Quantifying the tree crown ©

The main functions of the crown of a tree are:

to display actively photosynthesising leaves most efficiently to radiant energy and
to provide for the renewal of the leaves.

The size of a tree crown has a marked effect on, and is strongly correlated with, the growth of the tree and of its various parts. Measurement of crown parameters must obviously concern the forester.

The dimensions of tree crowns most commonly measured are:

diameter or width
depth or length
- height to the lowest green branch or complete green whorl
- crown ratio
crown sectional area
crown surface area
crown volume

Crown diameter or width

For many species, particularly conifers occurring in open-grown, even-aged stands, crown diameter is closely correlated with stem dbhob. For example, Leech (1984) established that crown width for open-grown radiata pine in South Australia is linearly related to tree dbhob and that the relationship is independent of site quality. For 109 observations, the weighted estimate of the relationship established was:

		CW  =  0.7544  +  0.2073 d
		      (0.0848)    (0.0032)
		where CW is crown width (m) and d is dbhob (cm)

. Leech's model can be used as the base for an index of stand density called Crown Competition Factor (CCF) which he suggests may be useful in studies of the growth and yield of radiata pine because the CCF is independent of age and site.

Because of the strong correlation between crown diameter and dbhob, tree volume tables based on crown diameter (in place of dbh) can be compiled for use with aerial photographs.

Crown diameter measurement

on the ground - project the edges of the crown vertically to ground using a plumb bob, crownometer, optical prism, etc. Readings can be recorded to 0.1 m. For some purposes, subjective judgement of the projection may be sufficient - record such estimates to the nearest metre.
on aerial photographs. Accuracy of measurement is dependent on image scale, film resolution and individual ability.

Results from (1) are sometimes greater than those from (2) because with photo measurements, only that part of the crown visible from above is measured, i.e. parts of the crown may be obscured by branches of other trees, etc.

Usually, crown diameter assessment is based on measurement of two or more diameters. Very small individual branches and minor crown irregularities are usually ignored.

The ratio of crown diameter to tree dbhob is called the CROWN RATIO. This ratio is of silvicultural interest and is used in photogrammetry to deduce tree dbhob from crown width.

Crown depth or length

Crown depth or length is usually determined with a direct or indirect measuring height instrument. Crown length is sometimes expressed as Green Crown % or "crown length ratio". (Do not confuse with "crown ratio" which is the ratio of crown diameter and dbhob of the bole).

Green crown % = 100 l/h

where l is the length of the living crown (m) and h is total height of the tree (m)

Note that l can be measured either to the lowest live branch-whorl (upper crown length) or to the lowest live branch, excluding epicormics (lower crown length).

Crown sectional area

If the crown is regular in outline, calculate sectional area from the measured diameter. If irregular, plumb the edges to ground, measure several axes, and plot and determine the area by planimeter. Repeating this process for a number of trees in a stand gives one estimate of crown closure.

Crown surface area and volume

The surface area and volume of a tree crown are derived from crown diameter and length assuming some solid shape appropriate to the particular tree, e.g. conic, parabolic, hemispherical.

The most active photosynthetic area in a tree crown comprises the young leaves at or near the crown surface. Thus, the surface area of the crown should be a useful parameter in growth prediction.

Usually, the crown of conifers and young hardwoods is modelled as a cone, its surface area being derived from the formula:

			CA  	=  pi*dbL/2
	where	CA 	=  crown surface area (m2)
			 db  	=  diameter at the base of the crown (m)
			  L   	=  sloping length from apex to base of crown (m)

Similarly, crown volume should be a useful predictor of tree growth. It, too, is modelled as a cone in conifers and young hardwoods, its volume being derived from the formula:

		CV 	=  pi*db^2*CD/12
	where   CV 	=  crown volume (m3)
			CD  	=  crown depth (m)
	      and   db 	=  as before.

Crown closure (= canopy closure)

Crown closure is a concept applied to even-aged stands or to the upper canopy level (dominant / codominant stratum) of uneven-aged stands. It is expressed as a proportion or percent of the ground area covered by the vertical projection of the tree crowns.

Crown closure is an approximate indicator of stand density. For this reason, it is an important variable in the estimation of stand volume from aerial photographs and in evaluating silvicultural operations and ecological conditions. It has a significant influence on snow pack accumulation and snow melt.

Instruments used to measure crown closure include the crownometer, spherical densiometer (Lemmon 1957 ), and moose horn (Robinson 1947 ; Garrison 1949 ).

Note: Use the term crown density to refer to the total bulkiness of tree crowns, which is the concept usually intended in its use in silviculture. Crown density incorporates both crown closure and foliage quantity.

Crown weight

Crown weight is important in biomass studies and in assessing the weight of slash left after logging operations. Crown weight can be related to dbhob, site and stand density but only limited information of this type is available for Australian forest stands. Some years ago, a concerted effort was made by the Division of Forest Research, C.S.I.R.O. to fill this gap.

Siemon et al. (1980) examined the effect of thinning on the distribution and biomass of foliage in the crown of 23-year-old radiata pine in Green Hills State Forest, Tumut, N.S.W. Regression equations relating needle dry weight to branch cross-sectional area were developed and used to examine the distribution of foliage of various ages within the crowns. They found that:

In the upper crown (70%-80% height decile and above), the percentage of 1-year foliage increased acropetally from 52% to 75% whereas that of 2-year and 3-year and older foliage decreased slightly (28% to 22%) and markedly (20% to 3%) respectively.
In the middle and lower crown, i.e. all deciles below the 70-80% decile, the distribution of foliage across age classes was approximately constant at 37% (1-year), 28% (2-year) and 36% (3-year and older). The combined biomass of one- and 2-year leaves within the whole crown averaged 73% of total leaf biomass under all thinning regimes.
Though stand density had little effect on proportionate distribution of foliage by position in the crown or leaf age, the total amounts of foliage varied greatly.
Total foliage biomass ranged from 4.9 to 11.3 tonnes/ha and annual foliage production from 2.4 to 4.3 tonnes/ha in stands of mean stand density ranging from 15 m2/ha (biomass) or 21 m2/ha (annual production) to 46 m2/ha.

A reasonable total leaf biomass for Pinus forest is 5 to 6 tonnes dry weight/ha. This estimate is obviously conservative for good quality radiata pine stands in Australia (and New Zealand) particularly stands with basal area exceeding 30 m2/ha.

http://online.anu.edu.au/Forestry/mensuration/CROWN.HTM
Cris.Brack@anu.edu.au
Tue, 7 Jan. 1997