Table 1. Soluble foliar polyamines in the current year’s foliage of white spruce (top) and paper birch (bottom) collected in early June (pre-heatwave) and late August (post-heatwave) of 2016 and 2017. Polyamine content is expressed as nmol g-1 DW for putrescine, spermidine, and spermine. Data are mean (n = 5-8) ± SE. Letters indicate statistical differences (p ≤ 0.05) among treatment groups on a given date based on Tukey’s HSD post hoc test.