Characterization of radiation regimes in nonrandom forest canopies: theory, measurements, and a simplified modeling approach

We used field measurements and Monte Carlo simulations of canopy gap-size d istribution and gap fraction to examine how beam radiation interacts with c lumped boreal forest canopies of aspen (Populus tremuloides Michx,), black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) . We demonstrate that the Beer-Lambert law can be modified to accommodate t ransmission of radiation through a clumped forest canopy as a function of p ath length or sun zenith angle. Multiband Vegetation Imager (MVI) measureme nts and Monte Carlo simulations showed that values of the zenith element cl umping index (Omega(e)(0)) are typically between 0.4 and 0.5 in jack pine a nd black spruce and 0.65 in aspen. Estimates of LAI obtained from MVI measu rements of the canopy gap fraction and adjusted for canopy clumping and bra nch architecture yielded LAI values of 3.0 in jack pine, 3.3 in aspen, and about 6.0 in black spruce. These LAI estimates were within 10-25% of direct measurements made at the same sites. Data obtained with the MVI, along wit h numerical simulations, demonstrated that assumptions of random foliage di stributions in boreal forests are invalid and could yield erroneous values of LAI measured by indirect techniques and false characterizations of atmos phere-biosphere interactions. Monte Carlo simulations were used to develop a general equation for beam radiation penetration as a function of zenith a ngle in clumped canopies, The essential measurements included stem spacing, crown diameter, crown depth, and within-crown gap fraction.