Biotic Stress and Yield Loss

of light interception). Algorithms for predicting the quantity of light available to andabsorbed within mixed canopies and the subsequent amount of carbon assimilatedhave been described in detail elsewhere.20, 23–26Because the quantity of radiation intercepted by each species is a primary determinantof growth under potential production conditions, a critical component ofcrop–weed competition models is how competition for light in mixed canopies isquantified. Several authors assumed that the fraction of radiation absorbed by aspecies (F a,i ) can be quantified based on the fraction of the total canopy LAI thatspecies i occupies weighted by its extinction coefficient (k i). 26–29 Therefore:k iLAI iF a,i [13.4](k iLAI i)∑ ni 1where the denominator is the sum of LAI for all n species in the canopy weighted bytheir respective extinction coefficients. Others have used similar approaches usingslightly different definitions of efficiency of interception. 23, 25 Although Equation13.4 makes excellent theoretical sense, to my knowledge this relationship has neverbeen tested.A true test of Equation 13.4 may not be possible because measuring the quantityof light absorbed by each species within a mixed canopy would be extremely difficult,if not impossible. However, we may be able to obtain an indirect test by measuringthe quantity of light not absorbed by each species. For example, we canmeasure the quantity of radiation incident above the canopy (I o), at the soil surface(I s , or at any point within the mixed canopy), at the soil surface following removal ofthe crop (I w ), and at the soil surface following removal of the weed (I c). The quantityof radiation intercepted by the mixed canopy is then represented by the differenceI o I s. We may then estimate the fraction of radiation intercepted by the crop and theweed using:Ic F a,c Is I Iand [13.5]oF a,w I w IsIo Iswhere the subscripts c and w represent the crop and weed, respectively. Methodologyfor measuring incident light and estimating the extinction coefficient in monoculturecrop stands is well established in the literature. 30 Therefore, if the leaf area index ofeach species within the mixed canopy is measured at the time radiation interceptionmeasurements are taken, estimates of F a,iobtained using Equation 13.5 can bedirectly compared with those obtained using Equation 13.4.Under the assumption that Equation 13.4 is correct, a number of canopys