Diverging Responses of Two Subtropical Tree Species (Schima superbaandCunninghamia lanceolata) to Heat Waves

The frequency and intensity of heat waves (HWs) has increased in subtropical regions in recent years. The mechanism underlying the HW response of subtropical trees remains unclear. In this study, we conducted an experiment with broad-leavedSchima superba(S. superba) and coniferousCunninghamia lanceolata(C. lanceolata) seedlings to examine HW (5-day long) effects on stem water transport, leaf water use efficiency (WUE), morphology and growth, and to elucidate differences in the responses of both species. Our results indicated that HWs can significantly reduce hydraulic conductivity in both species.C. lanceolataexperienced significant xylem embolism, with the percentage loss of conductivity (PLC) increasing by 40%, whileS. superbashowed a non-significant increase in PLC (+25%). Furthermore, HW also caused a reduction in photosynthesis rates (A(n)), but transpiration rates (Tr) increased on the 5th day of the HW, together leading to a significant decrease in leaf WUE. From diurnal dynamics, we observed that the HW caused significant decrease ofS. superba A(n)only in the morning, but nearly the all day forC. lanceolata. During the morning, with a high vapor pressure deficit (VPD) environment, the HW increasedTr, which contributed a lot to latently cooling the foliage. In comparing the two tree species, we found that HW effects onS. superbawere mostly short-term, with leaf senescence but limited or no xylem embolism. The survivingS. superbarecovered rapidly, forming new branches and leaves, aided by their extensive root systems. ForC. lanceolata, continued seedling growth initially but with subsequent xylem embolism and withering of shoots, led to stunted recovery and regrowth. In conclusion, apart from the direct thermal impacts caused by HW, drought stress was the main cause of significant negative effects on plant water transport and the photosynthetic system. Furthermore,S. superbaandC. lanceolatashowed clearly different responses to HW, which implies that the response mechanisms of broad-leaved and coniferous tree species to climate change can differ.