Responses of tree species to heat waves and extreme heat events

Abstract

The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

Drought stress and carbon assimilation in a warming climate: Reversible and irreversible impacts

“…Leaf temperature, water status and stomatal opening are interconnected in a complex manner (Valladares and Pearcy, 1997;Feller, 2006;Reynolds-Henne et al, 2010;Feller and Vaseva, 2014;Teskey et al, 2015). Increased abscisic acid levels cause closure of stomates and reduce leaf cooling by transpiration.…”

“…Additionally climate change models predict more frequent and more severe extreme events such as drought phases or heat waves (Schär et al, 2004;Fuhrer et al 2006;Mittal et al, et a., 2014;Fischer and Knutti, 2015;Teskey et al, 2015). Drought as well as elevated ambient temperature may cause considerable impacts on crop plants by disturbing essential physiological functions including photosynthesis, mineral nutrient acquisition, long-distance transport via xylem and phloem, interactions between organs as well as yield quantity and quality (Loreto and Centritto, 2008;Gilgen and Buchmann, 2009;Ji et al, 2010;Jentsch et al, 2011;Cottee et al, 2014;Molina-Rueda and Kirby, 2015;Ramya et al, 2015;Xu et al, 2015).…”

Stomatal conductance increases with rising temperature

“…4,5 Therefore, the effect of temperature on stomata is often considered indirect, through changes in plant water status, photosynthesis or vapor pressure deficit (VPD). Very little is known about the direct effect of temperature on stomatal conductance (g s ) 6 which may exist independently from indirect mechanisms. 7,8 Results of experiments that examined the direct dependence of stomatal conductance on temperature have not been consistent.…”

Plants and Drought in a Changing Climate

“…While higher CO 2 concentrations may boost plant growth and simultaneously help plants conserve water, the co-occurrence of hot conditions during drought could exacerbate plant stress, and potentially lead to increased damage to tissues and higher rates of mortality [65][66][67]. This increase in plant stress could occur even if drought frequency remains constant and have major impacts on forest structure and functioning in a hotter world [68].…”

“…Concurrent elevations in temperature during drought are likely to increase VPD further, as well as increase metabolic needs of plants [66]. Although experimental evidence is still limited, the existing studies find that hot droughts will kill trees faster [76], suggesting that in the future, even droughts of short duration may prove lethal, leading to more tree killing events even when drought frequency stays constant [68].…”