- Ward, James C.
USDA Forest Products Laboratory, Madison, WI
Lumber containing wetwood, or sinker heartwood, cannot be dried as rapidly as lumber with normal wood. To determine why wetwood dries more slowly, measurements of longitudinal gas permeability (LGP) were made in sapwood, heartwood, and wetwood from white fir (Abies concolor) and aspen (Populus tremuloides and P. grandidentata). The LGP values were then compared with drying times, drying rates, and anatomical characteristics of matched wood samples. Sapwood had highest average LGP values (11 to 38 Darcys) and fastest drying rates. Heartwood had lowest average LGP values (0.2 to 0.8 Darcys) than heartwood.
Scanning electron micrographs (SEM) show that low LGP values and slow drying rates for heartwood and wetwood are due largely to aspiration of bordered pits in white fir tracheids and to tyloses formation in aspen vessels. Scanning electron microscopy suggests that wetwood drying rates may be further reduced by deposits of bacteria and extraneous material that block transverse moisture diffusion and increase moisture holding capacity of the wood. Concurrently, wetwood bacteria may increase LGH by destroying tori in aspirated pits of white fir and by aborting tyloses development in aspen.