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.

Xylem traits are critical plant functional traits associated with water transport, mechanical support, and carbohydrate and water storage. Studies on the xylem hydraulic efficiency–safety tradeoff are numerous; however, the storage function of xylem parenchyma is rarely considered. The effects of a substantial number of xylem traits on water transport, embolism resistance, mechanical support, storage capacity and nonstructural carbohydrate (NSC) content were investigated in 19 temperate broadleaf species planted in an arid limestone habitat in northern China. There was no xylem hydraulic efficiency–safety tradeoff in the 19 broadleaf species. The total parenchyma fraction was negatively correlated with the fiber fraction. Embolism resistance was positively correlated with indicators of xylem mechanical strength such as vessel wall reinforcement, vessel wall thickness and fiber wall thickness, and was negatively related to the axial parenchyma fraction, especially the paratracheal parenchyma fraction. The paratracheal parenchyma fraction was positively correlated with the ratio of the paratracheal parenchyma fraction to the vessel fraction. In addition, the xylem NSC concentration was positively related to the total parenchyma fraction and axial parenchyma fraction. There was a storage capacity–embolism resistance tradeoff in the xylem of 19 broadleaf species in arid limestone habitats. We speculate that the temperate broadleaf species may show a spectrum of xylem hydraulic strategies, from the embolism resistance strategy related to a more negative P50 (the water potential corresponding to 50% loss of xylem conductivity) to the embolization repair strategy based on more paratracheal parenchyma.

Stomatal regulation serves as an important strategy for plants to adapt to drought. However, the understanding of how complexes of plant-functional traits vary along the continuum from isohydry to anisohydry remains insufficient. In this study, we investigated a proxy of the degree of iso/anisohydry—the water potential at stomatal closure—and a series of functional traits of leaves and branches in 20 temperate broadleaf species planted in an arid limestone habitat in northern China. The results showed that the water potential at stomatal closure was significantly correlated with many functional traits. At the anisohydric end of the spectrum, species had a higher leaf carbon content and vein density, a greater stomatal length, a thicker lower leaf epidermis, higher embolism resistance, higher wood density, a greater Huber value, a greater ratio of fiber wall thickness to xylem lumen diameter, a larger proportion of total fiber wall area to xylem cross-sectional area, a lower water potential at the turgor loss point (TLP), a smaller relative water content at the TLP, a lower osmotic potential at full turgor and a smaller specific leaf area. It is concluded that a continuum of coordination and trade-offs among co-evolved anatomical and physiological traits gives rise to the spectrum from isohydry to anisohydry spanned by the 20 tree species, and the anisohydric species showed stronger stress resistance, with greater investment in stems and leaves than the isohydric species to maintain stomatal opening under drought conditions.

Enunciating the carbon storage across various types of forests is a precondition for comprehending forest ecosystem carbon cycling. However, previous studies regarding forest carbon storage were primarily conducted in the general temperature zones, with a limited understanding of carbon storage in different forest types within climate transition zones. In this study, we employed biomass models to explore the carbon storage in three types of natural secondary forests (Pinus armandii forest, Quercus aliena forest, and Q. aliena–P. armandii mixed forest) in the transition zone between the northern subtropical and warm temperate regions of China. Furthermore, we used the variance decomposition analysis and random forest model to determine the key factors influencing carbon storage in three types of natural secondary forests. Our results indicated that the carbon storage of wood and soil layers in the Q. aliena–P. armandii mixed forest was significantly higher than that in the P. armandii and Q. aliena forests. Total carbon storage was ranked as follows: Q. aliena–P. armandii mixed forest (266.09 t/ha) > P. armandii forest (222.89 t/ha) > Q. aliena forest (212.46 t/ha). Our results also revealed that carbon storage of the wood layer was jointly regulated by environmental factors, plant physiological characteristics, and soil properties, while soil carbon storage was mainly affected by soil properties. These results highlight the significant advantages of mixed conifer–broadleaf forests in carbon storage, emphasizing the importance of mixed natural secondary forests in carbon cycling and ecosystem services. This study provides scientific evidence for enhancing forest carbon sink functions and developing forest conservation and management policies to combat climate change.

Pit membranes in between neighboring conduits of xylem play a crucial role in plant water transport. In this review, the morphological characteristics, chemical composition and mechanical properties of bordered pit membranes were summarized and linked with their functional roles in xylem hydraulics. The trade-off between xylem hydraulic efficiency and safety was closely related with morphology and properties of pit membranes, and xylem embolism resistance was also determined by the pit membrane morphology and properties. Besides, to further investigate the effects of bordered pit membranes morphology and properties on plant xylem hydraulic functions, here we modelled three-dimensional structure of bordered pit membranes by applying a deposition technique. Based on reconstructed 3D pit membrane structures, a virtual fibril network was generated to model the microflow pattern across inter-vessel pit membranes. Moreover, the mechanical behavior of intervessel pit membranes was estimated from a single microfibril’s mechanical property. Pit membranes morphology varied among different angiosperm and gymnosperm species. Our modelling work suggested that larger pores of pit membranes do not necessarily contribute to major flow rate across pit membranes; instead, the obstructed degree of flow pathway across the pit membranes plays a more important role. Our work provides useful information for studying the mechanism of microfluid flow transport across pit membranes and also sheds light on investigating the response of pit membranes both at normal and stressed conditions, thus improving our understanding on functional roles of pit membranes in xylem hydraulic function. Further work could be done to study the morphological and mechanical response of bordered pit membranes under different dehydrated conditions, as well as the related microflow behavior, based on our constructed model.