Wood Anatomy Properties and Global Climate Change Constraints of Forest Species from the Natural Forest of Mozambique

Abstract

Mozambique’s natural forests are increasingly affected by climate change, deforestation, and unsustainable exploitation, threatening both biodiversity and rural livelihoods. This study examines the wood anatomical characteristics of five commercially important tree species—Spirostachys africana Sond., Afzelia quanzensis Welw., Millettia stuhlmannii Taub., Pterocarpus angolensis DC., and Colophospermum mopane (J. Kirk ex Benth.) J. Léonard—to assess their vulnerability to drought, cyclones, and floods. The aim is to enhance current knowledge regarding their wood anatomy and to clarify how these anatomical traits could help to identify species most vulnerable to climate extremes. Wood samples were collected from native forests and analyzed in laboratories in Brazil and Portugal using standardized anatomical methods according to IAWA guidelines. The results show that Afzelia quanzensis, Millettia stuhlmannii, Pterocarpus angolensis, and Colophospermum mopane have solitary vessels with vestured pits and thick-walled fibers, which improve hydraulic conductivity and drought resistance. Colophospermum mopane shows the greatest anatomical adaptation to climatic stressors. By contrast, Spirostachys africana has narrow, grouped vessels and thin walls, indicating higher susceptibility to embolism and limited resilience. Cyclone resistance is associated with higher wood density and parenchyma abundance, which enhance mechanical stability and recovery. Flood resilience, however, appears to depend more on leaf and root adaptations than on wood anatomy alone. These findings highlight the role of wood structure in climate adaptability and underline the urgency of integrating anatomical data into forest management strategies to support the conservation and sustainable use of Mozambique’s forest resources.

1. Introduction

Mozambique is a country located in southeastern Africa, bordered by the Indian Ocean to the east. Most of the country lies within the tropical zone, and the climate ranges from tropical to subtropical. It is predominantly hot throughout the year, with two main seasons: a summer rainy season, occurring from October to March, with average temperatures of 31 °C; and a dry winter season, extending from April to September, with average temperatures of approximately 25 °C. Annual precipitation varies widely across the country, from less than 400 mm in Gaza Province to around 2000 mm in Zambezia Province [1].

Mozambique has a total surface area of 799,380 km² and a population of approximately 28.8 million. Over half of its territory—about 41 million hectares—is covered by forests, predominantly miombo woodlands. Of this forested area, approximately 27 million hectares are classified as production forests, while 13.2 million hectares are designated as conservation areas [2]. Despite its wealth of natural resources and its position as one of the most significant areas of native forest in Southern Africa, Mozambique remains one of the world’s poorest countries. More than half of the population lives below the poverty line, particularly in rural areas, where 8 out of 10 of the country’s poor reside. Forests provide crucial support for daily subsistence in these regions. It is estimated that 80% of the rural population depends historically on forests to meet their basic needs [3]. In addition to providing essential materials (such as construction wood, food, medicinal plants, fuelwood, and recreational resources) forest products can contribute up to 93% of the annual cash income of rural households [4]. Thus, forests play a pivotal role in Mozambique’s economy, particularly in rural development. The forestry sector contributes approximately USD 330 million annually to the national gross domestic product (GDP), generates around USD 200 million in foreign exchange through exports [3,5], and provides employment to a significant portion of the population. For example, according to ref. [6], an estimated 3 million people (roughly 15% of the population) are engaged in the semi-formal charcoal trade. In addition to high-value timber species (used for export, construction, and furniture) and low-value timber (primarily fuelwood), Mozambican forests provide vital ecosystem services of both local and global importance. These include the provision of non-timber forest products, water regulation, carbon sequestration, soil erosion prevention, and the conservation of biodiversity. Mozambique’s diverse ecological zones—ranging from tropical rainforest to dry woodland and savanna—support a wide range of forest types across approximately 62 million hectares of natural forest. The three most ecologically and economically significant forest formations are miombo, mopane, and mecrusse forests [7].

1.1. Main Forest Formations

1.1.1. Miombo Woodland

Miombo woodland represents the most extensive formation of tropical seasonal woodland and dry forest in Africa, spanning large areas of southern and central Africa. In Mozambique, miombo forests cover approximately two-thirds of the total forested area, especially north of the Limpopo River [2,3]. Though often regarded as ecologically uniform due to the dominance of species from the family Fabaceae (notably the genera Brachystegia, Julbernardia, and Isoberlinia), miombo woodland is characterized by high levels of biodiversity and endemism, including a rich avifauna and several global biodiversity hotspots. These forests typically occur in semi-arid regions of northern and central Mozambique and exhibit slow growth due to limited nitrogen and phosphorus availability [8]. The tree layer averages over 500 trees per hectare and rarely forms a fully closed canopy (canopy cover ranges between 40% and 80%) [9,10]. Predominant in Mozambique, miombo woodland is an important source of raw material for the timber industry, which commonly exploits three high-value species from the family Fabaceae associated with this ecosystem: Afzelia quanzensis Welw., Pterocarpus angolensis DC., and Millettia stuhlmannii Taub. These species are classified as first-class commercial timber.

1.1.2. Mopane Forest

Mopane forest, covering approximately 3.1 million hectares, is the second most extensive forest type in Mozambique. It predominates in semi-arid, low-altitude floodplains such as those of the Limpopo and Zambezi Rivers. Adapted to arid conditions and high temperatures, mopane forest occurs primarily in areas 300–900 m above sea level, with sandy to clayey soils and annual rainfall between 370–700 mm. Average temperatures range from 16 °C to 29 °C [7,11]. This forest type is largely dominated by a single species, Colophospermum mopane (J. Kirk ex Benth.) J. Léonard (family Fabaceae, subfamily Caesalpinioideae), which occurs in pure or nearly pure stands with an average tree density exceeding 480 trees per hectare. C. mopane is also classified as a first-class commercial timber species [12,13].

1.1.3. Mecrusse Forest

Mecrusse forest occupies an estimated 843,000 hectares, approximately 3% of Mozambique’s total forest area, primarily in the southern region [3,14]. This forest type is dominated by the evergreen species Androstachys johnsonii Prain LC. (family Euphorbiaceae), which generally comprises 80% to 100% of the forest canopy in dense stands, often with over 1000 trees per hectare [7,13].

1.2. Higly Valued Wood Forest Species

According to Ali et al. [15], Mozambique’s forest biodiversity includes 118 wood species listed in the National Forestry Rules and Guidelines, classified according to their commercial value as precious, first-, second-, third-, and fourth-class species. Among the most sought after and widely used are the precious Spirostachys africana Sond. (family Euphorbiaceae) and several first-class species such as Afzelia quanzensis, Millettia stuhlmannii, Pterocarpus angolensis, and Colophospermum mopane. These species are highly valued not only for their superior wood quality but also for their wide geographic distribution and the substantial volumes harvested and traded within Mozambique [7,15,16,17].

1.2.1. Spirostachys africana

Spirostachys africana Sond. (Common names: Sândalo (Po, Fr), Tambotie (En)) is a deciduous woodland tree commonly found near water sources, such as drainage lines and rivers, and in poorly drained, brackish soils. The aromatic wood is aesthetically appealing, featuring a fine grain and coloration that ranges from golden brown to deep reddish-brown, often interspersed with dark streaks due to contrasting growth layers. The pale yellow sapwood is sharply distinguished from the heartwood. The timber is considered hard, heavy, highly durable, and resistant to insect attack. Despite its density, it possesses good workability and responds well to machining. Highly valued as a premium decorative hardwood in Africa, it is widely used in the manufacture of luxury furniture, as well as for carvings, turned objects, and small specialty items. Its strength also makes it suitable for use in gun and arrow stocks. However, it is unsuitable as firewood due to its toxic smoke, which can induce headaches, nausea, and diarrhea, particularly if meat roasted over its coals is consumed [18,19,20]. Beyond its timber value, S. africana holds considerable importance in traditional medicine, attributed to its wide range of biological activities, including antimicrobial, antibacterial, antidiabetic, antimalarial, antioxidant, cyanogenic, anthelmintic, and larvicidal properties. It is used in the treatment of ailments such as diabetes, headaches, skin conditions, dental caries, diarrhea, malaria, and respiratory infections [20,21,22]. Despite these benefits, the species contains toxic compounds—particularly concentrated in the bark and latex—which can be harmful if not administered carefully. For instance, while the bark is used to treat stomach pain, excessive dosages may cause internal organ damage. Traditionally, the latex has been used to stupefy fish, facilitating their capture [19,20,21].

1.2.2. Afzelia quanzensis

Afzelia quanzensis Welw. (Common names: Chanfuta (Po), Pod Mahogany (En), Doussié (Fr)) is a deciduous species native to lowland forests across Mozambique, thriving in hot and dry regions characteristic of miombo woodland. It typically grows on deep, well-drained sandy soils with low fertility. The species is drought resistant, frost sensitive, does not tolerate waterlogging, and is slow growing under colder conditions [15,23]. The heartwood varies from golden to reddish brown with lighter patches, while the sapwood is a pale yellowish white and distinctly demarcated. The color tends to darken with age. The grain is interlocked, with a uniform medium to coarse texture, and natural luster [15,18]. As with other Afzelia species, A. quanzensis is noted for its dimensional stability, low shrinkage during drying, and excellent durability. It requires no preservative treatment, even in permanently humid environments or regions prone to fungal, insect, or marine borer infestations. Although somewhat challenging to work due to its density and interlocked grain, it machines well with appropriate tools and techniques. Pre-boring is recommended for nails and screws, while gluing typically presents no issues [15,18,23,24]. Classified as a first-class timber, A. quanzensis is suitable for purposes where strength, durability, and stability are required. The species is logged in its entire native range and used extensively for high-quality joinery, both indoor and outdoor furniture, heavy construction, flooring, bridge and dock structures, boat building, mine props, railway sleepers, exterior millwork, carvings, and inlays. Locally, it is also favored for musical instruments. In traditional medicine, extracts from the leaves, bark, and roots are used to address various health conditions [23,24,25].

1.2.3. Millettia stuhlmannii

Millettia stuhlmannii Taub. (Common names: Jambire (Po), Panga Panga (En), Wenge (Fr)) is one of Africa’s most valuable hardwoods on the international market. It is native to East Africa, occurring in deciduous woodlands up to 900 m in altitude, particularly in areas with high rainfall and moist riverine soils. The species thrives in sunny locations with fertile, moisture-retentive but well-drained soils, and it can also tolerate seasonally inundated forests. It forms symbiotic relationships with nitrogen-fixing soil bacteria, contributing to soil fertility [26,27]. The wood has a very peculiar color and design and is therefore highly appreciated and valued on the market. The heartwood is deep black-brown (fibers) with alternating whitish bands (axial parenchyma), creating a characteristic ‘partridge breast’ figure on tangential surfaces. This is sharply demarcated from the pale yellow sapwood. The grain is typically straight, the texture ranges from fine to medium, and the wood contains abundant gum deposits. Due to its exceptional quality, M. stuhlmannii is widely harvested in the wild, serving as one of Mozambique’s most significant timber exports, primarily processed in China. The timber presents high dimensional stability, is extremely dense, hard, and resistant to fungi, borers, and termites, making it highly suitable for humid environments [15,26,27,28,29]. Although difficult to work due to its density and the presence of silica and extractives, it can be successfully machined with stellite and tungsten-carbide tipped tools. The wood’s toxic constituents, including isoflavones, saponins, and alkaloids, may cause respiratory and skin irritation. It can be polished to a fine surface, but this should be done carefully to avoid splinters. Pre-boring before nailing and screwing is needed and the wood holds nails well. The wood can be rotary cut for veneer, but prior intensive steaming is needed. Air-drying is relatively fast and stable; kiln-drying must be slow to prevent checking. Gluing and varnishing can be problematic, but the use of fillers improves results [15,26,27]. Given its hardness, durability, and decorative appeal, the wood is ideal for indoor/outdoor high-end flooring, furniture, joinery, heavy construction, shipbuilding, railway sleepers, mine props, and musical instruments. Traditional medicinal uses include the treatment of stomach pain via root decoctions [15,26,27].

1.2.4. Pterocarpus angolensis

Pterocarpus angolensis DC. (Common names: Umbila (Po, Fr), African Teak, Bloodwood, Kiaat (En)) is a deciduous tree native to Eastern and Southern Africa, particularly in miombo woodland. It thrives in dry woodland and bushveld regions with well-defined wet and dry seasons, at elevations from sea level to 1650 m and annual rainfall above 500 mm. Preferring deep sandy or well-drained rocky soils, the species tolerates low fertility and is valued for its nitrogen-fixing capacity, which enhances soil quality. Is often used to soil conservation and dune fixation [15,30,31,32]. The species is fire tolerant, with thick bark enabling saplings to survive high temperatures (up to 450 °C), making it suitable for enrichment planting in fire-prone areas [30,31]. The timber is widely utilized across the continent and is considered second only to Ocotea bullata (Burch.) Baill. in terms of quality for furniture-making [31,33]. The preference for this timber is due to the heartwood ranging from pale to dark reddish brown, often displaying streaks, and it is aromatic, durable, and resistant to termites and borers. The texture is medium to coarse, and the grain is irregular and interlocked, which enhances the natural aesthetic. The wood is easy to work, glue, and polish (although irregular grain may produce some difficulties in planning), as well as to peel and slice, being suitable for carving and turning. Once dried, it is dimensionally stable and exhibits minimal shrinkage. Applications include high-quality furniture, flooring, joinery, veneer, marine construction, and musical instruments. The red gummy sap, which exudes upon cutting, is rich in tannins and is traditionally used as a dye [15,30,31,34]. Medicinally, P. angolensis bark and roots are widely used for their antioxidant, anti-inflammatory, and wound-healing properties in treating a range of ailments including malaria, stomach disorders, eye problems, headache, skin conditions, diarrhea, asthma, schistosomiasis, and tuberculosis [30,31,34,35].

1.2.5. Colophospermum mopane

Colophospermum mopane (J. Kirk ex Benth.) J. Léonard (Common names: Chanato (Po), Mopane, African Teak, Bloodwood, Kiaat (En)) is a deciduous or semi-deciduous tree and the dominant species in the mopane forest formation. It thrives in hot, dry, low-lying savanna regions, often on alluvial or colluvial soils, and is tolerant of both alkaline and poorly drained substrates. While highly drought resistant, it is sensitive to frost. Like other legumes, it forms nitrogen-fixing symbioses, allowing it to grow in nutrient-poor soils. As a versatile and valuable species, it is frequently harvested from the wild for a wide array of uses. In Mozambique, exports of this timber to China have significantly contributed to the reduction of the species’ forested area [36,37,38,39]. The heartwood is medium to dark reddish brown with black streaks, clearly demarcated from the pale yellow sapwood. The fine uniform texture and the interlocked grain give it a natural luster. The high classification (first-class) of C. mopane timber is primarily attributed to its exceptional hardness and density (1000–1200 kg·m³), durability, and resistance to termites and borers, making it highly suitable for heavy-duty outdoor applications such as railway sleepers, bridge construction, and mine props. It is also a popular choice for construction of traditional structures, buildings, flooring, furniture, tool handles, turned objects, and fence posts. Due to its extremely high density, it is difficult to work and requires powerful tools and careful machining. Though its use as firewood is prohibited by law, it remains a popular source of charcoal due to its excellent combustion properties [18,37,38,39,40,41]. Mopane is also valued for its medicinal applications; leaves, roots, and bark are used traditionally to treat ailments such as eye infections, wounds, and gastrointestinal disorders [42]. Additionally, it plays a significant socio-economic role as the host of the Gonimbrasia belina Westwood caterpillar, known as the mopane worm. This edible insect is rich in protein (47.5%) and is consumed fresh or dried, providing an important source of food and income for rural communities [42,43].

1.3. Environmental and Human Pressure Affecting Forest Trees

Despite its ecological and economic importance, Mozambique’s forest heritage is increasingly threatened by environmental and anthropogenic stressors, and its sustainability is at considerable risk. Mozambique possesses one of the most important tracts of native forest in Southern Africa, with substantial commercial value and a strong dependency link between forest ecosystems and rural communities. For many rural populations, forests provide the sole means of subsistence and opportunities for income generation [4,5,16,17,44]. Nevertheless, in recent years, a combination of factors has led to substantial depletion of these natural resources. Firstly, the extent of forest plantations in Mozambique remains limited and is insufficient to meet current demand. Consequently, native forests continue to be the primary source of hardwoods for both domestic and international markets. These forests are being critically overexploited through selective logging, often to the point of near exhaustion of key species [2,7,45,46]. This is particularly concerning given reports indicating low growth rates in Mozambique’s native forests, ranging from only 0.5 to 1.6 m³/ha/year [18]. A notable case is Millettia stuhlmannii, a highly valued timber species internationally, with a restricted worldwide distribution but with significant occurrence in Mozambique. Current harvesting levels are unsustainably high, leading to rapid depletion of many stands [27,47]. Deforestation and forest degradation (driven by overexploitation, agricultural expansion, and urban development) have greatly diminished Mozambique’s forestry potential, both in terms of forest area and available timber volume. According to the 2018 National Forest Inventory, approximately 267,000 hectares of forest were lost annually between 2003 and 2013, representing a historical deforestation rate of 0.79% [2]. Secondly, this declining trend in forest cover is projected to worsen due to the anticipated impacts of climate change. Southern Africa is among the world’s most vulnerable regions, with Mozambique ranking as one of the most climate-sensitive countries [48,49,50]. This vulnerability is exacerbated by several factors: (i) the country’s downstream position relative to major African river systems; (ii) a long coastline hosting key urban centers; (iii) location within the Intertropical Convergence Zone; (iv) widespread poverty; (v) heavy reliance on natural resources; and (vi) inability to implement adaptive measures effectively [3,48,50].

Climate change projections for southern Africa project future increased unpredictable rainfall, more frequent droughts, rising temperatures, and heightened incidence of extreme climate and weather events such as periods of extreme drought, floods and cyclones [51,52,53,54,55,56,57]. Moreover, climate change impacts are expected to accelerate in the short term, outpacing the implementation of effective mitigation measures, potentially becoming a major driver of biodiversity loss in Africa [58,59]. In this context, a key concern for tropical forests is how increased frequency of extreme weather events will affect species distribution and adaptation trends, to ensure rational use (by the population) and management (by governments) of these forest resources. Effective, evidence-based forest management requires accurate and accessible data. The vulnerability of trees to climate-related events is largely influenced by their wood anatomy, which can be affected by various factors and interacting mechanisms. For example, several authors highlight that the survival of trees under conditions of water stress is highly dependent on the structural adaptation of xylem tissues to, on the one hand, ensure adequate water supply and, on the other, prevent vessel cavitation [60,61,62,63,64]. In this context, researchers emphasize the importance of various anatomical features, such as vessel arrangement, size, and distribution, type of perforation plates (simple, scalariform, or foraminate), presence of vestured pits, type and amount of axial parenchyma, among others [65,66,67]. In light of the dual challenges posed by climate change and existing knowledge gaps, it is essential to update and consolidate existing knowledge on the wood anatomy of tree species. While southern Africa benefits from considerable climate-related research, data on the wood properties of many Mozambican forest species remain scarce, fragmented, or outdated, hindering informed decision making. The main objectives of this study are therefore to: (i) enhance the current knowledge base regarding the wood anatomy of key forest species in Mozambique, and (ii) clarify how the anatomical traits of these species influence their ecophysiological responses to environmental stress and identify those that are most vulnerable to climate extremes. This information will ultimately help to fill knowledge gaps and support early intervention strategies, such as identifying suitable species for reforestation efforts that will increase the resilience of Mozambique’s forests.

2. Materials and Methods

The five species used in this study are native to Mozambican forests and belong to the group of the country’s most important timber species (

Table 1. List of the species studied.

Scientific Name	Family	Common Name	Forest Type	Commercial Class
Spirostachys africana	Euphorbiaceae	Tambotie	Near the water	Precious
Afzelia quanzensis	Fabaceae	Pod Mahogany	Miombo	First
Millettia stuhlmannii	Fabaceae	Panga Panga	Miombo	First
Pterocarpus angolensis	Fabaceae	Umbila	Miombo	First
Colophospermum mopane	Fabaceae	Mopane	Mopane	First

). All of them are highly valued not only for their superior wood quality but also for their wide geographic distribution and the substantial volumes harvested and traded within Mozambique. The wood material was collected by the Mozambican team and processed using traditional anatomical techniques in laboratories located in Brazil and Portugal. The wood samples comprise partial cross-sections taken from mature trees of unknown age growing in natural forests in Mozambique, felled for harvesting in 2024.

For each species, three trees were randomly chosen. Spirostachys africana trees were harvested in Inhamitanga forest, Cheringoma and Marromeu districts, in the province of Sofala. Afzelia quanzensis trees were harvested in Licuáti forest, Matutuine district, in the province of Maputo. Pterocarpus angolensis trees were harvested in Temane, Inhassoro district, in the province of Inhambane. Millettia stuhlmannii and Colophospermum mopane trees were harvested in Catapú, Cheringoma district, in the province of Sofala. Cross-sections were obtained at a height of 0.3 to 0.5 m above ground level. From these, blocks corresponding to mature wood were extracted and used for anatomical analysis of each species. All samples were free from defects. After air-drying up to a constant weight, which corresponds to a wood moisture content of around 12%, the wood blocks were sawn into parallelepiped shapes along the three principal anatomical planes of the wood: transverse, radial, and tangential.

In the laboratory, the wooden blocks were progressively sanded with sandpaper of varying grit sizes (from 120 to 1500). Special care was taken to apply minimal pressure and avoid burning the wood due to friction between the wood and the sandpaper. This process continued until a surface quality suitable for anatomical observation was achieved; that is, until all imperfections resulting from the wood cutting process were removed and all anatomical characteristics of its cellular elements could be identified. Initial assessment of general wood characteristics was conducted via visual inspection using a 10× hand lens. Fiber and vessel element biometry was performed on isolated cells obtained from small wood fragments that were macerated using Franklin’s method, which involves equal volumes of glacial acetic acid and hydrogen peroxide at 60 °C for 48 h [68].

For histological sectioning, 1 cm³ specimens were prepared and immersed in 10% ethylenediamine for 24 h. After washing, the specimens were stored in a solution of equal parts glycerin and absolute alcohol. Histological sections 15–20 µm thick were cut using a Leica SM2000 sliding microtome. These sections were stained with safranin, mounted on glass slides with glycerin, and photographed using a Coleman binocular microscope equipped with a 12-megapixel digital camera. Selected images were digitally enhanced for contrast and clarity using InPixio Studio Photo Editor. Anatomical descriptions were made according to the terminology recommended by the “Standard List of Characters Suitable For Computerized Hardwood Identification” [69], following the guidelines of the International Association of Wood Anatomists (IAWA) Committee [70], regarding the characters suitable for hardwood identification. For each species, they were identified macroscopically the growth ring boundaries and the porosity, as well the vessel arrangement and grouping, tyloses, axial parenchyma and rays. At the microscopic level, in the transverse section, the vessel arrangement and grouping were analyzed, the vessel frequency and tangential diameter of the lumina vessel were measured, and the types of axial parenchyma and rays were characterized. In the tangential section, the type of perforation plates, types of rays (uniseriate, biseriate, and multiseriate), and their frequency were identified. In the radial section, the composition of the rays was analyzed (homocellular/heterocellular/procumbent) and the presence of crystals was identified. The fiber and vessel length were measured in individualized cells (maceration). For the lumina diameter and fiber and vessel length, at least 120 measurements per feature were taken. For each of these characteristics, the arithmetic mean and the corresponding standard deviation were calculated. The wood density (at 12% of moisture content, MC) was calculated as the ratio of the weight to the volume of the samples at 12% MC. This condition was achieved by pre-conditioning the samples in a climate chamber at 20 °C and 60% relative humidity until a constant weight was reached.

3. Results

This section provides detailed wood anatomical descriptions for each of the five species, organized into macroscopic and microscopic characteristics.

3.1. Spirostachys africana Sond.

3.1.1. Macroscopic Features

In the transverse section, Spirostachys africana exhibits numerous small vessels, imperceptible to the naked eye and visible only under 10× magnification. Some vessels appear as voids, occasionally filled with reddish or brown organic substances (Figure 1). Vessels are frequently arranged in long lines, radial multiples (2–4), aligned with the rays. Tyloses are occasionally present. Growth ring boundaries are distinct to the naked eye, demarcated by darker fibrous zones. The parenchyma is sparse or indistinct, even under 10× magnification. In the radial and tangential sections, vascular lines are narrow and only observable with a magnifying lens; they are straight and continuous, some containing black or brown inclusions (Figure 2). Tyloses within the vessels display a notable brightness. Ray apertures are barely discernible, even under magnification.

3.1.2. Microscopic Features

The wood is diffuse-porous. Vessels are predominantly grouped in radial multiples of 2–4, with rounded outlines and alternate intervessel pits (Figure 3). Solitary vessels represent only 21.2% of the total. The vessel frequency averages 49 vessels/mm² (very numerous). Vessel element length averages 344.50 µm, including the appendix (min: 279.18 µm, max: 401.90 µm, s = 36.86 µm), and the average tangential diameter of the lumina is 77.69 µm (min: 50.84 µm, max: 104.24 µm, s = 14.20 µm). Some vessels contain reddish or dark brown gelatinous substances. Perforation plates are simple, occasionally slightly oblique (Figure 4). Most vessels exhibit an apical appendix at least one end (Figure 5). The axial parenchyma is apotracheal, diffuse, and/or diffuse-in-aggregates, forming a reticulate pattern with ray lines. Prismatic crystals occur within axial parenchyma cells. Rays are abundant (>10/mm), exclusively uniseriate, non-storied, and predominantly composed of procumbent cells, often obstructed by reddish or brown organic substances. Prismatic crystals are also present in some ray cells (Figure 6). Fibers are non-septate with simple to minutely bordered pits, averaging 724.32 µm in length (min: 551.85 µm, max: 913.07 µm, s = 93.41 µm).

3.2. Afzelia quanzensis Welw.

3.2.1. Macroscopic Features

In the transverse section, vessels are visible to the naked eye, ranging from small to medium in size, mostly empty, some filled with tyloses. They are predominantly solitary, occasionally found in groups of two or three (Figure 7), and are diffusely distributed, though sometimes forming subtle diagonal patterns. Growth rings are distinct to the naked eye due to a darker fibrous zone and the presence of apotracheal axial parenchyma in narrow marginal lines, features that suggest potential for dendrochronological applications. The paratracheal axial parenchyma is lozenge-aliform, occasionally confluent. In some growth rings, the parenchyma forms a thick, confluent band. Vascular lines are visible to the naked eye, shallow but clearly defined, and often contain dark reddish or black organic substances (Figure 8).

3.2.2. Microscopic Features

The wood is diffuse-porous, with circular vessels, mostly solitary, though occasionally twinned with 2 or 3. Vessels are mostly empty but can be obstructed by shiny tyloses or organic inclusions (Figure 9). A subtle diagonal vessel arrangement relative to the ray lines is sometimes observed. The average tangential diameter of vessel lumina is 161.47 μm (min: 103.50 μm, max: 236.93 μm, s = 36.66 μm), and the average length of vessel elements is 261.72 μm (min: 206.64 μm, max: 319.45 μm, s = 37.99 μm). Vessel frequency is low, with fewer than 9 vessels/mm². Perforation plates are simple; intervessel pits are alternate, vestured, and polygonal in shape. The paratracheal axial parenchyma is lozenge-aliform, sometimes with confluent extensions, and the apotracheal parenchyma is present in marginal bands consisting of two cells per strand. Some axial parenchyma cells contain orange-colored substances or circular, apparently organic deposits (Figure 10). Rays are usually multiseriate with three rows of cells, occasionally uniseriate (Figure 10), occurring at a frequency of 3–11 rays/mm. Rays are homocellular, composed entirely of procumbent cells, some of which store organic material. Fibers are non-septate, thin-walled, with simple to minutely bordered pits, and average 986.04 μm in length (min: 765.47 μm, max: 1182.81 μm, s = 99.02 μm). Prismatic crystals are present in chambered axial parenchyma cells and occasionally within fibers (Figure 11). No storied structure is observed.

3.3. Millettia stuhlmannii Taub.

3.3.1. Macroscopic Features

The most distinctive macroscopic feature of Millettia stuhlmannii wood in the transverse section is the presence of alternating dark and light bands, resulting from the arrangement of parenchyma (light-colored) and fibers (dark-colored). Vessels are visible to the naked eye, mostly solitary and empty, although some are obstructed by tyloses or yellowish organic substances (Figure 12). Tangential vessel distribution is observed, but growth ring boundaries are indistinct or absent. Vascular lines are clearly visible in both tangential and radial sections; they are deep and filled with dark material, tyloses, or greenish-yellow substances (Figure 13). Rays are storied and easily distinguishable under magnification in tangential sections. Radial lines are well defined under magnification, contrasting with the dark, continuous, and evenly spaced fibrous bands.

3.3.2. Microscopic Features

The wood is diffuse-porous. Over 90% of vessels are solitary, rounded in outline, and occasionally twinned in groups of two or three. Many contain gum or other deposits (Figure 14), also visible in tangential sections. The average tangential diameter of vessel lumina is 223.61 μm (min: 99.53 μm, max: 323.82 μm, s = 71.46 μm), and the average length of vessel elements is 227.16 μm (min: 191.92 µm, max: 278.82 µm, s = 21.20 μm) (Figure 15). Vessel frequency does not exceed an average of 6 vessels per mm². Perforation plates are simple and the intervessel pits are alternate and vestured. The paratracheal axial parenchyma is arranged in wide, visibly storied tangential bands, lozenge-aliform to confluent in shape (Figure 16). The widest parenchyma bands often fully encircle vessels. The axial parenchyma bands are marginal (or seemingly marginal). Rays are multiseriate, typically 3–4 cells wide in their broadest tangential extension, homocellular with all cells procumbent, occurring at a frequency of 7–10 rays per mm. Fibers are very thick-walled, non-septate, with simple pits and an average length of 1312.57 μm (min: 826.01 μm, max: 1812.58 μm, s = 223.76 μm). A strongly storied structure is evident, with rays, axial parenchyma, vessel elements, and fibers all displaying storied organization. Prismatic crystals are present, located in chambered axial parenchyma cells (Figure 17).

3.4. Pterocarpus angolensis DC.

3.4.1. Macroscopic Features

The wood of Pterocarpus angolensis is characterized by highly prominent growth rings, clearly visible to the naked eye and measuring up to 8–9 mm in width. These rings are marked by darker fibrous latewood zones and the confluence of axial parenchyma throughout the ring (Figure 18). In the transverse section, vessels are small but visible without magnification, typically empty, though some contain tyloses. The axial parenchyma is sparse in the earlywood but abundant in the latewood. Ray lines are fine, continuous, and evenly spaced, observable only under 10× magnification. Rays exhibit storied arrangement in tangential sections, although this is only perceptible under magnification. Vascular lines are distinct and easily visible in both radial and tangential longitudinal sections (Figure 19).

3.4.2. Microscopic Features

The wood is semi-ring-porous, with distinct growth ring boundaries. Larger vessels tend to be concentrated at the beginning of the growth ring, while smaller vessels are found toward the end, coinciding with the formation of a darker fibrous latewood zone. Vessels are almost exclusively solitary (over 90%), circular in outline, and mostly empty (Figure 20). Vessel frequency averages fewer than 6 vessels per mm². The average tangential diameter of vessel lumina is 148.22 µm (min: 103.89 µm, max: 206.66 µm, s = 30.97 µm), and the average vessel element length is 212.55 µm (min: 173.81 µm, max: 262.45 µm, s = 24.16 µm). Perforation plates are simple, and intervessel pits are alternate, vestured (Figure 21). The paratracheal axial parenchyma is aliform, with thin extensions in earlywood and confluent in latewood, forming long tangential bands that merge with marginal parenchyma. Some vessels are surrounded by vasicentric parenchyma. In the tangential section, both ray and parenchyma cells exhibit storied arrangement. Rays are homocellular and composed entirely of procumbent cells, mostly uniseriate and occasionally biseriate, with a frequency of 10–15 rays per mm. Fibers are non-septate, with simple to minutely bordered pits, and an average length of 991.90 µm (min: 781.73 µm, max: 1150.84 µm, s = 119.03 µm). A storied structure is present throughout the tissue, with all elements (rays, axial parenchyma, vessel elements, and fibers) being storied. Prismatic crystals occur in vertical series within the fibers (Figure 22).

3.5. Colophospermum mopane (J. Kirk ex Benth.) J. Léonard

3.5.1. Macroscopic Features

The vessels of Colophospermum mopane are small and difficult to observe, even under magnification. They are predominantly solitary, some containing tyloses or filled with amber-colored organic substances (Figure 23). Growth rings are not prominent in transverse section, though in some cases they can be distinguished by a slight increase in fiber wall thickness. Ray lines are thin, well-defined, and continuous. In the tangential section, vascular lines are difficult to distinguish with the naked eye but become visible under magnification, often containing reddish translucent material. Rays are not storied and darker bands caused by growth layers may be present (Figure 24). The axial parenchyma is indistinct, even under magnification.

3.5.2. Microscopic Features

The wood is diffuse-porous. Vessels are round in outline, predominantly solitary and some twinned with two or three. Some are obstructed by brown organic substances, with an average frequency of 10–12 vessels per mm² (Figure 25). The average tangential diameter of the vessel lumina is 91.37 µm (min: 67.78 µm, max: 145.95 µm, s = 19.64 µm), and the average vessel element length is 203.25 µm (min: 120.95 µm, max: 295.69 µm, s = 47.37 µm). Intervessel pits are small, alternate, and vestured. Perforation plates are simple and slightly oblique (Figure 26). The paratracheal axial parenchyma is vasicentric and aliform (lozenge-shaped), surrounding some vessels, and confluent. Rays are mostly multiseriate with three cells in width, though some uniseriate rays are also present. They are homocellular, consisting entirely of procumbent cells, and occur in frequencies of 5–10 rays per mm. Prismatic crystals are found in chambered axial parenchyma cells, forming longitudinal series (Figure 27). Fibers are non-septate, with very thick walls and reduced lumina, simple pits, and an average length of 701.63 µm (min: 492.95 µm, max: 902.96 µm, s = 128.33 µm). Storied structure is absent.

4. Discussion

4.1. Summary of Wood Anatomical Features

To facilitate comparison between species,

Table 2. Summary of qualitative anatomical features of five tropical wood species from Mozambique.

Feature	Spirostachys africana	Afzelia quanzensis	Millettia stuhlmannii	Pterocarpus angolensis	Colophospermum mopane
Perforation plates	Simple	Simple	Simple	Simple	Simple
Inter-vessel pits	Alternate	Alternate	Alternate	Alternate	Alternate
Vestured pits	No	Yes	Yes	Yes	Yes
Vessels arrangement	Multiple (2–4)	1–3	1–3	1	1–3
Vessels frequency/mm2	49	<9	<6	<6	11
Vessels diameter (µm)	78	161	223	148	91
Axial parenchyma	Difuse	Lozenge-aliform, confluent, marginal	Lozenge-aliform, confluent, band, marginal	Aliform, confluent, band	Vasicentric, lozenge-aliform, confluent
Rays width	1	1–3	3–4	1–2	1–3
Rays cell composition	All procumbent	All procumbent	All procumbent	All procumbent	All procumbent
Storied structure	Absent	Absent	Rays, axial parenchyma, vessels, fibers	Rays, axial parenchyma, vessels, fibers	Absent
Mineral inclusions	Yes	Yes	Yes	Yes	Yes
Wood density (kg.m−3) *	810	840	870	600	1080

* Source: Meier E. (2015). WOOD! Identifying and Using Hundreds of Woods Worldwide. The Wood Database [18].

provides a summary of the key qualitative anatomical characteristics described in the previous sections for the five tropical wood species from Mozambique.

4.2. Relationships Between Wood Anatomy and Species Vulnerability to Extreme Weather Events

4.2.1. Drought Resilience

Climate is widely acknowledged as the primary determinant of plant distribution ranges [39], with water availability and temperature being two of the most influential climatic variables for tree development. However, research has shown that mean annual temperature correlates more strongly with plant functional traits than mean annual precipitation. The relatively weak correlation with precipitation may reflect the complex relationship between rainfall and actual water availability to plants [71]. At a global scale, studies have emphasized that the efficiency of water transport from the roots to the crown is as important, if not more, than the amount of precipitation itself [37,71,72]. Water uptake efficiency, governed by xylem hydraulic properties, is essential for maintaining photosynthesis, especially under prolonged water stress such as that found in many regions of Mozambique. Structural adaptation of xylem tissue helps prevent cavitation (embolism) and allows trees to maintain hydraulic function under drought conditions is vital for the growth and trees survival. These hydraulic properties result from a combination of xylem traits and wood anatomical features [60,61].

In the angiosperm, water transport is carried out by vessels whose diameter, length, and inter-vessel pit characteristics critically influence both hydraulic conductivity and resistance to embolism [62,63,64]. According to the Hagen–Poiseuille law, the theoretical hydraulic conductivity of a xylem vessel is proportional to the fourth power of its diameter [64]. Therefore, wider vessels significantly increase water transport efficiency [73,74,75]. This is especially advantageous in species from arid environments, where trees must maximize water uptake when available. However, larger vessels also increase susceptibility to xylem embolism. This risk can be mitigated if the vessels are surrounded by thick-walled fibers, which provide mechanical reinforcement against implosion under negative pressure [61,62,63,64,73,74]. Thus, resistance to embolism is generally associated with high wood density due to increased cell wall thickness of the fiber and vessel, and lower fiber lumen.

Another critical anatomical trait affecting water flow within trees is the nature of vessel perforation plates (vertical connectivity between vessel elements) and intervessel pits (horizontal connectivity). The vessel perforation plates are openings in the end wall of a vessel element where the primary cell walls and middle lamella have been hydrolyzed, allowing water flow between neighboring vessel elements. Simple perforation plates, with a single opening, are more efficient for water transport than scalariform plates, which have multiple elongated openings that increase resistance. Hence, simple perforation plates are considered advantageous in dry environments [65,66].

Intervessel pits also play a significant role. The bordered intervessel pits are small openings in the vessel wall, where the secondary cell wall was not deposited over the primary wall, permits water flow between adjacent vessels. Vestured pits are bordered pits with protuberances from the secondary cell wall of the pit chamber or outer pit aperture, and are thought to enhance hydraulic performance and safety [67]. They may reduce vulnerability to air seeding, improve embolism resistance, promote safety of water transport, and mitigate the effects of drought-induced xylem failure [67,76,77,78,79]. Vestured pits are especially common in species from deserts and tropical seasonal woodlands [76]. This combination of vestured pits and simple perforation plates is associated with highly efficient hydraulic systems in trees growing under water-limited conditions.

Based on the anatomical features observed in this study (see Table 2), Afzelia quanzensis, Millettia stuhlmannii, Pterocarpus angolensis, and Colophospermum mopane share several adaptive traits that enhance drought resilience:

• A low frequency of relatively large, solitary vessels that allow high water flow;
• Thick-walled fibers surrounding vessels, mostly solitary, reinforcing them against embolism;
• Simple perforation plates and vestured pits, optimizing hydraulic efficiency under drought stress.

These traits suggest that all four species are well adapted to warm and arid environments across a range of tropical ecological zones. Notably, C. mopane stands out as particularly drought resilient and is projected to be less impacted by climate change, even under severe global warming scenarios [37]. Its performance is further supported by its high wood density (often exceeding 1000 kg·m⁻³), indicating a substantial investment in xylem allocation to fiber walls that contributes to mechanical strength of the conduits to withstand implosion under conditions of limited water availability. Thus, the distribution of these four species is not expected to be significantly affected by increased exposure to water stress resulting from projected climate changes. Even if the edaphoclimatic conditions in some regions where these species currently occur become more limiting in terms of water availability, their multiple adaptive traits that enhance drought resilience are expected to allow them to persist and continue growing in those areas.

By contrast, Spirostachys africana presents a less favorable anatomical profile. Although it has a higher vessel frequency, these vessels are narrower and grouped (radial multiples of 2 to 4), with thin walls and no vestured pits. Such characteristics limit water transport efficiency and increase vulnerability to embolism. The lack of surrounding supportive fibers in grouped vessels further compromises resistance under water stress. The reduced hydraulic conductance from embolism can in turn result in a drop of xylem water potential and more vessels becoming embolized (runaway embolism) when these are grouped and in contact with each other, eventually leading to tree mortality from hydraulic failure [60,80,81,82]. These features suggest that S. africana is not only less efficient in water flow, but more susceptible to embolism problems, therefore is poorly adapted to dry and hot conditions. Similar conclusions were drawn from studies on drought-driven tree mortality in South Africa’s Kruger National Park [83]. In this way, contrary to what is expected for the other species under study, S. africana may undergo a significant reduction in its distribution range under the climate projections for Mozambique, becoming restricted to areas with adequate water supply, such as drainage lines and riverbanks

Mozambique’s climate is shaped by subtropical anticyclones of the Indian Ocean, the Intertropical Convergence Zone, thermal depressions of southern Africa, and southern cold fronts, all contributing to the frequency of strong cyclones and floods. These events, some of them cyclical while others occasional, are expected to become more frequent due to climate change. Cyclones are natural phenomena with a significant capacity for destruction across multiple levels, including forest ecosystems. Nevertheless, certain species are more vulnerable than others to these climatic events [37,49,55,56,57,84].

It is widely accepted that density is positively correlated with greater mechanical resistance of materials. Moreover, the increased whole-plant mechanical support provided by higher xylem density enhances resistance to stem breakage and, consequently, reduces mortality caused by external forces (i.e., wind) [85,86,87]. All five species analyzed exhibited relatively high wood density values, with C. mopane surpassing 1000 kg·m⁻³. S. africana, A. quanzensis, and M. stuhlmannii also show high densities between 800 and 900 kg·m⁻³. P. angolensis is somewhat less dense (~600 kg·m⁻³), which may render it slightly more vulnerable to wind damage.

The axial parenchyma also plays a key role in post-cyclone recovery by storing non-structural carbohydrates, which can later be mobilized to help trees withstand periods of stress [88,89,90]. After defoliation, which commonly follows cyclonic events, the tree’s photosynthetic capacity is disrupted, leading to carbon starvation. During such periods, trees rely on stored reserves to sustain metabolic activity [91,92,93]. Thus, species with a high proportion of axial parenchyma, reflecting greater investment in non-structural carbohydrates, exhibited the lowest mortality rates. Among the species studied, A. quanzensis, M. stuhlmannii, P. angolensis, and C. mopane demonstrated both a high quantity and diversity of axial parenchyma types. In the absence of stem breakage, these species may possess an enhanced capacity to withstand the detrimental effects of cyclones. By contrast, S. africana contains minimal axial parenchyma, limited to the apotracheal diffuse type, indicating a potentially greater vulnerability to cyclone-induced stress.

4.2.2. Flooding Resilience

Flood adaptation in trees is largely determined by traits in leaves, roots, and bark, rather than wood anatomy. Features like adventitious or aerial roots, pneumatophores, denticeles, and aerenchyma formation are central to flood tolerance, but are beyond the scope of this study [88,89,94,95].

Anatomically, the main feature contributing to flood resilience is the presence of axial parenchyma, which can store non-structural carbohydrates to support metabolism during periods of root asphyxia. Species such as A. quanzensis, M. stuhlmannii, P. angolensis, and C. mopane may thus have some capacity to withstand temporary flooding. S. africana, due to its low parenchyma content, appears less adapted. However, its frequent occurrence near water bodies and in brackish soils suggests it may possess root- or leaf-level adaptations not captured by this anatomical analysis, which may even overlap with or mask the effects of axial parenchyma. As such, wood anatomy alone is insufficient to fully assess flood vulnerability, and further multidisciplinary studies are warranted.

5. Conclusions

In this study, five species from Mozambique’s native forests were anatomically characterized. These species hold significant economic, social, and environmental value, yet face numerous anthropogenic and natural threats. Overall, the species exhibited diffuse-porous wood with indistinct growth ring boundaries. P. angolensis was the only species presenting semi-ring-porous wood with distinct growth ring boundaries. All species possessed a low frequency of vessels (<11 per mm²) with simple perforation plates, alternate and vestured intervessel pits, and mostly solitary vessel arrangements, except in S. africana, where vessels were predominantly in multiples (2 to 4 in a radial pattern) and pits were non-vestured. The average tangential vessel diameter ranged from 91 µm to 223 µm, with vessel length between 203 µm and 262 µm. In S. africana, vessels were narrower (average 78 µm), longer (average 335 µm), and more frequent (49 per mm²).

All species had a high proportion of axial parenchyma, arranged in various types such as vasicentric, lozenge-aliform, confluent, banded, and marginal. However, in S. africana, the amount of axial parenchyma was lower and limited to diffuse and/or diffuse-in-aggregates types. The rays were homocellular, composed entirely of procumbent cells, and often stored organic substances. Rays were usually multiseriate (3–4 cells wide), though uniseriate rays also occurred. In S. africana, rays were exclusively uniseriate. Fibers were non-septate, with simple to minutely bordered pits, and had average lengths ranging from 701 µm in C. mopane to 1312 µm in M. stuhlmannii. P. angolensis and M. stuhlmannii exhibited storied structure in rays, axial parenchyma, vessels, and fibers, whereas the remaining species showed no evidence of storied organization. All species showed the presence of mineral inclusions and/or organic substances within vessels, rays, or axial parenchyma cells.

Regarding the relationship between wood anatomical traits and species vulnerability to extreme weather events, the results were inconclusive in the context of flooding. It was not possible to establish a direct relationship between anatomical features and flooding vulnerability. By contrast, with respect to cyclones, all species showed anatomical characteristics that provide a minimum level of resistance to strong winds. They all exhibited high wood density and substantial investment in reserve substances stored in axial parenchyma, which can be mobilized following crown destruction caused by cyclonic events. In this regard, C. mopane is noteworthy, with an average wood density exceeding 1000 kg.m⁻³. Concerning drought resistance, A. quanzensis, M. stuhlmannii, P. angolensis, and C. mopane appear well adapted to warm, arid environments. The fibers and vessels of these species possess features that promote high water-use efficiency and reduced vulnerability to xylem embolism. Therefore, climate change may not pose a significant threat to their long-term survival, contrary to initial concerns. This is not the case for S. africana. Its anatomical features indicate lower water-use efficiency and a markedly higher susceptibility to xylem embolism. Consequently, this species is expected to be more adversely affected by climate change, and its distribution may need to be confined to habitats with reliable water availability or, at the very least, limited dry periods.