A team of researchers led by Bi Irie Ghislain Tré of Nangui Abrogoua University, working with colleagues in Côte d’Ivoire and France, measured 550 mahogany trees across three natural stands spanning the country’s major agro-ecological zones. The sites could hardly be more different in character. Pouniakele, in the northern Sudanian savanna, receives about 1,200 millimetres of rain a year and is the driest and most disturbed of the three, with pastures and agricultural exploitation pressing in on the trees. Kouassi-N’Dawa, in the east, is a sacred forest on a hill where human activity is deliberately limited by cultural tradition. Koumokro, in the centre-east, is a community forest sitting in the climatic transition zone between subtropical and humid tropical conditions, receiving roughly 1,300 millimetres of annual rainfall. Measurements were collected between January 2022 and June 2024, all on standing trees in their natural habitat, using non-destructive techniques.

The methodological design is what gives the study its power. Rather than treating all trees as a single population, the researchers sorted them into three diameter-based cohorts that serve as proxies for developmental stages: young trees between 5 and 25 centimetres in diameter, non-exploitable adults from 25 to 40 centimetres, and exploitable adults of 40 centimetres or more, the size class targeted by selective cutting for charcoal. On every tree, eleven traits were recorded, ranging from the classic dendrometric measures of trunk diameter, total height, bole height and crown spread to a suite of foliar characters including leaf length and width, petiole length, the number of leaflets per leaf, and the length and width of individual leaflets. This dual focus on architecture and foliage allowed the team to track how different parts of the tree’s form respond to different pressures across the lifespan.

The statistical treatment was equally rigorous. After standardisation, the data were subjected to analysis of variance to detect differences among sites within each cohort, then to hierarchical cluster analysis to group trees with similar morphology, and finally to discriminant function analysis to identify which traits did the best job of separating those groups. Bartlett’s test of sphericity and the Kaiser-Meyer-Olkin measure confirmed the datasets were suitable for multivariate work. The results were strikingly consistent: at every developmental stage, and again when all 550 trees were pooled, the analyses resolved three distinct morphological groups, and leaf length and leaflet length emerged as the most discriminating traits across all stages of life.

Some of the site-level patterns were unexpected. Pouniakele, despite being the driest and most degraded site, produced trees with the largest mean diameters in every cohort. Kouassi-N’Dawa and Koumokro, by contrast, yielded trees with significantly larger leaves. In the young cohort, ten of the eleven measured variables differed highly significantly among sites, with juveniles at Kouassi-N’Dawa reaching the greatest total heights, an apparent response to light competition in the shaded sacred forest. Among non-exploitable adults, Koumokro dominated seven variables, posting the tallest mean height at 13.54 metres and the widest crown spread at 8.5 metres, while Pouniakele trees paradoxically combined the thickest trunks, averaging 33.12 centimetres, with the lowest values for six other traits.

Perhaps the most intriguing result is a reversal in the ontogenetic trajectory of diameter. Among juveniles, the gradient ran from thickest at Koumokro to thinnest at Pouniakele. By maturity, that order had flipped, with Pouniakele trees topping the rankings. The researchers interpret this inversion as evidence that the survivors at the harsh northern site prioritise radial growth to enhance mechanical stability and resource storage after episodes of water stress and pruning. Supporting this reading, the coefficient of variation for crown spread surged from 25.6 percent in young trees to 51.9 percent in exploitable adults, a signature of individual trees each following their own adaptive path through a mosaic of microhabitats and human disturbances.

The pooled analysis of all trees crystallised the picture into three site-scale morphotypes. Group 1 contained the giants of the sample, with a mean diameter of 42.9 centimetres, a total height of 14.37 metres and a crown spread of 9.53 metres, but the shortest leaves at 28.3 centimetres. Group 3 sat at the opposite pole: small trees averaging just 10 centimetres in diameter and 6.6 metres in height, yet bearing the largest leaves, up to 37.7 centimetres long. The geographic distribution of these morphotypes was anything but random. At drought-prone Pouniakele, 60.5 percent of trees belonged to the big-trunk, small-leaf Group 1, a combination that favours water conservation and mechanical resilience. At Kouassi-N’Dawa and Koumokro, where regeneration is denser and competition for light fiercer, the small-stature, large-leaf Group 3 dominated, reflecting an investment in foliar area to fuel rapid height growth in crowded stands. At Pouniakele, that morphotype was almost entirely absent, accounting for just 1.2 percent of trees.

The study’s two founding hypotheses fared differently. The first, that morphological heterogeneity would be greatest at the juvenile stage, was confirmed: coefficients of variation showed young trees to be the most variable, consistent with high initial phenotypic plasticity. The second, that natural selection would progressively filter this diversity into a converged adult form, was refuted. Even the exploitable adults remained strongly differentiated, with seven of eleven parameters differing significantly among sites. This persistence of structure into old age suggests that savanna mahogany maintains multiple viable adaptive strategies simultaneously rather than marching toward a single optimal body plan. The authors frame this as an evolutionary bet-hedging strategy, a reservoir of resilience against environmental fluctuation, though they caution that the coexistence of several phenotypic groups could also represent a transitional phase before a stable adaptive optimum emerges under recurrent disturbances such as bushfires and pruning.

The practical implications reach well beyond evolutionary theory. Because leaf length and leaflet length proved reliably discriminant at every stage, foresters now have simple, measurable phenotypic criteria for selecting superior mother trees for seed collection, a critical step for plantation development that could relieve pressure on natural stands. The identification of vulnerable growth stages also helps target protection efforts. At the same time, the authors are candid about the limits of their work. The study relies exclusively on morphology, so it cannot yet say whether the observed morphotypes reflect genetic differentiation between populations or purely environmental plasticity, and the cohort design compares different individuals rather than following the same trees over time. Combining the morphometric approach with neutral genetic markers, and mounting long-term monitoring of marked cohorts, are the obvious next steps. What is already clear, however, is that conserving this species means conserving its diversity of forms, not just its count of individuals, and that a sacred forest, a community woodland and a battered northern savanna each preserve a different chapter of the mahogany’s adaptive story.

Subject of Research: Ontogenetic variation in morphological traits of Khaya senegalensis across three savanna sites in Côte d'Ivoire

Article Title: Ontogenic dynamics of phenotypic diversity in savanna mahogany (Khaya senegalensis) across three sites in Côte d’Ivoire

Article References: Tré, B. I. G., Koffi, K. G., Kouonon, L.-C., Yao, Y. E. P., Pereda-Loth, V., & Sié, R. S. (2026). Ontogenic dynamics of phenotypic diversity in savanna mahogany (Khaya senegalensis) across three sites in Côte d’Ivoire. Discover Plants, 3(1), Article 408. https://doi.org/10.1007/s44372-026-00883-8

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00883-8

Keywords: Khaya senegalensis, savanna mahogany, phenotypic plasticity, ontogeny, Côte d'Ivoire, morphological variability, tree conservation, CITES, mother tree selection, drought adaptation, leaf morphology, sustainable forestry

The technical approach combined straightforward field measurements with accessible laboratory methods. Ambient concentrations of fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide, and nitrogen oxides were monitored five days a week over three months using EPAM-7500 particle monitors and Aeroqual Series 500 gas analyzers, with instruments positioned about one meter above the ground to capture pollutants originating from vehicle exhaust. Sampling sessions rotated purposively among the roads during morning, afternoon, and evening peak-traffic windows, and two-hour means were computed from one-minute averages. Leaf samples, meanwhile, were collected in triplicate from the traffic-facing side of each tree at a standardized height of 1.9 meters, always taking the third leaf from the tip in the phyllotaxis sequence—a choice that ensures every measured leaf sits at the same developmental stage, neither too young nor too old.

In total, 300 leaves per site were gathered between May and July 2024, during the early rainy season and at least 48 hours after any rainfall, to guarantee uniform hydration and comparable surface conditions. Leaf length and breadth were measured with a metric ruler, while surface area was quantified by tracing each leaf onto graph paper and counting full and partial squares. For the microscopic work, the team used the so-called lasting impressions method: a section of the leaf surface is coated with clear nail polish, allowed to dry in successive layers, and then peeled away with forceps to produce an exact replica of the epidermis. These peels were mounted in glycerin, photographed under a Leica DM750 light microscope, and analyzed in the open-source ImageJ software to count stomata and calculate stomatal density and total stomatal area.

The results paint a picture of stress written in plant tissue. At the high-traffic sites, leaf dimensions shrank in the more sensitive species. Azadirachta indica, the familiar neem, showed a mean leaf area of 20.0 square centimeters at the control site but only 11.5 square centimeters at Arterial Road III, a statistically significant decline. Senna siamea followed the same pattern, dropping from 13.0 square centimeters at the control to 8.17 square centimeters at the busiest road. In contrast, Albizia lebbeck and Khaya senegalensis maintained relatively stable macroscopic traits across sites, and in some cases even recorded their largest leaves at high-traffic locations—a counterintuitive result the study links to compensatory growth, in which tolerant species expand their photosynthetic surface to offset pollution-induced losses in efficiency, or to nitrogen-rich pollutants such as NO2 inadvertently acting as a fertilizer for species with high tolerance thresholds.

The microscopic data were even more striking. Stomata, the adjustable pores on leaf surfaces that regulate gas exchange, responded sharply to pollution. Albizia lebbeck recorded a stomatal density of 296.7 stomata per square millimeter at the control site but only 63.0 at Arterial Road II. Khaya senegalensis fell from 206.7 per square millimeter at the control to just 20.0 at Arterial Road I, where its total stomatal area collapsed from 35,446 square millimeters to 5,071.7. Senna siamea showed significant reductions in both metrics across all arterial roads, while Azadirachta indica reached its lowest stomatal density, 31.0 per square millimeter, at Arterial Road III. Reduced stomatal density is widely interpreted as a defensive strategy: by closing down the number of openings, a leaf limits the entry of gaseous toxins and particulates, though the price is reduced capacity for photosynthesis and cooling.

To disentangle which pollutants were driving these changes, the study employed multiple regression analysis together with variance inflation factor (VIF) diagnostics, a statistical tool that quantifies how strongly predictor variables are intercorrelated, with values above 10 signaling problematic multicollinearity. Here the findings became species-specific. For Khaya senegalensis, NO2 produced a VIF of 31.077 and PM2.5 a VIF of 25.016 for micro-morphological traits, both far above the threshold, indicating that these two pollutants are tightly intertwined with the observed stomatal changes in that species. For macro traits, NO2 again dominated, with a VIF of 15.764 in Khaya senegalensis. Carbon monoxide, by contrast, consistently showed low VIF values between 1.358 and 1.429, suggesting it acts relatively independently and exerts minimal direct influence on leaf structure. PM10 showed only mild associations across the board.

The regression results align with established mechanisms of pollution injury. Nitrogen dioxide, a signature component of vehicular exhaust, triggers oxidative stress in plant tissues that can lead to stomatal dysfunction, while fine particles deposit on leaf surfaces and inside stomatal pores, physically obstructing gas exchange and reducing photosynthetic efficiency. The strong co-occurrence of NO2 and PM2.5 in urban air, driven by shared emission sources such as traffic, means the two often act in concert. For Khaya senegalensis, carbon monoxide and PM10 emerged as significant influences on stomatal traits, consistent with evidence that prolonged carbon monoxide exposure disrupts cellular respiration and stress responses, and that coarse particles cause mechanical damage to leaf surfaces.

Not every species responded the same way, and that variation is precisely what makes the findings useful. Azadirachta indica and Senna siamea displayed greater sensitivity, with significant reductions in leaf area and stomatal traits, marking them as promising bioindicators—living sensors whose anatomy registers pollution levels that would otherwise require expensive instrumentation to detect. Khaya senegalensis, despite showing the strongest statistical association with NO2 and PM2.5, maintained comparatively stable macroscopic traits and total stomatal area at moderately polluted sites, suggesting genuine tolerance mechanisms. Senna siamea’s muted stomatal response may reflect protective traits such as thick cuticular layers and efficient antioxidant defenses, reinforcing its reputation as a resilient urban tree.

The practical implications extend well beyond Winneba. In rapidly urbanizing regions of West Africa and other developing areas where continuous air-quality monitoring networks are sparse, roadside trees could serve as a low-cost, distributed biomonitoring infrastructure. The study recommends prioritizing Khaya senegalensis and Albizia lebbeck in roadside planting schemes to strengthen the ecological buffering capacity of urban corridors, while integrating micro-morphological assessments into greenbelt management to guide species selection. The author acknowledges limitations, including a short sampling window, the absence of physiological and soil data, and uneven species presence across roads. Even so, the work demonstrates that the anatomy of an ordinary street tree can carry a detailed, quantifiable archive of the air it breathes—and that reading that archive may be one of the most accessible tools cities have for understanding their own pollution.

Subject of Research: Morphological and anatomical responses of roadside trees to vehicular air pollution

Article Title: Air pollution and plant responses: a study on morphological and anatomical changes in roadside trees along traffic-dense roads

Article References: Nkansah, F. K. (2025). Air pollution and plant responses: a study on morphological and anatomical changes in roadside trees along traffic-dense roads. BMC Environmental Science, 2(1), Article 16. https://doi.org/10.1186/s44329-025-00031-9

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00031-9

Keywords: air pollution, roadside trees, leaf morphology, stomatal density, PM2.5, nitrogen dioxide, biomonitoring, urban greening, vehicular emissions, Ghana, Khaya senegalensis, plant anatomy