Filling the “vertical gap” between canopy tree species and understory shrub species: biomass allometric equations for subcanopy tree species

Subcanopy tree species are an important component of temperate secondary forests. However, their biomass equations are rarely reported, which forms a “vertical gap” between canopy tree species and understory shrub species. In this study, we destructively sampled six common subcanopy species ( Syringa reticulate var. amurensis (Rupr.) Pringle, Padus racemosa (Lam.) Gilib., Acer ginnala Maxim., Malus baccata (Linn.) Borkh., Rhamnus davurica Pall., and Maackia amurensis Rupr. et Maxim.) to establish biomass equations in a temperate forest of Northeast China. The mixed-species and species-specific biomass allometric equations were well fitted against diameter at breast height (DBH). Adding tree height (H) as the second predictor increased the R 2 of the models compared with the DBH-only models by –1% to + 3%. The R 2 of DBH-only and DBH-H equations for the total biomass of mixed-species were 0.985 and 0.986, respectively. On average, the biomass allocation proportions for the six species were in the order of stem (45.5%) > branch (30.1%) > belowground (19.5%) > foliage (4.9%), with a mean root: shoot ratio of 0.24. Biomass allocation to each specific component differed among species, which affected the performance of the mixed-species model for particular biomass component. When estimating the biomass of subcanopy species using the equations for canopy species (e.g., Betula platyphylla Suk., Ulmus davidiana var. japonica (Rehd.) Nakai, and Acer mono Maxim.), the errors in individual biomass estimation increased with tree size (up to 68.8% at 30 cm DBH), and the errors in stand biomass estimation (up to 19.2%) increased with increasing percentage of basal area shared by subcanopy species. The errors caused by selecting such inappropriate models could be removed by multiplying adjustment factors, which were usually power functions of DBH for biomass components. These results provide methodological support for accurate biomass estimation in temperate China and useful guidelines for biomass estimation for subcanopy species in other regions, which can help to improve estimates of forest biomass and carbon stocks.