Metabolite tissue collection and processing
Leaf samples were harvested in mid-June one week prior to the first heatwave event of 2016 and 2017. Late-August samples were harvested three days after the final heatwave of 2016 and 2017 for metabolite analysis. Fresh tissue of 2-4 healthy mature birch leaves or one spruce branchlet from the current year’s growth was removed from each plant (n = 4-6 plants per treatment) using a razor blade. The foliage was immediately put into liquid nitrogen and then transferred to a -20C freezer to await processing. The samples were freeze-dried, ground using a mini Wiley Mill (Thomas Scientific, Swedesboro, NJ) and then passed through a 0.85 mm sieve (#20 mesh). The dried samples were stored at -20C until analysis.