Experimental warming and drought treatments reduce physiological activities and increase mortality of Pinus koraiensis seedlings

This study was conducted to investigate physiological responses and mortality of P. koraiensis seedlings under warming and drought treatments. In May 2016, 90 P. koraiensis seedlings (aged 2 years) were planted in each plot (n = 20), and exposed to a combination of + 3 °C warming and − 30% drought, with 5 replicates. Net photosynthetic rate (A), stomatal conductance (g_s), transpiration (E), and maximal photochemical efficiency of photosystem II (F_v/F_m) were measured from April 2017 to December 2018. Seedling mortality was measured eight times during the study period at irregular intervals. A, g_s, and E decreased by − 21.99%, − 34.58%, and − 33.6% under the warming treatment, and by − 5.82%, − 11.03%, and − 8.56% under the drought treatment, respectively, in response to decreasing soil water content and increasing soil and leaf temperature. There was no significant difference in the overall F_v/F_m by the warming and drought treatments. The long-term reduction in photosynthesis by the warming treatment might cause carbon starvation, resulting in a 7.43-fold increase in seedling mortality. Moreover, under the drought treatment, seedling mortality was unaffected since its effects on A were occasional and small as compared to the warming treatment. Due to the unusually high temperature in summer of 2018, leaf temperature was 38.28 °C and seedlings were exposed to temperature above 45 °C for 10.7 h under the warming treatment. F_v/F_m in August 2018 decreased sharply by − 11.79% and seedling mortality increased by 15.31-fold during summer, under the warming treatment. Thus, leaf damage by severe heat stress may have triggered a rapid increase in seedling mortality.