Measuring seasonal dynamics of leaf area index in a mixed conifer-broadleaved forest with direct and indirect methods

doi:10.3724/SP.J.1258.2014.00079

Abstract

Abstract:

Aims Leaf area index (LAI) is a commonly used parameter for quantifying canopy structure and can be quickly measured by indirect optical methods in a forest stand, but few studies have evaluated the accuracy of optical methods to estimate seasonal variations of LAI in a mixed conifer-broadleaved forest. The aims of this study are to (1) develop a practical field method for directly measuring seasonal variations in LAI for mixed conifer-broadleaved forest; (2) evaluate the accuracy of optical methods (digital hemispherical photography (DHP) and LAI-2000 plant canopy analyzer) for measuring the seasonality of LAI; and (3) determine how much the accuracy of estimating the seasonality of LAI can be improved by using optical methods after correcting for influencing factors (e.g., woody materials and clumping effects within a canopy).
Methods The seasonal variations of LAI in a mixed broadleaved-Korean pine (Pinus koraiensis) forest were estimated from litterfall and used to evaluate optical LAI (effective LAI, L_e) measurements using the DHP and the LAI-2000 plant canopy analyzer. We corrected a systematic error due to incorrect automatic photographic exposure for DHP measurements. In addition to optical L_e, we also measured the seasonality of other major factors influencing the determination of LAI, including woody-to-total area ratio (α), clumping index (Ω_E) and needle-to-shoot area ratio (γ_E).
Important findings The LAI from different methods all showed a unimodal form, and peaked in early August. Effective LAIs from the optical methods underestimated LAI throughout the growing seasons (from May to November). L_e from DHP underestimated LAI by an average of 55% (ranging from 50% to 59%) and from LAI-2000 plant canopy analyzer by an average of 27% (ranging from 19% to 35%). The accuracy of L_e from DHP after correcting for the automatic exposure, α, Ω_E and γ_E was greatly improved, but the LAI was underestimated by 6%-15% (with mean value of 9%) from May to November. In contrast, the accuracy of L_e from LAI-2000 plant canopy analyzer after correcting for the α, Ω_E and γ_Ewas also greatly improved, the difference between corrected L_e from LAI-2000 plant canopy analyzer and observed LAI was less than 9%. The results from our study demonstrate that seasonal variations in LAI in mixed conifer-broadleaved forests can be optically measured with high accuracy (85% for DHP and 91% for LAI-2000 plant canopy analyzer), as long as corrections are made for the influences of woody materials and foliage clumping on the measurement.

Key words: clumping effects, digital hemispherical photography (DHP), LAI-2000 plant canopy analyzer, leaf area index, litter collection method, needle life span, woody-to-total area ratio

LIU Zhi-Li, JIN Guang-Ze, ZHOU Ming. Measuring seasonal dynamics of leaf area index in a mixed conifer-broadleaved forest with direct and indirect methods[J]. Chin J Plant Ecol, 2014, 38(8): 843-856.

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Figures/Tables 10

Table 1 Composition and specific leaf area (SLA) of major tree species in a 9 hm2 typical mixed broadleaved-Korean pine forest plot at Liangshui in the Xiaoxing’an Mountains, China

主要树种 Major tree species	密度 Density (trees·hm^-2)	平均胸径 Mean DBH (cm)	胸高断面积 Basal area (m²·hm^-2)	相对优势度 Relative dominance (%)	比叶面积 SLA (cm²·g^-1)
红松 Pinus koraiensis	133	42.81	24.15	57.09	83.79 (3.74)
臭冷杉 Abies nephrolepis	101	16.17	3.01	7.11	80.80 (5.43)
云杉 Picea spp.	20	18.99	1.06	2.51	59.41 (9.70)
紫椴 Tilia amurensis	81	13.35	3.01	7.11	243.59 (14.26)
色木槭 Acer mono	238	7.73	2.43	5.74	305.04 (50.08)
风桦 Betula costata	67	13.02	2.04	4.82	199.79 (11.55)
裂叶榆 Ulmus laciniata	108	7.73	1.48	3.50	261.40 (5.65)
水曲柳 Fraxinus mandschurica	45	12.54	1.27	3.00	338.36 (13.63)
其他 Others	1 580	2.96	3.85	9.10	-
总计 Total	2 373	7.41	42.30	100.00	-

Fig. 1 Distribution of litter traps and contour map of a 9 hm2 typical mixed broadleaved-Korean pine forest plot at Liangshui in the Xiaoxing’an Mountains, China.

Table 2 Needle life span for evergreen needle tree species

树种 Tree species	最大值 Maximum	最小值 Minimum	平均值±标准偏差 Mean ± SD
红松 Pinus koraiensis	3.48	2.69	3.08 ± 0.25
臭冷杉 Abies nephrolepis	4.63	3.11	3.68 ± 0.52
云杉 Picea spp.	4.35	3.51	3.91 ± 0.27

Fig. 2 Seasonal changes of increases in proportion of the total leaf area for major tree species in the growing season in the mixed broadleaved-Korean pine forest (mean ± SD). A, deciduous broadleaf tree species. B, evergreen needle tree species.

Fig. 3 Seasonal changes of leaf area index (LAI) for major tree species in the mixed broadleaved-Korean pine forest (mean ± SD). A, deciduous broadleaf tree species. B, evergreen needle tree species.

Fig. 4 Seasonal changes of woody-to-total area ratio and clumping index in the mixed broadleaved-Korean pine forest (mean ± SD).

Table 3 Seasonal dynamics of needle-to-shoot area ratio for evergreen needle tree species in the mixed broadleaved-Korean pine forest (mean ± SD)

月份 Month	红松 Pinus koraiensis	臭冷杉 Abies nephrolepis	云杉 Picea spp.
May	1.46 ± 0.22	1.19 ± 0.22	1.44 ± 0.34
June	1.49 ± 0.24	1.21 ± 0.30	1.47 ± 0.43
July	1.69 ± 0.38	1.11 ± 0.30	1.29 ± 0.33
Aug.	1.78 ± 0.45	1.16 ± 0.34	1.31 ± 0.33
Sept.	1.72 ± 0.28	1.10 ± 0.23	1.28 ± 0.35
Oct.	1.65 ± 0.36	1.07 ± 0.21	1.26 ± 0.29
Nov.	1.65 ± 0.26	1.10 ± 0.23	1.30 ± 0.36
平均值 Mean	1.63	1.13	1.34
变异系数 CV (%)	7.3	4.6	6.3

Fig. 5 Seasonal dynamics of leaf area index (LAI) in a mixed broadleaved-Korean pine forest estimated by using different methods (mean ± SE, n = 64). The true LAI is derived from litter collection; effective LAI from DHP is calculated directly by DHP software; effective LAI from LAI-2000 is calculated directly by C2000 software; Corrected DHP LAI is derived by correcting effective LAI from DHP for the automatic exposure, woody-to-total area ratio, clumping index and needle-to-shoot area ratio; Corrected LAI-2000 LAI is derived by correcting effective LAI from LAI-2000 for the woody-to-total area ratio, clumping index and needle-to-shoot area ratio.

Fig. 6 Seasonal dynamics of the difference between the litter collection leaf area index (LAI) and the optical LAI including both DHP and LAI-2000 (mean ± SE, n = 64). A is for the DHP method. B is for the LAI-2000 method. Difference (%) = (litter collection LAI - DHP or LAI-2000 LAI) / litter collection LAI × 100%, DHP or LAI-2000 LAI all includes effective LAI and corrected LAI (the correction scheme is the same as that in Fig. 5).

Fig. 7 Seasonal dynamics of the difference between the leaf area index (LAI) from DHP and the LAI from LAI-2000 (mean ± SE, n = 64). Difference (%) = (Le from LAI-2000 - LAI from DHP) / Le from LAI-2000 × 100%, LAI from DHP includes Le from DHP (uncorrected) and corrected LAI from DHP that calculated by Le from DHP after correcting for only the automatic exposure (corrected).

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