Abstract
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-leaved Schima superba (S. superba) and coniferous Cunninghamia 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. lanceolata experienced significant xylem embolism, with the percentage loss of conductivity (PLC) increasing by 40%, while S. superba showed a non-significant increase in PLC (+25%). Furthermore, HW also caused a reduction in photosynthesis rates (An), 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 of S. superba An only in the morning, but nearly the all day for C. lanceolata. During the morning, with a high vapor pressure deficit (VPD) environment, the HW increased Tr, which contributed a lot to latently cooling the foliage. In comparing the two tree species, we found that HW effects on S. superba were mostly short-term, with leaf senescence but limited or no xylem embolism. The surviving S. superba recovered rapidly, forming new branches and leaves, aided by their extensive root systems. For C. 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. superba and C. lanceolata showed clearly different responses to HW, which implies that the response mechanisms of broad-leaved and coniferous tree species to climate change can differ.
Highlights
Human activities continue to affect the environment, and increasingly frequent, intense and longer-lasting heat waves (HWs) are expected with ongoing climate warming [1,2]
Treatment, the percentage loss of conductivity (PLC) of S. superba increased by 25%, which was, not significantly different from that of the control group (p = 0.206), whereas the PLC of C. lanceolata was significantly increased by 40% from 12.5% (p = 0.006) (Figure 3c)
HWs caused no significant differences in HV for the two species (Figure 3f), they did significantly decrease the Ks and Kl of C. lanceolata, but not significantly in S. superba, where the KS of S. superba decreased by 40% from 0.195 g m−1 s−1 MPa−1 and the Kl decreased by 47%, while the KS of C. lanceolata decreased by 66% from 0.407 g·m−1 s−1 ·MPa−1 the decreased by 75% (Figure 3d,e)
Summary
Human activities continue to affect the environment, and increasingly frequent, intense and longer-lasting heat waves (HWs) are expected with ongoing climate warming [1,2]. A HW is commonly defined as a period with excessively hot days, usually combined with dry atmospheric and surface soil conditions [3,4,5]. Such extreme thermal climatic events can exert stronger selective pressures on organisms than gradual changes in temperature [6,7,8,9] and could negatively impact the function and survival of plants [10,11,12,13]. There is evidence that subtropical HWs can cause a significant reduction in carbon sequestration [18]
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