Advances in Wood Anatomy: Cutting-Edge Techniques for Identifying Wood and Analyzing Its Structural Modifications

1. Introduction

Wood, a natural lignocellulosic polymer, plays several important roles in trees, including water conduction, structural support, and nutrient storage [1]. The xylem consists of different cell types and structures that have evolved to fulfil these functions effectively [2]. Over the course of their lifetime, trees continuously adapt their xylem structure in response to environmental and mechanical stresses [3]. Understanding these adaptations is key to forest science and provides insights into the resilience, health, and growth patterns of trees.

The structure of the xylem, which is influenced both by species-specific traits and environmental conditions, can reveal information about a tree’s ecological and physiological responses [4]. Linking anatomical traits to the ecological functions of trees helps provide a deeper understanding of how trees have adapted to different environmental conditions over time. Extreme environmental conditions such as frost, drought, flooding, and pest infestation, as well as mechanical stresses such as wind, snow, and mechanical injury, can lead to the formation of atypical wood structures [5]. These include reaction wood, wound wood, and various anomalies in the annual rings, such as false rings (intra-annual density fluctuations (IADFs)) and frost rings [6,7,8]. These structures can be detected and analyzed in depth to understand the acclimation mechanisms of trees and to evaluate how such modifications impact wood quality and technological properties.

Indeed, climate change poses a significant threat to forest ecosystems worldwide, altering key environmental factors such as temperature, precipitation patterns, and the frequency and intensity of extreme weather events [9]. These changes can lead to changes in wood anatomy that affect the growth and resilience of trees. By studying these anatomical modifications, researchers can predict how forests will respond to future climate scenarios and develop strategies to mitigate the negative effects.

This Special Issue presents a collection of studies that focus on the anatomy of wood and the modifications of wood structures in response to different factors. By integrating different scientific approaches, the research presented here provides a comprehensive overview of how wood structures are affected by environmental and mechanical factors.

Moreover, the selected studies use advanced techniques and methods to analyze wood anatomy, such as advances in machine learning, which provide more efficient and objective tools to identify wood species than the time-consuming traditional methods that often rely only on expert knowledge.

This SI aims to highlight the interdisciplinary nature of wood anatomy applications as well as the importance of considering wood anatomical studies of fundamental science in more applied research on forest management and wood technology.

2. Contributions Overview

The Special Issue “Wood Anatomy and Evaluation of Wood Structures and Their Modifications” features seven case studies by 38 authors from countries including the Republic of Korea, China, Canada, Brazil, and Croatia. This collection offers studies where wood anatomy is considered from different viewpoints, with a focus on wood structural adaptations in relation to species evolution, plant organ function, leaf habits, environmental stress, and site-specific conditions. Moreover, the use of advanced technologies for wood classification and identification is presented as a robust tool for ensuring proper species identification and optimizing the use of timber resources.

3. Summary of Main Outputs and Findings

3.1. Wood Identification

Wood identification is a crucial element of wood research that has an impact on various areas, such as forestry, nature conservation, and industrial applications. The studies in this category present a powerful combination of traditional anatomical methods and advanced machine learning techniques that provide insights into species identification.

Novel research using convolutional neural networks (CNNs) for conifer species classification has shown that architectures such as VGG16, ResNet50, and GoogLeNet can achieve over 90% classification accuracy in distinguishing species. The study highlights the importance of extending datasets, especially by including latewood images, which significantly improve classification performance. The results show that extended datasets lead to more stable results and faster convergence during training. This underlines the critical role of data pre-processing in optimising CNN performance in wood identification. This progress indicates the potential for the development of more accessible automatic wood species identification systems and promises to improve classification accuracy in future applications [10].

Within the context of new computer-technology approaches to wood identification, another study presents an advanced method using a one-class support vector machine (OCSVM). This method, which requires only positive samples during training, is particularly favorable for the recognition of rare wood species. The study shows that the combination of this approach with cross-sectional image analysis and a pre-trained VGG16 model for feature extraction is a powerful tool for the accurate identification of wood species, especially in scenarios where few or no negative samples are available. This emphasizes the need for advanced algorithms that are able to cope with the inherent variability and complexity of wood structures, especially in commercial and conservation contexts (He et al.) [11].

Advances in computer technology for species identification still benefit from the use of images of microscopy views of wood sections obtained with traditional procedures, analyzed with reference to the International Association of Wood Anatomists lists of microscopic features [12] and, increasingly often, featuring the application of quantitative wood anatomy. A traditional comparative study on the anatomical characteristics of Acacia mangium and Acacia hybrid grown in Vietnam shows how bark morphology and traits like vessel diameter and fibre length are crucial for distinguishing the two species and assessing their wood quality. The results show that A. mangium generally has a rougher bark and larger lumina conduits, while A. hybrid has a smoother bark and larger rays, providing a solid basis for species identification through anatomical observation. The study suggests that these anatomical characteristics can be effectively used to assess wood quality to aid in the selection and management of Acacia species in plantation forests (Savero et al.) [13].

3.2. Structural Variations

Understanding the structural variations in wood is crucial for improving wood quality, optimizing forest management, and gaining insights into the evolutionary adaptations of tree species. The studies focus on variations in wood traits in different species and under different environmental conditions. These variations are analyzed in relation to the evolutionary background, plant organ and leaf habits, and environmental stresses.

He et al. [11] reported on the structural differences in xylem vessels and parenchyma cells in tree species with different evolutionary background. These were analysed in six species, namely Michelia macclurei Dandy, Cinnamomum camphora (L.) presl, Erythrophleum fordii Oliv, Melaleuca leucadendron L., Parashorea chinensis Wang Hsie, and Tectona grandis L.F. The study shows that more evolved species typically have wood with improved traits in terms of both the efficiency and safety of water transport, achieved through vessels with larger lumina and thicker walls in earlywood, and a lower vessel density in latewood. In addition, significant differences in the distribution of starch grains and the density of pits in ray parenchyma cells are observed between primitive and evolved species, reflecting the adaptive strategies of trees over time. This study highlights evolutionary trends towards the optimization of quantitative anatomical traits to better adapt to specific environmental conditions. These results provide a deeper understanding of how wood anatomy is shaped during species evolution and its role in trees’ functional efficiency at the base and the efficiency of their survival strategies.

In the paper by Dutra et al. [14], wood anatomy variations and related functionality are explored in relation to the role of organs (stems and roots) and leaf habit (deciduous, semi-deciduous, and evergreen) in 15 species from the Brazilian Cerrado. The study shows that roots generally have a higher proportion of parenchyma cells and a lower proportion of fibers than stems, suggesting that these organs are adapted to different functional roles. The study also reveals species-specific differences in anatomical features related to leaf habits regarding the proportion of root rays, which was higher in evergreen than deciduous species. The authors conclude that organ type and leaf habit have a valuable influence on the capacity of xylem storage, and thus on plant growth patterns and their responses to the limiting and challenging conditions of the Cerrado in a climate change scenario.

In addition to wood anatomical traits, environmental stresses can also alter lignin distribution in the cell walls. The effects of mechanical stress caused by gravity in leaning-grown trees of the Taxodium hybrid Zhongshanshan were studied, focusing on tracheid morphology, size, and lignin distribution in the cell walls. The research demonstrates that tracheids in compression wood undergo significant structural changes, including a decreased size, thicker cell walls, and the absence of the secondary inner wall layer (S3). These modifications are essential, providing the mechanical support and stability required by the tree under stress. The study also highlights that stress-induced modifications in lignin deposition occur more rapidly than changes in cell morphology. Understanding these stress-induced variations is crucial for optimizing the use of fast-growing species like Taxodium hybrid in areas prone to physical stress, ensuring their stability and longevity [15].

Finally, the effect of cambial age, site, and growth conditions on variations in tracheid length was studied in Pseudotsuga menziesii. The study identifies significant differences in tracheid length between earlywood and latewood, as well as within and between different sites. This suggests that site-specific factors, such as climate and soil conditions, play a critical role in determining wood structure. The research also explores the correlation between tracheid length and growth patterns, revealing weak associations or no association. This suggests that other factors, possibly genetic or environmental, may have a more significant influence on tracheid development. These findings are essential for understanding the factors that contribute to wood uniformity and quality, providing valuable guidance for silvicultural practices aimed at optimizing wood production [16].

4. Conclusions

Wood anatomy is an important area of forest science that provides deep insights into the growth, adaptation, and responses of trees to environmental challenges. The studies presented in this SI provide insights into the sources of variations in wood anatomical traits and possible consequences for tree growth that can impact the correct strategies for forest management and conservation, as well as wood quality and industrial applications. The integration of advanced technologies such as CNNs and machine learning algorithms into wood anatomy research represents a significant advance, enabling more accurate and efficient species identification.

The application of the quantitative wood anatomy approach is also useful to quantify the complexity and adaptability of trees in response to different external and internal factors. Understanding structural changes in wood is important for predicting the impact of climate change on forests, optimising wood quality, and reconstructing past environmental conditions.

The advances in wood identification and insights into structural variations provide a framework for improving wood research and its applications. A holistic approach to the study of wood anatomy is crucial for predicting the impact of climate change on forests and making informed decisions in forest management, in conservation strategies, and for industrial applications in which the sustainable and efficient utilization of wood resources is a major target.

The multidisciplinary nature of the application of wood anatomical studies confirms the need for continued interdisciplinary collaboration where the application of advanced techniques plays a key role, especially in the context of climate change altering tree growth and wood quality.