Correction of stand variable estimates obtained by the stand reconstruction technique: can stump information improve the predictions?

Information on thinned tree stumps was included in a stand reconstruction technique to test possible improvements in the estimates of stand variables (aboveground biomass, total stem volume, stem volume growth and stand density). Thirty sample trees and one hundred and sixty-eight stumps of the Sakhalin fir Abies sachalinensis (F. Schmidt) Mast., the Ezo spruce Picea jezoensis (Siebold & Zucc.) Carrière, and Glehn’s spruce Picea glehnii (F. Schmidt) Mast., were collected in six stands of pure tree species within the Hitsujigaoka Experimental Forest in Hokkaido, Japan. Stem analysis data and census data both gathered in 2013 from six stands were used to estimate stand variables in the past. Then, the stand variables were estimated by the stand reconstruction technique, with and without the stump information and subsequently compared in terms of prediction accuracy. In other words, the reconstructed values were statistically compared with the observed values obtained from censuses between 1988 and 2013. The results showed that the accuracy of the estimated variables can be improved by alleviating underestimation after adding old stumps. Without adding data on the stumps, the percentage error of the estimates of the stand variables varied within ± 20 % of the observed values. By including the stumps, the percentage error of the estimates of the same stand variables generally fell within ± 15 % for the years after 1997. The 95 % confidence intervals of the estimated means by the bootstrap method suggested that adding stumps does not always improve the prediction in stand density; but generally, improves the predictions on aboveground biomass, stem volume and stem volume growth. Overall, dramatic changes in the aboveground biomass and stand density through thinning operations were reproduced better, although the amount of improvement is sometimes minimal, by incorporating information on the stumps for all 3 species examined.