Empirical models for estimating volume and biomass of poplars on farmland in Sweden

There is growing interest in establishing short-rotation poplar plantations in Sweden, primarily as a future feedstock for bioenergy. Cost-efficient planning and effective management of these plantations would be greatly facilitated by robust equations for estimating the taper, volume and height of poplar trees, and the biomass of poplar stumps and roots. Thus, the aim of the studies this thesis is based upon were to construct and evaluate such equations. Six equations were constructed: a stem taper equation, a stem volume equation, three stem height equations, and a stump and root biomass equation. The constructed taper and volume equations were compared with five and six published equations, respectively. The taper and volume equations are based on measurements of 69 and 72 trees, respectively, at 37 sites, the height equations on measurements of 117 trees at 40 sites, and the stump and root biomass equations on measurements of 72 stumps at six sites. All sites were located in central and southern Sweden (lat. 55-60° N). The mean age of the stands was 21 years (range 6-43 years) for the taper, volume and height analyses and 20 years for the stump/root biomass analysis. The results confirm many previous findings that exponential equations provide the best descriptions and predictions of taper. A complex exponential equation presented by Kozak provided the best fit to the collected data. However, the constructed (polynomial) equation is recommended when a simpler model is required and larger bias is acceptable. Equations developed by Eriksson and Scott, and the constructed equation for estimating volume, provided the best fits to the volume data, but a constructed new equation is recommended for simplicity, as it only includes three parameters. The diameter-height models were developed using a mixed model approach. The models included the stand variables mean diameter and age, and were improved by including a random site effect (to address variations among sites and provide locally calibrated estimates) which increased the R2 value from 0.87 to 0.96. The stump and root biomass equations were based on a power equation with DBH as an independent variable. The R2 values were 0.93 and 0.80 for dry weight estimates of stumps and roots, respectively. The mean dry stump weight represented 21% of the mean stem weight. The total dry weight per hectare for stumps and for roots amounted to 34.9 and 12.0 tons, respectively