Estimating tree crown size

I co-authored an academic paper in 2005 that summarised research undertaken to explore the relationship between a tree’s stem diameter and its crown (or canopy) diameter 1.  Out of my 60 or so publications, it has been one of the most popular among forest scientists (e.g. Google Scholar citations).

Tree crown diameters

It was fascinating to discover that statistically there was a very good relationship (scientists would refer to a correlation from a regression analysis) between stem diameter and crown diameter.  We decided to explore this further by calculating the ratio between the two, we called it the z ratio (= crown diameter ÷ stem diameter).  We then plotted this z ratio against stem size.  You can see the result on the graph below for nine common European broadleaved trees.

Crown diameters for different tree species

Tree crown and stem diameter ratio graph
Crown diameter: stem diameter relationship for nine broadleaved tree species. The z ratio (y axis) is crown diameter divided by stem diameter; the dbh (x axis) is stem diameter at breast height (measured at 1.3m). Click to enlarge graph

The graph highlights some very interesting growth patterns and difference between different species:

  • Common walnut (Juglans regia) has the largest crown diameter at any given stage in its stem size.  When a walnut stem is 15 cm in diameter its crown can be estimated to be 5m wide.  Foresters can use that knowledge to design walnut plantations: e.g. if they plant their walnut trees 5m apart, their crowns will not compete until their stem diameter is 15 cm (which will take about 15 years from planting in the UK).
  • Sweet chestnut (Castanea sativa), like walnut, has a very large crown while it is young (with a small stem size).  Unlike walnut however, as its stem size increases, the ratio with its crown diameter decreases rapidly to the point after 35cm in diameter, when it has the smallest crown diameter for any of the nine tree species assessed.
  • Sycamore (Acer pseudoplatanus) has the most consistent crown to stem ratio while it grows.

basal area per hectare (G, m2 ha-1)The data can be used to plan tree spacings and to calculate basal area.  For example: for walnut with a stem diameter of 0.60m, its crown diameter is 13.27m, and its z ratio is 22.12.   Using the equation (left) for estimating basal area per hectare (G, m2 ha-1) tells us that there would be 57 trees per hectare with a basal area of 16.1 m2 ha-1.

These findings can be used beyond tree spacings and calculating basal area; they can also be used to help in:

  • planning thinning regimes (how many trees to remove in a growing plantation and when)
  • planning stand density (how many trees to retain in a forest stand at any given size)
  • assisting in managing mixed conifer-broadleaved stands
  • estimating branchwood  and woodfuel volumes
  • maintaining free-growth silvicultural systems, and
  • in urban tree planning by arboriculturists and landscape gardeners (e.g. designing and managing tree avenues).

Gabriel Hemery


Reference

1 Hemery, G.E., Savill, P. & Pryor, S.N. (2005).  Applications of the crown diameter – stem diameter relationship for different species of broadleaved trees. Forest Ecology and Management 215, 285-294. View abstract

6 thoughts on “Estimating tree crown size

  1. Another very good article with plenty of practical applications for the professional forestry worker.
    As a volunteer verifier for the Ancient Tree Hunt it would also be useful for assessing the condition of the ancient trees but as I noted that the paper with the actual methods etc, costs a small fortune. Not that I begrudge you your due as this sort of research takes a lot of money to conduct. Even the simplest research can cost what appears to be a disproportionate amount of money hence why a lot of the interesting stuff doesn’t get done because there is no profitable outcome.

    So my question is this. Is there a simple formula that can be used by me in the field to calculate the crown/trunk ratio of a particular tree species and then gauge (calculate) the actual crown size and compare it to the expected crown size so that any degredation due to declining condition could be better identified? This could be used as an extension to the rest of the parameters currently used to asses the condition of the ancient trees we survey in the UK which include the usual stag heading, hollowing trunk and branches as well as tree fungi of various sort..
    Also will you be putting together a blog on the various types of tree fungi and other parasitic tree growths? This is one area of trees that I find most interesting along with lichens.
    All information would be greatly appreciated.

    1. Thanks Kevin

      Your comment regarding the inaccessibilty of published research is spot on and one of the main inspirations for my blog. Unfortunately the payment that journals charge does not come back to the scientist/authors. The way it works is that you spend many days or even years gathering evidence, days or weeks crunching data, and days or weeks writing the draft text. There then follows a long wait while anonymous peer scientists review the paper and the editor of the journal has his/her say. Usually there will be more work required to make the paper acceptable for publication. Then, unless the author pays a considerable fee to the journal, it is only accessible to academic institutions or others that have paid for a subscription. So, the benefits to the scientists/authors are certainly not monetary, only an ability to advance scientific understanding in your chosen field and perhaps to build a research record for your career and credit for your institution.

      As to your question I will do some thinking on this for you. There is an issue where the natural dieback of trees with age will skew the simple relationship I describe. However, as you suggest for those trees that are not veterans it would be interesting to use this data to assess tree health. When I get time I will look at the original data and see what I could produce.

      Regarding fungi, the simple answer is yes.

  2. Many thanks for the feedback. Looking forward to the next instalment on fungi and also any info you come up with regards canopy size and tree health.

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