The ecophysiological mechanisms underlying the pattern of bimodal elevational distribution of evergreen tree species remain incompletely understood. Here we used leaf economics spectrum (LES) theory to explain such patterns. We measured leaf economic traits and constructed an LES for the co-existing 19 evergreen and 15 deciduous species growing in evergreen broad-leaved forest at low elevation, beech-mixed forest at middle elevation and hemlock-mixed forest at high elevation in Mao'er Mountain, Guangxi, Southern China (25°50′N, 110°49′E). Leaf economic traits presented low but significant phylogenetic signal, suggesting trait similarity between closely related species. After considering the effects of phylogenetic history, deciduous species in general showed a more acquisitive leaf strategy with a higher ratio of leaf water to dry mass, higher leaf nitrogen and phosphorous contents, higher photosynthetic and respiratory rates and greater photosynthetic nitrogen-use efficiency. In contrast, evergreen species exhibited a more conservative leaf strategy with higher leaf mass per area, greater construction costs and longer leaf life span. With the elevation-induced decreases of temperature and soil fertility, both evergreen and deciduous species showed greater resource conservation, suggesting the increasing importance of environmental filtering to community assembly with increasing elevation. We found close inter-specific correlations between leaf economic traits, suggesting that there are strong genetic constraints limiting the independent evolution of LES traits. Phylogenetic signal increased with decreasing evolutionary rate across leaf economic traits, suggesting that genetic constraints are important for the process of trait evolution. We found a significantly positive relationship between primary axis species score (PASS) distance and phylogenetic distance across species pairs and an increasing average PASS distance between evergreen and deciduous species with increasing elevation, implying that the frequency of distantly related evergreen and deciduous pairs with wide spreading of leaf economic values increases with increasing elevation. Our findings thus suggest that elevation acts as an environmental filter to both select the locally adapted evergreen and deciduous species with sufficient phylogenetic variation and regulate their distribution along the elevational gradient based on their coordinated spreading of phylogenetic divergence and leaf economic variation.

To explore the interactive effects of wind and light on growth and morphogenesis, poplar saplings were subjected to two different levels of wind and light. Morphogenesis, growth, water relations and gas exchange were examined in a factorial experiment. The results showed that wind reduced both shoot height and root growth but did not affect shoot diameter or root/shoot ratio, while light promoted both shoot and root growth, and the root/shoot ratio. However, light and wind consistently increased stem taper. Wind and light reduced leaf water potential and increased stem xylem cavitation at midday. Lateral shade decreased root and stem hydraulic conductance, while wind reduced stem hydraulic conductance without influencing root hydraulic conductance. Wind and light reduced the vulnerability of poplar saplings to xylem cavitation. Lateral shade strongly reduced the CO ₂ assimilation rate and stomatal conductance. Wind reduced the assimilation rate at midday under full sunlight. In summary, both light and wind increased stem taper, although the two environmental factors had opposite effects on the growth of poplar saplings. Lateral shading reduced the growth of shoots and roots of poplar saplings by restricting assimilation, and light promoted greater allocation of carbon to roots than to shoots, and to a lower rather than upper portion of the stem, leading to increased root/shoot ratio and stem taper. At the sapling stage, wind reduced shoot height and root growth primarily by interrupting water balance, and also induced thigmomorphogenic responses, such as increased taper. Finally, lateral shading did not suppress the thigmomorphological responses to wind.

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.

Two broadleaf evergreen canopy species (Schima superbaandEngelhardia 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.superbaregenerates its leaves in February, whileE.roxburghianaregenerates its leaves in May. The mothThalassodes quadrariaattacked the two broadleaf evergreen species in March to April, and the newly produced leaves were removed forS.superbabut not forE.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 inS.superbaduring 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 ofS.superba. In contrast,E.roxburghianaexperienced 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 impactedS.superbathanE.roxburghiana.

How the mortality and growth of tree species vary with the iso-anisohydric continuum and xylem vulnerability is still being debated. We conducted a precipitation reduction experiment to create a mild drought condition in a forest in the Baotianman Mountains, China, a sub-humid region. Three main sub-canopy tree species in this region were examined. After rainfall reduction, Lindera obtusiloba showed severe dieback, but two other co-occurring species did not show dieback. The water potential at stomatal closure of Dendrobenthamia japonica, L. obtusiloba and Sorbus alnifolia was −1.70, −2.54 and −3.41 MPa, respectively, whereas the water potential at 88 % loss in hydraulic conductivity of the three species was −2.31, −2.11 and −7.01 MPa, respectively. Taken together, near-anisohydric L. obtusiloba with vulnerable xylem was highly susceptible to drought dieback. Anisohydric S. alnifolia had the most negative minimum water potential, and its xylem was the most resistant to cavitation. Isohydric D. japonica conserved water by rapidly closing its stomata. Ultimately, the hydraulic safety margin (HSM) of L. obtusiloba was the smallest among the three species, especially in precipitation-reduced plots. In terms of the stomatal safety margin (SSM), L. obtusiloba was negative, while S. alnifolia and D. japonica were positive. Of the two species without dieback, rainfall reduction decreased growth of D. japonica, but did not influence growth of S. Alnifolia; meanwhile, rainfall reduction led to a decrease of non-structural carbohydrates (NSCs) in D. japonica, but an increase in S. alnifolia. It is concluded that HSM as well as SSM allow interpreting the sensitivity of the three sub-canopy species to drought. The drought-induced dieback of L. obtusiloba is determined by the interaction of stomatal behaviour and xylem vulnerability, and the species could be sensitive to climate change-caused drought although still in sub-humid areas. The isohydric/anisohydric degree is associated with NSCs status and growth of plants.