水保所知识产出(1956---)知识库

Automatic and accurate measurements of the leaf area index (LAI) can provide information about key parameters
of vegetation structure in an ecosystem. Little attention, however, has been paid to LAI dynamics for shrub
canopies in both arid and semiarid regions compared to forests and crops. One of the main sources of error when
measuring shrub LAI from the indirect method is the woody part. The influence of woody parts on LAI esti-
mations can be expressed as the woody-to-total area ratio, α ; there are also relatively few studies on α value of
shrubs, especially the dynamic changes of α . This study used a new cost-effective tool, the fisheye webcam, to
derive daily LAI and α in the fields of Caragana korshinskii and Salix psammophila on the Loess Plateau in China.
The LAI data were compared with measurements acquired by an LAI-2200 Plant Canopy Analyzer and direct
methods. The LAI-2200 data were consistent with the effective plant area index (PAI eff ) estimated using upward-
pointing webcams (UPWs) (R 2 = 0.89, root mean square error (RMSE) = 0.21) but not downward-pointing
webcams (DPWs) (R 2 = 0.52, RMSE = 0.68). Shrub PAI eff derived from the DPWs was much smaller than
PAI eff derived from the UPWs over the season (y = 0.48x–0.03, R 2 = 0.79, RMSE = 0.85). LAI derived from the
direct methods was strongly linearly correlated with the PAI minus WAI obtained from upward digital hemi-
spherical photography (R 2 = 0.94, RMSE = 0.62 for C. korshinskii and R 2 = 0.99, RMSE = 0.31 for
S. psammophila). This result indicates that the upward digital hemispherical photography for deriving PAI and
WAI performs good, which can be utilized to calculate LAI accurately. The woody-to-total area ratio ( α ) of de-
ciduous shrub canopies was a dynamic parameter that varied throughout the season, decreasing as the canopy
leaves developed and increasing when they fell. α = 1 for the leaf-off season and < 0.1 when the foliage peaked. α
differed little between the two shrub canopies (R 2 = 0.95, RMSE = 0.07), and it could be used to convert PAI to
LAI for C. korshinskii and S. psammophila. The UPWs demonstrated excellent potential for continuous LAI mea-
surement for the two shrub canopies, but the DPWs should be further tested before use. The fisheye-webcam is
inexpensive and can easily monitor LAI at field scale and can be used to evaluate LAI products derived from
remote sensing.