Standard and non-standard measurements of wood density: how big is the breach?

A crucial functional trait related to plant strategies is wood density. Wood density is determined as the ratio between the wood dry mass and its fresh volume. Standard laboratory methods for determination of wood density are the norm, but other non-standard methods can also be used, especially in the field. At present, we do not know how accurate these non-standard methods might be. This research compared standard and non-standard methods for determination of wood density while taking into consideration particular plant growth forms that are often neglected in wood density studies (e.g. vines, short shrubs).

Wood density was estimated and compared using the standard methods (water displacement for volume and oven drying at 101 °C for 72 h for mass) and non-standard methods (geometric for volume and oven drying at 70 °C for 72 h for mass) for 153 species with different growth forms and taxonomic orders.

Across species and growth forms, wood density did not vary as a result of the drying temperature. However, wood density was underestimated on average by 5.36 % when the volume determination was done using the geometric instead of the water-displacement method. The s.d. of wood density was also significantly higher when using the geometric method for volume determination. These differences in wood density estimation were not altered by growth form, taxonomic order or the size of the woody sample.

It is statistically reliable to estimate wood density following the drying of samples at 70 °C for 72 h. The difference in the amount of water that remains in the wood after drying between 70 and 101 °C for 72 h is negligible. However, the geometric method of determination of wood volume is likely to incur a 'positive bias' of overestimation attributable to error in the estimation of wood density.