For this FACTivity, you will answer the question: What is the relationship between tree height and the diameter at breast height (DBH)? In other words, when trees get taller, does the DBH get smaller, larger, or stay the same? Materials: Flexible tape measure or a length of string or rope and a yard stick Data log ( included ) Sample bar chart ( included ) Topics: Active Forest Management, Diameter at Breast Height (DBH), Measuring, Tree Inventory.