Photo credit: HSBC Bank community involvement project participants

Introduction

A dendrometer band placed around a tree. (Credit: SERC Staff)

We can measure the circumference (and calculated diameter) of a tree by wrapping a diameter tape¹ around its trunk. This gives a coarse measurement of the tree's size, but comparing measurements taken this way over short times involves several potential sources of error. These measurements are suitable for comparison over longer intervals of several years. One sort of comparison is to determine the rate of growth.

For precisely measuring changes in the circumference of a tree over shorter intervals, a simple, inexpensive device is a manual dendrometer band. Dendrometer bands allow us to make accurate observations of seasonal, weekly, or even daily variations in a tree's circumference. Our method is adapted from the system described in Keeland and Young (2007).

A dendrometer band is a narrow stainless-steel band that wraps around the trunk of a tree. A stainless-steel spring connects the ends of the band and keeps the band fixed tightly to the trunk. Once installed on a tree, the band remains in place and can last without rusting or deformation for many years.

A digital caliper. (Credit: SERC Staff)

The ends of the band overlap in a way that allows measurement of the distance between one end and a fixed point near the other end. That distance, or gap, between these points changes as the tree's circumference fluctuates and can be measured very accurately with a precision caliper. By comparing measurements of the gap width, we can track even small changes over very short periods.

Note that the dendrometer band doesn't measure the actual circumference of the tree – it allows us to measure change in the circumference. The actual circumference is measured with a diameter tape and recorded when the band is being installed. This gives a baseline measurement.

Because the digital calipers used to measure the gap are very precise (accurate to ±0.01 mm) the dendrometer system enables us to observe in detail the timing and rate of tree growth. These observations allow us to relate how growth responds to changing environmental influences such as light, available moisture and temperature.

¹ A diameter tape (or 'D-tape') is a measuring tape with two scales, one on each side. One side, called the circumference side, measures the distance directly with a familiar scale. The other side, called the diameter side, has a scale with values of the circumference side divided by pi (π) and is used to measure the diameter indirectly. So, for example, the 10cm mark on the diameter side corresponds to the 31.4 cm mark on the circumference side. (That is, the circumference of a circle with a diameter of 10 cm is approximately 31.4 cm.)