Modelling and measuring vertical light absorption within grass-clover mixtures

The vertical profile of leaf area, leaf dispersion, and light partitioning and absorption were studied during two regrowth periods in mixtures of pere nnial ryegrass and two white clover cultivars differing in leaf size under cutting management without fertiliser nitrogen. A triangular leaf area dens ity function with height gave a good description of the vertical leaf area profile for both species. Leaf dispersion was studied by analysing inclined point quadrat data. Calculated leaf dispersion factors of both species wer e linearly correlated with downward cumulative LAI of the mixture and appea red to be the principal cause for variation in the extinction coefficient ( k) with canopy height. This relationship was negative for clover and positi ve for grass, corresponding with a shift from regular leaf dispersion in th e top layers to a clumped dispersion in the bottom layers for clover and th e reverse pattern for grass. Measured light profiles could be exactly mimic ked with a modified version of a general multi-layer light competition mode l by incorporating for both species leaf dispersion as a function of downwa rd cumulative LAI in combination with fitted dispersion-free values of k wh ich only reflect the leaf-angle distribution. Competitive success of clover over grass for light absorption in this study was, next to its greater con tribution to total LAI and a more planofile leaf-angle distribution, relate d to its higher position in the mixture where maximum leaf area density occ urred and regular leaf dispersion in the top layers of the canopy. These la st two characteristics were especially manifest in the large-leaved clover. (C) 1999 Elsevier Science B.V. All rights reserved.