Tree crown

Tree crown
Forest Measurement and Modelling.

The size of a tree crown is strongly correlated with the growth of the tree. The crown displays the leaves to allow capture of radiant energy for photosynthesis. Thus, measurement of the tree crown is often made to assist in the quantification and understanding of the growth of the tree. The biomass of the crown and the quantity and quality of the branch material however is also of direct interest to ecological studies and research into the effects of trees on pollution. The crown also has great visual impact. This impact is important in urban forests because it will affect the perceived beauty of a tree.

Tree crowns are highly variable. Their general shape varies from relatively dense conoids for young and healthy trees of an excurrent habit to wide open hemispherical shapes for older trees of a deliquescent habit. The parameters selected for measurement will depend on the selected tree and the reason for measurement.

Crown width
(Sectional area)

Crown ratio
The width of a crown can be measured by projecting the edges of the crown to the ground and measuring the length along one axis from edge to edge through the crown centre. Unless the crown has a regular shape, the width measured will depend on the axis selected for measurement. If the crown width is being used to estimate sectional area (used in crown surface area and volume calculations) two axes are normally selected and averaged:
  • The diameter of the maximum and minimum axis of the crown;
  • The diameter of the maximum axis and the axis at 90 degrees;
  • The diameter of any two axes at 90 degrees to each other.

The two diameter measurements are averaged using an arithmetic mean (most common) or a geometric mean (for highly elliptical boles).

If the crown is highly irregular, the projected points from a number of crown edges should be plotted on scaled graph paper. A planimeter can be used to estimate sectional area.

If aerial photographs of an appropriate scale are available, crown width can be measured on the photograph and expanded by the photograph scale. Measurements of width from photographs often underestimate the width estimated from the ground because parts of the crown are too small for resolution or are obscured by other trees.

If the crown is clearly visible from the ground, a Criterion laser or ground-based photograph can be used to estimate the width of the axis perpendicular to the instrument. When using a photograph, the slope distance from the camera to the crown (D) and the focal length of the camera (f) define the scale of the photograph at the crown. The crown width is calculated as:

There is a strong linear relationship between crown width and bole diameter. This relationship, expressed as a ratio of crown width to diameter at breast height, is called the crown ratio.

Crown depth
Crown depth is the length along the main axis from the tree tip to the base of the crown. This length is normally determined using a hypsometer to determine the heights of the two points.

The base of the crown may be defined by:
  1. The lowest complete branch whorl or major branch that forms part of the canopy (used in calculating upper crown length)
  2. The lowest live branch, excluding epicormics or water shoots (used in calculating lower crown length)
The depth of the crown is often expressed as:
  • Crown length ratio (crown length divided by total tree height)
  • Green crown percent (Crown length ratio expressed as a percentage)

Crown surface area Crown surface area is a surrogate for the area available for leaves to capture radiant energy from the sun or atmospheric gasses and pollution. As the most actively photosynthetic leaves are the young leaves near the crown periphery, crown surface area is a useful index of growth.

Crown surface area (Ca) is calculated assuming the crown is a solid geometric shape with a measured crown depth (L) and crown width (D). One of three regular shapes are assumed.

These calculations ignore the surface at the base of the crown.

Crown volume
Enclosed volume
Crown volume is calculated in a similar way to crown surface area. Crown volume (Cv) is estimated from the crown width (D) and crown depth (L) after assuming one of three regular geometric shapes:

Twigs and leaves in the crown
Crown biomass
Detail about the surface area of the leaves, mineral and nutrient content, and the mass of leaves, twigs and branches within the crown are occasionally measured for a diverse range of needs.

These needs include:
  • Ecological studies
  • Estimation of the palatability of leaves to arboreal fauna
  • Estimation of growth and site
  • Atmospheric pollution mitigation (particularly urban trees)
  • Carbon sequestration estimation
  • Correlating reflectance from airborne or remote scanners with ground parameters

There are three major ways of estimating these details for a specific tree:
  • Weighing: Fell the tree and separate the leaves, twigs and branches. Measure the fresh or green weight of these components and the dry weight (and mineral content etc. if required) of a sub-sample of components. The ratio of dry : fresh weight is used to estimate the original tree's total mass. A variety of sampling approaches designed to estimate an unbiased ratio of dry : fresh weight have been used (Snowdon), but the most common approach is probably an ad hoc sample from components when they are initially separated.

  • Regression: Measure the diameter of the major branches one or two centimetres from the junction with the stem (immediately below the zone of rapid taper). Select a sample of these branches, determine the dry weight of the components for each sampled branch, and derive a correlation. Common relationships fitted include natural log of mass and branch diameter raised to a power. There is some suggestion that the regressions fitted are unique to each tree and time - coefficients change as the tree ages or moves into different growth stages as well as with changes in site and health.

  • Variable probability sampling: A powerful sampling approach that selects one (or more) places to cut a disk of wood and one (or more) terminal branches using a probability of selection proportional to size. Valentine et al (1984) and Gregoire et al (1995) published a method that utilised variable probability sampling systems for estimating the biomass of components of trees. They concluded that this method was easier to implement in the field than other sampling systems, in addition to being efficient and able to produce unbiased estimates of the mean and variance of the biomass estimates.

Crown health Tree health is not simply quantified. An index of health may be derived from a range of continuous and ordinal variables including:
  • Average elongation of twigs (over past two or three years)
  • Percentage of crown dieback
  • Presence of large dead branches and hollows
  • Presence of fungi fruiting bodies
  • Presence of termites or other nominated insects
  • Presence of mistletoe
  • Foliage discolouration
  • Crown shape (regular is healthier)

[temp.htm] Revision: 6/1999