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

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_E was 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.