This post is part of the series Simulating Climates in Growth Chambers.

We are especially interested in plant response to stressful events.

We wanted to measure response to extreme heat using fluorescence, so we needed to superimpose a heat wave. The question became then: what does a realistic heat wave look like? The definition of a heat wave is “a prolonged period of excessively hot weather”. Definitions vary with country and region, and depend on the baseline climate of the area. Heat waves are often defined in terms of the impact they have on human health, not on plants. The World Meteorological Organization defines a heat wave as a sequence of >5 days where the daily maximum exceeds the average maximum by 5 °C (Wikipedia). De Boeck et al. (2010)¹ analysed

characteristics of Western European heat waves. They defined a heat wave as a period of 7 days where temperatures reached higher than 90% of T_max, where T_max is the daily maximum determined from weather data and is a different number for each day of the year. The reference period is 1961-1990, so daily weather station data for 1961-1990 were needed for their method. We had neither the time nor the data for such an approach, but found several references to a duration of between 5 and 7 days for a heat wave.

Because of the background climate in our experiments, I resorted to the following solution. At the warmest time of the season (week 15), the warm (A) and cool (B) phases were swapped, resulting in a warm period of six days (15A+16A). Temperatures were increased by 5°C to make this six-day period “hot” instead of just warm. All plants were well-watered before the start of the heat wave (Fig. 6, pine dry, week 16: even the dry treatment received a full dose of water). Although in nature heat waves have drought stress associated with them, we were afraid to create conditions which were lethal rather than stressful. We simply couldn’t afford to lose half of the plants.

Temperature regimes during a six day heat wave were planned to otherwise minimally disrupt the background climate (MAT 11 °C).

Plants were carefully observed for signs of stress. Lodgepole pine plants had already set bud and survived without any noticeable damage, and chlorophyll fluorescence was measured as a measure of stress on day 6 of the heat wave. Interior spruce plants were still actively growing and suffered visibly from this heat wave, with needles browning and tips dying. As a result, chlorophyll fluorescence measurements were completed on day 4 of the heat wave, after which the heat was reduced by 5°C to reduce plant death and data loss. In fact, spruce plants not subjected to either heat waves or drought also suffered from high temperatures in week 15-16, as evidenced by browning of needles.