Wood Specific Gravity: Variation with Height, Implications for Biomass

The specific gravity (SG) of wood is a measure of the amount of structural material a tree species allocates to support and strength. Wood SG has traditionally been a forester’s index of wood properties. In recent years, however, wood SG has been increasingly measured by ecologists exploring the functional traits of plants and by conservationists estimating global carbon stocks. While these developments have expanded our knowledge and sample of woods, the methodologies employed to measure wood SG have not received as much scrutiny as SG’s ecological importance.

In the recently published Wood Specific Gravity Variation With Height And Its Implications For Biomass Estimation, Michael Wiemann and G. Bruce Williamson demonstrate a nondestructive method of accurately estimating SG and subsequently determining woody biomass in standing trees. The authors accomplished this by determining wood SG in the field, a feat that hasn’t always been easy.

Wiemann says, “On?site measure­ments provide true values, eliminating potential biases from geographic variation and nonrandom samples in databases. However, determining SG in the field is time consuming and difficult because wood samples must be obtained by fell­ing trees or extracting cores.”

Historically, felling trees was widely employed by foresters, but in the mid?1800s Max Pressler invented the increment borer. To­day, extracting cores is generally preferable to felling trees as conservation and ongoing research demand a nondestruc­tive sampling methodology. In the report, the authors propose a method to estimate whole tree SG when complete tree harvest is impractical or impossible.

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Cross sections from an Acer saccharum (sugar maple) from Wisconsin, showing change in shape, size, and stem integrity with height. Borer is on the right.