The field of computational modelling has advanced significantly in recent years as information technology, in-field data and plant modelling software have been cleverly combined to develop greater confidence in models, including a visual twist!

Using simulations has the benefit of allowing researchers and industries to quickly test ideas before committing to long-term trials, Inigo Auzmendi* and Jim Hanan** write.

While in-orchard research is still required to gather the data needed to draw conclusions, simulations can help to point out the potential for further research to better understand the effects of orchard design and management on production levels.

The QAAFI team for horticultural plant physiology and modelling is collaborating with horticultural researchers in the Queensland Department of Agriculture and Fisheries (DAF) to better understand how orchard design and management practices affect the growth and yield of macadamia trees. This work is part of the National Tree Crop Intensification in Horticulture program (AS18000).

The team uses the simulation of the annual growth of individual tree organs above ground, that is: stems, leaves and fruits, to mimic the growth of a virtual tree over a growing season. The orchard environment is replicated by simulating a group of trees to better understand the effects of planting density (trees per hectare) on yield (tonnes of nut-in-shell, NIS, per ha).

Figure 1. Visual 3D representation of a virtual macadamia orchard at around six years after planting,
before simulating tree growth and yield during one season. The ground surface (grey) corresponds
to the central tree. The canopy includes stems (dark grey) and leaves (green). Image adopted from Auzmendi and Hanan (2020b). 

Planting density

Initially, the team created a virtual macadamia tree to study carbohydrate distribution and branch autonomy. In the present article, they describe the results of simulating orchard yield by ‘growing’ a virtual tree surrounded by several other trees, reproducing the orchard light environment (see Figure 1).

With an aim of the AS18000 Program macadamia research to increase understanding and knowledge of how planting density influences yield, the simulations complement field planted trials by enabling testing of a wider range of planting densities. These results could be later used to conduct a comparison analysis of associated profitability (yield/quality) and sustainability.

Simulating orchard yield

Using a virtual canopy created from in-orchard tree measurements, the model simulates processes influenced by external factors such as the light environment of individual leaves, as well as internal plant processes like photosynthesis and carbohydrate distribution. The virtual orchard grows day by day throughout a growing season, in a similar fashion to a real orchard. By placing several trees together in a virtual orchard scenario, the result of the interaction of several factors produces a simulation of the orchard yield under varying planting density scenarios.

By simulating the growth of individual organs and then combining them at both the individual tree and orchard levels, the team was able to investigate trends that may seem counterintuitive or difficult to explain. For example, higher planting density increased orchard yield if trees were not planted too close together. When planting density was too high, a yield decrease was observed (see Figure 2).

Figure 2. Relationship between simulated yield and planting density in virtual macadamia orchards.
Macadamia growth during one season was simulated with seven different planting densities ranging from 300 to 1600 trees per hectare. 

Planting density implications and future work

The simulation results show that planting trees at higher densities does not always lead to increased yield per hectare. Beyond the actual numbers, the trend simulated and presented in this article demonstrates that an orchard is undoubtedly more than just a collection of organs or trees, and that the dynamics of the entire orchard system have a significant impact on yield outcomes.

In an experimental orchard with three different planting densities established in the DAF Bundaberg Research Facility, current work focuses on discovering which planting density has the highest yield potential or the highest yield at the lowest cost. The process includes applying different management practices that have successfully been trialled in-orchard to increase yield, in the context of the already known and simulated limitations of increased planting density.

The simulation results will be compared with in-field results from the macadamia planting systems trial in Bundaberg, aiming to maximise the synergy between these studies. The comparison will also allow testing the potential of computational simulations for input into orchard design and management practices. With this in mind, future planting density recommendations should consider not only these simulations, but also specific orchard characteristics such as cultivar, soil and climate details that our current model does not account for.

While this project is expected to demonstrate the possibilities of simulation as a tool for increasing the understanding of orchard density to benefit both research and industry, there are several other questions to which simulations may be applied in the future. These include developing an understanding of the differences in architecture (branching patterns, shoot length, leaf area, flowering, etc), light interception and yield between cultivars, tree shapes and sizes.

This work commenced as a part of the Transforming Subtropical/tropical Tree Crop Productivity (AI13004) project and is continuing as a component of research within the National Tree Crop Intensification in Horticulture Program (AS18000), funded by the Hort Frontiers Advanced Production Systems Fund, part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation, with co-investment from Queensland’s Department of Agriculture and Fisheries and Queensland Alliance for Agriculture and Food Innovation – The University of Queensland, and contributions from the Australian Government.

*University of Queensland research fellow Inigo Auzmendi undertakes research on improving management practices in avocado, macadamia and mango. He is focused on studying architecture, vegetative vigour, crop load and light interception using functional-structural plant modelling to understand the interactions between management practices, environmental factors, plant carbon balance and growth. Contact: Inigo Auzmendi. E-mail: i.auzmendi@uq.edu.au Phone: 07 344 32702.

**QAAFI honorary principal fellow Jim Hanan undertakes multi-disciplinary collaborative research developing mathematical, computational and visualisation approaches and techniques that facilitate the study of genetics, physiology, morphogenesis and ecology at the scale of cells, individual plants and insects. These developments are used to increase our understanding, and as a tool in applied research and education.

First published in the Autumn 2023 News Bulletin of the Australian Macadamia Society.