Estimates of live-tree carbon stores in the Pacific Northwest are sensitive to model selection

The results of our study demonstrate that model-selection error may introduce 20 to 40% uncertainty into a live-tree carbon estimate, possibly making this form of error the largest source of uncertainty in estimation of live-tree carbon stores. The effect of model selection could be even greater if models are applied beyond the height and DBH ranges for which they were developed.Model-selection uncertainty is potentially large enough that it could limit the ability to track forest carbon with the precision and accuracy required by carbon accounting protocols. Without local validation based on detailed measurements of usually destructively sampled trees, it is very difficult to choose the best model when there are several available. Our analysis suggests that considering tree form in equation selection may better match trees to existing equations and that substantial gaps exist, in terms of both species and diameter ranges, that are ripe for new model-building effort.The rapid increase in atmospheric carbon dioxide (CO2) concentration is a major contributor to primarily anthropogenic global warming [1]. International agreements such as the Kyoto Protocol require participating nations to reduce CO2 and other greenhouse gas emissions. To implement such commitments, countries must produce nation wide inventories of carbon (C) sources and sinks. Forests can be both C sources and sinks, so there is interest in exploring forest C sequestration to offset anthropogenic CO2 emissions [2]. However, before sequestration potential can be assessed, the magnitude of forest C sources and sinks must first be determined.Live trees are a significant C storage pool in United States of America (US) forests, ranking second behind soil C [3,4]. Live-tree C is often estimated from regression equations that relate biomass (or volume subsequently expressed as biomass using density conversion factors) to some easily measured tree dimension obtained from inventory data, such as DBH (diameter a