Evaluating Growth Patterns and Soil Dynamics of the Endangered African Rosewood (Pterocarpus erinaceus Poir.) in Contrasting Ecological Regions in Ghana
Abstract
Pterocarpus erinaceus (Poir.) is an endemic and endangered species of the Guinea-Sudano-Sahelian zones currently overexploited for timber, fodder, and medicinal purposes. Therefore, plantations are critical to meeting the high demand in the international market while reducing pressure on natural stands, but its growth performance across different ecological zones is not well documented. Thus, we assessed the influence of ecological zones and soil properties on the survival and growth performance of P. erinaceus. Data were collected from 12 subplots of P. erinaceus stands in the Dry Semi-Deciduous Fire Zone (DSFZ) and 16 subplots in the Moist Semi-Deciduous North West Zone (MSNW). We assessed the survival and growth performance of P. erinaceus using one-way ANOVA, variation in soil properties using the Generalized Linear Model, and the influence of soil properties on P. erinaceus growth characteristics via correlation analysis. We found that the percentage survival of P. erinaceus was similar in both ecological zones. However, P. erinaceus demonstrated a high level of variation in growth attributes, growing taller with deeper canopies in wetter conditions and larger crown spread in drier conditions. The aboveground biomass carbon stocks of P. erinaceus stands were similar in both ecological zones, highlighting the potential role of the species as a carbon sink in the ecological zones. Soil organic matter, nitrogen, and calcium contents were higher in soils of the MSNW stands than those of the DSFZ stands. However, potassium, magnesium, sodium, and phosphorus contents were stable across the ecological zones. We concluded that wetter conditions enhance the growth performance of P. erinaceus as a timber species.
1. Introduction
African rosewood (Pterocarpus erinaceus Poir.) is a medium-size deciduous tree belonging to the Fabaceae family [1–3]. It is native to the semi-arid Guinneo-Sudano-Sahelian Forest zones of West Africa with presence in about 8 countries within the West African subregion due to its ability to thrive in a varied range of environmental conditions [2–5]. Its superior wood qualities make it one of the most valuable and preferred timber species for the manufacture of valuable artifacts in Asia and China in particular stemming a rapidly increasing demand for its wood [1, 6]. It has therefore emerged to become one of the most heavily traded tropical hardwood species on international markets and a major source of revenue and foreign exchange for many West African economies. Apart from its contributions to national economies, P. erinaceus provides many socioeconomic and cultural benefits to local communities who exploit it for timber and nontimber products [1–3, 5–7].
Despite its significant socioeconomic importance, recent rises in global demand and commercial value for its wood contributed to increased exploitation and consistent increase in its international trade [8, 9]. West Africa is currently the largest producer and supplier of African rosewood globally with large volumes of the exported timber sourced from illegal sources [6]. According to Dumenu and Bandoh [8], African rosewood supply from Africa to China experienced a 700% increase since 2010. Kossi et al. [2] also reported that the West Africa region produced about 80% of the total volume of African rosewood exported to China in 2016. This high pressure exerted on the populations of P. erinaceus makes it one of the most exploited, heavily traded, and threatened tropical hardwood species globally [1, 7]. These anthropogenic pressures in addition to the eminent threat of climate change raise concerns on the sustainability of African rosewood in many African countries where climate impacts are very prominent [5, 7, 10].
In Ghana, P. erinaceus is ecologically distributed across the Forest-Savanna Transition, Sudan Savanna, and Guinea Savanna Forest zones but present in ten out of its sixteen administrative regions [3, 8]. The species was traditionally largely exploited for fodder, medicine, charcoal, fuelwood, construction of musical instruments, and manufacture of farm tools [3]. However, recent rising demand for African rosewood timber in China has increased the exploitation and international trade. According to Obiri et al. [3], between 2003 and 2013, an estimated 111,110 m3 of rosewood was exploited in Ghana. The authors further reported that export volumes of the species have been experiencing a consistent increase in recent years with estimates of 40,998.7 m3, 81,958.7 m3, and 10,021.2 m3 recorded in 2013, 2016, and 2019, respectively, with wood from illegal sources contributing significantly to this trend. The country has therefore emerged to become the second leading suppliers of African rosewood timber to China in Africa and fourth globally [8]. A recent national inventory in 2021 revealed a 50% reduction in stem numbers and volume of P. erinaceus species compared to the 2013 estimate [3]. It is therefore one of the most exploited and threatened species in Western Africa and Ghana.
The present trend of P. erinaceus exploitation and global trade has raised serious concerns about its sustainability with many advocating the need to adopt appropriate measures to protect the species from the eminent threat of anthropogenic disturbance [8, 11]. These advocacies have influenced major trading countries in West Africa to impose harvesting and export bans in recent years [4, 7, 8, 11]. In Ghana, conservation efforts for the species have ranged from the imposition of intermittent harvesting and export bans, restoration of natural stands, plantation establishment, adoption of export quotas, community sensitization, and imposition of minimum felling limits [2, 8]. This was followed by the registration and enlistment of the species in Annex II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and change of conservation status to globally endangered on the red list of the International Union for Conservation of Nature (IUCN) [5, 8].
Despite these interventions, illegal exploitation, smuggling, and trade of P. erinaceus continue to grow in the major producing countries in Africa [4]. For example, while Saibu [12] reports incidence of illegal exploitation of P. erinaceus in the Mole National Park, Treanor [13] reports that import values reported by the Chinese government from Ghana were 3.5 times higher than export values recorded in Ghana. Anthonio and Antwi-Boasiako [6] have however argued that despite these challenges, the current commercial value of P. erinaceus presents a huge prospect for economic transformation and livelihood improvement under appropriate conservation strategies in many African countries whose economies are largely natural resources dependent [6]. It also has the potential for ecological balances in the fragile savanna ecosystems despite the imminent threats of climate change [3, 5, 8, 11]. There is therefore an urgent need to develop strategies for the sustainable management of this species.
In response to these calls, many have recognized population restoration through the establishment of African rosewood plantations as a strategic measure to restore the rapidly declining population and ensure the sustainable management of the species [11]. However, many authors have argued that the success of these interventions requires an in-depth understanding of the species biology, ecology, survival, and growth performance [5]. Also, species-site matching has been widely recognized to be crucial for tree survival, growth, and general success of reforestation program [7, 14]. However, despite the establishment of P. erinaceus plantations across different ecological zones of Ghana with unique climate, soil properties, and species interactions, limited empirical studies have examined the nexus between soil properties and survival and growth performance of P. erinaceus under different environmental conditions. Ansah et al. [15] assessed variation in seed set, seed morphological and chemical traits, germinability, and seedling growth of P. erinaceus in Ghana, but data on the growth performance of the species beyond the seedling stage in the field and the role of edaphic factors rarely exist. This knowledge is however critical for the successful establishment of African rosewood plantations and general ecological balances in the forest ecosystem.
This study, therefore, assessed the influence of ecological zones and soil properties on the survival and growth performance of P. erinaceus. The study specifically sought to (a) assess the survival and growth performance of P. erinaceus in two ecological zones in Ghana, (b) assess the aboveground biomass carbon stocks and soil organic carbon of P. erinaceus stands in the two ecological zones, (c) evaluate physicochemical properties of soils under P. erinaceus stands in the two ecological zones, and (d) determine the effects of soil characteristics on the growth performance of P. erinaceus. The study aimed to provide a holistic understanding of the relationship between P. erinaceus, ecological zones, and soil conditions. The study potentially contributes to the development of evidence-based conservation and management strategies for the sustainable management of P. erinaceus and general environmental sustainability and biodiversity conservation in Ghana.
2. Methods
2.1. Study Area Description
The study was conducted in the Jimira (Latitude 6° 36′ 16″ North and Longitude 1° 55′ 18″, 347 msl) and Yaya Forest Reserves (Latitude 7° 27′ 29″ North and Longitude 2° 5′ 33″ West, 331 msl) in the Moist Semi-Deciduous North West (MSNW) and Dry Semi-Deciduous Fire Zone (DSFZ) ecological zones, respectively (Figure 1). The Jimira Forest Reserve is under the Nkawie Forest District in the Ashanti Region while Yaya Forest Reserve is under the Sunyani Forest District in the Bono Region of Ghana. Jimira Forest Reserve, designated as Forest Management Unit (FMU) 37, has an area of 6285 ha and the forest is located within the Atwima Mponua District, which has a total population of 155,254 inhabitants [16]. The Yaya Forest Reserves falls within the Wenchi and Sunyani Municipalities, with a total population of 124,758 and 193, 595 inhabitants, respectively. The reserve has a total area of 5,136 ha and it is designated as FMU 18. Tree species such as Triplochiton scleroxylon, Celtis mildbraedii, and Terminalia superba predominate in the Jimira Forest Reserve while Khaya ivorensis, Ceiba pentandra, and Cola gigantea are the dominant trees in the Yaya Forest Reserve. However, due to forest degradation and deforestation, significant portions of Yaya Forest Reserve have been converted to teak (Tectona grandis) and Cedrela (Cedrela odorata) plantations. Both reserves experience a bimodal rainfall with a major peak in May/June and a minor peak in October. The mean annual temperature ranges from 25.4°C to 26.2°C for both reserves. The annual precipitation over the period 2015–2021 was 1,281.45 mm–1,746.42 mm, with a mean of 1,403.61 mm for Yaya Forest Reserve, and 1,313.09 mm–1,893.17 mm, with a mean of 1,599.54 mm at Jimira Forest Reserve [16]. The soils of the Jimira Forest Reserve are basically metamorphosed sediments of the Lower Birimian formation. The predominant soil type at Yaya and Jimira Forest Reserves is forest ochrosol which is referred to as Haplic Ferralsols and it is rich in soil organic matter with a characteristic dark brown colour in the A-horizon and reddish brown colour below the A-horizon due to ironstone concretions [17]. The ochrosols are generally well-drained and slightly acidic or neutral.
2.2. Sampling Design and Data Collection
P. erinaceus was selected for this study due to the recent high exploitation pressure on the species, its endemic nature and conservation status, and the rarity of robust data to support plantation and conservation efforts [2, 8, 9]. Two ecological zones where P. erinaceus stands were planted on the same day were randomly selected from a list of eight trial sites provided by the local offices of the Forestry Commission of Ghana. The selected stands were established in 2015 at Yaya and Jimira Forest Reserves; thus, stands were eight years old at the time of the assessment. There were 3 ha P. erinaceus stands at Yaya Forest Reserve and 4 ha stands at Jimira Forest Reserve; 4 subplots measuring 25 m × 25 m were established in each hectare per ecological zone, making a total of 16 plots in the MSNW and 12 subplots in the DSFZ. The stands of P. erinaceus in both reserves were monocultures, planted at 4 m × 4 m. In each subplot, the circumference of each tree was measured (at 1.3. meters above the ground) with a tape measure in centimeters and later converted to diameter values while the total height (m) was measured with a hypsometer (Haglöf Vertex IV and Transponder). Crown dimensions (length, breadth, depth, radius, and diameter) (m) of P. erinaceus trees and ground litter depth (cm) were measured in each subplot. Two samples of undisturbed soil samples were collected at each sampling point between the depth of 1–15 cm and 15–30 cm from the soil wall after digging for bulk density determination. The samples were collected at the opposite corners of each plot using bulk density cylinders. Soil samples for chemical analysis were collected using a soil auger. Five (5) samples per plot were collected at 0–15 cm and 15–30 cm from the four corners of each 1 ha plot and the middle, pooled together for each layer, thoroughly mixed, and subsampled for chemical analysis.
2.3. Soil Data Processing and Chemical Analysis
In order to determine the pH of the soil, a slurry of 1 : 1 dirt to solution was created by combining a 10-gram sample of dried soil with 10 ml of distilled water. After that, the pH was determined using a pH meter calibrated with buffer solutions at pH 4 and pH 7 . Ammonium acetate technique (1 NH4OAc) was applied to assess exchangeable cations and related bases in the soil [18]. Ten (10) grams of each soil sample was mechanically shaken for two hours with 100 ml of 1.0 N NH4OAc and a 10-milliliter aliquot of the extract was used to estimate extractable Ca, Mg, K, and Na. A calibration curve was utilized to convert the emission intensities into the corresponding concentrations in the soil extract after the intensities were determined with a calibrated flame photometer. The values of exchangeable acidity and bases were added up to determine the effective cation exchange capacity (ECEC). The Walkley-Black chromic acid wet oxidation method was used to analyse the carbon content of the soil [19]. A solution of concentrated H2SO4 in a 2 : 1 ratio and 1 N (0.1667 M) K2Cr2O7 was used to oxidise a 2-gram sample of soil organic matter. The quantity of digested Cr in the mixture was then used to compute the organic carbon concentration, and excess Cr2O72- was titrated using 0.5 N ferrous sulphate solution. The Kjeldahl method was applied to assess available nitrogen (%) [20]. Using concentrated sulfuric acid and Kjeldahl catalysts—a combination of one-part selenium, ten parts copper (II) sulphate, and one hundred parts sodium sulphate—nitrogen in soil organic matter was converted into ammonium. For the oxidation process, ten grams of air-dried soil was utilized. Using extra caustic soda (40% NaOH), ammonia liberated from the ammonium sulphate was collected, distilled, and titrated with HCl (0.1 M). Using the Bray-1 method, which involved dissolving 5 grams of dry soil in 25 ml of Bray-1 solution, available phosphorus was measured. Using a spectrophotometer and the ammonium-molybdate technique, which uses ascorbic acid as a reducing agent, the phosphorus concentration of the extract was measured at 650 nm.
We used the hydrometer approach to find the soil particle size distribution. Fifty-one (51) grams of air-dried soil was placed in a beaker along with a mixture of 50 ml of Calgon solution and 100 ml of deionized water. The suspension was briskly stirred, let to settle for half an hour, and then well mixed. Using a calibrated hydrometer, the quantities of sand, silt, and clay were calculated, and the soil was categorised into textural groups based on the USDA soil triangle. We estimated soil bulk density (BD) as BD (g cm−3) = [(X − Y)/V], where X and Y stand for the weights of the empty trays and oven-dried soils in trays, respectively, and V is the volume of the bulk density cylinder.
2.4. Data Analysis
3. Results
3.1. Survival and Growth Performance of P. erinaceus in Different Ecological Zones
The mean survival percent of P. erinaceus was 37.6% and 43.6% for DSFZ and MSNW, respectively, and the differences were statistically insignificant (p = 0.560). P. erinaceus was 17% taller with 11% deeper crowns in the MSNW than the DSFZ (Table 1). Contrarily, mean crown breadth, diameter, and radius of P. erinaceus and mean litter depth were 17%, 10%, 10%, and 83% greater, respectively, in the DSFZ than the MSNW. However, mean DBH and crown length were similar in the two ecological zones (Table 1).
| Parameter | Ecozone | F value | p value | |
|---|---|---|---|---|
| DSFZ | MSNW | |||
| DBH (cm) | 6.11 ± 0.08 | 6.14 ± 0.06 | 0.09 | 0.760 |
| H (m) | 3.10 ± 0.05 | 3.63 ± 0.04 | 78.65 | <0.001 |
| Canopy depth (m) | 1.80 ± 0.04 | 2.03 ± 0.03 | 20.54 | <0.001 |
| Crown length (m) | 1.78 ± 0.04 | 1.69 ± 0.04 | 2.71 | 0.100 |
| Crown breadth (m) | 1.73 ± 0.04 | 1.48 ± 0.03 | 23.00 | <0.001 |
| Crown diameter (m) | 1.73 ± 0.04 | 1.57 ± 0.03 | 10.77 | 0.001 |
| Crown radius (m) | 0.87 ± 0.02 | 0.79 ± 0.02 | 10.77 | 0.001 |
| Litter depth (cm) | 2.58 ± 0.05 | 1.41 ± 0.04 | 313.89 | <0.001 |
3.2. Aboveground Biomass and Carbon Stocks of P. erinaceus in Different Ecological Zones
The mean biomass of P. erinaceus was 11.374 ± 0.651 Mg C ha−1 in DSFZ and 13.16 ± 0.563 Mg C ha−1 in the MSNW. In terms of carbon stocks, the mean biomass of P. erinaceus was 5.687 ± 0.651 in DSFZ and 6.58 ± 0.563 in the MSNW. The one-way ANOVA results showed that there were no significant differences between the ecological zones in relation to aboveground biomass (p = 0.309) and carbon stocks (p = 0.309).
3.3. Physicochemical Properties of Soils under P. erinaceus Stands in Different Ecological Zones
Soil moisture content was significantly different across the ecological zones (p = 0.009), but remained similar in each soil depth (Figure 2; SI Table 1). Soil organic matter varied across the ecological zones (p = 0.004), and there was a significant interaction between ecological zone and soil depth (p = 0.011), indicating that the differences in soil organic matter content between soils under P. erinaceus stands in DSFZ and the MSNW ones varied with soil depth. Specifically, the MSNW had 50% higher SOM compared to DSFZ (Figure 2). The proportion of sand (p = 0.001) and silt content (p < 0.001) differed across the ecological zones but remained similar in relation to soil depth, while clay content remained similar across the ecological zones and soil depth (SI Table 1). The soils were predominantly sand or loamy sand in the DSFZ and loamy sand, sandy loam, or sand in the MSNW.
With regard to soil chemical properties and nutrient concentrations, the concentration of N varied significantly with soil depth (p = 0.002) and ecological zone (p = 0.030), with a significant interaction effect (p = 0.035), indicating that variation in N concentration between the two ecological zones depends on the soil depth (Figure 2). Specifically, N content in the topsoils of the MSNW stands was at least 60% higher than N content in topsoils or subsoils of DSFZ. The nutrients K, P, Mg, and Na remained similar in the different ecological zones and soil depth while Ca was 77% higher in the MSNW compared to the DSFZ (p = 0.011). However, soil pH did not vary with ecological zone, soil depth, or their interaction (SI Table 1). This indicated that soil pH remained relatively consistent from the surface to deeper layers, irrespective of ecological conditions.
3.4. Effect of Soil Characteristics on the Growth Performance of P. erinaceus
Soil moisture and N contents of topsoils (0–15 cm) positively correlated with tree canopy depth (Table 2). However, K content was negatively correlated with tree canopy length, breadth, diameter, and radius while Mg content was negatively correlated to canopy breadth. Soil organic matter content was positively related to tree height. With regard to subsoils (15–30 cm), Mg concentration was negatively correlated to canopy breadth (r = −0.924, p = 0.003), DBH (r = −0.825, p = 0.022), and radius (r = −0.825, p = 0.022).
| Soil parameter | Growth parameter | ||||||
|---|---|---|---|---|---|---|---|
| DBH (cm) | H (m) | CDP (m) | CL (m) | CB (m) | CD (m) | CR (m) | |
| Moisture content (%) | 0.359 (0.429) | 0.681 (0.092) | 0.893 (0.007) | −0.495 (0.259) | −0.657 (0.109) | −0.616 (0.141) | −0.616 (0.141) |
| N (%) | −0.146 (0.754) | 0.784 (0.37) | 0.818 (0.024) | −0.370 (0.414) | −0.500 (0.253) | −0.500 (0.253) | −0.500 (0.253) |
| K (mg·kg−1) | 0.363 (0.423) | 0.069 (0.884) | 0.344 (0.449) | −0.868 (0.011) | −0.817 (0.025) | −0.853 (0.015) | −0.853 (0.015) |
| Mg (mg·kg−1) | −0.321 (0.482) | 0.113 (0.810) | 0.709 (0.075) | −0.405 (0.367) | −0.714 (0.033) | −0.714 (0.071) | −0.714 (0.071) |
| SOM (%) | −0.075 (0.873) | 0. 791 (0.034) | 0.595 (0.159) | −0.465 (0.293) | −0.369 (0.426) | 452 (0.308) | −0.452 (0.308) |
- CDP is canopy depth (m), CL is crown length (m), CB is crown breadth (m), CD is crown diameter (m), and CR is crown radius (m). Only significant correlations are shown.
4. Discussion
4.1. Survival and Growth Performance of P. erinaceus in Different Ecological Zones
The recorded mean survival percent of P. erinaceus was 37.6% in the DSFZ and slightly higher at 43.6% in the MSNW, but the differences were insignificant. These findings suggest that despite the differing environmental conditions in the DSFZ and MSNW, P. erinaceus exhibited a consistent level of survival possibly due to the fact that the species may possess adaptive traits that enabled it to thrive in both ecological zones despite differences in moisture availability and competition [22, 23]; Adji et al. [3–5]. For example, physiological traits such as deep root systems, efficient water use strategies, or drought resistance mechanisms possibly enabled P. erinaceus to survive in both ecological zones [22]. Furthermore, existing literature suggests P. erinaceus tolerates a mean annual rainfall of 600–1500 mm and temperature of 15–35°C, giving the species the ability to regenerate and grow in almost all climatic zones of Western Africa [7], which possibly explains its similar survival in the two ecological zones. Additionally, studies on Pterocarpus species have shown that they have the ability to thrive in different climatic and environmental conditions [7]; Adji et al. [2–5]. Specifically, Pterocarpus indicus has been recognized for its resilience in different environmental conditions [24]. It is imperative to note the low level of survival (<45%) of the species recorded in this study. This observation is possibly due to the weak growth of young seedlings and their young flexible stem, which probably makes them vulnerable to wildfires, pests, trampling of animals, and adverse climatic conditions [2]. That notwithstanding, poor tending operations may account for the low survival of the species (personal observation).
P. erinaceus were taller with deeper canopies in the MSNW, reflecting the species’ adaptability to the wetter conditions and potentially more fertile soils [7, 25]. In other words, P. erinaceus exhibited faster growth in response to increased moisture and nutrient availability. The richer soils found in the MSNW might support increased height and deeper canopy growth, possibly highlighting the potential of P. erinaceus to exploit available resources in diverse ways based on ecological conditions [25]. Similarly, existing literature suggests that moisture availability leads to the formation of dense vegetation which triggers growth in height to exploit the limited available light while in drier conditions, trees invest in root development at the expense of height growth in order to exploit the limited available water [23, 25]. Variations in tree heights and canopy structure have implications for resource exploitation and this has been well documented in the context of forest structure and biodiversity (e.g., [25, 26]). For example, taller trees with deeper canopies can provide unique niches for various species and influence forest microclimate [27].
We found that P. erinaceus stands in the DSFZ had broader crowns and deeper litter on the plantation floor than the MSNW ones (Table 1). This observed phenomenon in the DSFZ P. erinaceus stands is possibly a strategy the species has adopted to maximize resource capture in drier conditions [22, 23]. The broader crown possibly provides shade and conserves moisture and influences light interception, which may reduce competition from understory vegetation [28]. The deeper litter depth may be due to greater litterfall and slower rate of decomposition in drier environments [29, 30], which has implications for organic matter management, nutrient recycling, seedling establishment, and soil properties. That notwithstanding, it may serve as fuel for wildfires which are very common in the ecological zone.
Interestingly, the mean diameter at breast height (DBH) and crown length were similar in both ecological zones (Table 1), suggesting that, despite the ecological differences, certain growth parameters remain consistent between the DSFZ and MSNW. The similarities in DBH and crown length may indicate that P. erinaceus possibly allocates its resources differently in response to the ecological zones while maintaining an overall comparable growth pattern. This uniformity in DBH and crown length in the two ecological zones has implications for forest management and timber production, as these attributes are often considered when assessing the commercial value of timber species [31]. Additionally, DBH is a crucial parameter for estimating tree age and growth rates. Other studies have shown that DBH can be an indicator of tree vitality and carbon storage [32]. Therefore, the observed similarity in DBH possibly supports the notion that P. erinaceus maintains relatively consistent growth rates across different ecological zones. That notwithstanding, long-term temporal analysis may further deepen our understanding of the growth patterns of P. erinaceus.
4.2. Aboveground Biomass and Carbon Stocks of P. erinaceus in Different Ecological Zones
The similarity in mean biomass carbon stocks of P. erinaceus between the DSFZ and MSNW has ramifications for carbon sequestration efforts and forest management [7]. Its ability to accumulate similar aboveground biomass in different ecological zones highlights its adaptability and suggests its potential for use in afforestation and reforestation projects aimed at enhancing carbon sequestration [7]. Thus, Pterocarpus erinaceus may play a significant role in sequestering carbon in both DSFZ and MSNW forest zones and possibly hold a broader relevance in the context of global climate change mitigation, given the species capacity to thrive in a wide range of environmental conditions [7]. That notwithstanding, variations in soil properties, microclimates, and other ecological factors which exist within broader ecological zones may influence biomass and carbon stocks. Moreover, forests are dynamic ecosystems, and biomass and carbon stocks can change over time due to various factors such as tree growth, disturbances, and climate variability [33]. Therefore, long-term monitoring is essential to track changes in the P. erinaceus stands.
4.3. Physicochemical Properties of Soils under P. erinaceus Stands in Different Ecological Zones
The results indicate that soil moisture content was significantly different across the ecological zones, possibly due to differences in rainfall patterns, evaporation rates, and moisture-holding capacity of the soils [34]. The MSNW experiences higher rainfall, and hence greater moisture content is expected. Variations in soil moisture content affect tree growth, root development, and overall ecosystem dynamics [35]. For example, environmental factors such as temperature, rainfall, and soil nutrients influenced variations in morphological traits of P. erinaceus in Ghana [15]. Thus, tailoring planting strategies to soil conditions is critical to the success and growth of tree species, including P. erinaceus, in different ecological zones [55]. The fact that moisture content remained similar across different soil depths within each ecological zone suggests that the distribution of soil moisture is influenced primarily by ecological factors associated with the broader zone, rather than variations with soil depth [36].
Soil organic matter (SOM) is a critical component of soil health, influencing nutrient availability, soil structure, and carbon sequestration [37]. Soil organic matter content varied significantly across the ecological zones, with an interaction between ecological zone and soil depth suggesting that differences in SOM content between the ecological zones varied with soil depth. The MSNW P. erinaceus stands had greater SOM (50%) than the DSFZ ones. We hypothesize that the observed differences are due to varying rates of organic matter input, decomposition, and mineralization, all of which can be influenced by factors such as vegetation type, climate, and land management practices [38, 39]. Soil organic matter acts as a reservoir for essential nutrients, including nitrogen, phosphorus, and micronutrients [39]. The higher SOM content in the MSNW P. erinaceus stands may contribute to a greater potential for increased nutrient availability, potentially supporting higher plant productivity in this zone evidenced by the greater heights and deeper crowns reported in this study [38].
The dominant soils were sand or loamy sand in the DSFZ P. erinaceus stands and loamy sand, sandy loam, and sand in the MSNW P. erinaceus stands; this is similar to the soil types reported by Ansah et al. [15] across different locations where the species is found. We found variations in the proportion of sand and silt content across the ecological zones but noted that these proportions remained similar in relation to soil depth. Soil particle size distribution is fundamental to understanding soil properties, including water-holding capacity, nutrient availability, and soil structure [40]. The variations in sand and silt content may have implications for water infiltration, drainage, and root development [41]. The consistency of clay content across ecological zones and depths indicates that this particular soil component remains relatively stable possibly due to geological factors and parent material [42]. Clay particles can influence soil’s cation exchange capacity, which plays a role in nutrient availability to plants. Soil texture influences water retention and drainage, affecting the availability of water and nutrients to plant roots. Thus, the differences in sand and silt content can influence the growth of P. erinaceus in these ecological zones.
The study revealed that soil pH was similar irrespective of ecological zone or soil depth, which indicates that these factors and their interaction did not significantly affect soil pH [43]. Soil pH is a critical determinant of soil health and it influences various biological and chemical processes, including nutrient availability, microbial activity, and plant growth [15, 44]. Therefore, the consistent soil pH has implications in relation to the composition and activity of soil microbial communities which influences nutrient cycling, organic matter decomposition, and soil structure [44]. Nitrogen and calcium contents were significantly higher in the MSNW P. erinaceus stands than the DSFZ ones possibly as a result of differences in microbial activity, nutrient cycling, parent material, and weathering processes. MSNW P. erinaceus stands had more organic matter and possibly higher microbial activity, which can lead to increased nitrogen and Ca levels [45]. The observed differences in Ca and N concentrations have the potential to affect the health and development of vegetation [15]. Calcium plays a critical role in nutrient recycling and soil structure and it is crucial for cell wall structure and membrane function, while nitrogen is essential for protein synthesis and overall plant growth. Higher calcium levels in the MSNW P. erinaceus stands may contribute to increased plant growth and litter production in this zone, with cascading effects on nutrient cycling and ecosystem function [46]. The stability of K, P, Mg, and Na concentrations suggests that these elements were consistently recycled and maintained in the ecosystem, contributing to the sustainability of nutrient cycles.
4.4. Effect of Soil Characteristics on the Growth Performance of P. erinaceus
The positive correlation between canopy depth and soil moisture content in the topsoil suggests that soil moisture shaped P. erinaceus canopy development, and the positive correlation between canopy depth and soil N content indicates that the species benefited from higher nitrogen levels in the topsoil [47]. This relationship may be linked to the nutrient requirements of P. erinaceus for growth, which has implications for ecosystem nutrient cycling [47]. The negative correlations between K and Mg content with various canopy attributes may suggest that trees with larger canopies are more efficient at nutrient uptake and thus deplete available nutrients in the topsoil [48]. Trees with expansive canopies may require more nutrients to support their growth and maintenance [49]. The positive correlation between soil organic matter content and tree height suggests that taller P. erinaceus trees thrive in areas with higher organic matter content in the topsoil, highlighting the role of organic matter in nutrient cycling and tree growth. Organic matter acts as a reservoir for essential nutrients and can enhance soil fertility, structure, and water retention. Mg is an essential nutrient for plant growth, and its availability can influence the health and productivity of vegetation. The negative correlations between canopy breadth, diameter, and radius with soil Mg content in the subsoil indicate that larger canopies are associated with lower magnesium concentrations at greater soil depths. This suggests that tree canopies may influence nutrient distribution and mobility within the soil profile [50]. It also highlights that soil depth can play a significant role in shaping tree canopy characteristics and nutrient availability. Ecosystem resilience may depend on how trees adapt to nutrient limitations in deeper soils, which can affect long-term ecosystem stability [51]. The fact that soil properties influenced canopy characteristics is crucial for sustainable land management and conservation.
4.5. Policy and Management Implications of Findings
P. erinaceus demonstrated comparable survival levels across the different ecological zones in spite of differences in climatic conditions, indicating its potential suitability for widespread cultivation across diverse ecological zones [7]. This means the potential for successful establishment of P. erinaceus plantations in different ecological zones exists, and that has implications for policy formulation [5, 15, 52]. Policymakers and land managers may explore the establishment of agroforestry systems or cultivation plots across various ecological zones to encourage the growth of P. erinaceus and boost its economic value [5, 52]. Furthermore, given that wetter conditions and nutrient availability suggest better growth performance, policy interventions and management practices can be tailored to suit specific ecological zones to enhance their establishment, optimize their growth and productivity, and improve the conservation of the species [15, 53]. For example, implementing site-specific soil amendment, mulching, organic matter management, shading strategies, and irrigation based on local conditions may enhance the performance of P. erinaceus. Recognizing the ability of P. erinaceus to survive in diverse ecological conditions and accumulate similar biomass carbon stocks reveals its potential role in ecosystem restoration, climate change mitigation, and biodiversity conservation efforts [7, 52, 53]. Thus, integrating P. erinaceus cultivation into restoration and land rehabilitation programs may contribute to ecological resilience and habitat restoration.
5. Conclusion
Pterocarpus erinaceus demonstrated the ability to survive and adapt to different ecological conditions, as evidenced by variations in growth characteristics between wetter and drier ecological zones. Understanding these ecological variations is critical for sustainable forest management, conservation, and restoration efforts, which will ensure the long-term survival and prosperity of this valuable tree species. Furthermore, the species exhibited similar aboveground biomass carbon storage, highlighting its potential capacity for carbon sequestration in different ecological zones. P. erinaceus demonstrated the ability to thrive in different soil moisture conditions, which makes it a versatile species in the landscape. Soil physical and chemical properties influenced the growth performance of P. erinaceus. Particularly, soil moisture, organic matter, and Mg and N contents moderated the canopy attributes and total height of trees. Taken together, our results have implications for the restoration of P. erinaceus stands, carbon sequestration, and soil fertility management [13, 36, 46, 47, 54–56].
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Authors’ Contributions
Michael Asigbaase conceptualised and designed the study, acquired, processed, and analysed the data, supervised the implementation of the study, and drafted and reviewed the manuscript. Seidu Adams, Daniel Adusu, Collins Ayine Nsor, Samuel Kumi, Daniel Akoto Sarfo, Paul Kofi Nsiah, and Selina Adutwumwaa Acheamfour contributed substantially to the design of the study, data analysis and presentation, and drafting and review of manuscript. All authors have read and approved the final manuscript.
Acknowledgments
We are grateful to the staff of the local offices of Forestry Commission for their support during the data collection process. We appreciate the technicians at the Faculty of Renewable Natural Resources, KNUST, for their assistance during laboratory analysis.
Open Research
Data Availability
All the datasets on which the conclusions are based are included within the article and supplementary information file. Upon request, further information about the data will be provided by the corresponding author.
