Tree Structure & Light Capture
The woody components of a tree (branches and trunk) make up the majority of the above ground biomass, and serve two important functions: 1) as structures to orient leaves to enable maximum light capture, and 2) as “tubes” along which water, nutrients and sugars are transported among the leaves, flowers, fruits, and roots.
The shape and distribution of branches defines tree structure. The structure of a tree is governed by physiological processes within the tree and environmental factors, both of which determine the extent to which pruning and training practices can be used to achieve ideal conditions for fruit and nut production. In this section, we will review the basic processes that govern tree growth and architecture. With an understanding of basic tree biology, anyone can go out and make observations of trees, deduce important facts about how that tree grows annually, and predict where flowers and fruit are produced in the canopy.
Watch these animations for a overview of the patterns of tree growth:
Tree Growth1, Tree Growth 2
Basic Branch Anatomy
Trees are primarily composed of dead woody tissue that provides structural support for leaves and fruit. Embedded within the woody support tissue are points of living tissue, called meristem, which produce new branches, leaves, and flowers. Apical and lateral meristem are two primary types of meristem in trees that determine architecture, and play different roles in determining the overall shape of a tree canopy.
- Apical meristem is found at the tips of growing shoots, while
- Lateral meristem is found in buds that occur along branch edges (Figure 8).
Apical Dominance & Branch Growth Patterns
The presence of apical meristem at the tip of a branch suppresses development of lateral meristem further down the branch. The suppression of lateral meristem by apical meristem is called apical dominance. Since apical dominance suppresses lateral meristem growth, the majority of lateral meristem buds do not go on to develop into shoots. However, if the apical meristem is removed, it eliminates apical dominance in that branch and lateral meristem will develop into new shoots during the following growing season. The Pruning and Training section of the website contains more information about the relationship between apical dominance and common pruning and training techniques in new and fully established trees.
Constraints on Branch Growth Imposed by Limited Resources
The distribution of resources within a branch is another important determinant of branch anatomy. Although some of the glucose produced by photosynthesis is transported to the roots, most glucose travels only short distances to local sinks (including nearby fruit and growing wood). As a result of apical dominance, the majority of available resources in a branch are allocated to shoot tip extension and growth at the expense of lateral branch production. Because sugars are primarily taken up and used for growth in nearby tissues, branches are tapered and no lateral branch will exceed the diameter of the primary branch from which it is produced.
Branch Anatomy & Apical Dominance Combine to Determine Tree Architecture
The overall form of a tree can be described based on simple observations of the pattern and distribution of branch growth along the main trunk. Specifically, the relative positions of branches as they are produced each year, the number of shoots produced per branch each year, and the angle at which branches are produced from the primary stem. Subtle changes in any of these factors can result in dramatic differences in overall tree architecture (Figures 9 & 10). Opposite branches are produced in successive pairs on opposite sides of the main stem. Both opposite and spiral arrangements minimize self-shading of lower leaves by upper leaves. Almost all tree fruit and nut crop species currently grown in California follow a spiral branch placement pattern, although branch angle and apical dominance vary substantially.
There are two important factors which determine overall tree structure. First, the number of lateral shoots produced per year, or the strength of apical control, has an important impact on overall canopy structure. A tree with weaker apical dominance will produce more lateral shoots per year and result in a dense canopy (Fig 10a), whereas a tree with stronger apical dominance will produce fewer lateral shoots per year and a less dense, and open, canopy (Fig 10b). Second, trees that produce branches at very narrow angles have an upright growth habit and a vertical canopy (Fig 10b, c and e). Intermediate branch angles result in a rounder intermediate tree shape common in fruit and nut tree crops (a, d and g). In contrast, a combination of very wide branch angles and less rigid branches results in a downward “weeping” growth habit (Fig 10f).