The effect of the calculation method, plot size, and stand density on the top height estimation in natural spruce-fir-broadleaf mixed forests

Abstract

Stand dominant height or top height (TH) is a crucial variable for the measurement of potential site productivity, forest growth and yield projection, biomass estimation, structure stratification, and determination of development stages. There are variable definitions of TH, and it is affected by calculation method and stand characteristics. These issues have been examined for pure forests, but there is a knowledge gap on the estimation of TH for natural mixed forests with heterogenous and complex structure. We examined the effect of the calculation method, plot size, and stand density on the TH estimation in natural spruce-fir-broadleaf mixed forests in northeast China. The data were from 12 one-hectare plots with 10,195 height measurements. There were 14 calculation methods derived from the combination of four TH estimation methods (Conventional estimate; Adjusted largest trees; U-estimator, Sub-plot estimation) and four optional tree selection strategies with the consideration of tree species and tree size. Sample plots, with an area ranging from 0.01 ha to 0.50 ha with the interval of 0.01 ha, were randomly selected, and repeated 200 times with a bootstrapping method. In total there were 120,000 subplots generated from the 12 one-hectare plots. Results revealed that there was a strong correlation among the estimate methods, but the TH estimates were significantly different with the difference ranging from 0.02 m to 9.49 m. A threshold of plot size for stabilizing the TH estimate ranged from 0.05 ha to 0.1 ha. Stand density had complicated effects on TH estimations. The uncertainty for plot area and stand density was quantified. There were weak correlations between TH and stand volume, biomass and the periodic basal area increment, but moderate and high correlations with stand arithmetic and weighted mean height, respectively. We recommend the adjusted tallest trees method irrespective of tree species as the optimum for practical application for mixed forests.