Abstract:In estimating aboveground forest biomass (AGB), three sources of 15 error that interact and propagate include: (1) measurement error, the quality of the 16 tree-level measurement data used as inputs for the individual-tree equations; (2) 17 model error, the uncertainty about the equations of the individual trees; and (3) 18 sampling error, the uncertainty due to having obtained a probabilistic or 19 purposive sample, rather than a census, of the trees on a given area of forest land. 20Monte Carlo simulations were used to examine measurement, model and 21 sampling error, and to compare total uncertainty between models, and between a 22 phase-based terrestrial laser scanner (TLS) and traditional forest inventory 23 instruments. Input variables for the equations were diameter at breast height, total 24 tree height (defined the height from the uphill side of the tree to the tree top) and 25 height to crown base; these were extracted from the terrestrial LiDAR data. 26Relative contributions for measurement, model and sampling error were 5%, 70% 27 and 25%, respectively when using TLS, and 11%, 66% and 23%, respectively 28 when using the traditional inventory measurements as inputs into the models. We 29 conclude that the use of TLS can reduce measurement errors of AGB compared 30 to traditional measurement approaches. 31Keywords: Model error; sampling error; measurement error; Pacific Northwest 32 33