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 ~12-27◦C, July
~13-28◦C, August
~13-27◦C) except for heatwave days,
where heatwave rooms were raised by 10◦C 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).