Since 2013, the Small Tree High Productivity Initiative has been researching ways to improve the productivity of several tree crops through better canopy management of high density orchard systems. This article provides an update on several aspects of the mango research and its implications for mango production.
Planting Systems Trial
A Planting Systems Trial was set up at Walkamin Research Station (WRS) in 2013 to investigate how three mango varieties ‘Keitt’, ‘Calypso’, and ‘NMBP 1243’ can be grown at high (1250 trees per hectare), medium (416 trees per hectare) and low (208 trees per hectare) planting densities when trained as either conventional or single leaders (both on and off a trellis). Since our last report in the September 2016 issue of Mango Matters, the trial has flowered and cropped, giving us some early insights into the effects of the pruning and density treatments on productivity.
The first harvest of the three year old trees was completed in January 2017 and planting density, canopy training and variety all influenced tree productivity. The most productive trees (as measured in yield per kg per m3 canopy volume) were the single leader trained trees planted at medium density (Table 1). However, when productivity was expressed as yield (tonnes per hectare), the high density trees had over five times the yield of the low density planting (Table 1).
Light interception is the percentage of sunlight captured by the canopy and calculated on a per hectare basis. Increasing the percentage of light intercepted by a crop, has the potential to increase total photosynthesis and productivity per hectare. In many tree crops, the optimum light intercepted per hectare has been shown to be up to 90%. The amount of light intercepted per hectare depends on factors such as tree planting density (row and tree spacing), canopy size and shape.
Light interception was measured in a range of commercial orchards with ‘Kensington Pride’ trees (1 to 30 years old) growing in the Mareeba/Dimbulah district. In these orchards, light interception reached a maximum of 67% in 25 to 30 year old trees with canopy volume of about 15,000 m3 per hectare, after which, neither canopy volume nor light interception increased (as shown in Figure 1a—Baseline) in the commercial orchards. Tree yields increased with light interception up to (approximately) 20 tonnes per hectare at 67% light interception (Figure 1b).
Figure 1 a, b and c show light interception (as a percentage of the total light available) in ‘Kensington Pride’ trees from the baseline survey (red dots) and the combined data of ‘Calypso’, ‘Keitt’ and ‘NMBP1243’ from the Planting Systems Trial at different planting density and canopy training systems (other symbols), in relation to (a) Canopy volume per hectare, (b) Yield per hectare and (c) Yield per unit of canopy volume.
The efficient use of intercepted light depends on how light is distributed throughout the canopy with factors such as canopy density, training, tree height and shape influencing light distribution.
Our light distribution studies are investigating how intercepted light is distributed within the canopy. In other crops, light levels within the canopy have been shown to influence fruit quality, size and colour and leaf photosynthesis. Another experiment looking at light distribution within commercial ‘Kensington Pride‘ orchard trees has shown large variations in light distribution within the canopy, with many leaves in the shade. Conversely, the outer canopy may be receiving too much light, at times, possibly causing photo-inhibition as indicated by sun burn on leaves and fruit. By reducing shade and letting more light in to the canopy we can optimise the number of leaves exposed to light and increase the overall carbon production for growth and yield.
Another experiment investigated the distribution of light in trees that had been pruned as either standard pruning, heavy pruning, window pruning and not pruned. This study found the most even light was seen in heavily pruned trees followed by window pruned and commercially pruned, with the non-pruned trees having the poorest light distribution and heaviest shaded internal canopies (Figure 2).
Fruit blush colour was highest in the window pruned trees followed by the normally pruned, heavily pruned and then non-pruned trees where light distribution was lowest. The largest fruit (size and weight) were found in commercially pruned trees.
In the Planting Systems Trial at Walkamin Research Station, three year old, conventionally trained trees, planted at medium density, had significantly more light at ground level and in the bottom 25% of tree, than other training systems. This reflects the lack of foliage in this area due to high skirting practices in conventional pruning systems (Picture 1).
However, higher in the canopy, at 75% of tree height, single leader trained trees on a trellis planted at high density had significantly more light across the canopy than conventionally trained trees (Picture 2). Free standing central leader trained trees had similar light distribution to those on a trellis (Picture 3). This shows that some of the canopy training systems being evaluated in the Planting Systems Trial are displaying better light distribution and have potential to optimise light better than in conventional canopies.
Evidence for the importance of mango canopy training system is not only reflected by the interception and distribution of light in the single leader trained trees, but also by a greater number of inflorescences. For example, at two years old there were 1.21 inflorescences per m3 per volume of canopy in single leader compared to 0.26 inflorescences per m3 in the conventional training systems.
There are many factors that influence canopy productivity which need to be considered when designing tree canopies for high productivity at high planting densities. This project is helping us to understand some of these factors and the interrelationships that exist between these factors. It is important to remember that the data outlined here is only for the first few years of production and continued measurement of these factors in the Planting Systems Trial at Walkamin Research Station can help us to further understand what happens to yield and light relations under the different planting densities and pruning regimes as the trees mature.
Acknowledgments: The Small Tree High Productivity Initiative is an initiative of the Queensland Government. Major partners include the Department of Agriculture and Fisheries (DAF), DAF’s research alliance with The University of Queensland (Queensland Alliance for Agriculture and Food Innovation), and the NSW Department of Primary Industries.
A key element of this initiative has been co-funded by Hort Innovation using the across horticulture levy and voluntary contributions from DAF and matching funds from the Australian Government through the Hort Innovation project “Transforming tropical/subtropical tree crop productivity”. We are especially grateful to Hort Innovation and the various associated industries and horticultural businesses for their support for this initiative.
We would also like to thank the mango growers Edward Balzarolo, Raimond Bin and Steven Schincariol for access to their orchards for this research and the farm-staff and casuals for help at WRS.
Article submitted by Peter Rigden, Ian Bally, Paula Ibell, Anahita Mizani, Mahmud Kare and Carole Wright.