Description

How we measure tree growth

Most measurements of tree diameter are made using a tape measure wrapped around the trunk at a standard height above ground.  This measure is called the 'diameter at breast height' (DBH).  However, the use of the tape measure is subject to several errors, such as inconsistent placement of the tape on the tree and the limited resolution of the measuring tape.  The errors are large enough that annual tree growth can't be reliably estimated.  Because of these uncertainties, trees are typically only remeasured every few years.  A consequence of this long interval is that it is hard to relate the change in diameter to the many variations in weather and other environmental factors that can occur between measurements.

An alternative method, using manual band dendrometers, can provide the consistent, accurate and short-term measurements needed.  A dendrometer band consists of a long strip of stainless steel that wraps around the trunk, held tightly against the trunk with a strong steel spring.  One end of the band overlaps with a rectangular open section on the other end, revealing a gap where one can see the trunk.  As the tree grows in circumference the band end recedes and the gap in the open section grows wider.  The increase in the width of the gap is equivalent to the increase in the tree circumference.  And, as the gap width can be measured very precisely (to within 0.01 mm, with a digital caliper) the dendrometer band provides a way to make detailed and repeatable measures of a tree's size and growth over many years.

We intended to use the dendrometers primarily to supplement the typical tape measure approach with annual observations.  But the method may be used to measure tree sizes at many time scales: weekly, daily and even hourly.  We discovered that changes in tree size are sensitive to different environmental effects at each of these scales.  At each scale, the trees show coordinated changes in diameter – all sizes of trees in an area show the same pattern of change, although the pattern in the smaller trees is reduced compared to the pattern in the large ones.   Over a year the total growth is most sensitive to the amount of rainfall in the early spring.  And as precipitation changes from year to year, total growth also varies widely. Within a year, the timing and amount of growth seems to depend on the trees' access to light – trees with well-illuminated crowns start growth sooner and grow more than do those are shaded.  Over many days the tree diameter is very sensitive to changes in soil moisture.  During a dry period, the diameter change may slow down, or even stop, if the drought continues long enough.  A tree stem's diameter even fluctuates on a daily schedule – they are larger in the early morning and shrink somewhat during the day.  Overall, we've found trees are far more sensitive than previously imagined – they can provide a useful barometer of forest responses to climate factors.

Since we began our dendrometer measurements in 2009, the Forest Ecology lab has expanded the use of this method to projects focused on many different factors at SERC and to comparisons of tree growth among other sites, such as those in the Smithsonian Forest Global Earth Observatory (ForestGEO).

Make and use your own dendrometer bands

Read the instructions on how to make a dendrometer band, which include how to install the bands and make measurements and how to calculate estimates of tree size and growth.  You will also find an example dataset on the growth of several trees at SERC using dendrometer bands, and some historic data on local climate if you want to explore.