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.