The “MaluSim” Apple Carbon Balance Model—Validation and Utilization

“MaluSim” is a simplified carbon balance model to estimate seasonal patterns and sums of tree dry matter production, distribution and fruit development.  The model is a “big leaf” tree model (all leaves, all fruit, all roots, etc. treated as one organ each) with a daily time step.  Shoot growth is based on shoot numbers, growth rates and fraction of shoots growing. Daily photosynthesis is calculated from daily absorbed radiation by a daily canopy light response.  Respiration is summed from each organ based on seasonal specific respiration, organ weight or area and driven by temperature.  Dry matter is partitioned to organs based on demands if there is no limitation, but has different competitive sink strengths if carbon is limiting.  The inherent competitiveness is set as: shoots>fruit>roots=structure.  A fruit abscission submodel is based on a fruit abscission vs growth rate relationship. Seasonal patterns suggest the greatest carbon limitation to the crop to occur at 2-3 weeks after bloom. Validation studies indicate that the dry matter production component is robust and that leaf area and light interception is realistic.  Carbon balance at thinning time is well correlated with natural drop and response to chemical thinners and is being used to help growers predict thinner efficacy.