Current climate change has made heat waves more likely as both the temperature mean and variability are increasing (Schär et al. 2004). Several well-documented heat waves have occurred during the past years such as those of 2003 (Europe), 2010 (Russia) and 2012 (North America), and the likelihood of such major events is expected to increase 5 to 10-fold within the next 40 years (Barriopedro et al. 2011). Heat stress in plants is usually observed when tissue temperatures exceed 40°C, a threshold that is fairly stable across biomes. Such excessive temperatures affect plant metabolism in multiple ways, ultimately reducing growth and reproduction. This seems at odds with the reported lack of significant single-factor effects of heat waves in several ecological studies on heat waves (Hoover et al. 2014, De Boeck et al. 2016). Here we examine how these seemingly contrasting notions can be reconciled. The fundamental issue is that air temperature is often considered a direct indicator of heat stress, while many other environmental conditions also determine leaf temperature. The extent to which variables other than air temperature can affect heat stress is discussed by making use of both an energy balance model and field data.
Results/Conclusions
Our results show that, for air temperatures of 40°C, leaf temperatures may differ 10°C and more depending on radiation, wind speed, relative humidity and plant water status. Surprisingly, low relative humidity, which is generally considered unfavorable for plant survival as it promotes water loss, leads to avoidance of the most extreme cases of overheating because the associated strong evaporative cooling reduces leaf temperature well below air temperature. The effects of high wind speed are likewise counterintuitive. By promoting close coupling between the plant and the air, high wind speeds make it impossible for leaf temperatures to greatly exceed air temperatures, thus providing protection against extreme heat, especially under high radiation. In other words, heat waves characterized by extreme air temperatures may pose little danger under some atmospheric conditions, but could be lethal in other cases.
Extreme events have always shaped ecosystems as they set the environmental limits for species occurrence. By elucidating the complex impact of heat extremes, the current study contributes to understanding the consequences of future extreme event regimes for species and eventually the novel communities which are likely to arise under regimes with more frequent and more intense extreme events.