Going nuts: simulating macadamia’s carbon allocation in Australia

Understanding how trees allocate resources to canopy and fruit formation is fundamental for managing orchards. Macadamia trees are evergreen, can grow up to 19 m tall and 13 m wide and produce fruits for 40-60 years. Seven species are native to Australia, two of which are commercially produced mainly in Australia, South Africa, Kenya and USA (especially Hawaii). 

Drs Inigo Auzmendi and Jim Hanan at the University of Queensland propose a new model called Autonomous Units Carbon Allocation Model (AUCAM) to simulate the dynamic process of carbon allocation of macadamia trees. The researchers revealed how leaf and fruit growth different in the inner and outer zones of the canopy as they simulated tree growth in 3D. The scientists previously published plant growth model of avocados which predicted the developmental timing of the leaf sink–source transition stage.

Auzmendi and Hanan used a functional-structural plant model in the open-access L-studio software. The model is L-system based as it captures the plant architecture in a string of modules or L-string. The tree architecture was based on a Macadamia integrifolia tree which was planted in 2004 at an orchard in Beerwah (QLD, Australia).

The model simulated daily photosynthetically active radiation (PAR) and net carbon assimilation for each leaf and estimated potential growth for each organ (e.g. leaf area, shoots, fruits). The plant architecture was divided into autonomous units in terms of carbon allocation as no autonomy, units growing from the main trunk, branches and branchlets. Fruit growth was based on field observations.

Auzmendi and Hanan found high yield variability between outer and inner zones of the canopy when branchlets were considered to be an autonomous unit. Leaves within the inner canopy did not have a big influence on the tree carbon supply. The carbon supply itself varied during the season mostly due to fluctuations of intercepted light. The rate of fruit growth was slow at the beginning of the season and once again slowed down after secondary flush budburst. 

The scientists explain, “when shoots start growing, new leaves compete with fruits for carbon resources and their shade reduces the light intensity reaching older leaves, therefore further reducing the size and, therefore, the potential sink that those fruits could have in the future”. 

They conclude, “our simulations highlight the importance of the specific times of competition between fruit and vegetative growth, as has been already observed in macadamia with time of pruning and fruit growth”. 

Operating a 312 tree per hectare orchard costs around $3,000 to $3,500 per hectare per year and harvesting, de-husking, drying, and storing is an additional $1,000 to $1,500 per hectare. As Macademia integrifolia is a vulnerable species in Australia, Auzmendi and Hanan’s model can help both commercial producers and it also can also inform conservation planning.