Small Tree High Productivity Initiative—2017 results update

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).

Table 1. The influence of training system and planting density on the combined average yield per canopy volume (kg per m3) and yield per hectare (tonnes per hectare) of three varieties (Keitt, Calypso, and NMBP 1243) in three year old trees from the Planting Systems Trial at Walkamin Research Station.

Note: Figures marked with the same letter are not significantly different to each other at the 95% confidence level. These results indicate high density planting and canopy training can significantly increase yields early in the life of an orchard.

Light Interception

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.

Figure 1a. In the Planting Systems Trial, more light was intercepted per hectare in the high density planted trees than the medium and low density planted trees, with three year old trees planted at high density intercepting approximately the same light as seven year old conventional ‘Kensington Pride’ trees in a commercial orchard.

Figure 1a. In the Planting Systems Trial, more light was intercepted per hectare in the high density planted trees than the medium and low density planted trees, with three year old trees planted at high density intercepting approximately the same light as seven year old conventional ‘Kensington Pride’ trees in a commercial orchard.

Figure 1b. Yield (tonnes per hectare) increased with increasing light interception. Commercial ‘Kensington Pride’ trees reached a maximum yield of 20 tonnes per hectare at about 67% light interception, after which yield declined. In the high density planted trees, yields have exceeded 20 tonnes per hectare with less light interception, and have potential to improve as the high density canopies fill out their allotted space.

Figure 1b. Yield (tonnes per hectare) increased with increasing light interception. Commercial ‘Kensington Pride’ trees reached a maximum yield of 20 tonnes per hectare at about 67% light interception, after which yield declined. In the high density planted trees, yields have exceeded 20 tonnes per hectare with less light interception, and have potential to improve as the high density canopies fill out their allotted space.

Figure 1c. When we compared light interception with yield (kg per m3 canopy), we found that the training system could further increase the light intercepted per hectare for any given canopy volume. This has the potential to further increase productivity in high density orchards with well-trained canopy systems.

Figure 1c. When we compared light interception with yield (kg per m3 canopy), we found that the training system could further increase the light intercepted per hectare for any given canopy volume. This has the potential to further increase productivity in high density orchards with well-trained canopy systems.

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).

Figure 2. Average light distribution at four heights in the canopy of commercial ‘Kensington Pride’ trees with four different pruning techniques.

Figure 2. Average light distribution at four heights in the canopy of commercial ‘Kensington Pride’ trees with four different pruning techniques.

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).

Picture 1. A three year old, conventionally trained, Calypso mango tree, after pruning from the Planting Systems Trial at Walkamin Research Station. Note that there is more light in the bottom part of the tree and heavy shading in the upper canopy. 

Picture 1. A three year old, conventionally trained, Calypso mango tree, after pruning from the Planting Systems Trial at Walkamin Research Station. Note that there is more light in the bottom part of the tree and heavy shading in the upper canopy. 

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.

Picture 2. Four year old ‘Keitt’ mango trees trained as single leaders, on high density trellis in the Planting Systems Trial at Walkamin Research Station. Even distribution of light throughout the canopy with minimal shading of leaves can optimize light distribution in the canopy and increase the photosynthetic efficiency of leaves.

Picture 2. Four year old ‘Keitt’ mango trees trained as single leaders, on high density trellis in the Planting Systems Trial at Walkamin Research Station. Even distribution of light throughout the canopy with minimal shading of leaves can optimize light distribution in the canopy and increase the photosynthetic efficiency of leaves.

Picture 3. Three year old ‘NMBP 1243’ mango tree trained as a single leader, from the Planting Systems Trial at Walkamin Research Station. Even distribution of light throughout the canopy can optimize light distribution in the canopy and increase the photosynthetic efficiency of leaves.

Picture 3. Three year old ‘NMBP 1243’ mango tree trained as a single leader, from the Planting Systems Trial at Walkamin Research Station. Even distribution of light throughout the canopy can optimize light distribution in the canopy and increase the photosynthetic efficiency of leaves.

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.