Modeling effects of soil, climate, and silviculture on growth ring specific gravity of Douglas-fir on a drought-prone site in Western Washington

Because wood specific gravity (SG), also known as relative or basic density, is correlated with many physical and mechanical properties it is a widely used indicator of wood quality. Furthermore, because SG measures dry weight per unit green volume and dry wood contains about 49% carbon and 20 MJ/kg energy, SG is useful for converting tree volume to carbon and energy content equivalents. Although many studies have investigated variables affecting SG, few have developed models to predict SG at the local level from age, silvicultural treatment, and growing environment variables. This study developed a four-parameter logistic model with mixed effects to predict the specific gravity of annual growth rings (RSG) from forty six 76-year-old Douglas-fir (Pseudotsuga menziesii, Mirb. Franco) trees sampled from a thinning and biosolids fertilization experiment on a drought-prone site in Western Washington. RSG for up to sixty rings per tree was measured with X-ray densitometry on strips taken from disks removed from the top of the first 5 m log. Inter-annual RSG was modeled using treatment variables; local temperature, precipitation, and soil moisture deficit as growing environment variables; and ring number, area, width, and radius to the ring as tree variables. Ring age, ring area, March to May mean temperature, July soil moisture deficit, and whether or not biosolids had been applied were important predictors of RSG on this site. The equation for RSG can be easily converted to carbon or energy content equations by multiplying by the dry wood carbon and energy content factors.