Gas exchange of Pinus densiflora seedlings is reduced by the combined effect of high summer temperatures and cumulative heat under open-field warming.

Pinus densiflora Siebold and Zucc. (Korean pine) is one of the dominant coniferous species on the Korean Peninsula and is reportedly vulnerable to climate change. We investigated how P. densiflora seedlings respond to seasonal warming at an open-field nursery in Seoul, South Korea. Using infrared heaters, the ambient temperatures of four seedling groups were raised by 4 °C at different periods of the growing season: control (C; no artificial warming), constant warming throughout the growing season (W; 15 April-October), warming during summer only (WS; 1 June-31 August), and warming during spring and fall (WSF; 15 April-31 May and 1 September-15 October). Linear mixed-effects models were used to assess the effects of treatments, time of assessment and season on the gas exchange and leaf pigment contents of the needles. The results showed that net photosynthetic rate was reduced by W from July to October, with the most pronounced decrease in October. The responses of transpiration rate and stomatal conductance were quicker and stronger than that of net photosynthetic rate, showing reduction from June, with the most pronounced decrease in October under W and in July to August under WS, thereby improving the water-use efficiency of the seedlings. Across treatments, the reduction in physiological activities was most pronounced in the W treatment, highlighting the substantial negative impact of combined high temperatures and accumulated heat on the gas exchange of the seedlings. The decrease in the chlorophyll-to-carotenoid ratio further indicated that the seedlings were experiencing stress. Overall, our findings indicate that P. densiflora seedlings are more adversely affected by warming during summer than in spring and fall, with prolonged warming leading to greater impacts. Although increased water-use efficiency suggests the potential for adaptation to warming in P. densiflora, a long-term decline in gas exchange is anticipated under projected climate change.