Plant material and growth conditions
A detailed description of the plant material and growth conditions is described in Gagne, Smith, McCulloh, in press). Briefly, 1 to 2-year-old paper birch and 3-year-old native Wisconsin white spruce saplings were grown in the Biotron Laboratory and Controlled Environment Research Center on the University of Wisconsin campus for two growing seasons (June-Oct 2016, April-Sept 2017). Daily temperature and water availability were altered to test their effects on survival and growth of the saplings. Additionally, the atmospheric CO2concentration in the rooms was elevated from ambient conditions to 700 µmol CO2 mol−1 to simulate a future intermediate emission projection scenario. Two greenhouse rooms were established in 2016; one received monthly heatwaves while the other served as the control non-heatwave room. In the second year, we doubled the number of rooms and split the plants from 2016 accordingly into their respective rooms. Temperatures in the non-heatwave rooms were based on 30-year weekly averages from Hayward, WI, USA, a location central to the distribution of these species in northern WI. All greenhouse rooms received the same temperature regime (daily temperature flux: June ~12-27C, July ~13-28C, August ~13-27C) except for heatwave days, where heatwave rooms were raised by 10C for 8 continuous days in the middle of June, July, and August. To alter the watering regime, half of the plants in all rooms were well-watered, and the other half received less water. In 2016, the well-watered birch received ~1.3 L wk-1 and the reduced-water birch received ~0.65 L wk-1; the well-watered spruce received 300 mL wk-1 and the reduced-water received 150 mL wk-1. In 2017, the volumes increased to account for the increase in plant size to ~4.8 L wk-1 and ~2.4 L wk-1for the birch and 600 mL wk-1 and 300 mL wk-1 for the spruce. From this point forward, the four treatment groups are referred to as: control (C ; well-watered, no heatwave); drought (D ; reduced-watered, no heatwave); heat (H ; well-watered, heatwave) and heat+drought (HD ; reduced-watered, heatwave).