测量时期对建立两种方法测定值间的相关关系是否存在显著影响,最后构建适于校正不同森林类型不同时期自动曝光对DHP测量LAI产生误差的经验模型.结果表明: 4种森林类型4个时期内,在自动曝光设置下DHP测量的LAI比LAI-2200测量值低估20%～49%;森林类型对构建两种方法测量LAI值的经验模型不存在显著影响,而测量时期存在显著影响.本研究构建的A、B两种分类经验模型,分别适用于校正4种森林类型在6和9月、7和8月DHP测量的LAI.经分类经验模型校正后,DHP测量4种森林类型4个时期的LAI值提高了45%～79%,测量精度可提高到83%～94%.通过DHP和LAI-2200测量LAI值间的经验模型,可有效校正自动曝光对DHP测量LAI的影响,极大地提高其测量精度,为使用DHP快捷、高效地测量不同森林类型的LAI及其季节动态提供技术支持.     

Measuring and modelling plant area index in beech stands

For some beech (Fagus sylvatica L.) stands with different stand densities the plant area index (PAI) was measured by means of a Licor LAI-2000 plant canopy analyser. The stands are located on the slopes of a valley in south-west Germany and had been treated by different types of silvicultural management (heavy shelterwood felling, light shelterwood felling, control plot). The analyser was used (a) to investigate the light conditions on plots of the same thinning regime, (b) to quantify the differences between the different treatments and (c) to obtain absolute values of PAI for interdisciplinary research. PAI was measured at three different phenological stages (leafless, leaf-unfolding and fully leafed season in 2000) and was found to be about 5.2 for the fully developed canopy on the control plots, 3.2 on the light fellings and about 2.0 for the heavy fellings. In the leafless period PAI was between 1.1 (control) and 0.4 (heavy felling). Measurements made in summer 2000 and summer 2002 were compared, and showed an increase of PAI, especially on the thinned plots. Measurements of photosynthetically active radiation (PAR) above and below the canopy in combination with measured PAI were used to apply Beers Law of radiation extinction to calculate the extinction coefficient k for different sky conditions and for the different growing seasons on the control plots. The extinction coefficient k for the beech stands was found to be between 0.99 and 1.39 in the leafless period, 0.62 to 0.91 during leaf unfolding and between 0.68 and 0.83 in the fully leafed period. Using PAR measurements and the k values obtained, the annual cycle of PAI was modelled inverting Beers Law.