Extreme climate events during summer inhibit foliar gas exchange of Larix kaempferi seedlings

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Gwang-Jung Kim, Hyeonji Kim, Hyung-Sub Kim and Yowhan Son, Department of Environmental Science and Ecological Engineering, Korea University, Seoul, Korea, Republic of (South), Heejae Jo, Ojeong Resilience Institute, Korea University, Seoul, Korea, Republic of (South), Min Seok Cho and Nam Jin Noh, Forest Technology and Management Research Center, National Institute of Forest Science, Pocheon, Korea, Republic of (South)

Background/Question/Methods
Extreme climate events such as heat waves, drought, and heavy rainfall are occurring frequently and intensely according to climate change. Such extreme events may induce irreversible damages to the ecosystem, especially for plants. Larix kaempferi, which is one of the major afforestation species in South Korea, is known for being highly vulnerable to climate change, but its responses to extreme stresses have not been reported clearly. Here, we investigated the physiological responses of Larix kaempferi seedlings to extreme heat, drought, and heavy rainfall in an open field experiment. In May 2020, we planted 88 1-year-old seedlings of Larix kaempferi in each 1.5 × 1.0 m plot. Treatments consisted of three temperature levels (control, +3 ℃, and +6 ℃) and three precipitation levels (control, drought, and heavy rainfall) with three replicates. We manipulated the temperature and precipitation treatments from July to August 2020 and measured foliar gas exchange parameters (net photosynthetic rate, transpiration rate, stomatal conductance, a ratio of intercellular to ambient CO2 concentrations) and total chlorophyll content of seedlings every 2 weeks.
Results/Conclusions
The temperature manipulation significantly decreased foliar gas exchange of Larix kaempferi seedlings, while there was no significant effect by the precipitation manipulation. Net photosynthetic rate (µmol m-2 s-1, mean ± SD) was lowest in the +6 ℃ treatment (3.39 ± 1.06), followed by the control (3.87 ± 0.98) and the +3 ℃ treatment (4.01 ± 1.09). Other foliar gas exchange parameters were lowest in the +6 ℃ treatment, followed by the +3 ℃ treatment and the control. The high temperature lowers CO2 transport by damaging photosystem II and Rubisco in C3 plants, and thus, reduced the intercellular CO2 concentration in the heated plots. Since stomata respond to such CO2 concentration in leaves, gas exchange in the heated plots was likely to decline along with stomatal closure. On the other hand, temperature manipulation significantly increased the total chlorophyll content. The high temperature accelerates the pigment biosynthesis in leaves and thereby increased the chlorophyll content. Our results show that extreme climate events, especially extreme heat, could trigger the constraint of CO2 uptake, stomatal closure, and photosynthetic decline in Larix kaempferi seedlings, and the inhibition of gas exchange may further degrade the quality of seedlings.