Abstract
Drought-induced tree mortality has been observed worldwide. Nevertheless, the physiological mechanisms underlying this phenomenon are still being debated. Potted Robinia pseudoacacia and Platycladus orientalis saplings were subjected to drought and their hydraulic failure and carbon starvation responses were studied. They underwent simulated fast drought (FD) and slow drought (SD) until death. The dynamics of their growth, photosynthesis, water relations and carbohydrate concentration were measured. The results showed that during drought, growth and photosynthesis of all saplings were significantly reduced in both species. The predawn water potential in both species was ~ -8 MPa at mortality. The percentage loss of conductivity (PLC) was at a maximum at mortality under both FD and SD. For R. pseudoacacia and P. orientalis, they were >95 and ~45 %, respectively. At complete defoliation, the PLC of R. pseudoacacia was ~90 % but the trees continued to survive for around 46 days. The non-structural carbohydrate (NSC) concentrations in the stems and roots of both FD and SD R. pseudoacacia declined to a very low level near death. In contrast, the NSC concentrations in the needles, stems and roots of P. orientalis at mortality under FD did not significantly differ from those of the control, whereas the NSC concentrations in SD P. orientalis stems and roots at death were significantly lower than those of the control. These results suggest that the duration of the drought affected NSC at mortality in P. orientalis. In addition, the differences in NSC between FD and SD P. orientalis did not alter mortality thresholds associated with hydraulic failure. The drought-induced death of R. pseudoacacia occurred at 95 % PLC for both FD and SD, indicating that hydraulic failure played an important role in mortality. Nevertheless, the consistent decline in NSC in R. pseudoacacia saplings following drought-induced defoliation may have also contributed to its mortality.
Highlights
In recent years, climate change has caused drought resulting in extensive woodland tree mortality and forest decline worldwide (Anderegg and Anderegg 2013; Bouche et al 2014; Gaylord et al 2015)
slow drought (SD) P. orientalis survived for 1 month longer than SD R. pseudoacacia (101.97 ± 4.14 days and 132.03 ± 5.51 days for R. pseudoacacia and P. orientalis, respectively) (Tables 2 and 3)
The saplings of R. pseudoacacia were leafless within 1 month but survived for another ~45 days
Summary
Climate change has caused drought resulting in extensive woodland tree mortality and forest decline worldwide (Anderegg and Anderegg 2013; Bouche et al 2014; Gaylord et al 2015). The causes, consequences, climatic thresholds and models of drought-induced tree mortality have been under close study (Breshears et al 2009; Martínez-Vilalta et al 2012; Anderegg et al 2013; McDowell et al 2013; Gustafson 2014; Mitchell et al 2014; Gu et al 2015). Their underlying physiological mechanisms are still being debated (Anderegg et al 2012; Sevanto et al 2014; Meir et al 2015). Two physiological responses to drought are defined as follows (Pangle et al 2015): (i) hydraulic failure—low water potential due to decreased soil water content or increased transpiration rate impeding long-distance water transport and causing cavitation embolism or dehydration; (ii) carbon starvation—stomatal closure and restricted carbon translocation, uptake and assimilation during prolonged drought
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