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Different tree species have various strategies in response to different drought types. Black locust (a deciduous tree) and arborvitae (an evergreen tree) present different physiological death processes when exposed to fast− (FD) or slow−drought (SD) stress. In this study, black locust shed its leaves when subjected to drought, underwent hydraulic failure and carbon depletion regardless of FD or SD. However, arborvitae retained its needles when exposed to drought, only suffered hydraulic failure under FD, while it experienced both hydraulic failure and carbon depletion under SD.

Two broadleaf evergreen canopy species (Schima superba and Engelhardia roxburghiana) with different phenologies in a subtropical region of southern China were used to determine the influence of leaf phenology on the impact of an insect pest attack. S. superba regenerates its leaves in February, while E. roxburghiana regenerates its leaves in May. The moth Thalassodes quadraria attacked the two broadleaf evergreen species in March to April, and the newly produced leaves were removed for S. superba but not for E. roxburghiana. The young trees were artificially defoliated to imitate an insect pest attack during March 2014. Nonstructural carbohydrate (NSC) and growth measurements and a retrospective analysis based on the radial growth of mature trees were conducted in January 2015. The results showed that NSC concentrations decreased in S. superba during canopy rebuilding, and the subsequent defoliation severely inhibited leaf and shoot growth, prevented NSC restoration in roots and stem xylem, and caused high mortality. The insect outbreaks reduced the radial growth of S. superba. In contrast, E. roxburghiana experienced less growth retardation, lower mortality, and normal radial growth. Thus, taking phenology-dependent variation in NSCs into consideration, defoliation and insect pest outbreaks more negatively impacted S. superba than E. roxburghiana.

Background Despite the importance of root hydraulics, there is little research on the in situ dynamic responses of embolism formation and embolism repair of roots distributed in different soil depths in response to different water regimes. Results The vessel diameter, hydraulic conductivity, and vulnerability to cavitation were in the order of deep root > shallow root > branch. The midday PLC of shallow root was the highest in the dry season, while the midday PLC of deep root slightly higher than that of branch with no significant difference in the two seasons. The capacity of embolism repair of roots was significantly greater than that of branch both in dry season and wet season. The xylem pressure was in the order of deep roots > shallow root > branch, and it was negative in most of the time for the latter two in the dry season, but positive for both of the roots during the observation period in the wet season. The NSC and starch content in roots were significantly higher than those in branches, especially in the dry season. In contrast, roots had lower content of soluble sugar. Conclusions The relatively stable water condition in soil, especially in the deep layers, is favorable for the development of larger-diameter vessels in root xylem, however it cannot prevent the root from forming embolism. The mechanism of embolism repair may be different in different parts of plants. Deep roots mainly depend on root pressure to refill the embolized vessels, while branches mainly depend on starch hydrolysis to soluble sugars to do the work, with shallow roots shifted between the two mechanisms in different moisture regimes. There is theoretically an obvious trade-off between conducting efficiency and safety over deep roots, shallow roots and branches. But in natural conditions, roots do not necessarily suffer more severe embolism than branches, maybe due to their root pressure-driven embolism repair and relatively good water conditions.

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