High temperatures and droughts following winter dormancy can negatively affect seedling growth and mortality. An open-field experiment was conducted to study the growth and mortality of Larix kaempferi seedlings in response to spring warming and drought treatments and to determine whether seedlings could regain their growth capability once the treatments were discontinued. In May 2020, 1-year-old seedlings were exposed to four treatments: control, warming-only, drought-only, and the combined warming and drought. Drought treatment reduced the seedling height and root collar diameter and increased the mortality rate. The combined warming and drought treatments had the highest mortality rates, followed by the drought, control, and warming treatments. However, after the cessation of the treatments, the combined warming and drought treatments increased seedling height, root collar diameter, and individual seedling biomass because the high mortality rate relaxed competition among seedlings. This suggests that the effects of low competition on the surviving seedlings may mitigate the negative effects of warming and drought on seedling growth. Our study demonstrates that despite the high mortality and decreased growth during the treatment period, seedlings subjected to combined high temperature and drought stress showed short-term high levels of growth compared to seedlings subjected to a single stress.

Excessively high temperatures and droughts after winter dormancy can affect the physiological responses of plant seedlings. In the present study, an open-field experiment was conducted to investigate the impact of spring warming and drought treatments on the short-term physiological responses of 1-year-old Larix kaempferi seedlings. The warming treatment was designed to increase the air temperature by 4 °C compared to that of the temperature control plots and was carried out for seven days each in the second and fourth week of May 2020. Moreover, the drought treatment was designed to completely block precipitation for four weeks in May 2020. After the first warming treatment period, stomatal conductance, transpiration rate, and net photosynthetic rate decreased by 35.16%, 29.53%, and 13.34% in the temperature warming plots compared to those in the temperature control plots, respectively. After the resting stage, stomatal conductance and transpiration rate increased by 101.47% and 72.80% in the temperature warming plots compared to those in the temperature control plots, respectively. Stomatal conductance, transpiration rate, and net photosynthetic rate tended to decrease in the drought treatment. The total chlorophyll content did not change under the warming treatment, but it increased by 20.29% in the drought treatment plots compared to that in the precipitation control plots in the fourth week; this may have resulted from chlorophyll hormesis. Furthermore, the correlation and principal component analyses showed that seedling physiological responses and environmental conditions were closely related. We found that spring warming and drought treatments can reduce stomatal conductance, transpiration rate, and net photosynthetic rate, thus affecting seedling growth. This study is expected to be the basis for more in-depth studies on the effects of warming and drought treatments on the growth and phenology of L. kaempferi seedlings.