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Transpiration is a key process that couples the land-atmosphere exchange of water and carbon, and its maximum water transport ability affects plant productivity. Functional traits significantly influence the maximum transpiration rate; however, which factor plays the dominant role remains unknown. SAPFLUXNET dataset, which includes sap flux density of diverse species worldwide, provides fundamental data to test the importance of photosynthetic and hydraulic traits on maximum tree-level sap flux density (J s_max ). Here, we investigated variations in J s_max of 2194 trees across 129 species using data from the SAPFLUXNET dataset, and analysed the relationship of J s_max with photosynthetic and hydraulic traits. Our results indicated that J s_max was positively correlated with photosynthetic traits at both leaf and tree level. Regarding hydraulic traits, J s_max was positively related to xylem hydraulic conductivity (K s ), leaf-specific hydraulic conductivity (K l ), xylem pressure inducing 50 % loss of hydraulic conductivity (P 50 ), xylem vessel diameter (V dia ), and leaf-to-sapwood area ratio (A l A s ). Random forest model showed that 87 % of the variability in J s_max can be explained by functional traits, and hydraulic traits (e.g., P 50 and sapwood area, A s ) exerted larger effects on J s_max than photosynthetic traits. Moreover, trees with a lower sapwood area or depth could increase their sap flux density to compensate for the reduced whole-tree transpiration. J s_max of the angiosperms was significantly higher than that of the gymnosperms. Mean annual total precipitation (MAP) were positively related to J s_max with a weak correlation coefficient. Furthermore, J s_max showed a significant phylogenetic signal with Blomberg's K below 0.2. Overall, tree species with acquisitive resource economics or more efficient hydraulic systems show higher water transport capacity, and the efficiency of xylem hydraulic system rather than the demand for carbon uptake predominantly determines water transport capacity., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Subtropical broadleaved forests play a crucial role in supporting terrestrial ecosystem functions, but little is known about their belowground soil fungal communities despite that they have central functions in C, N, and P cycles. This study investigated the structures and identified the drivers of soil fungal communities in subtropical deciduous and evergreen broadleaved forests, using high-throughput sequencing and FUNGuild for fungal identification and assignment to the trophic guild. Fungal richness was much higher in the deciduous than in the evergreen forest. Both forests were dominated by Ascomycota and Basidiomycota phyla, but saprophytic fungi were more abundant in the deciduous forest and ectomycorrhizal fungi predominated in the evergreen forest. Fungal communities had strong links to plant and soil properties. Specifically, plant diversity and litter biomass were the main aboveground drivers of fungal diversity and composition in the deciduous forest, while host effects were prominent in the evergreen forest. The belowground factors, i.e., soil pH, water content, and nutrients especially available P, were identified as the primary drivers of soil fungal communities in the broadleaved forests. Co-occurrence network analysis revealed assembly of fungal composition in broadleaved forest soils was non-random. The smaller modularity of the network in the deciduous forest reflects lower resistance to environment changes. Concluding, these results showed that plant community attributes, soil properties, and potential interactions among fungal functional guilds operate jointly on the divergence of soil fungal community assembly in the two broadleaved forest types.

The phenomenon of overyielding in species-diverse plant communities is mainly attributed to complementary resource use. Vertical niche differentiation belowground might be one potential mechanism for such complementarity. However, most studies that have analysed the diversity/productivity relationship and belowground niche differentiation have done so for fully occupied sites, not very young tree communities that are in the process of occupying belowground space. Here we used a 5–6 year old forest diversity experiment to analyse how fine-root (<2 mm) production in ingrowth cores (0–30 cm) was influenced by tree species identity, as well as the species diversity and richness of tree neighbourhoods. Fine-root production during the first growing season after the installation of ingrowth cores increased slightly with tree species diversity, and four-species combinations produced on average 94.8% more fine-root biomass than monocultures. During the second growing season, fine-root mortality increased with tree species diversity, indicating an increased fine-root turnover in species-rich communities. The initial overyielding was attributable to the response to mixing by the dominant species, Pseudotsuga menziesii and Picea abies, which produced more fine roots in mixtures than could be expected from monocultures. In species-rich neighbourhoods, P. abies allocated more fine roots to the upper soil layer (0–15 cm), whereas P. menziesii produced more fine roots in the deeper layer (15–30 cm) than in species-poor neighbourhoods. Our results indicate that, although there may be no lasting overyielding in the fine-root production of species-diverse tree communities, increasing species diversity can lead to substantial changes in the production, vertical distribution, and turnover of fine roots of individual species.

The mechanism whereby tree species richness and identity affect the production of fine roots (≤ 2 mm) in forests remains controversial. Complementarity effects (via resource partitioning and facilitation, CEs) and selection effects (that is, dominant of species with particular traits, SEs) are the two hypotheses to explain biodiversity effects on ecosystem functions. This study aimed to (1) examine how tree species diversity affects fine root production and (2) disentangle the complementarity effect and the selection effect on the relationship between biodiversity and fine root production. A total of 60 tree clusters with 15 combinations of diversity gradients consisting of 1–4 tree species (Pinus massoniana, Choerospondias axillaris, Cyclobalanopsis glauca and Lithocarpus glaber) were established in subtropical forests. The sequential soil core and ingrowth core methods were used in each cluster to measure fine root biomass and productivity. Fine root production increased with increase in tree species richness. The biodiversity effects on fine root production mostly resulted from CEs. In the nongrowing season, in most cases, the CE on biomass was positive and became stronger as richness increased, but the opposite situation was observed in the growing season. The strong positive and negative effects of the proportions of C. glauca and L. glaber in the tree clusters on fine root biomass, CEs and SEs, suggest the coordinated action of species diversity and identity in modulating biodiversity effects on belowground processes. [ABSTRACT FROM AUTHOR]

Aims: There is evidence that different facets of biodiversity such as species richness (SR), phylogenetic diversity (PD) and functional diversity (FD) can modulate ecosystem functioning via niche-complementarity or mass-ratio effects, but the support for these hypotheses on fine root biomass remains unclear. Methods: In a tree diversity gradient in subtropical forests (ranging from 2 to 10 tree species), we identified the species identity of fine roots (≤ 2 mm in diameter) using a molecular method. This allowed computation of PD as well as trait-based FD and community-weighted means (CWM) metrics, which were then tested as predictors of fine root biomass. Results: Three metrics (SR, CWM of specific root length and PD) were positively related to root biomass. SR was the best predictor of fine root biomass and its predicting ability depended on stand density. The predicting power of CWM of specific root length varied with the soil nutrient status. Conclusions: Overall, the results supported both tested hypotheses (mass-ratio and niche-complementarity) that seem to be non-mutually exclusive. Furthermore, our findings emphasize that stand density and soil nutrient status are the important variables driving fine root biomass and trait-values. [ABSTRACT FROM AUTHOR]

Context: Reliable estimates of wood density (WD) within individual trees could maximize the value of Pinus massoniana for specific end-use. Aim: We examined and quantified the axial patterns of WD in trees with different social status in the stands. Methods: Wood disks were sampled at the bottom, breast height, and middle of each 1-m sections from 108 stems, harvesting from three social classes in subtropical forests. A mixed-effects model was designed to quantify axial variation. Results: The WD at different height was significantly different from the whole-stem WD (WWD) except the relative height of 0.1. An overall decrease of 133.8 kg m in WD was found from stem base to top. WD was significantly influenced by relative heights, tree age, and social class. WD of each relative height in mature trees was significantly higher than that of younger trees. Tree social class can affect WD development in the axial direction at age classes 2 and 3. Combining the fixed plus random effects, the final model explained 91 % of the observed variation in WD. Conclusion: The WD development patterns in the axial direction vary considerably among tree age, diameter at breast height, and social class. To distinguish and supply timber for specific end-uses, we should use the axial variation in disk WD (DWD) instead of WWD directly. The accurate predictions of WD provided by the model could be used to optimally classify logs into different product classes and maximize economic benefits. We can use DWD at the relative height of 0.1 instead of WWD of a single tree. [ABSTRACT FROM AUTHOR]

Background:Root decomposition is an important process that contributes to forest ecosystem carbon (C) flux and nutrient cycling. Aims:To determine how tree species affected fine root (≤ 2 mm) decomposition rates and nitrogen (N) release. Methods:Fine root samples of the coniferousPinus massoniana, the deciduousChoerospondias axillarisand the broadleaved evergreenCyclobalanopsis glaucatree species were collected from three secondary successional subtropical forests (P.massoniana–Lithocarpus glabermixed forest,C.axillarisforest andC.glauca–L.glabermixed forest) in southern China. Samples were incubated in situ, using the litterbag method for a period of 500 days to measure mass loss and changes in fine root C and N concentrations. Results:C.axillarisexhibited the fastest decomposition rate (k) during the 500 day decomposition period, followed byP.massonianaandC.glauca. Fine root mass loss was positively correlated with initial N concentrations and negatively correlated with initial C:N ratios. Initial fine root C concentrations had a significant effect on N release, but the effect varied over time. Annual fine root production released ca. 16.2–44.2 kg N ha−1through decomposition. Conclusions:Fine root mass loss during decomposition differed considerably among tree species. The chemical quality of fine roots was a key driver in the rates of decomposition. Soil properties partly accounted for the differences in decomposition rates. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Tree mycorrhizal associations drive soil N cycling pattern in subtropical forests. • Soil nitrate N decreased with increasing ectomycorrhizal (ECM) tree dominance. • ECM tree and associated fungi reduce soil N mineralization and nitrification rates. • High NAGase activity and C:N ratio but low pH was found in ECM-dominated soils. Trees and their associated arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi drive carbon (C) and nitrogen (N) cycling patterns. However, the mechanisms underpinning this relationship and the degree to which mycorrhiza mediate the effects of forest tree composition on soil N cycling outside of the temperate zone remain unclear. Here, we conducted field surveys to establish a natural gradient with increasing ECM tree dominance in subtropical forests, and explored the effects of tree mycorrhizal associations on soil N cycling. Across this mycorrhizal gradient, we quantified soil available N, microbial biomass N, net N transformation rates, as well as N -acquiring enzyme activities, litterfall biomass and quality, and soil pH during the one-year growing season. We found that with increasing ECM tree dominance, the dissolved organic N and nitrate N decreased, while the microbial biomass N increased linearly. Soils dominated by AM trees had 1.8–2.3 times larger mineral N contents, net N mineralization rate, and net nitrification rate than ECM-dominated stands, suggesting a rapid N cycling in AM-dominated stands. ECM trees and their associated fungi reduce the net N mineralization rate in topsoil mainly by producing recalcitrant litterfall (i.e., higher C:N) and by secreting β- N -acetylglucosaminidase to deplete N directly from soil organic matter, which together increased N limitation for free-living decomposers. The low pH and high C:N ratio in ECM-dominated soils inhibit the proliferation of ammonia-oxidizers and thus decrease the net nitrification rate. Our results demonstrate that the increasing ECM tree dominance increase soil N -acquiring enzyme activity and C:N ratio but decrease pH value, all of them critically mediating soil N availability. Consequently, by altering the relative abundances of tree mycorrhizal associations shifts in forest composition under global changes and plantation establishment can be expected to result in altered soil N cycling. [ABSTRACT FROM AUTHOR]