Measuring tree height: a quantitative comparison of two common field methods in a moist tropical forest

1. Tree height is a key variable for estimating tree biomass and investigating tree life history, but it is difficult to measure in forests with tall, dense canopies and wide crowns. The traditional method, which we refer to as the 'tangent method', involves measuring horizontal distance to the tree and angles from horizontal to the top and base of the tree, while standing at a distance of perhaps one tree height or greater. Laser rangefinders enable an alternative method, which we refer to as the 'sine method'; it involves measuring the distances to the top and base of the tree, and the angles from horizontal to these, and can be carried out from under the tree or from some distance away. 2. We quantified systematic and random errors of these two methods as applied by five technicians to a size-stratified sample of 74 trees between 5.7 and 39.2m tall in a Neotropical moist forest in Panama. We measured actual heights using towers adjacent to these trees. 3. The tangent method produced unbiased height estimates, but random error was high, and in 6 of the 370measurements, heights were overestimated by more than 100%. 4. The sine method was faster to learn, displayed less variation in heights among technicians, and had lower random error, but resulted in systematic underestimation by 20% on average. 5. We recommend the sine method for most applications in tropical forests. However, its underestimation, which is likely to vary with forest and instrument type, must be corrected if actual heights are needed.