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.

Fitting and comparing three sets of additive biomass models for prediction of biomass or carbon stocks of natural and planted Quercus variabilis Blume forests. To make the sum of estimated values from biomass models of various components of a tree equal to estimated tree total biomass for Quercus variabilis Blume (cork oak) forests in North China, single-tree additive biomass models were developed. 100 trees from 100 plots in North China were felled to obtain biomass of aboveground components, and roots of 19 of those trees were extracted for measurement of root biomass. After Box–Cox transformations of variables, two sets of independent component biomass models with a dummy variable to define stand origin were separately built using linear mixed effects analyses (one set of models with site as a random factor; the other set without any random factor). Then three methods were compared to force additivity of those models: sums of linear mixed effects models, sums of linear models, and simultaneous equation fits based on linear models. Model parameters were estimated by ordinary least squares (OLS) or seemingly unrelated regression procedures (SUR). Coefficients of determination (R ²), root mean square error (RMSE), confidence interval of predictions (CI), residuals plots and histograms of residuals indicated that models fitted with sums of linear mixed effects models were the least biased and most precise at estimating total aboveground biomass. Further testing for the linear mixed effects models with jackknife validation and prediction sum of squares (PRESS) statistics indicated that the additive biomass models can be used to predict biomass or carbon stocks of cork oak forests in North China within specific tree diameter at breast height and height ranges.

Sapindus mukorossi is widely distributed in tropical and subtropical areas of southern China; the seed kernel oil is potential biodiesel material, and the saponins extracted from fruit pericarp are very valuable efficient natural surfactants. Therefore, S. mukorossi is an ideal tree species for developing forestry bioenergy and multiple other products. In this study, 42 S. mukorossi fruits from mother trees were collected from 39 distinct locations in 12 Chinese provinces to infer fruit and seed trait responses to environmental factors. Canonical correspondence analysis (CCA) was conducted using 21 horticultural fruit traits and 10 environmental factors that represented different climatic and geographic conditions throughout southern China. CCA revealed well-developed patterns of natural phenotypic variation, and insight into the ecological factors that are potentially important in shaping this variation. The results presented here further elucidate the natural distribution and ecological adaptations of wild S. mukorossi resources, which will be valuable for S. mukorossi cultivation by helping identify ideal planting areas. The germplasm resources with extensive morphological variation can also contribute to S. mukorossi breeding in the future by helping develop new cultivars with high saponin yield.

Understanding the response characteristics of fine roots to soil drought of different degrees is essential for revealing the ecological adaptability of trees to different water environments and diverse plant resource absorption strategies. This study focused on a Chinese white poplar (Populus tomentosa) plantation stand, which gradually experienced the process of deep soil drying. In 2019 and 2021, by measuring the fine-root length density (FRLD), mean root diameter (MRD), specific root length (SRL), and root tissue density (RTD) of 1920 root samples and continuously monitoring the soil water content (SWC) in 0–600 cm soil layers, we explored the response characteristics of fine-root distributions and morphological traits relative to soil drought of different degrees. The results showed that P. tomentosa primarily changed the fine-root vertical distribution rather than the total amount of fine roots for coping with soil drought of different degrees. Shallow soil drought induced more fine-root distributions in the deep soil layer, while drought in both shallow and deep soil further aggravated this trend. Shallow soil drought restrained shallow fine-root growth, yet deep soil drought promoted deep fine-root growth. The very deep fine roots (400–600 cm) were more sensitive to soil drought than shallow fine roots. The shallow soil drought significantly increased the SRL of very deep fine roots; in contrast, when deep soil drought also occurred, the MRD and SRL significantly increased and decreased, respectively. In addition, fine-root morphological traits exhibited significant vertical spatial and temporal variation. MRD increased and then decreased, and the RTD gradually decreased with depth, while SRL had an increased trend in the very deep soil layer (400–600 cm). When the rainy season came, MRD and SRL increased and decreased, respectively. In conclusion, when facing gradual deep soil drying, P. tomentosa will use a large range of rooting patterns to meet the water demand of the canopy. These patterns range from “drought tolerant strategies” by distributing more fine roots in the deeper soil layer where water is abundant to “drought tolerant strategies” by changing very deep fine-root morphological traits to improve water-absorbing and transporting efficiencies. Our findings provide insight into the ecological adaption strategy of tree root systems relative to soil drought of different degrees in arid and semi-arid regions and provide crucial theoretical support for developing water management technologies to cope with deep soil drying under climate change.