Tree Species Diversity Enhances Woody Biomass Production by Promoting Xylem Fiber Cell Number and Wall Thickness

The positive influence of biodiversity on ecosystem functioning—such as the commonly observed “high productivity” in mixed-species forests—has been extensively confirmed in numerous studies in recent years (Zheng et al. 2024). For instance, evidence indicates that incorporating broad-leaved tree species into pure coniferous forests can improve resource utilization efficiency and enhance the overall productivity of the forest ecosystem (Liu et al. 2024). The scientific community has gradually recognized that biodiversity likely affects fundamental physiological processes by altering microenvironmental conditions (e.g., light and moisture) and biotic interactions (Ray et al. 2024). However, previous research on biodiversity has primarily focused on the individual, community, or ecosystem level (Werner et al. 2024). Our current understanding of how biodiversity influences microscopic processes remains very limited, which restricts the ability to establish a mechanistic link between biodiversity and forest ecosystem functioning at mulitiple scales. Xylem cellular traits act as a critical mechanistic bridge connecting biodiversity changes to forest biomass accumulation, as woody biomass production is ultimately determined by the quantity and structural characteristics of xylem cells, exploring the response of these cellular traits to biodiversity gradients is thus key to disentangling the biodiversity-productivity relationship at a fine scale. The radial growth of trees and the accumulation of biomass are intrinsically processes driven by cambial activity, involving the production of xylem cells (Wu et al. 2016). This process encompasses key stages such as cell division, enlargement, secondary wall thickening, and lignification, which collectively determine the final morphology of the wood structure (Rathgeber et al. 2016). However, the response mechanisms of these processes across gradients of biodiversity remain unclear, limiting our understanding of how diversity influences the mechanisms of wood formation. Consequently, advancing biodiversity research from a macroscopic perspective down to the cellular scale is a critical step for deeply understanding and predicting how biodiversity loss impacts forest ecosystem functioning under global change. A recent study by Zhang et al. (2025) addresses this issue by investigating how tree diversity enhances woody biomass production through changes in xylem fiber cell number and wall thickness, this study presents novel cellular-level evidence supporting the positive relationship between biodiversity and ecosystem productivity in forest ecosystems. In this study, the researchers leveraged the large-scale Biodiversity-Ecosystem Functioning (BEF-China) experiment platform in subtropical China. They conducted repeated micro-sampling of 72 individual trees throughout the growing season. Using techniques such as semi-thin sectioning and polarized light microscopy, they observed the anatomical structure of xylem fiber cells (including number, size, and wall thickness), vessel characteristics, and cambial phenology. This approach enabled dynamic tracking of cambial phenology and cell development. Woody stems are formed by the accumulation of secondary xylem tissue, which contains fiber cells with limited radial elongation and thick lignified secondary cell walls (Zhu and Li 2024). The researchers found that compared to trees grown in monoculture (Neighbour Species Richness, NSR = 0), the woody biomass production of trees grown with four different species (NSR = 4) increased by an average of 19%–29% (Figure 1a). Correspondingly, the cell number increased by 3.7%–6.8% across species (Figure 1a), and the cell wall thickness increased by 9.2%–20.6% (Figure 1a). However, the cell diameter only increased by 1.4%–2.0% (Figure 1a). Overall, neighbourhood diversity significantly increased the number and wall thickness of xylem fiber cells but did not increase cell diameter. Notably, the researchers later expanded the plot scale, and the experimental results remained consistent at both the neighbourhood and plot scales, which provides empirical support for the consistency of the observed diversity effects and improves the reliability of the study's conclusions. Based on the continuous observation of the wood anatomical structure and developmental dynamics, the researchers developed a structural equation model (SEM) with species as a random intercept. This model is the core analytical method used in this artical, which is employed to quantify the direct or indirect causal pathways of neighborhood species richness (NSR) and neighborhood competition index (NCI) on woody biomass production. It is important to note that the SEM-derived pathways in this study are model-dependent interpretive results rather than direct demonstrations of causal relationships between variables. The model was iteratively optimized by the authors until it met specific statistical criteria, including a non-significant Fisher's C test (p > 0.05) indicating good model fit to the data, all retained paths being statistically significant, and validation via dseparation tests to ensure no critical paths were omitted. Specifically, their model confirmed that a higher NSR has a significant positive effect on the number of xylem fiber cells and the thickness of the cell wall. These two factors are the core driving forces for wood biomass production. In addition, a higher NSR also leads to significant changes in the phenology of the cambium, including an earlier initiation of cambium activity and a prolonged duration of cell wall thickening. However, these changes did not result in an increase in biomass, possibly due to the trade-off between the total duration and rate of cell wall thickening(Figure 1b). A higher NCI was associated with a reduction in total vessel area, yet it indirectly contributed to increased cell wall thickness (Figure 1b). Notably, the researchers also found that the earlier initiation of cambial activity and extended duration of wall thickening associated with higher NSR did not directly promote increased woody biomass production. This result reveals the complexity of the diversity effect pathway: NSR exerts a direct positive regulatory effect on xylem fiber cell number and wall thickness, which are the core drivers of biomass accumulation, while the phenological shifts induced by NSR do not form an effective indirect pathway for biomass promotion. Overall, NSR exhibited a positive effect on woody biomass production, whereas NCI showed a weakly negative influence. In summary, Zhang et al. (2025) have made an important advance in elucidating the microphysiological basis of the biodiversity-ecosystem functioning relationship, and this study highlights the crucial role of cellular-level processes in regulating the diversity effect on forest biomass production. Meanwhile, this study also has certain limitations and unresolved uncertainties. The research is restricted to the subtropical forest ecosystem of the BEF-China platform, and the applicability of its conclusions to other forest types remains to be verified. In addition, the study only focuses on the anatomical and phenological characteristics of xylem, and lacks in-depth exploration of the physiological and molecular regulatory mechanisms underlying the changes in these traits. Therefore, based on the core findings of this study, the subsequent work should focus on these directions. Firstly, through experiments, verify whether the hormone signals directly mediate the regulation of the richness of neighboring species on the number and thickness of xylem fibers. Previous studies have demonstrated that cambial development is regulated by a complex hormonal network, including auxin, gibberellin, ethylene, cytokinins, and brassinosteroids, each playing distinct roles in cell division, elongation, and differentiation (Ding et al. 2024). Other studies have shown that the cambium development of poplar trees is regulated by auxin and gibberellin signals (Hu et al. 2022). Therefore, researchers can incorporate these hormone factors into experiments in the future to continue exploring the mechanism. Secondly, as this study was conducted in a subtropical forest, further research in boreal, temperate, or dry tropical forests is necessary to assess the generality of these mechanisms. Thirdly, introduce continuous monitoring of microclimate factors such as canopy light interception and temperature fluctuations, and directly correlate the environmental changes caused by the neighborhood with the dynamic development of the xylem, thereby strengthening causal inference rather than merely remaining at the description of correlation. This study was supported by the Guangdong Provincial Natural Science Foundation Project (2025A1515012280). The authors declare no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analysed during the current study. No new data were generated or analyzed in this commentary.