Excessive Accumulation of Chinese Fir Litter Inhibits Its Own Seedling Emergence and Early Growth—A Greenhouse Perspective

Litter accumulation can strongly influence plants’ natural regeneration via both physical and chemical mechanisms, but the relative influence of each mechanism on seedling establishment remains to be elucidated. Chinese fir (Cunninghamia lanceolata) is one of the most important commercial plantations in southern China, but its natural regeneration is poor, possibly due to its thick leaf litter accumulation. We used natural and plastic litter to study the effects of Chinese fir litter on its own seedling emergence and early growth, as well as to assess whether the effect is physical or chemical in nature. Results showed that high litter amount (800 g·m−2) significantly reduced seedling emergence and the survival rate for both natural and plastic litter. Low litter amount (200 g·m−2) exerted a slightly positive effect on root mass, leaf mass, and total mass, while high litter amount significantly inhibited root mass, leaf mass, and total mass for both natural and plastic litter. Root-mass ratio was significantly lower, and leaf-mass ratio was significantly greater under high litter cover than under control for both natural and plastic litter. Although the root/shoot ratio decreased with increasing litter amount, such effect was only significant for high litter treatment for both natural and plastic litter. Seedling robustness (aboveground biomass divided by seedling height) decreased with increasing litter amount, with high litter treatment generating the least robust seedlings. Because plastic and natural litter did not differ in their effects on seedling emergence and growth, the litter layer’s short-term influence is primarily physical. These data indicated that as litter cover increased, the initial slightly positive effects on seedling emergence and early growth could shift to inhibitory effects. Furthermore, to penetrate the thick litter layer, Chinese fir seedlings allocated more resources towards stems and aboveground growth at the expense of their roots. This study provided experimental evidence of litter amount as a key ecological factor affecting seedling development and subsequent natural regeneration of Chinese fir

Seedling emergence and early growth of Chinese fir under different light levels and seed positions: implications for natural regeneration

Chinese fir, Cunninghamia lanceolata (Lamb.) Hook. (Taxodiaceae), is an evergreen conifer primarily distributed in southern China. This species exhibits very poor natural regeneration, possibly due to low light and a thick litter layer. To improve the understanding of the natural regeneration capacity of Chinese fir, in this study, we conducted a shade house experiment to determine the optimum light requirements and seed positions for seedling emergence and early growth. The experiment involved five light levels (100%, 60%, 40%, 15%, 5% of full sunlight) and four seed positions (1 cm beneath the soil surface without litter, on the soil surface without soil–seed contact, on the soil surface and covered with litter, and 1 cm beneath the soil surface and covered with litter). Seedling emergence was highest at 5%–15% sunlight, whereas seedling height, root length, root mass, stem mass, leaf mass, and total mass were highest at 60% sunlight. For each light level, seed position significantly affected emergence and growth. The above-litter position inhibited seedling emergence and survival, while the below-litter position favored seedling emergence and early growth, particularly under high light levels. Based on these results, to enhance natural regeneration of Chinese fir, we recommend periodical thinning to increase light into the understory after successful seedling emergence. We also recommend sowing seeds deeper into the litter to improve soil contact and moisture conditions

Responses of Chinese fir and Schima superba seedlings to light gradients: Implications for the restoration of mixed broadleaf-conifer forests from Chinese fir monocultures

Although Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations are widely grown for timber production in southern China, they have low biodiversity and provide limited ecosystem services. To address this problem, C. lanceolata are increasingly mixed with broadleaf Schima superba Gardn. & Champ. (Theaceae). The success of these mixed plantations relies on introducing each species in the appropriate sequence, which requires understanding how tree species respond to light variations. We therefore compared S. superba and C. lanceolata seedling light tolerance in shaded houses under five light gradients (5%, 15%, 40%, 60%, and 100% sunlight). Our findings showed that S. superba seedlings exhibited greater net height increment (ΔHt), net diameter growth (ΔDia), leaf area, root mass, stem mass, leaf mass, and total mass under low light conditions (15% sunlight). However, as sunlight increased, these growth variables became higher in C. lanceolata seedlings. With more sunlight, both species experienced a drop in height to diameter ratio (HDR), and specific leaf area (SLA), but an elevated root to shoot ratio. Additionally, under the same light levels, S. superba seedlings exhibited greater leaf area and root to shoot ratio than C. lanceolata seedlings. Our results suggested that S. superba might be more suitable for underplanting beneath a heavy canopy due to its shade-tolerant traits. In contrast, C. lanceolata was less shade-tolerant, having an optimum seedling growth under full sunlight. These findings suggest that underplanting S. superba seedlings in C. lanceolata monoculture plantation (i.e., underplanting regeneration approach) could be a better silvicultural alternative than simultaneously planting both seedlings

Seed rain and soil seed bank in Chinese fir plantations and an adjacent natural forest in southern China: Implications for the regeneration of native species

The natural regeneration of native broadleaved species underneath forest monoculture plantations is important to recover ecosystem functions and to mitigate adverse environmental effects. To understand how seed rain and soil seed bank facilitate natural regeneration, we surveyed their density and composition in a monoculture Chinese fir plantation, a mixed Chinese fir-broadleaf plantation, and an adjacent natural broadleaved forest for two years in southern China. Twenty-eight species (16 families) were in seed rain, and 45 species (27 families) were in soil seed bank. Seed rain density did not differ significantly across stands; however, the number of taxa in seed rain was highest in the mixed plantation and lowest in the natural forest. Seed bank density was significantly higher in the mixed plantation than in the other stands (p < .05). The Sorensen similarity index of species composition between seed sources and aboveground vegetation were relatively low (<.50). The seeds of various native tree species were common in the seed bank of the plantations, indicating that seed rain and seed bank played an important role in native forest regeneration. We recommend that managers interested in sustainable forestry should take into consideration the presence of existing soil seed bank when developing their management strategies. In addition, with regard to forest regeneration process, we also recommend supplementation of the species composition by direct seeding or planting of desired species

Brain age prediction using the graph neural network based on resting-state functional MRI in Alzheimer's disease

IntroductionAlzheimer's disease (AD) is a neurodegenerative disease that significantly impacts the quality of life of patients and their families. Neuroimaging-driven brain age prediction has been proposed as a potential biomarker to detect mental disorders, such as AD, aiding in studying its effects on functional brain networks. Previous studies have shown that individuals with AD display impaired resting-state functional connections. However, most studies on brain age prediction have used structural magnetic resonance imaging (MRI), with limited studies based on resting-state functional MRI (rs-fMRI).MethodsIn this study, we applied a graph neural network (GNN) model on controls to predict brain ages using rs-fMRI in patients with AD. We compared the performance of the GNN model with traditional machine learning models. Finally, the post hoc model was also used to identify the critical brain regions in AD.ResultsThe experimental results demonstrate that our GNN model can predict brain ages of normal controls using rs-fMRI data from the ADNI database. Moreover the differences between brain ages and chronological ages were more significant in AD patients than in normal controls. Our results also suggest that AD is associated with accelerated brain aging and that the GNN model based on resting-state functional connectivity is an effective tool for predicting brain age.DiscussionOur study provides evidence that rs-fMRI is a promising modality for brain age prediction in AD research, and the GNN model proves to be effective in predicting brain age. Furthermore, the effects of the hippocampus, parahippocampal gyrus, and amygdala on brain age prediction are verified

Photoinduced Production of Chlorine Molecules from Titanium Dioxide Surfaces Containing Chloride

Titanium dioxide (TiO2) is extensively used with the process of urbanization and potentially influences atmospheric chemistry, which is yet unclear. In this work, we demonstrated strong production of Cl-2 from illuminated KCl-coated TiO2 membranes and suggested an important daytime source of chlorine radicals. We found that water and oxygen were required for the reactions to proceed, and Cl-2 production increased linearly with the amount of coated KCl, humidity of the carrier gas, and light intensity. These results suggested that water promotes the reactivity of coated KCl via interaction with the crystal lattice to release free chloride ions (Cl-). The free Cl- transfer charges to O-2 via photoactivated TiO2 to form Cl-2 and probably the O-2(-) radical. In addition to Cl-2, ClO and HOCl were also observed via the complex reactions between Cl/Cl-2 and HOx. An intensive campaign was conducted in Shanghai, during which evident daytime peaks of Cl-2 were observed. Estimated Cl-2 production from TiO2 photocatalysis can be up to 0.2 ppb/h when the TiO2-containing surface reaches 20% of the urban surface, and highly correlated to the observed Cl-2. Our results suggest a non-negligible role of TiO2 in atmospheric photochemistry via altering the radical budget.Peer reviewe