Tree growth and leaf morphological measurements
To assess tree size at the beginning of the experiment, height
(h ) and the basal stem diameter (d ) of each tree were
measured on July 17 just prior to defoliation and the onset of drought
treatment. We measured d at 4.5 cm above the soil surface andh from the soil surface to the apical meristem. For each tree,
initial d 2h was calculated; this metric
is known to be highly and linearly correlated with aboveground biomass
in young P. tremuloides (Stevens et al., 2007). Towards the end
of the experiment, on August 22, finald 2h of each tree was measured. On August
27, stems, petioles, and leaves of all trees were separately harvested.
Fresh leaf area (without petioles) was measured for each tree using an
LI-3100 (Li-Cor Biosciences, Lincoln, NE, USA). Stems, petioles, and
leaves were then oven-dried and the total above-ground tree dry weight
(including stems, petioles, and leaves), leaf weight ratio (LWR, total
leaf dry weight/ total above-ground tree dry weight), stem weight ratio
(SWR, total stem weight/ total above-ground tree weight) and the
specific leaf area (SLA, total fresh leaf area/ total leaf dry weight)
were quantified.