Three mature leaves of different individual leaf area and from different tree heights were randomly selected
per measurement plot. Fresh leaf area was measured shortly after leaf collection with a Li-3000 Leaf Area Meter (Li-COR Biosciences, Lincoln, NE, USA). Leaves of plots of the same genotype were merged, oven dried at 70 °C and their combined dry mass determined by weighing. A measure for individual leaf area (cm2) was obtained by averaging the aforementioned assessed fresh leaf areas per genotype (n = 12). Leaf nitrogen (N) concentrations were determined by dry combustion (with a NC-2100 element analyzer, Carlo selleckchem Erba Instruments, Italy) of a subsample of the grounded dried leaves for each genotype and for both GS1 and GS2. The phenological onset and ending of GS2 was monitored by observing the apical buds of four selected trees per measurement plot during spring and autumn 2011. The timing of spring bud flush (day of the year; DOY) selleck kinase inhibitor was defined at a stage according to the following: “Bud sprouting, with a tip of the small leaves emerging out of the bud scales, which could not be observed individually” (based on UPOV, 1981). The timing of bud set (DOY), accompanied by the end of leaf production and
the end of height growth was set at the time when the “apical bud was present but not fully closed, bud scales were predominantly green and no more rolled-up leaves were present” (Rohde et al., 2010). The length of the growing season (days) was then defined as the period in between Interleukin-3 receptor these well-defined phenological stages. A detailed description of phenological observations on poplar can be found in Pellis et al. (2004). Wood characteristics were determined for six out of the 12 genotypes (i.e. Bakan, Grimminge,
Koster, Oudenberg, Skado and Wolterson). After GS2, in January 2012, wood samples were taken from five trees in each of the eight measurement plots per genotype (n = 40). 2-cm-long specimens were cut from the main stem at a height of 5–7 cm from the base of the current-year shoot and were stored at −20 °C. Thin (approx. 7 mm) disks were cut from these 2-cm-long samples for scanning with a flatbed scanner. The disk area was then determined semi-automatically in Matlab (7.12.0, 2011 Mathworks, Natick, MA, USA) on the scans. The exact thickness of the disks was measured with a Mitutoyo digital caliper and, by multiplication with the measured disk area, the fresh volume was derived. The disks were oven-dried for 48 h at 103 °C, from which wood density (kg m−3) and moisture content (%) were derived.