Analyzing the Uncertainties in Use of Forest-Derived Biomass Equations for Open-Grown Trees in Agricultural Land

Quantifying carbon in agroforestry trees requires biomass equations that capture the growth differences (e.g., tree specific gravity and architecture) created in the more open canopies of agroforestry plantings compared with those generally encountered in forests. Whereas forest-derived equations are available, equations for open-grown trees are not. Data from destructively sampled open-grown trees in the Northern Great Plains were used to examine the uncertainties in the use of forest-derived equations for open-grown trees. Three species, representative of major morphological types of agroforestry trees, were studied: green ash, Austrian pine, and eastern redcedar. Forest-derived equations provided good estimates of trunk biomass at lower diameter ranges but, as diameter increased, resulted in overestimation up to 40% for individual trees. Across the full diameter ranges, individual tree branch biomass was underestimated by 29-82%, depending on species and equation source (regional or nonregional). Although open-grown trunk and branch biomass curves diverged down and up, respectively, from their forest-derived counterparts, those for the whole tree tended to converge, albeit significantly above the forest-derived curves. Whole-tree biomass for individual trees was underestimated by at least 18%. To correct the biases, we studied the adjustment factor of forest- to open-grown tree biomass. It shows a power function with diameter. On a whole-tree basis, it was evaluated as a constant (1.2) independent of species and diameters. Application of this constant factor adjusted the biomass underestimation of three-species-mixed plantation by forest-derived equations from 2110 4.6%, providing a cost-efficient approach to use forest-derived equations for open-grown trees in agriculture land.