Bark content variations and the production of Populus deltoides as a source of bioenergy under temperate climate conditions

Plot design and harvesting Twelve sampling plots (16 m × 16 m) in three P. deltoides plantations were established based on systematic random design. To minimize edge effects, surrounding rows were not considered during sampling. The age of the stands was 18-20 years old. In each sampling plot, the DBH (diameter at breast height 1.3 m above the ground) of the individual trees was measured with a caliper in two perpendicular directions and the mean DBH determined. Tree height was measured by Haglöf-Vertex IV hypsometer. Based on the DBH and height measurements, 10 DBH classes from 15 to 42 cm (3 cm intervals) were established. The value of each DBH class represented the central value (i.e., class 15 included all DBH from 12.5 to 17.5 cm). In each DBH class, one representative tree was selected and harvested for a total of 10 P. deltoides trees. Measurements of bark percentagesThe stems of harvested trees were marked and cut into 2 m-segments. The mid-length diameter of each segment was measured outside the bark in two perpendicular directions with a caliper to determine the mean diameter. A 5 cm-thick disc was cut from the middle of each segment. A total of 123 discs were obtained and brought to the laboratory. All the discs were arranged into 2-cm wide diameter classes. The value of each disc class represents the central value (i.e., class 20 included all discs whose diameters ranged from 19.5 to 20.5 cm). Bark was separated from the wood using a peeler knife for each disc. Fresh bark and wood were weighted separately, oven-dried at 80 °C until constant weight, and the oven-dry weight measured. The bark percentage of each disc was considered as bark percentage of a 2 m-segment for fresh and dry weight. Finally, the bark percentage of the whole stem in each DBH class was calculated by adding the 2 m-segments.

样地设计与采样收获
基于系统随机抽样设计，在3个美洲黑杨（P. deltoides）人工林中设置12个16 m × 16 m的采样样地。为最小化边缘效应，采样过程中不考虑样地周边的林行。供试林分林龄为18~20年。
在每个采样样地内，使用游标卡尺沿两个垂直方向测定单株树木的胸径（DBH，即距地面1.3 m处的树干直径），并计算平均胸径；树木树高采用Haglöf-Vertex IV型测高仪进行测定。基于测定得到的胸径与树高数据，划分出10个胸径级，范围为15~42 cm，组距为3 cm；每个胸径级的取值为组中值（例如15 cm级包含所有胸径介于12.5~17.5 cm的个体）。每个胸径级选取1株标准木进行砍伐采样，最终共获取10株美洲黑杨样本。

树皮占比测定
将砍伐获取的树干进行标记，并切割为2 m长的木段。使用游标卡尺沿两个垂直方向测定每个木段去皮后的中段直径，计算得到平均直径。从每个木段的中部截取厚度为5 cm的圆盘试样，共获取123个圆盘试样并带回实验室。将所有圆盘试样按直径划分为2 cm间隔的径级，每个径级的取值为组中值（例如20 cm级包含所有直径介于19.5~20.5 cm的圆盘试样）。对每个圆盘试样，使用削皮刀将树皮与木质部分离；分别称量鲜树皮与鲜木材的重量，随后将其置于80 ℃烘箱中烘干至恒重，测定其烘干重量。将每个圆盘试样的树皮占比视为对应2 m木段的鲜重与干重树皮占比。最终，通过累加各2 m木段的树皮占比，计算得到每个胸径级对应的整株树干树皮占比。