Modelling and measuring vertical light absorption within grass–clover mixtures

Abstract

The vertical profile of leaf area, leaf dispersion, and light partitioning and absorption were studied during two regrowth periods in mixtures of perennial ryegrass and two white clover cultivars differing in leaf size under cutting management without fertiliser nitrogen. A triangular leaf area density 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 were linearly correlated with downward cumulative LAI of the mixture and appeared to be the principal cause for variation in the extinction coefficient ( k) with canopy height. This relationship was negative for clover and positive for grass, corresponding with a shift from regular leaf dispersion in the top layers to a clumped dispersion in the bottom layers for clover and the reverse pattern for grass. Measured light profiles could be exactly mimicked with a modified version of a general multi-layer light competition model by incorporating for both species leaf dispersion as a function of downward cumulative LAI in combination with fitted dispersion-free values of k which only reflect the leaf-angle distribution. Competitive success of clover over grass for light absorption in this study was, next to its greater contribution to total LAI and a more planofile leaf-angle distribution, related to its higher position in the mixture where maximum leaf area density occurred and regular leaf dispersion in the top layers of the canopy. These last two characteristics were especially manifest in the large-leaved clover.