Abstract
As a widely used ground-based optical instrument, the LAI-2000 or LAI-2200 plant canopy analyzer (PCA) (Li-Cor, Inc., Lincoln, NE) is designed to measure the plant effective leaf area index (Le) by measuring the canopy gap fraction at several limited or discrete view zenith angles (VZAs) (usually five VZAs: 7, 23, 38, 53, and 68°) based on Miller’s equation. Miller’s equation requires the probability of radiative transmission through the canopy to be measured over the hemisphere, i.e., VZAs in the range from 0 to 90°. However, the PCA view angle ranges are confined to several limited ranges or discrete sectors. The magnitude of the error produced by the discretization of VZAs in the leaf area index measurements remains difficult to determine. In this study, a theoretical deduction was first presented to definitely prove why the limited or discrete VZAs or ranges can affect the Le measured with the PCA, and the specific error caused by the limited or discrete VZAs was described quantitatively. The results show that: (1) the weight coefficient of the last PCA ring is the main cause of the error; (2) the error is closely related to the leaf inclination angles (IAs)—the Le measured with the PCA can be significantly overestimated for canopies with planophile IAs, whereas it can be underestimated for erectophile IAs; and (3) the error can be enhanced with the increment of the discrete degree of PCA rings or VZAs, such as using four or three PCA rings. Two corrections for the error are presented and validated in three crop canopies. Interestingly, although the leaf IA type cannot influence the Le calculated by Miller’s equation in the hemispheric space, it affects the Le measured with the PCA using the discrete form of Miller’s equation for several discrete VZAs.
Highlights
Rings? (3) How can the error be eliminated? The aims of this study were to show: (1) why the discrete view zenith angles (VZAs) may affect leaf area index (Le) measured with the plant canopy analyzer (PCA); (2) how the discrete VZAs affect
The leaf inclination angles (IAs) type cannot influence Le calculated using Miller’s equation in hemispheric space, it may affect Le measured with the PCA using the discrete form of Miller’s equation at several discrete VZAs in this study
A theoretical deduction is presented to prove why the discrete VZAs can affect Le measured with the PCA: both the weight coefficient of the last PCA ring and the foliage G value are two main reasons
Summary
The leaf area index (LAI), defined as one half of the leaf area per unit ground area [1], is an essential parameter for controlling mass and energy exchanges between the forest and creativecommons.org/licenses/by/ 4.0/). LAI measurement is important in the fields of plant, ecology, and vegetation remote sensing [5,6,7]. As an indirect method of measuring the LAI, ground-based indirect instruments, such as the LAI-2000 or LAI-2200 plant canopy analyzer (PCA, LI-COR, Lincoln, Nebraska USA), AccuPAR (Decagon Devices, Inc., Pullman, Washington, USA), Tracing Radiation and Architecture of Canopies (TRAC, 3rd Wave, Ontario Canada), digital hemispherical photographs (DHPs), and light detection and ranging (LiDAR), have been widely applied in LAI measurement for much of the past 30 years [7,8,10,11] (Figure 1)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
