A Modified Self-thinning Equation to Describe Size/Density Relationships for Defoliated Swards

Use of the self-thinning rule to describe size/density compensation (SDC) in defoliated swards is examined. It is shown that defoliation related variation in leaf area and associated morphogenetic changes in plant structure necessitate slope corrections, designated Ca and Cr , respectively. The theory predicts that reduced leaf area in more heavily defoliated swards will result in SDC at slopes more negative than -3/2 (variable leaf area SDC), and that there will be a transition to -3/2 (constant leaf area) SDC at higher herbage mass. Empirical data from previous experiments with Lolium perenne L. and Medicago sativa L. are examined, and appear to confirm the theoretical predictions, including the slope change at the point of transition from variable to constant leaf area SDC. This transition point, designated di , is subject to interspecific variation and is related to the mature shoot size of a particular species. Some applications of this theory are discussed, and in particular a sward productivity index is proposed.