Abiotic stresses could have complex and diverse effects on the growth and development of forest trees. In this review, we summarized the responses of morphological, physiological and anatomical traits in woody plants to abiotic stresses, including drought, flood, extreme temperature, salinity, heavy metal, microplastics and combined stresses, especially from the xylem perspective. Under most abiotic stress, xylem hydraulic conductivity decreases, which is associated with leaf stomatal regulation and the inhibition of aquaporin (AQP) activity. Meanwhile, woody plants regulate the size and morphology of their roots and leaves to balance water absorption and transpiration. The anatomical traits are also altered, such as denser leaf stomata, narrower conduits and thicker cell walls. In addition, different stresses have unique effects, such as flood-induced adventitious roots and aeration tissues, forest fire-induced irreversible xylem damage, low temperature-induced tissue freezing, salt stress-induced hinderance of ion absorption and heavy metal-induced biological toxicity. Under stresses of drought, flooding and heavy metals, woody plants’ growth may occasionally be promoted. The effects of combined stress on the physiological, morphological and anatomical traits of woody plants are not simply additive, with the related mechanism to be further studied, especially in natural or near-natural conditions.

Forests are vital ecosystems that are increasingly threatened by environmental stress; exploring the possible trade-off between hydraulic safety and efficiency in plant xylem is crucial to understanding their environmental adaptation strategies. However, to date, there is no consensus whether such trade-offs exist among and within species. To better comprehend the mechanism of xylem water transport, in this review, we summarized previously published work on xylem hydraulic safety and efficiency trade-off from the inter-species, intra-species, and intra-tree perspectives and its influencing factors. We gathered data on xylem hydraulic safety and efficiency and their related anatomical traits, i.e., conduit diameter and inter-conduit pit membrane thickness, from a total of 653 plant species analyzed in 80 published papers. At the inter-species level, we confirmed that there is a weak hydraulic safety and efficiency trade-off. For gymnosperms and herbaceous species, the observed trade-off is stronger. At the intra-species level, the hydraulic safety and efficiency trade-off was found in individuals of the same species investigated in the literature. At the intra-tree level, there is a trade-off between hydraulic safety and efficiency for leaves, stems, and roots, and we confirmed the vessel widening hypothesis, i.e., vessel diameter in the outer wood increases from the top to the bottom of the tree. Additionally, pit membrane thickness increases as the tree height decreases, thus increasing the xylem hydraulic efficiency and affecting the trade-off. Finally, we discussed the environmental factors affecting the trade-off between hydraulic safety and efficiency in the xylem, such as plant habitats, temperature, rainfall, altitude, and soil. Further investigations of the bordered pit membrane from the three-dimensional perspective would be useful to understand the hydraulic safety and efficiency trade-off at the nanoscale.

Pinus thunbergii and Euonymus japonicus are two species commonly found in arid and semi-arid areas; however, their responses in terms of physiological traits and soil properties under drought and cadmium (Cd) stress are not clear. In this study, we carried out single and combined stress treatments consisting of drought and Cd on saplings of P. thunbergii and E. japonicus and investigated the responses in terms of the physiological traits and soil properties of both species. For both species, under single Cd stress, Cd was observed in both the xylem and phloem, while the root Cd²⁺ flow rate fluctuated at different levels of Cd stress. Under both single and combined stress, as the stress level increased, the abscisic acid (ABA) content of the leaves and roots increased significantly, while the indole-3-acetic acid (IAA) content of the leaves and roots decreased significantly. Moreover, the non-structural carbohydrate (NSC) content of the leaves, stems, and roots, as well as the leaf chlorophyll content, decreased significantly. Under drought stress, the xylem water potential and hydraulic conductivity significantly decreased, which was exacerbated by Cd stress; this led to a more significant decrease in water potential and hydraulic conductivity under the combined stresses. Meanwhile, no significant changes in the conduit lumen diameter and double-wall thickness were observed, except for the double cell wall thickness of the P. thunbergii tracheid, which increased. In addition, both the single stresses and the combined stress of drought and Cd induced significant changes in the soil properties of the two species, i.e., the ammonium nitrogen, nitrate nitrogen, and effective phosphorus of the soil increased significantly, and the increase in content was more significant under combined stress. The diversity of the soil microbial community of P. thunbergii saplings significantly increased, while no change was found in its microbial community abundance under the single stresses and combined stress; however, the diversity and abundance of the soil microbial community in E. japonicus saplings showed the opposite pattern, which indicates that the effect of Cd on soil microorganisms is more significant than the effect of drought. The activity of sucrase and catalase in P. thunbergii soil fluctuated under the single stress and combined stress when compared, and the activity of sucrase in the soil of the E. japonicus species decreased. However, its catalase activity increased significantly under the single drought and Cd stress and combined stress when compared. We found that the combined stresses exacerbated the effects of the single stress in both species. Our study provides more detailed information on the responses in terms of the physiological traits and soil properties of the two species under single and combined stress consisting of drought and Cd.