The Effect of Tree Spacing on the Growth and Biomass of Wattle Trees in Northwestern Ethiopia

Abstract

The wattle tree (Acacia mearnsii) is gaining importance as an exotic species in northwestern Ethiopia, providing ecological, environmental, and economic benefits, especially for fuelwood and charcoal production. This study aimed to investigate the effect of tree spacing on the growth and biomass of wattle trees. The study employed a randomized complete block design with three replications across three spacing treatments (0.5 m × 0.5 m, 1 m × 1 m, and 1.5 m × 1.5 m) in the Fagita district. Tree height and diameter measurements were taken at 12, 18, and 30 months post-planting from stands established in 2021. The results showed significant differences in tree height among the spacing treatments at 18 and 30 months. The closest spacing (0.5 m × 0.5 m) yielded the highest average tree heights of 32.12 cm, 84.86 cm, and 302.98 cm at 12, 18, and 30 months, respectively. At 18 months, the largest average diameter (1.22 cm) was found in the narrowest spacing (0.5 m × 0.5 m), whereas at 30 months, the widest spacing (1.5 m × 1.5 m) recorded the largest diameter (1.51 cm). Throughout the study, height, diameter, and average aboveground biomass exhibited an inverse relationship with spacing, with this effect diminishing as trees aged. Tree spacing significantly impacted average aboveground biomass at 18 months, with the densest spacing (0.5 m × 0.5 m) yielding the highest average aboveground biomass (1.97 kg at 18 months and 2.41 kg at 30 months). Average aboveground biomass increased as the trees matured. These findings suggest that closer spacing of A. mearnsii can enhance biomass production, positioning it as a promising candidate for energy generation. Leveraging these insights can optimize resource utilization while supporting global energy demands and reforestation initiatives aimed at carbon sequestration.

1. Introduction

Planted forests are crucial for conservation, carbon sequestration, and as a sustainable source of fuelwood and industrial timber [1]. Acacia mearnsii De Wild (Fabaceae), commonly known as black wattle, is native to Australia and is widely utilized for various products and services. These include fodder, apiculture, fuel, fiber, timber, and tannin production. It also provides services such as erosion control, shade or shelter, nitrogen fixation, soil improvement, ornamental value, and use in intercropping systems [2]. While A. mearnsii is considered invasive in South Africa, its substantial above-ground biomass (AGB) and the high costs of removal have led to a recommendation for a ’novel ecosystems’ approach as a practical management strategy [3]. The study further revealed that reducing the canopy of A. mearnsii could increase grass production while also enhancing carbon sequestration [3]. Additionally, studies have shown that wood from A. mearnsii grown in seed production areas (SPA) and clonal population areas (CPA) could serve as a viable raw material for pulp and paper production [4].

Acacia mearnsii was introduced to Ethiopia in 1990 to support short-rotation forestry and address urban firewood shortages caused by deforestation [5,6]. Originally classified as Acacia decurrens, it was later reidentified as Acacia mearnsii [7]. Its cultivation has expanded in Ethiopia’s humid, high-altitude agroclimatic zones due to its rapid growth and short rotation period of 4–5 years [5,8]. A. mearnsii plantations were first established as public forest initiatives by the government and later evolved into private ventures for charcoal production [9]. A study has highlighted the substantial benefits of A. mearnsii plantations, including increased incomes, job creation, and improved social well-being. These plantations have also significantly improved degraded soils, thereby enhancing natural capital [10]. Further research indicates that A. mearnsii tree-based farming systems are effective in land restoration and boosting farmer incomes [11].

The growing demand for fuelwood in both urban and rural areas of Ethiopia has led to deforestation, declining forest cover, and biodiversity loss [12]. A study report estimates that Ethiopia’s total demand for wood products will rise to 158 million cubic meters annually by 2033 [13]. Wood fuel, including both fuelwood and charcoal, will remain the primary forest product consumed [13]. The 2015 Forest Sector Review projected that Ethiopia needs an additional 7.2 million hectares of plantations to close the wood fuel supply gap [14]. A. mearnsii plays a crucial role in this context, with its wood and bark serving as significant sources of energy, consumed at a large scale for combustion and charcoal production [15]. Energy transition plans focusing on developing local wood energy sectors have identified Acacia mearnsii as the primary species for this purpose due to its potential suitability [16].

Studies on forest biomass serve multiple purposes, including assessing forest growth to evaluate its energy production potential [17]. Forests have also gained attention for their role in combating climate change, due to their capacity to absorb carbon and reduce CO₂ levels, thereby helping to address the increasing concentrations of greenhouse gases, especially carbon dioxide [18]. Rising bioenergy demand boosts forest carbon stocks through afforestation efforts and more intensive management practices compared to scenarios without bioenergy utilization [19]. Promoting both wood-based bioenergy and forest sequestration can enhance carbon storage while simultaneously preserving natural forests [19]. In this context, it is important to highlight the role of short-rotation plantations in biomass production, considering the economic, social, and environmental factors involved. Silvicultural management remains a significant gap in forest productivity in the study area of Ethiopia [20]. Tree spacing plays a vital role in optimizing resource availability, greatly impacting plant growth rates, wood quality, and production costs [21]. Optimal spacing is determined by the ability to produce the maximum yield with the desired size, shape, and quality. However, it varies based on the plant species, site conditions, and the genetic potential of the reproductive material used [22].

Research has explored the relationship between tree spacing, growth, biomass, and carbon stock. For instance, research on Acacia mangium has shown that narrower spacings, such as 2.0 × 2.0 m, produce smaller trees but with a higher proportion of dry biomass concentrated in the trunk, thereby increasing their energy potential [23]. In Tectona grandis, the maximum height of 3.4 m was achieved at a spacing of 1.8 × 1.8 m, while the largest diameter at breast height (9.18 cm) was observed at a spacing of 2 × 2 m [24]. Similarly, Eucalyptus grandis produced the highest biomass yield of 325.1 t ha⁻¹ under denser spacing, demonstrating that closer planting can substantially enhance biomass accumulation [25]. Likewise, studies on Schizolobium parahyba var. amazonicum showed that tighter spacings resulted in a 50% higher carbon stock (19.43 Mg ha⁻¹) compared to wider spacings [26], indicating that spacing affects not only growth but also the ecological role of carbon sequestration. Biomass production also varies with tree age; denser spacings promote higher biomass yields in younger trees, while older trees often experience better individual growth in wider spacings [23,25]. This emphasizes the complexity of spacing effects, underscoring the necessity for customized approaches that take into account the species involved and the intended management goals.

Canopies with a high leaf area index (LAI) effectively capture more light, boosting photosynthesis and enhancing energy availability for plant growth [27]. When aiming for timber production for energy purposes, dense spacing is generally recommended to maximize biomass yield per unit area within the shortest possible timeframe [28]. Forest management, therefore, aims to identify species with ample environmental adaptability, high productivity, and excellent energy-generation potential [19,29]. It has been shown that the aboveground biomass of A. mearnsii trees can be estimated using conventional variables, such as diameter at 1.3 m above ground (d) and total tree height, without the need for cutting and weighing new trees under similar conditions [30]. Therefore, this study aims to evaluate the effect of tree spacing on the growth and biomass of A. mearnsii trees.

2. Materials and Methods

2.1. Study Area

The study was conducted in the Fagita-Lekoma district (10°57′23″–11°11′21″ N and 36°40′01″–37°05′21″ E) in the Awi zone, Amhara National Regional State, Ethiopia (Figure 1 and Figure A1). The Fagita-Lekoma district covers an area of 653.39 km² and is bordered to the south by Banja-Shikudad, to the west by Guangua, to the north by Dangla, and to the east by the West Gojjam zone [9]. The district has an elevation ranging from 1887 to 2902 m above sea level with temperatures fluctuating between 22 °C and 26 °C, and it receives an average annual rainfall of 1770 mm [31]. The Fageta district was chosen due to its status as the primary region for growing A. mearnsii. The topography of the district is rugged and undulating, and land use in the district is dominated by a mixed crop-livestock system [32]. Over the past decade, Acacia mearnsii plantations have become increasingly widespread in the district, predominantly established by converting croplands and grazing areas [33]. The major soil types in the Fagita Lekoma district are Vertisols, Nitosols, Cambisols, and Acrisols [34,35]. The primary land uses are cropland (52.3%), forest land (25.6%), grassland (21.3%), and villages (0.8%), with significant conversion to A. mearnsii woodlots [36].

2.2. Experimental Design

Pot filling was carried out using a standard nursery substrate composed of forest soil, compost, and sand in a 3:2:1 ratio. Seeds were collected from healthy trees within the same local area and sown in nursery pots, selecting the strongest seedlings for the trial. The experimental site, located at an elevation of 2761 m, falls within the moist subtropical agro-climatic zone [37] and was previously used exclusively for cropland. The experimental A. mearnsii stands were established in July 2021 using four-month-old seedlings. The experiment was conducted using a randomized complete block design, featuring three replications and three spacing treatments per replication (0.5 × 0.5 m, 1 × 1 m, and 1.5 × 1.5 m). The total area of the experiment was 0.50 ha, with each plot measuring 40 m². A 1 m gap was left between replications, and a 0.5 m gap was maintained between plots within each replication.

2.3. Data Collection

From each plot, covering an area of 40 m² and consisting of three replications and three different spacing groups, the heights of a total of 654 individual trees were measured at 12, 18, and 30 months after planting. Additionally, the diameters were measured at 18 and 30 months after planting from stands established in 2021. The aboveground biomass (AGB) was estimated for each tree using the best-fit allometric models for A. mearnsii, specifically developed for the same study area in a previous study [30].

Estimated aboveground biomass = a + b ∗ (Height × Diameter)^c

(1)

where:

• a = 1.92;
• b = 0.02;
• c = 1.68.

2.4. Statistical Analysis

Statistical analyses, including descriptive statistics, were employed to calculate the mean and standard error. A one-way ANOVA was conducted to evaluate the impact of tree spacing, the explanatory variable, on tree height, diameter, and aboveground biomass. Correlation analysis was performed to examine the relationship between tree spacing and the dependent variables. Data from each repetition were combined after verifying the homogeneity of experimental error variances using Levene’s Test and Bartlett’s Test. The R Studio statistical software version 4.4.2 package was utilized to perform the statistical computations.

3. Results

3.1. The Effect of Spacing on the Growth of Acacia mearnsii Tree Species

The study found that tree spacing had a significant effect on the height of A. mearnsii at 18 and 30 months after planting (p < 0.05) (

Table 1. This presents the mean values, standard errors, p-values, and F-values for height of A. mearnsii at different spacings and months after planting.

Months	Spacing (m)	Mean (cm)	Standard Error	F	p-Value
12	0.5 × 0.5	32.12	0.85	1.42	0.24
	1 × 1	28.98	1.41
	1.5 × 1.5	28.43	2.41
18	0.5 × 0.5	84.86	3.07	5.14	0.01 **
	1 × 1	63.16	3.96
	1.5 × 1.5	74.71	9.48
30	0.5 × 0.5	302.98	6.99	3.30	0.04 **
	1 × 1	264.89	10.07
	1.5 × 1.5	291.17	22.99

p-value with asterisk (**) is significantly different (p < 0 05).

and

Table A1. ANOVA table of tree spacing and height of A. mearnsii tree.

Months	Source of Variation	SS	df	MS	F	p-Value	F Crit
12	Between Spacing	485.80	2.00	242.90	1.42	0.24	3.02
	Residuals	56,577.50	331.00	170.93
	Total	57,063.30	333.00
18	Between Spacing	27,719.40	2.00	13,859.72	5.14	0.01	3.02
	Residuals	1,117,963.00	415.00	2693.89
	Total	1,145,682.40	417.00
30	Between Spacing	75,075.90	2.00	37,537.96	3.30	0.04	3.02
	Residuals	3,738,704.70	329.00	11,363.84
	Total	3,813,780.70	331.00

SS = Sum of Squares; MS = Mean Square; and df = Degrees of Freedom.

). The highest average tree heights (32.12 cm, 84.86 cm, and 302.98 cm) were observed in the closest spacing of 0.5 × 0.5 m at 12, 18, and 30 months after planting, respectively (Table 1). In contrast, during 12 months after planting, the lowest mean height (28.43 cm) was recorded at the widest spacing of 1.5 × 1.5 m. The study also found that at 18 and 30 months after planting, the mean heights of trees in the widest spacing of 1.5 × 1.5 m (74.71 cm and 291.17 cm) were greater than those in the 1 × 1 m spacing (63.16 cm and 264.89 cm), as shown in Table 1.

The study found that tree spacing had no significant effect on the diameter of A. mearnsii (

Table 2. This presents the mean values, standard errors, p-values, and F-values for the diameter of A. mearnsii at different spacings and months after planting.

Months	Spacing (m)	Mean (cm)	Standard Error	F	p-Value
18	0.5 × 0.5	1.22	0.04	0.31	3.02
	1 × 1	1.08	0.05
	1.5 × 1.5	1.17	0.11
30	0.5 × 0.5	1.45	0.05	0.23	3.02
	1 × 1	1.24	0.08
	1.5 × 1.5	1.51	0.19

and

Table A2. ANOVA table of tree spacing and diameter of A. mearnsii tree.

Months	Source of Variation	SS	df	MS	F	p-Value	F Crit
18	Between Spacing	105.13	2	52.56	1.17	0.31	3.02
	Residuals	18,618.38	415	44.86
	Total	18,723.5	417
30	Between Spacing	2.11	2	1.06	1.49	0.23	3.02
	Residuals	233.35	329	0.71
	Total	235.47	331

). At the early growth stage (18 months), trees in the narrowest spacing (0.5 × 0.5 m) had a larger average diameter (1.22 cm) compared to those in the wider spacings of 1 × 1 m and 1.5 × 1.5 m, which had mean diameters of 1.08 cm and 1.17 cm, respectively. In the later growth stage (30 months), trees in the widest spacing (1.5 × 1.5 m) had a greater mean diameter (1.51 cm) compared to those in the narrower spacings of 0.5 × 0.5 m and 1 × 1 m, which had mean diameters of 1.24 cm and 1.45 cm, respectively. This suggests that the influence of spacing on tree diameter changes with age.

The study found that trees planted at the closest spacing (0.5 × 0.5 m) consistently exhibited greater height across all growth stages of A. mearnsii (Figure 2a–c). However, at the later growth stage (30 months), trees with the widest spacing (1.5 × 1.5 m) exhibited a higher mean height compared to those in the second widest spacing (Figure 2a–c). Additionally, trees in the widest spacing (1.5 × 1.5 m) also had the largest mean diameters at the later growth stages (30 months) (Figure 3a,b).

At all growth stages, tree height, diameter, and aboveground biomass exhibited an inverse relationship with spacing, with this effect diminishing as the trees aged (

Table 3. Correlation^® between spacing and tree height, diameter, and aboveground biomass.

		Height (m)
	All Months	12 Months	18 Months	30 Months
Spacing	−0.06	−0.10	−0.12	−0.09
		Diameter
Spacing	−0.05		−0.06	−0.04
		Aboveground biomass
Spacing	−0.07		−0.11	−0.09

).

The study found a proportional relationship between tree height and diameter across all spacing treatments, with the relationship becoming stronger as the A. mearnsii trees aged (Figure 4).

3.2. Aboveground Biomass of A. mearnsii in Different Spacing and Ages

The study showed that the highest average aboveground biomass (1.97 kg and 2.41 kg) was recorded in reduced spacing of 0.5 × 0.5 m, and the lowest average aboveground biomass (1.94 and 2.16) was recorded in wider spacing of 1 × 1 m at 18 and 30 months after plantings, respectively (

Table 4. This presents the mean values, standard errors, p-values, and F-values for the estimated aboveground biomass of A. mearnsii at different spacings and months after planting.

Months	Spacing (m)	Mean (KG)	Standard Error	F	p-Value
18	0.5 × 0.5	1.97	0.01	3.92	0.02 **
	1 × 1	1.94	0.00
	1.5 × 1.5	1.95	0.01
30	0.5 × 0.5	2.41	0.05	2.76	0.07
	1 × 1	2.16	0.05
	1.5 × 1.5	2.34	0.12

p-value with asterisk (**) is significantly different (p < 0 05).

). The average aboveground biomass (1.95 kg and 2.34 kg) recorded in spacing of 1.5 × 1.5 m was higher than the average aboveground biomass (1.94 and 2.16) recorded in spacing of 1 × 1 m, both at 18 and 30 months after plantings. The average above-ground biomass increases as the trees mature. The study revealed that spacing had a significant effect on the aboveground biomass of A. mearnsii at 18 months after planting (p < 0.05) (Table 4 and

Table A3. ANOVA table of tree spacing and aboveground biomass of A. mearnsii tree.

Months	Source of Variation	SS	df	MS	F	p-Value
18	Between Spacing	0.07	2.00	0.03	3.92	0.02
	Residuals	3.50	415.00	0.01
	Total	3.56	417.00
30	Between Spacing	2.82	2.00	1.41	2.76	0.07
	Residuals	168.24	329.00	0.51
	Total	171.06	331.00

).

The box plot showed that the total aboveground biomass was higher in narrow spacing both at 18 and 30 months after planting and increased as age increased (Figure 5a,b).

4. Discussion

At all growth stages, the tree height, diameter, and aboveground biomass of the A. mearnsii tree showed an inverse relationship with spacing. This suggests that densely planted wattle trees grow taller primarily due to intensified competition for light. Supporting this, a previous study has demonstrated that reduced planting spacings enhance radiation use efficiency and biomass yield, highlighting spacing as a crucial factor in optimizing growth in A. mearnsii [38]. The study also revealed that the relationship between growth and spacing changed as the trees aged. During the early growth stages, seedlings planted closer together faced intense competition for sunlight, while competition for underground nutrients was less pronounced. However, as the trees matured, underground nutrient availability became the primary limiting factor, making wider spacings more favorable for achieving improved growth [39]. This suggests that the competitive dynamics associated with spacing evolve as the trees mature. This dynamic shift aligns with research suggesting that young plants require different light intensities compared to mature ones, influencing their root establishment and nutrient uptake [40]. For example, the larger mean stem diameter of A. mearnsii trees at the widest spacing, compared to narrower spacings in the older growth stages, can be explained by this dynamic. These findings support research showing that as A. mearnsii trees mature, their diameter and height increase with wider spacing [41,42,43]. Additionally, they align with studies indicating that closer spacings in A. mearnsii result in smaller stem diameters and higher mortality [41].

The consistently highest mean aboveground biomass of A. mearnsii trees at the closest spacing across all growth stages indicates that biomass is more strongly affected by tree density per hectare than by the diameter of individual trees. These findings emphasize the importance of dense tree planting to maximize biomass production, supporting previous research that indicates Acacia mearnsii trees with tighter spacing yield the highest biomass [44]. Similarly, further investigations have revealed a positive relationship between biomass accumulation and the age of A. mearnsii trees, in addition to a decrease in biomass as the spacing between trees increases [25]. These insights are essential for optimizing A. mearnsii plantations for energy production in the region and align with research that underscores the potential of Acacia mearnsii for biomass fuel production [15]. In addition to mean biomass being larger in closer spacing, the associated increase in stand density results in increased biomass per hectare.

These results provide essential management guidelines for optimizing A. mearnsii plantation productivity for various purposes, particularly for energy production at both local and global levels. For energy-focused plantations, denser spacings can maximize biomass yields, making A. mearnsii a promising candidate for biofuel and renewable energy production. Additionally, the study highlights the ecological importance of understanding light and nutrient dynamics at different growth stages, informing sustainable management practices. The application of these findings could enhance sustainability, improve resource use efficiency, and support local livelihoods while addressing global energy and reforestation needs.

5. Conclusions

The study demonstrated that tree spacing has a significant effect on the height of Acacia mearnsii at 18 and 30 months after planting. The closest spacing of 0.5 × 0.5 m consistently resulted in the highest tree heights, with an average increase at each growth stage, while the widest spacing of 1.5 × 1.5 m showed the lowest mean height at 12 months. Additionally, the narrowest spacing (0.5 × 0.5 m) formed the largest average diameter in the early growth stage (18 months), while the widest spacing (1.5 × 1.5 m) led to the largest diameter in the older growth stage (30 months). The study also found an inverse relationship between spacing and tree height, diameter, and average aboveground biomass, with the effect diminishing as the trees aged. Spacing also had a significant impact on the average aboveground biomass of A. mearnsii at 18 months. Importantly, reduced spacing was associated with higher average aboveground biomass, suggesting that optimizing tree spacing can increase biomass production. Therefore, adopting closer spacing in A. mearnsii plantation management could enhance biomass production, contributing to sustainable energy generation and carbon sequestration. Further research is recommended to investigate the role of additional management strategies, such as tree-based farming systems, in optimizing the growth and biomass of A. mearnsii.