title: Effects of field plot size on prediction accuracy of aboveground biomass in airborne laser scanning-assisted inventories in tropical rain forests of Tanzania.
abstract: BACKGROUND
  Airborne laser scanning (ALS) has recently emerged as a promising tool to acquire auxiliary information for improving aboveground biomass (AGB) estimation in sample-based forest inventories. Under design-based and model-assisted inferential frameworks, the estimation relies on a model that relates the auxiliary ALS metrics to AGB estimated on ground plots. The size of the field plots has been identified as one source of model uncertainty because of the so-called boundary effects which increases with decreasing plot size. Recent research in tropical forests has aimed to quantify the boundary effects on model prediction accuracy, but evidence of the consequences for the final AGB estimates is lacking. In this study we analyzed the effect of field plot size on model prediction accuracy and its implication when used in a model-assisted inferential framework.
  
  
  RESULTS
  The results showed that the prediction accuracy of the model improved as the plot size increased. The adjusted R2increased from 0.35 to 0.74 while the relative root mean square error decreased from 63.6 to 29.2%. Indicators of boundary effects were identified and confirmed to have significant effects on the model residuals. Variance estimates of model-assisted mean AGB relative to corresponding variance estimates of pure field-based AGB, decreased with increasing plot size in the range from 200 to 3000 m2. The variance ratio of field-based estimates relative to model-assisted variance ranged from 1.7 to 7.7.
  
  
  CONCLUSIONS
  This study showed that the relative improvement in precision of AGB estimation when increasing field-plot size, was greater for an ALS-assisted inventory compared to that of a pure field-based inventory.
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, Oslo, NO 1432, Ås Norway.
authors.forename: Ernest William
authors.initials: EW
authors.lastname: Mauya
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, Oslo, NO 1432, Ås Norway.
authors.forename: Endre Hofstad
authors.initials: EH
authors.lastname: Hansen
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, Oslo, NO 1432, Ås Norway.
authors.forename: Terje
authors.initials: T
authors.lastname: Gobakken
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, Oslo, NO 1432, Ås Norway.
authors.forename: Ole Martin
authors.initials: OM
authors.lastname: Bollandsås
authors.affiliation: Department of Forest Mensuration and Management, Sokoine University of Agriculture, P.O. Box 3013, Morogoro ᅟ, Tanzania.
authors.forename: Rogers Ernest
authors.initials: RE
authors.lastname: Malimbwi
authors.affiliation: Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, Oslo, NO 1432, Ås Norway.
authors.forename: Erik
authors.initials: E
authors.lastname: Næsset
keywords: Aboveground biomass; Airborne laser scanning; Model-assisted estimation; Plot size