Remote Sensing of Leaf Area Index and Radiation Intercepted by Understory Vegetation

To estimate leaf area index (LAI), intercepted radiation, and other related characteristics of vegetation, ecologists often require separate consideration of the understory and overstory components of vegetation. In the open ponderosa pine (Pinus ponderosa) forests of east—central Oregon, bitterbrush (Purshia tridentata) and manzanita (Arctostaphylos patula) are major understory species. We constructed artificial canopies of these two species and determined whether remotely sensed properties of canopy reflectance might be used to estimate LAI and the fraction of photosynthetically active radiation intercepted (f I P A R ). Although the two species differed in their canopy structures and leaf characteristics, they showed similar light extinction coefficients, varying only from 0.52 for bitterbrush to 0.46 for manzanita as derived from the Beer—Lambert Law. The relationship between f I P A R and LAI was asymptotic at an LAI of 6 for both species. The normalized—difference vegetation index (NDVI) provided a good linear estimate of f I P A R (R 2 = 0.86 for manzanita and 0.83 for bitterbrush), and the simple ratio of near infrared to red (SR) permitted a linear estimate of LAI (R 2 = 0.86 for manzanita and 0.74 for bitterbrush) of this vegetation growing on soils with uniform reflectance properties. Aerial estimates of overstory and understory cover derived from aircraft or satellites provide a basis for assessing the proportional contribution of understory shrub cover to the total vegetation, given that basic relationships between spectral reflectance, LAI, and f I P A R have been established, as in this study.