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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. Study Implications: In recent decades, the rapidly increasing plantations in the Yellow River Delta of China as elsewhere have brought enormous economic and ecological benefits. However, afforestation in the Yellow River Delta is complex due to soil salinity and unique soil water and nutrient dynamics. The most extensive Robinia pseudoacacia plantations have shown a noticeable decline. Fine root dynamics affected by soil environments and stand age play a significant role in tree growth and stand productivity. Our study illustrates the general growth patterns of tree growth and fine root dynamics of R. pseudoacacia plantations under salt stress and related to soil environments (nutrients, moisture, salinity) in the Yellow River Delta, China. Young and middle-aged R. pseudoacacia plantations could reduce soil salt content although soil salt content and pH increased again after the plantations reached a certain age (42 years in this study). The increase in soil salt content greatly limited fine root biomass and tree growth. As a result, thinning stands at an appropriate time (after 32 years in this study) and relevant engineering measures to reduce soil salt content should be carried out to improve fine root growth and stand productivity. In addition, replacing old R. pseudoacacia stands that suffer severe decline and mortality with new plantations will be necessary for sustainable management of plantations and maintaining a reasonably high level of forest productivity in coastal saline-alkali areas.

Background: Natural regeneration is critically important for the sustainable management of artificial forests. Studies have been investigated for the effects of seedlings height distribution and ontogenetic adaptability in a 60-year-old Quercus acutissima artificial pure forest in Mount Tai. Results: The results showed that the height distribution of seedlings under the forest was pyramidal-shape from the year 2010 to 2017. In 2017, the density of seedlings under the forest was only 61.63% of that in 2010, and the number of individuals at each height class was lower than that in 2010, especially the individuals at 20-40cm height class only 32% of that in 2010. With the increase of seedling height under the forest, seedling biomass (SB), ground diameter (GD), root mass ratio (RMR), total root length (TRL), root volume (RV), root average diameter (RAD) and nonstructural carbohydrates (NSC) increased significantly, but leaf mass ratio (LMR), leaf area ratio (LAR), photosynthetic/non-photosynthetic tissue ratio (P/NP), specific root length (SRL) and specific root surface area (SRA) decreased significantly. Correlation analysis showed that SB was negatively correlated with LMR, LAR, P/NP, SRL and SRA, and positively correlated with RMR, TRL, RV, RAD and NSC. NSC was negatively correlated with LMR, LAR, P/NP, SRL and SRA. Conclusions: Therefore, with the increase of seedlings height, the increased carbon storage was helpful for the seedlings of Q. acutissima to survive under the forest for a long time, but the reduced ability of light interception, photosynthetic tissue ratio, water and nutrient absorption capacity contributed to their low survival ratio under the forest.

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