In 1998, a new protocol was instituted for measuring the diameters of every tree greater than 5 cm dbh in the coniferous and deciduous forest plots. Trees are usually measured in January. Exponential equations are used to estimate tree mass for each year. The goal is to estimate wood production for any given time period (usually one year) as the difference between two estimates of mass.
original data source http://lter.kbs.msu.edu/datasets/48