The use of forest-derived specific gravity for the conversion of volume to biomass for open-grown trees on agricultural land

Accounting for agroforestry contributions to carbon sequestration and cellulosic feedstock production requires biomass equations that accurately estimate biomass in open-grown trees. Since equations for open-grown trees are rare and developing these is expensive, existing forest-based equations are an attractive alternative for open-grown trees in carbon accounting and biomass modeling. How accurate this alternative is depends on how similar the key attributes, such as specific gravity, trunk shape, and crown architecture, are between open- and forest-grown trees. We evaluated the use of forest-derived specific gravity for conversion of volume to biomass for morphologically distinct open-grown species: green ash, ponderosa pine, and eastern redcedar. Trunk biomass was consistently and significantly underestimated from 6.3% to 16.6% depending on species, indicating open-grown trees have greater trunk specific gravity than forest-grown counterparts within the same geographic region; however a conclusive difference in branch specific gravity was not found between open- and forest-grown trees. Open-grown trees have greater trunk specific gravity, sharper trunk taper, and larger crown. When forest-based equations are used for trunk biomass of open-grown trees, the greater trunk specific gravity results in underestimation; however, the sharper trunk taper results in overestimation. Studies are needed to examine whether the underestimation could be offset by the overestimation and how the larger crown affects biomass estimation when forest-based equations are used for open-grown trees. Our results provide an essential understanding to interpret the biometric relationship of open- to forest-grown trees and to develop an efficient means how forest-based equations might be best modified for open-grown trees.