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Updating Canada’s National Forest Inventory with multiple imputations of missing contemporary data

Magnussen, Steen, Stinson, Graham, Boudewyn, Paul
The Forestry chronicle 2017 v.93 no.3 pp. 213-225
forest inventory, image interpretation, national forests, Ontario, Quebec, Saskatchewan
Canada’s National Forest Inventory (NFI) is facing an issue of spatial imbalance in photo interpreted data from 400 ha photo-plots available for estimation of state and change. Multiple imputations (MI) of missing data is therefore considered as a means to mitigate a potential bias arising from spatial imbalance, and—to a lesser degree— improve the precision relative to what can be achieved with the subset of plots having current data. In this study we explored MI with data from three study sites located in the provinces of Quebec, Ontario, and Saskatchewan. Specifically, we looked at state at time T₂ and change between T₁ and T₂ in cover-type area proportions and in per unit area stem volume. At each location we found significant T₁ differences in these attributes between plots with and without T₂ data. A MI procedure with 20 replications of stochastic model-based imputations of missing data was therefore effective as a way to mitigate a bias that would arise if T₂ inference was based exclusively on plots with T₂ data. Possible differences between the T₂ and T₁ photointerpretation, paired with no efficient stratification of disturbed and undisturbed plots, largely eliminated expected gains in precision from the MI boosting of the effective T₂ sample size. Despite recognized limitations, we recommend MI as an effective tool to counteract an emerging spatial imbalance in the NFI.