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The interactive effects of heat wave regimes, elevated CO₂ concentration, and drought on tree physiology and growth

(2015)
Author
Promoter
(UGent) and Robert O Teskey
Organization
Abstract
Our global climate is changing. In addition to rising CO2 concentrations and increasing average temperatures, our weather is also becoming more extreme. A key factor of this growing extremity are heat waves, which have been predicted to become more severe but also more frequent. Since heat waves are very often accompanied by drought, it is easy to imagine the devastating impact this can have, and already has had, on crop and forest ecosystems. Heat waves can impact a wide variety of tree functions such as photosynthesis, stomatal regulation, and carbon allocation. At the leaf level, photosynthesis is reduced, photo-oxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. Stomatal conductance has been found to increase at high temperatures in some species, which may be a mechanism for heat stress avoidance. At the whole tree level, heat stress has been shown to decrease growth and shift carbon allocation. We investigated this impact on Quercus rubra L. and Pinus taeda L. seedlings by applying repeated weeklong heat waves, during which temperatures could go up to 53°C and water availability was limited. We observed that occurrence of a +12°C heat wave significantly reduced net photosynthesis of Quercus rubra L. seedlings in comparison with a constant +3°C increase in temperature, but only in the afternoon. Morning net photosynthesis was observed to increase at the beginning of the heat wave. Stomatal conductance was higher at the beginning of the heat wave than before, but continuously decreased as the heat wave progressed. After the heat wave, stomatal conductance was even lower than before the heat wave. Since transpiration followed the same trend as stomatal conductance, we suggest that this was a heat stress avoidance mechanism of the seedlings. We furthermore observed that elevated [CO2] completely eliminated the negative effect of heat waves on Q. rubra biomass accumulation. In fact, Q. rubra in the heat wave treatment had increased stem height and diameter in the elevated [CO2] treatment, compared with all other treatments. The impact of drought was more dubious in our observations. While the combination of heat waves and drought clearly had a greater negative impact on gas exchange and biomass than either heat waves or drought alone, the difference (both relative and absolute) between seedlings exposed to a +12°C heat wave and seedlings exposed to a constant +3°C temperature elevation was smaller under drought conditions than under well-watered conditions. When we measured chlorophyll a fluorescence parameters, we even found that low soil moisture had no additional effect on Fv'/Fm' or PSII. We have found one putative example of acclimation to these heat waves, which was the reduced stomatal conductance and transpiration outside of the heat wave, and subsequent the increase in these parameters, supplemented by an increase in net photosynthesis, during the heat wave. These data suggest that the seedlings with previous heat wave exposure activated a stomatal regulation mechanism which increased stem water storage outside of the heat wave, to allow greater evaporative cooling during the heat wave, reflecting a mechanism to conserve water that prioritises survival over carbon acquisition.

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Citation

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Chicago
Bauweraerts, Ingvar. 2015. “The Interactive Effects of Heat Wave Regimes, Elevated CO₂ Concentration, and Drought on Tree Physiology and Growth”. Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
APA
Bauweraerts, I. (2015). The interactive effects of heat wave regimes, elevated CO₂ concentration, and drought on tree physiology and growth. Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium.
Vancouver
1.
Bauweraerts I. The interactive effects of heat wave regimes, elevated CO₂ concentration, and drought on tree physiology and growth. [Ghent, Belgium]: Ghent University. Faculty of Bioscience Engineering; 2015.
MLA
Bauweraerts, Ingvar. “The Interactive Effects of Heat Wave Regimes, Elevated CO₂ Concentration, and Drought on Tree Physiology and Growth.” 2015 : n. pag. Print.
@phdthesis{6843828,
  abstract     = {Our global climate is changing. In addition to rising CO2 concentrations and increasing average temperatures, our weather is also becoming more extreme. A key factor of this growing extremity are heat waves, which have been predicted to become more severe but also more frequent. Since heat waves are very often accompanied by drought, it is easy to imagine the devastating impact this can have, and already has had, on crop and forest ecosystems. Heat waves can impact a wide variety of tree functions such as photosynthesis, stomatal regulation, and carbon allocation. At the leaf level, photosynthesis is reduced, photo-oxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. Stomatal conductance has been found to increase at high temperatures in some species, which may be a mechanism for heat stress avoidance. At the whole tree level, heat stress has been shown to decrease growth and shift carbon allocation. We investigated this impact on Quercus rubra L. and Pinus taeda L. seedlings by applying repeated weeklong heat waves, during which temperatures could go up to 53{\textdegree}C and water availability was limited. We observed that occurrence of a +12{\textdegree}C heat wave significantly reduced net photosynthesis of Quercus rubra L. seedlings in comparison with a constant +3{\textdegree}C increase in temperature, but only in the afternoon. Morning net photosynthesis was observed to increase at the beginning of the heat wave. Stomatal conductance was higher at the beginning of the heat wave than before, but continuously decreased as the heat wave progressed. After the heat wave, stomatal conductance was even lower than before the heat wave. Since transpiration followed the same trend as stomatal conductance, we suggest that this was a heat stress avoidance mechanism of the seedlings. We furthermore observed that elevated [CO2] completely eliminated the negative effect of heat waves on Q. rubra biomass accumulation. In fact, Q. rubra in the heat wave treatment had increased stem height and diameter in the elevated [CO2] treatment, compared with all other treatments. The impact of drought was more dubious in our observations. While the combination of heat waves and drought clearly had a greater negative impact on gas exchange and biomass than either heat waves or drought alone, the difference (both relative and absolute) between seedlings exposed to a +12{\textdegree}C heat wave and seedlings exposed to a constant +3{\textdegree}C temperature elevation was smaller under drought conditions than under well-watered conditions. When we measured chlorophyll a fluorescence parameters, we even found that low soil moisture had no additional effect on Fv'/Fm' or PSII. We have found one putative example of acclimation to these heat waves, which was the reduced stomatal conductance and transpiration outside of the heat wave, and subsequent the increase in these parameters, supplemented by an increase in net photosynthesis, during the heat wave. These data suggest that the seedlings with previous heat wave exposure activated a stomatal regulation mechanism which increased stem water storage outside of the heat wave, to allow greater evaporative cooling during the heat wave, reflecting a mechanism to conserve water that prioritises survival over carbon acquisition.},
  author       = {Bauweraerts, Ingvar},
  isbn         = {9789059898066},
  language     = {eng},
  pages        = {XV, 238},
  publisher    = {Ghent University. Faculty of Bioscience Engineering},
  school       = {Ghent University},
  title        = {The interactive effects of heat wave regimes, elevated CO\unmatched{2082} concentration, and drought on tree physiology and growth},
  year         = {2015},
}