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).
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.
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.
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).
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