RESEARCH & EVENTS
Almond tree architecture - why does it matter ?
Dr Grant Thorp The New Zealand Institute for Plant and Food Research Ltd
UNDERSTANDING tree architecture is critical for breeding , selection and management of almond varieties needed for more productive growing systems :
• Tree architecture describes the natural growth habit of a tree .
• It describes how a tree grows in terms of branching and flowering habit .
• While pruning does not change tree architecture , pruning can be used to enhance key architectural traits of different varieties .
• The ideal is to allow the tree ’ s natural architecture to be expressed .
The global almond industry has developed around the concept of large-tree orchard systems . Trees are widely spaced in the orchard and pruned to promote the growth of several large scaffold branches . On maturity the trees form fully covered , closed canopies , with the crop restricted to the upper canopy . It is these closed canopies , with heavy internal shading , that limit ‘ Nonpareil ’ kernel yields to 3 – 4 t / ha . Other temperate tree crop industries , for example apple , have faced similar situations . They have found solutions to low productivity through the use of new varieties with architectural traits better suited to intensive planting systems , enabling improved light management and substantially higher yields . The AS18000 National Tree Crop Intensification in Horticulture Program for almonds , funded by Hort Innovation and led by Plant Food Research , is aiming to define these architectural traits and to exploit them in new growing systems . The overwhelming conclusion from this research undertaken in Australia and in California has been that practically all the existing sweet almond varieties grown are not well suited to more intensive growing systems . Of the varieties included in this research , only Shasta ® ( BA2 ) and Independence ® (‘ Alm-21 ’) trees appeared to be well suited to high-density growing systems . Both varieties are spur-bearing types and form relatively few , upright scaffold branches . When left unpruned in the nursery and planted in the field with a single trunk , they naturally produce a slender , narrow tree shape . Other standard varieties tested , including ‘ Nonpareil ’, ‘ Monterey ’, ‘ Carmel ’ and ‘ Price ’, and the newer varieties ‘ Vela ’, ‘ Carina ’, ‘ Maxima ’ and ‘ Capella ’, all produced several strong scaffold branches when planted as single-axis trees and left unpruned to express their natural growth habit . But even Shasta ® and Independence ® were not ideal because their strong spurbearing habit appeared to limit the number of fruiting sites and created large gaps in the tree canopies . A balance of medium-vigour lateral shoots and spurs appears to be the desired growth habit for consistently high yields . One of the advanced genotypes used in this research was chosen for its high yield efficiency and upright growth habit with a combination of spur and lateral fruiting wood . Plants of this genotype are now being grown in a high-density trial with new growing systems at the Almond Centre of Excellence in Loxton . Working with almond breeders in Australia , California and Spain , we have been developing systems to help quantify architectural traits and improve the efficiency of screening programmes . Options include :
• Evaluating seedling progeny on clonal rootstocks to give a more uniform response in terms of tree vigour and cropping ability .
• Screening young non-cropping trees while still in the nursery or in their first-leaf from planting which is when branching habit can first be observed ( Figure 1 ).
• Using first- and second-leaf trees to identify genotypes that naturally produce a strong primary trunk without dominant scaffold branching .
• Using second and third-leaf trees to record the survivability of axillary shoots growing in low light conditions , with high survivability being regarded as a positive trait ( Figure 2 ).
• Identifying genotypes that naturally produce long barren sections of wood , a negative trait which can first be observed in second and third-leaf trees ( Figure 3 ).
With these relatively simple techniques almond breeders will be encouraged to place a greater emphasis on tree architecture when they are selecting parents in their crossing programmes and when first screening seedling progeny from these crosses .
Figure 1 . Range of almond tree forms seen in first-leaf trees . Decurrent ( left ): when branches extend downward and the adult tree does not develop a strong central leader . Excurrent ( centre ): when trees form a symmetrical “ Christmas tree ” shape with a distinctive central leader . Columnar ( right ): when short lateral shoots form a narrow , compact tree canopy .
34 In A Nutshell - Winter 2023 Vol 24 Issue 2