Drought treatments
We cut trenches beneath the drip line of 24 trees (n=3 per block) on May 20-21, 2013 using methods modified from Devine & Harrington (2008). Trenches were cut to 33 cm deep using a ditch witch and the interior walls were lined with 0.15 mm thick polyethylene sheeting to restrict root growth and water access. Each trench was then backfilled with soil. We also removed snow within the same drip lines of trenched trees after every major snowfall during the winter of 2013. As described in Kolbet al . (2019), the study measured tree predawn water potential and the trenching treatment produced a level of water stress consistent with moderate drought, similar to previous uses of this method with ponderosa pine (McCullough & Wagner, 1987). We briefly summarized the results from Kolb et al . (2019) under Result section below.