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Extreme drought events are becoming frequent globally, resulting in widespread plant mortality and forest dieback. Although savanna vegetation cover ~20% of the earth’s land area, their responses to extreme drought have been less studied than that of forests. Herein, we quantified branch dieback, individual mortality and the associated physiological responses of four evergreen shrubs (Tarenna depauperate Hutch., Maytenus esquirolii (H. Lév.) C.Y. Cheng, Murraya exotica L., Jasminum nudiflorum Lindl.) in a savanna ecosystem in Southwest China to an incidence of extreme drought during 2019 and 2020. We found that 80–100% of the individuals of these species exhibited branch dieback, whereas individual mortality was only found in T. depauperate (4.5%). All species showed high resistance to stem embolism (P50, water potential at 50% loss of hydraulic conductivity ranged from −5.62 to −8.6 MPa), whereas the stem minimum water potentials reached −7.6 to ca −10.0 MPa during the drought. The low water potential caused high native embolism levels (percentage loss of hydraulic conductivity (PLC) 23–65%) in terminal branches, and the remaining stems maintained 15–35% PLC at the end of the drought. Large within-individual variations in stem vulnerability to embolism were detected, and shedding of vulnerable branches could be a mechanism for shrubs to reduce water and carbon consumption. Overall, the content of total nonstructural carbohydrates (NSC) and their components in the stem were generally comparable to or higher than those in the rainy season in three of the four species. Because the leaves were turgor-less for most time during the drought, high NSC levels during the drought could be due to recycling of NSC from dead branches or translocation from roots. Our results suggest high tolerance of savanna shrub species to extreme drought, which could be facilitated by high embolism resistance in some stems and shedding of vulnerable branches to maintain individual water and carbon balance.

Water use efficiency (WUE) is a key physiological trait in studying plant carbon and water relations. However, the determinants of WUE across a large geographical scale are not always clear, limiting our capacity to predict WUE in response to future global climate change. We propose that tree WUE is influenced by calcium (Ca) availability and precipitation. In addition, although it is well-known that transpiration is the major driving force for passive nutrient uptake, the linkage between these two processes has not been well-established. Because Ca uptake is an apoplastic and passive process that purely relies on transpiration, and there is no translocation once assimilated, we further developed a theoretical model to quantify the relationship between tree Ca accumulation and WUE using soil-to-plant calcium ratio (SCa/BCa) and tree WUE derived from δ13C. We tested our theoretical model and predicted relationships using three common tree species across their native habitats in Northern China, spanning 2300 km and a controlled greenhouse experiment with soil Ca concentrations manipulated. We found that tree WUE was negatively related to precipitation of the growing season (GSP) and positively with soil Ca. A multiple regression model and a path analysis suggested a higher contribution of soil Ca to WUE than GSP. As predicted by our theoretical model, we found a positive relationship between WUE and SCa/BCa across their distribution ranges in all three tree species and in the controlled experiment for one selected species. This relationship suggests a tight coupling between water and Ca uptake and the potential use of SCa/BCa to indicate WUE. A negative relationship between SCa/BCa and GSP also suggests a possible decrease in tree Ca accumulation efficiency in a drier future in Northern China.

Characterizing differences in key functional traits between simple-leaved (SL) and compound-leaved (CL) tree species can contribute to a better understanding of the adaptive significance of compound leaf form. In particular, this information may provide a mechanistic explanation to the long-proposed fast-growth hypothesis of CL tree species. Here, using five SL and five CL tree species co-occurring in a typical temperate forest of Northeast China, we tested whether higher hydraulic efficiency underlies potentially high photosynthetic capacity in CL species. We found that the CL species had significantly higher hydraulic conductance at the whole-branch level than the SL species (0.52 ± 0.13 vs 0.15 ± 0.04 × 10-4 kg m-2 s-1 Pa-1, P = 0.029). No significant difference in net photosynthetic rate (14.7 ± 2.43 vs 12.5 ± 2.05 μmol m-2 s-1, P = 0.511) was detected between these two groups, but this was largely due to the existence of one outlier species in each of the two functional groups. Scrutinization of the intragroup variations in functional traits revealed that distinctions of the two outlier species in wood type (ring- vs diffuse-porous) from their respective functional groups have likely contributed to their aberrant physiological performances. The potentially high photosynthetic capacity of CL species seems to require ring-porous wood to achieve high hydraulic efficiency. Due to its limitation on leaf photosynthetic capacity, diffuse-porous wood with lower hydraulic conductivity largely precludes its combination with the 'throw-away' strategy (i.e., annually replacing the stem-like rachises) of compound-leaved tree species, which intrinsically requires high carbon assimilation rate to compensate for their extra carbon losses. Our results for the first time show clear differentiation in hydraulic architecture and CO2 assimilation between sympatric SL and CL species, which contributes to the probing of the underlying mechanism responsible for the potential fast growth of trees with compound leaves.

Xylem vessels are used by most angiosperm plants for long-distance water and nutrient transport. Vessel length is one of the key functional traits determining plant water-transport efficiency. Additionally, determination of maximum vessel length is necessary for correct sample collection and measurements in hydraulic studies to avoid open-vessel and cutting-under-tension artifacts. Air injection and silicone injection (BLUESIL RTV141A and B mixtures) are two widely used methods for maximum vessel length determination. However, the validity of both methods needs to be carefully tested for species with different vessel lengths. In this study, we tested the air-injection and silicone-injection methods using eight species with different vessel lengths: short (1 m). We employed a novel approach using RTV141A injection without the RTV141B hardener as a reference method because RTV141A cannot penetrate inter-vessel pit membranes and is not prone to hardening/solidification effects during the injection process. The results revealed that the silicone-injection method substantially underestimated the maximum vessel length of all eight species. However, the air-injection method tended to overestimate the maximum vessel length in five out of eight species. The ratio of underestimation of the silicone-injection method was higher for species with longer vessels, but the overestimation of the air-injection method was independent of the vessel length. Moreover, air injection with different pressures—ranging from 40 to 300 kPa—resulted in comparable results. We conclude that the conventional silicone-injection method can underestimate the vessel length, whereas the air-injection method can overestimate the maximum vessel length, particularly for long-vessel led species. We recommend RTV141A-only injection for determining the maximum vessel length, and it can also be used to validate the use of the air-injection and conventional silicone-injection methods for a given species.

Habitat differentiation between polyploid and diploid plants are frequently observed, with polyploids usually occupying more stressed environments. In woody plants, polyploidization can greatly affect wood characteristics but knowledge of its influences on xylem hydraulics is scarce. The four Betula species in NE China, representing two diploids and two polyploids with obvious habitat differentiation, provide an exceptional study system for investigating the impact of polyploidization on environmental adaptation of trees from the point view of xylem hydraulics. To test the hypothesis that changes in hydraulic architecture play an important role in determining their niche differentiation, we measured wood structural traits at both the tissue and pit levels and quantified xylem water transport efficiency and safety in these species. The two polyploids had significantly larger hydraulic weighted mean vessel diameters than the two diploids (45.1 and 45.5 vs 25.9 and 24.5 μm) although the polyploids are occupying more stressed environments. As indicated by more negative water potentials corresponding to 50% loss of stem hydraulic conductivities, the two polyploids exhibited significantly higher resistance to drought-induced embolism than the two diploids (-5.23 and -5.05 vs -3.86 and -3.13 MPa) despite their larger vessel diameters. This seeming discrepancy is reconciled by distinct characteristics favoring greater embolism resistance at the pit level in the two polyploid species. Our results showed clearly that the two polyploid species have remarkably different pit-level anatomical traits favoring greater hydraulic safety than their congeneric diploid species, which have likely contributed to the abundance of polyploid birches in more stressed habitats; however, less porous inter-conduit pits together with a reduced leaf to sapwood area may have compromised their competitiveness under more favorable conditions. Contrasts in hydraulic architecture between diploid and polyploid Betula species suggest an important functional basis for their clear habitat differentiation along environmental gradients in Changbai Mountain of NE China.

To understand water-use strategies of woody bamboo species, sap flux density (Fd) in the culms of a woody bamboo (Bambusa vulgaris Schrader ex Wendland) was monitored using the thermal dissipation method. The daytime and night-time Fd were analyzed in the dry and rainy seasons. Additionally, diurnal changes in root pressure, culm circumference, and stomatal conductance (gs) were investigated to characterize the mechanisms used to maintain diurnal water balance of woody bamboos. Both in the dry and rainy seasons, daytime Fd responded to vapor pressure deficit (VPD) in an exponential fashion, with a fast initial increase in Fd when VPD increased from 0 to 1 kPa. The Fd and gs started to increase very fast as light intensity and VPD increased in the morning, but they decreased sharply once the maximum value was achieved. The Fd response of this woody bamboo to VPD was much faster than that of representative trees and palms growing in the same study site, suggesting its fast sap flow and stomatal responses to changes in ambient environmental factors. The Fd in the lower and higher culm positions started to increase at the same time in the morning, but the Fd in the higher culm position was higher than that of the lower culm in the afternoon. Consistently, distinct decreases in its culm circumference in the afternoon were detected. Therefore, unlike trees, water storage of bamboo culms was not used for its transpiration in the morning but in the afternoon. Nocturnal sap flow of this woody bamboo was also detected and related to root pressure. We conclude that this bamboo has fast sap flow/stomatal responses to irradiance and evaporative demands, and it uses substantial water storage for transpiration in the afternoon, while root pressure appears to be a mechanism resulting in culm water storage recharge during the night.

The lowland tropical rainforests in Xishuangbanna, Southwest (SW) China, mark the northern limit of Asian tropics. Fog has been hypothesized to play a role in maintaining rainforests and tropical crop production in this region, but the physiological mechanism has not been studied. The goals of this study were to characterize the seasonal dynamics in photosynthesis and to assess the potential for fog to mitigate chilling-induced photodamage for tropical trees and crops in Xishuangbanna. We measured seasonal dynamics in light-saturated net photosynthetic rate (Aa), stomatal conductance (gs), intercellular CO2 concentration, quantum yield of Photosystem II (Fv/Fm) and maximum P700 changes (Pm; indicates the amount of active PSI complex), as well as chilling resistance and fog (light/shading) effects on low temperature-induced decline in Fv/Fm and Pm for native tree and introduced lower latitude tree or woody shrub species grown in a tropical botanical garden. Despite significant decreases in Aa, gs, Pm and Fv/Fm, most species maintained considerably high Aa during the cool season (2.51– 14.6 μmol m −2 s −1 ). Shaded leaves exposed to seasonal low temperatures had higher Fv/Fm than sun-exposed leaves in the cool season. All species could tolerate 1.4 °C in the dark, whereas a combined treatment of low temperature and high light caused a distinctly faster decline in Pm and Fv/Fm compared with low temperature treatment alone. Because fog persistence avoids or shortens the duration of high light condition in the morning when the temperatures are still low, our results provide support for the hypothesis that fog reduces chilling damage to tropical plants in this region and thus plays a role in maintaining tropical rainforests and agriculture in SW China.

Despite considerable investigations of diurnal water use characteristics in different plant functional groups, the research on daily water use strategies of woody bamboo grasses remains lacking. We studied the daily water use and gas exchange of Sinarundinaria nitida (Mitford) Nakai, an abundant subtropical bamboo species in Southwest China. We found that the stem relative water content (RWC) and stem hydraulic conductivity (K(s)) of this bamboo species did not decrease significantly during the day, whereas the leaf RWC and leaf hydraulic conductance (K(leaf)) showed a distinct decrease at midday, compared with the predawn values. Diurnal loss of K(leaf) was coupled with a midday decline in stomatal conductance (g(s)) and CO(2) assimilation. The positive root pressures in the different habitats were of sufficient magnitude to refill the embolisms in leaves. We concluded that (i) the studied bamboo species does not use stem water storage for daily transpiration; (ii) diurnal down-regulation in K(leaf) and gs has the function to slow down potential water loss in stems and protect the stem hydraulic pathway from cavitation; (iii) since K(leaf) did not recover during late afternoon, refilling of embolism in bamboo leaves probably fully depends on nocturnal root pressure. The embolism refilling mechanism by root pressure could be helpful for the growth and persistence of this woody monocot species.