Fine roots (<2 mm in diameter) play a significant role in tree growth and stand productivity. However, knowledge of fine root dynamics in saline alkali soil remains limited. Using sequential soil core and ingrowth core methods, we assessed the dynamics of aboveground biomass (AGB) and production (AGP), fine root biomass (FRB), production (FRP) and turnover (FRT), and soil properties (water content [SWC], salt content [SSC], pH and nutrients) in 2-, 18-, 32- and 42-year-old black locust (Robinia pseudoacacia L.) stands in the Yellow River Delta, China. Corresponding to the unimodal growth pattern in AGP, the AGB of black locust rapidly increased until age 32 and then slowed down. In contrast, across all depths, FRB and FRP took a unimodal pattern with a decrease in growth around age 32, and FRT continually decreased with stand age. SWC and soil nutrients increased with stand age, whereas SSC and pH decreased with stand age until age 32 and then increased. Based on the correlation analysis, we may presume that in addition to the age effect, the decline of black locust in AGP is most likely caused by SWC, SSC, and pH stresses through the dysfunction in nutrient absorption by fine roots.

There is a wide range of naturally regenerated Morus alba in the declining Robinia pseudoacacia plantation of the Yellow River Delta. It is important to clarify the key mechanism of natural regeneration of M. alba for the transformation of declining R. pseudoacacia plantation. According to the death density of R. pseudoacacia, the plantation of R. pseudoacacia was divided into nondeclining, moderately declining, and severely declining forests. The structural characteristics of adult trees and seedlings of M. alba in different decline degrees forest were investigated. A pot experiment was conducted to study the seed germination and early seedling growth of M. alba in saline alkali soil and nonsaline alkali soil under different soil salt contents and light intensities. The results showed that the natural regeneration of M. alba was obviously affected by the decline of R. pseudoacacia plantation. With the increase of decline degree, M. alba density and seedling density first increased and then decreased, and were the highest in the medium decline plantation. Under full light intensity, the vigor index of M. alba seeds and the biomass of seedlings were significantly greater than those of 25% full light intensity. The germination rate and germination index under 1‰ soil salt content were significantly lower than those under 3‰, but the biomass of seedlings was on the contrary. The 1000-seed weight, seed germination, and seedling biomass of moderately declining R. pseudoacacia plantations were close to those of nonsaline alkali land, while significantly higher than those of nondeclining plantations, but the germination index of moderately declining R. pseudoacacia plantation was higher than that of nonsaline alkali land. Therefore, the germination ability of maternal trees in saline alkali land was higher than that in nonsaline alkali land under salt stress.

Elucidating the influence mechanisms of seed germination and seedling growth is important for revealing the natural regeneration of forest plantations. We collected the seeds from 58-year-old Quercus acutissima Carruth. forest, and the seeds were further divided into three classes: large, medium, and small, and sown under the forest gaps (I, 197.82 m²; II, 91.85 m², III, understory) to observe seed germination and early seedling growth. Precipitation in the study area and soil moisture content in the forest gaps were also observed during the trial period. The results showed that the precipitation in 2019 was similar to that in 2020; both were significantly lower than the precipitation in 2021. The difference in soil water content between gaps I and II was not significant, and both were significantly lower than III. The order of seedling emergence rate in gaps was II > III > I, but the minimum was almost close to zero in I. Large and medium seeds showed significantly greater emergence rate than small seeds. The seedlings of II had higher seedling height, ground diameter, ground diameter relative growth rate, seedling biomass, root surface area, and root volume than those of III. Large seeds had the highest ground diameter, ground diameter relative growth rate, biomass, root mass ratio, root shoot ratio, and root surface area. Correlation analysis showed that seedling biomass was significantly and positively correlated with root surface area and root volume, and significantly and negatively correlated with specific root length and specific root surface area. The regulation of soil moisture in the gap and the adaptability related to seed size were two key factors influencing the seed germination and early seedling growth of Q. acutissima.

Groundwater plays a significant role in influencing the growth and distribution of Robinia pseudoacacia L. plantations, with the largest planting area in the Yellow River Delta, by affecting the soil water–salt environment. This study aimed to clarify the mechanism of groundwater’s influence on the growth of R. pseudoacacia under different levels of groundwater mineralization (GWM) and groundwater depth (GWD). We simulated GWM of 0, 2 and 4 g L⁻¹, and GWD of 0.8, 1.3 and 1.8 m. As GWM increased, soil relative water content (SRWC) and soil salt (dissolved salt) content (SSC) increased; sapling biomass (SB), stem mass (SM), leaf mass (LM), photosynthesis characteristics (maximum net photosynthetic rate (P_n), stomatal conductance (g_s), intercellular CO₂ concentration (C_i), transpiration rate (E) and water use efficiency (WUE)) decreased; root mass (RM), root mass ratio (RMR) and root–shoot ratio (RSR) first increased then decreased; stem mass ratio (SMR) first decreased then increased; and leaf mass ratio (LMR) increased. As GWD increased, SRWC decreased, but SSC first increased then decreased; SB, RM, RMR, RSR, and photosynthesis characteristics increased; SM and LM first increased then decreased; and SMR and LMR decreased. SRWC and SSC were negatively correlated with SB and photosynthesis characteristics. SRWC was negatively correlated with RMR and RSR, whereas it was positively correlated with LMR. SSC was negatively correlated with SMR, whereas it was positively correlated with LMR. The first principal component, including SB, RM, and photosynthesis characteristics, was related to sapling growth. The second principal component, including RMR, SMR, and RSR, was mainly related to biomass allocation. In conclusion, GWM and GWD affected the soil water and salt content, which were key factors influencing the photosynthesis and growth of R. pseudoacacia. Adjustments in biomass allocation and photosynthesis were the main adaptive strategies of R. pseudoacacia to salt, drought, and flooding stress.

Pot experiments were performed to comparatively study the differences in 16 salt tolerance indices between the seedlings of six Salix matsudana clones under the stress of various concentrations of NaCl (0, 0.1%, 0.3%, 0.5%, and 0.7%), including the salt injury index, shoot fresh weight, root fresh weight, leaf water content, relative conductivity, malondialdehyde content, and antioxidant enzyme activity. The salt-tolerant clones and key indices of salt tolerance were selected. Transcriptome sequencing analysis was performed on the selected salt-tolerant and salt-sensitive clones under salt stress, and the links between the physiological indices of salt tolerance and gene expression were analyzed. Results: (1) Superoxide dismutase (SOD), peroxidase (POD), chlorophyll, and net photosynthetic rate were closely related to the salt tolerance of Salix matsudana at the seedling stage. The regression equation was constructed as follows: salt tolerance index (y) = 0.224x₁₀ + 0.216x₁₁ + 0.127x₁₂ + 0.191x₇ − 0.187 (x₁₀ = chlorophyll, x₁₁ = SOD, x₁₂ = POD, x₇ = net photosynthetic rate). (2) The number of differentially expressed genes between the seedlings of salt-tolerant and salt-sensitive clones varied with the time of exposure (0 h, 4 h, 12 h, and 24 h) to 200 mmol·L⁻¹ NaCl stress. The most differentially expressed genes in Sm172 were detected upon 24 h vs. 4 h of salt treatment, while the most in Sm6 were in the 24 h vs. 0 h comparison. Gene Ontology analysis and Kyoto Encyclopedia of Genes and Genomes analysis showed that several differentially expressed genes were involved in carotenoid biosynthesis and plant mitogen-activated protein kinase signaling pathways. The nine highly expressed transcription factor genes (Sm172-f2p30-2392, Sm172-f2p28-2386, Sm6-f8p60-2372, Sm6-f2p39-2263, Sm6-f16p60-2374, Sm6-f3p60-931, Sm6-f2p60-1067, Sm172-f3p54-1980, and Sm172-f3p54-1980) were closely correlated with the four key indices of salt tolerance. These genes could become genetic resources for salt tolerance breeding of Salix matsudana.

The distribution of vegetation in coastal wetlands is significantly influenced by soil properties. However, the mechanisms of how soil characteristics impact the physiological processes of Tamarix chinensis forests remain underexplored. This study examined changes in the soil physicochemical properties and structural attributes of natural T. chinensis forests in the Yellow River Delta with increasing distance from the shoreline. T. chinensis trees were classified into healthy, intermediate, and dying categories based on growth potential, and dynamic changes in salt ions and non-structural carbohydrates (NSCs) were investigated. Results indicated that increasing distance from the shoreline corresponded to decreased soil salinity and pH, and increased soil moisture. T. chinensis mortality rate decreased, while tree height and ground diameter increased with distance. Soil salt content was positively correlated with T. chinensis mortality, but negatively correlated with tree height and ground diameter. Trees with lower growth potential had higher Na⁺ but lower K⁺ and K⁺/Na⁺ ratio. Soil salt content was positively correlated with root and stem Na⁺, while soil moisture was positively correlated with leaf NSCs. These findings suggest that soil salt content and moisture significantly influence T. chinensis ion absorption and NSC accumulation, with sodium toxicity being a key factor in the spatial distribution of T. chinensis forests.