Photosynthetic responses of Pinus densiflora seedlings are affected by accumulated heat under artificial warming

Photosynthetic responses of Pinus densiflora seedlings to warming in different seasons were investigated. In March 2023, four temperature treatments with five replicates were conducted in an open-field nursery site located in Seoul, South Korea: constant warming (April 15th-October 15th), spring and fall warming (April 15th-May 31st and September 1st-October 15th), summer warming (June 1st-August 31st), and control. In each plot, 108 1-year-old P. densiflora seedlings were planted. The temperature of the warming plots was set to increase by 4°C compared to control plots using infrared heaters. Photosynthetic responses of seedlings were measured on the 10th of each month from May to October and linear mixed-effect models were used to analyze the effect of treatments on photosynthetic responses. Net photosynthetic rate was not affected by any of the treatments until June but decreased by 12.7% under summer warming compared to spring and fall warming in July. In August, both constant warming and summer warming treatments decreased the net photosynthetic rate by 18.0% and 12.3%, respectively, compared to the control. However, in September, following the cessation of the summer warming and the initiation of the fall warming treatment, seedlings only subjected to constant warming exhibited a significant reduction in net photosynthetic rate, with a 33.0% decrease compared to the control. This reduction increased to 41.2% in October, whereas summer warming and spring and fall warming treatments did not affect photosynthesis in the same month. This study indicates that warming might result in losses in plant photosynthesis, and those losses could be higher during summer. In addition, although the spring and fall treatment did not independently affect net photosynthetic rate of seedlings, the accumulated heat during spring and fall appears to have attributed to the photosynthetic reduction under the constant warming treatment.   * This research was carried out with the support of the Korea Forest Service Government (KFSG) as [Graduate School specialized in Carbon Sink].