Evaluating optical measurements of leaf area index against litter collection in a mixed broadleaved-Korean pine forest in China
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We evaluated the error caused by optical measurements of leaf area index using a direct method in a mixed broadleaf-coniferous forest in China.
Indirect optical methods to measure leaf area index (LAI) have been previously developed, but it is difficult to evaluate the accuracy of these methods in a mixed broadleaf-coniferous forest. In this study, the LAI in a mixed broadleaved-Korean pine (Pinus koraiensis) forest in China was estimated directly by litter collection (LAIlit) for the purpose of evaluating optical LAI measurements using digital hemispherical photography (DHP) and LAI-2000. With the DHP method, we corrected a systematic error due to incorrect automatic photographic exposure. With both DHP and LAI-2000 methods, we studied the influences of zenith angle selection schemes (0°–45°, 30°–60°, 45°–60° and 0°–75°) on the effective LAI (L e) measurement. In addition to optical L e, we also investigated other major factors influencing the determination of LAI, including woody-to-total area ratio (α), element clumping index (Ω E) and needle-to-shoot area ratio (γ E). A significant correlation (P < 0.01) was observed between optical (DHP and LAI-2000) and litter collection methods, but DHP L e underestimated LAIlit by 61 % on average based on different zenith angle ranges, and L e at 45°–60° agrees better with LAIlit (R 2 = 0.75, P < 0.01 and RMSE = 4.5), and the accuracy was enhanced by 21 % on average after considering α, Ω E and γ E and was further improved by 36 % after correcting for the error due to exposure. In contrast, LAI-2000 L e underestimated LAIlit by 32 % on average based on different zenith angle ranges, and L e in rings 1–3 is closer to LAIlit (R 2 = 0.80, P < 0.01 and RMSE = 2.1) than those in other rings (e.g., 3–4, 4 and 1–5), and after correcting for α, Ω E and γ E, the difference between LAI-2000 LAI and LAIlit was less than 6 %. Although DHP L e underestimated LAI-2000 L e by an average of 43 % at different zenith angle ranges, significant correlations between them were found (minimum r = 0.787, P < 0.01). We confirm the accuracy of the best estimates of LAI using DHP and LAI-2000 methods are to be over 94 % after considering woody materials and foliage clumping within shoots and the canopy. Meanwhile, the litter collection method is useful for estimating LAI in a mixed broadleaf-coniferous forest, if the specific leaf area for all major species and the average leaf age for evergreen coniferous species are known.
KeywordsLeaf area index Litter collection method Digital hemispherical photography (DHP) LAI-2000 Influence factors Mixed broadleaved-Korean pine forest
Author contribution statement
Conceived and designed the experiments: Guangze Jin. Performed the experiments: Zhili Liu, Yujiao Qi. Analyzed the data: Zhili Liu, Yujiao Qi. Wrote the paper: Zhili Liu, Jing M. Chen, Guangze Jin.
This work was financially supported by the Ministry of Science and Technology of China (No. 2011BAD37B01), the National Natural Science Foundation of China (No. 31270473); the Program for Changjiang Scholars and Innovative Research Team in University (IRT1054), and the Fundamental Research Funds for the Central Universities (2572014AA01).
Conflict of interest
The authors declare that they have no conflict of interest.
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