In forestry, a tree crown measurement is one of the tree measurements taken at the crown of a tree, which consists of the mass of foliage and branches growing outward from the trunk of the tree. The average crown spread is the average horizontal width of the crown, taken from dripline to dripline as one moves around the crown. The dripline is the outer boundary to the area located directly under the outer circumference of the tree branches. When the tree canopy gets wet, any excess water is shed to the ground along this dripline. Some listings will also list the maximum crown spread which represents the greatest width from dripline to dripline across the crown.Blozan, Will. 2004, 2008. The Tree Measuring Guidelines of the Eastern Native Tree Society. http://www.nativetreesociety.org/measure/Tree_Measuring_Guidelines-revised1.pdf Accessed March 4, 2013.Blozan, Will. 2006. Tree Measuring Guidelines of the Eastern Native Tree Society. Bulletin of the Eastern Native Tree Society, Volume 1, Number 1, Summer 2006. pp. 3–10.American Forests Measuring Guidelines. Accessed March 4, 2013. Other crown measurements that are commonly taken include limb length, crown volume, and foliage density. Canopy mapping surveys the position and size of all of the limbs down to a certain size in the crown of the tree and is commonly used when measuring the overall wood volume of a tree.
Average crown spread is one of the parameters commonly measured as part of various champion tree programs and documentation efforts. Other commonly used parameters, outlined in tree measurement, include height, girth, and volume. Additional details on the methodology of tree height measurement, tree girth measurement, and tree volume measurement are presented in the links herein. American Forests, for example, uses a formula to calculate Big Tree Points as part of their Big Tree Program that awards a tree 1 point for each foot of height, 1 point for each inch of girth, and point for each foot of crown spread. The tree whose point total is the highest for that species is crowned as the champion in their registry. The other parameter commonly measured, in addition to the species and location information, is wood volume. A general outline of tree measurements is provided in the article tree measurement, and more detailed instruction in taking these basic measurements is provided in "The Tree Measuring Guidelines of the Eastern Native Tree Society" by Will Blozan.
Largest recorded crown spreads Data on greatest crown spread is somewhat limited as this parameter is not measured as frequently as are tree height and trunk girth. The largest recorded is the "Monkira Monster" (Eucalyptus microtheca) located at the Neuragully Waterhole in southwestern Queensland, Australia, which measured 239 feet in 1954.Plant Champions – The Web Site. [6a The Thirteen Tree Species with the Broadest Crowns (Single Trunked).] Accessed March 8, 2013. A raintree (Albizia saman) in Venezuela was measured to have a crown spread of 207 feet in 1857. It reportedly is still living, but in poor health. A University of Connecticut siteUniversity of Connecticut. Samanea tubulosa (Benth.) Barneby & Grimes. Accessed March 8, 2013. suggests that in the wild they may have crown spreads up to 80 meters. Robert Van Pelt measured a crown spread of 201 feet of a kapok tree (Ceiba pentandra) at Barro Colorado Island, Panama, in 2003.
Crown spread methodologies
Cross-method The average crown spread is the average of the lengths of longest spread from edge to edge across the crown and the longest spread perpendicular to the first cross-section through the central mass of the crown. Crown spread is taken independent of trunk position. Spread should be measured to the tips of the limbs, not to "notches" in the crown shape, and at approximately right angles from each other.
Average crown spread = (longest spread + longest cross-spread)/2 The surveyor locates the point on the ground immediately below the branch tip on one end of the measurements and marks that position. He then moves to opposite side of the crown and locates the point under that branch tip. The spread along that line is the horizontal distance between those two positions. On steeply sloping ground (over 15 degrees) the taped distance between the two points can be corrected to a true horizontal by using basic trigonometry: horizontal distance = cos(inclination) × slope distance. The help of an assistant or the use of a laser rangefinder can greatly speed this process. When using a laser rangefinder, as in measuring tree height, several points on the far edge of the crown can be explored to find the furthest point. A laser rangefinder is also useful for measuring crown spread where one side of the crown is not easily accessible, such as a tree growing on a cliff side or other barrier.
Spoke method When using the spoke method, four or more measurements are taken from the outer dripline of the crown to the side edge of the trunk. The distance to the side edge of the trunk is for all practical purposes equal in length to the distance to the center of the trunk. If the measurement from the dripline of the crown to the side edge of the trunk is not level, then the measured length must be converted to a horizontal distance.
spoke length = cos(inclination) × (measured distance)
If the slope angle is less than 10 degrees the difference between the horizontal distance and the measured length will be less than 1.5% for slope distances less than 100 feet. These individual spoke lengths are averaged and this average is equal to half the average crown spread. The more spokes measured in the process, the more accurate the characterization of the average crown spread.
This is the preferred method of canopy researchers and is probably the most accurate, and can also be used to quantify crown area. On large trees it can be accomplished quickly with a laser rangefinder.
Another instance where a laser rangefinder and clinometer are useful is if the canopy is high off the ground. For example, white pine typically has a long bare stem, with branching beginning far up the trunk. In these cases a series of shots taken to the outer reaches of the branches standing at the side of the trunk can be used to calculate the length of spokes. In this case the angles will be steep, and the length of a spoke will be:
cos(inclination) × (laser-measured distance) = spoke length
Google Earth measurements With the increased availability of high-resolution air photos available through Google EarthGoogle Earth. https://www.google.com/earth/index.html Accessed March 8, 2013. http://www.ents-bbs.org/viewtopic.php?f=235&t=2388 Accessed March 8, 2013. crowns of individual trees can be distinguished, providing another option for measuring crown spread.NTS BBS, Multiple posts. April 2012. Google Earth to accurately measure canopy spread. http://www.ents-bbs.org/viewtopic.php?f=235&t=2388 Accessed March 8, 2013. Google Earth itself includes a ruler tool that can be used to measure diameters or spokes across the crown of the tree. Alternatively, the crown area can be measured and crown spread calculated from that value. EasyAcreageWildsoft. EasyAcreage for measuring are on a Windows PC. http://www.wildsoft.org/ Accessed March 8, 2013. is a Google Earth area measurement tool that calculates the area of any shape outlined on the Google Earth display. The perimeter of the tree can be traced on a satellite photo, and the software will calculate the area.
Average crown spread can be determined with a simple formula:
This area is the area of an equivalent circle. The measurement made through a remote sensing application may need to be checked in person.
LeverettLeverett, Robert T. February 20, 2013. Re: Revisiting Crown Area. http://www.ents-bbs.org/viewtopic.php?f=235&t=4996#p21920 Accessed March 4, 2013.Leverett, Robert T. February 20, 2013. Re: Revisiting Crown Area. http://www.ents-bbs.org/viewtopic.php?f=235&t=4996&start=10#p22023 Accessed March 4, 2013. has also provided four options for measuring the crown area through compass and clinometer surveys around the outer edge of the crown or through a combination of measurements from the edge of the crown and to the trunk, and those around the crown perimeter. All four circumscribe the crown area's drip-line with a polygon and divide the polygon into a series of adjacent triangles, measure the area of each triangle and sum them. One option, the Polygonal Method, measures each side of a triangle to compute its area. The second and third methods uses azimuths and one distance to the trunk calculate the area. The fourth method, the Azimuth Method, requires only azimuths and distance measurements from point to point around the crown perimeter.
Polygonal method The measurer walks the perimeter of the crown following the drip line fairly closely. Points are identified on the ground that represent the outline of the crown and marked in a way so that the next point is always visible from the prior one. For the first point, the distance to the center of the tree's trunk is measured along with the vertical angle to the point. Then the distance to the next exterior point is measured along with the vertical angle. The measurer moves to that next exterior point and repeats the process, continuing clockwise until the crown is encircled. The last leg of the first triangle becomes the first leg of the second triangle, and so on, so only the first triangle requires measurements for all three legs. The result is a series of adjacent triangles with the sides determined. The area of each triangle is computed from its sides, and the sum of the areas computed. Each triangle covers part of the crown area. The sum of the triangles equals the total projected crown area. The instruments needed include a laser rangefinder or a tape measure and a clinometer.
Azimuth method In the fourth method the measurer does not interact with the trunk or any internal point of the polygon. The measurer walks the perimeter shooting horizontal distances and azimuth to the next point until the crown's perimeter is circled. This is the simplest and most flexible method of the four. This method can also easily be used to measure the areas of other features encountered, for example, tree groupings or vernal ponds.
Maximum crown spread This is another measurement that is sometimes collected. Maximum crown spread is the maximum width of the crown along any axis from the drip-line on one side of the tree to the dripline on the opposite side of the tree.
Crown density The USDA Forest Service has published a guidance documentUSDA Forest Service. 2010. Phase 3 Field Guide – Crowns: Measurements and Sampling, Version 5.0, October, 2010 http://www.fia.fs.fed.us/library/field-guides-methods-proc/docs/2011/field_guide_p3_5-0_sec23_10_2010.pdf Accessed March 8, 2013. on field evaluations of a variety of crown characteristics beyond that of the normally taken basic measurements. Included are a series of definitions of terms, crown shape, crown density/foliage transparency, un-compacted live crown ratio, vigor class, and various dieback evaluations.