Comparison of decay classification, knife test, and two penetrometers for estimating wood density of coarse woody debris

Inventories of the necromass of coarse woody debris typically involve measurements of density (e.g., kilograms per cubic metre) on a sample of logs, with densities of other logs estimated based on assignment to decay classes. Here, we compare two new devices for assessing density of woody debris, a spring penetrometer and a dynamic penetrometer, with the traditional decay classification and knife test in terms of the strength of the relationship with measured density and the consistency in measurements by four different people. Our evaluation was conducted in a diverse tropical forest and involved only a brief training period in each method. Classifications or scores from all four methods were only weakly correlated with measured density, and consistency among technicians in the measurement-density relationship was highest for the dynamic penetrometer. Therefore, we conclude that when training time is limited and the sampled logs can reasonably be assumed to be representative of all of the logs (e.g., an inventory of one site at one time), it is best to simply assume that the average density of the sampled logs is representative of nonsampled logs. For inventories involving multiple people, limited training, and cases where the sample average is likely to be unrepresentative, we recommend the dynamic penetrometer.