Drought aggravates the negative effects of warming on the growth of drought-intolerant tree seedlings in temperate forests

Abstract Background How tree seedlings with contrasting drought tolerance respond to climate change remains largely unexplored, which will limit our ability to predict future forest composition and resilience under shifting climatic conditions. Here, we investigated the effects of warming and interannual precipitation variability on the photosynthetic properties, antioxidant enzyme activities, and growth of two dominant tree species ( Fraxinus mandshurica , a drought-intolerant species; Quercus mongolica , a drought-tolerant species) in a temperate forest through a two-year field warming experiment using an infrared heating system. Results The infrared treatment increased temperatures by 2 °C in the 0–10 cm soil layer and by 1.49 °C at 0.2 m above the surface (seedling height), accompanied by a 4.45% increase in vapor pressure deficit (VPD) at 0.2 m compared to ambient conditions. In a non-drought year, warming reduced photosystem II performance in both species and significantly increased malondialdehyde content by 67% in F. mandshurica seedlings. In a drought year, warming reduced the net photosynthetic rate by 12% in Q. mongolica and by 28% in F. mandshurica , and also decreased antioxidant enzyme activity in both species. Notably, the growth of Q. mongolica was not affected by warming in the non-drought year, whereas warming significantly suppressed the total biomass, root collar diameter increment, and height growth of F. mandshurica over two years, with reductions of 30%, 48% and 67% in the drought year, and 33%, 18% and 11% in the non-drought year, respectively. Increased VPD may be a key factor limiting the growth and physiological responses of F. mandshurica seedlings under warming conditions. Conclusions These findings indicate that the effects of warming are species-specific, with drought-intolerant species showing greater sensitivity to projected climate change. Their responses are further intensified under drought conditions, underscoring their heightened vulnerability in a warming climate.