1. Introduction
Biomass estimation is one of the most crucial steps in estimating carbon sequestration in different biomass components [
1,
2]. It is important to determine the future projections of carbon dioxide (CO
2) stored by biomass over the years [
3]. This information is useful in ecology as it provides insights into ecosystem structure and function, serves as an indicator for sustainability, and helps in assessing ecosystem productivity and the global carbon budget [
4,
5].
Biomass estimation can be achieved using different methods, including the direct application of allometric equations, the multiplication of stand volume by wood density, or the use of the biomass expansion factor, which is a measure of volume-to-biomass ratio [
4,
5,
6]. These methodologies have a common requirement for generalized allometric equations, although they differ in terms of predictor variables. The former method is often used for younger plants, while the latter is more appropriate for mature forest trees due to its rarity and adherence to ethical guidelines and regulations, aiming to mitigate any potential ecological impact on ecosystems [
4,
7].
There has been recent interest in the development of the allometric equation method for biomass estimation, which is a cost-effective technique and uses easily measurable variables such as diameter and height. Additionally, it has the potential to adapt to specific species, taking into account their unique characteristics and growth patterns [
8]. These specialized models can also include site-specific factors, such as soil type, climate, and topography, which can enhance the accuracy, efficiency, and conciseness of biomass estimates and reduce errors in carbon biomass stock assessments [
9,
10,
11,
12].
This study focuses on creating allometric equations for estimating biomass in different components of the argan plant (
Argania spinosa (L) Skeels), a vital species in Morocco supporting local economies and ecosystems. Argan trees, predominantly found in the 830,000-hectare Argan Biosphere Reserve (ABR), face challenges due to the increased demand for argan oil, impacting forest density and sustainability. To address this, Morocco has initiated the domestication of forest-origin species by establishing argan orchard perimeters in vulnerable areas [
13]. This innovative project, based around the Arganiculture concept, aims to meet national commitments for environmental protection and sustainability outlined in the nationally determined contributions (NDCs). Arganiculture is expected to enhance carbon storage, mitigate soil erosion, and sustain the agroforestry system, supporting both mitigation and adaptation efforts. The development of allometric equations to estimate Argania biomass holds immense potential for quantifying the carbon sequestration capabilities within established orchards. These data would significantly contribute to policy formulation, enabling informed decisions on land use, conservation strategies, and carbon offset initiatives [
14].
In earlier studies, Benzyane [
15] and Belghazi [
16] employed a semi-destructive method to estimate the biomass of argan trees in the Haha plateau (Essaouira region). Benzyane [
15] generalized the equation for mature trees considered as part of a high forest, while Belghazi [
16] focused on young stands classified as coppice pushed on old and already-well-established root systems. Determining the age of Argania trees in forest stands posed challenges; however, these studies relied on parameters like tree height and trunk circumference. The published biomass equations focus on large argan trees with diameters > 10 cm. No attempt has been made to explore the biomass estimation of argan trees at the establishment growing stage; these are considerably smaller and contribute little to the total stand biomass. In our current research, allometric equations focus on argan plants of known ages and are derived from nursery seedlings subsequently transplanted into the newly established Arganiculture orchards. By deriving allometric equations from nursery seedlings, this research aims to provide a more comprehensive understanding of the biomass of argan trees at the established growth stage, which can be identified as young argan plants. This approach contributes to a more accurate assessment of total stand biomass in argan forests. It enables more informed forest management and conservation strategies, improves carbon sequestration assessments, and contributes to better modeling of forest growth and development [
14].
Accordingly, the present study attempts to (1) develop allometric equations for young argan plants and their components (e.g., leaves, stem, small branches, roots) to estimate their biomass, (2) apply these equations to estimate the total biomass in selected studied orchards, and (3) convert the estimated biomass to total carbon dioxide storage.
4. Discussion
Allometric models were developed to estimate the biomass of different components. including stems, leaves, roots, and the total biomass of argan plants within recently established argan orchards perimeters across different regions within the Argan Biosphere Reserve. The studied plants were 2 to 6 years old, corresponding to the establishment growing stage (encompassing the lag phase and the very early part of the exponential growth phase). They exhibited diameters ranging from 0.33 to 7.91 cm and a height between 0.29 and 1.42 m. The mean root-to-shoot ratio obtained was 0.64. The root-to-shoot ratio indicates that there are on average 0.64 units of root per 1 unit of aboveground biomass across the plants in the study. The variation in the root-to-shoot ratio across diameter classes indicates that smaller plants allocate a larger proportion of their resources to belowground structures compared to larger plants.
Young plants with smaller stems may require larger root systems for stability and support in arid and semi-arid climates. This could be explained by the need to acquire resources such as water and nutrients in the soil [
33]. The ratio varies with environmental conditions such as precipitation, soil moisture, texture, and fertility and stand conditions such as age, height, forest type, or origin [
34,
35].
The study of carbon content in plant components identified differences in the mean carbon content of roots based on the age of the argan plants. In particular, the carbon content of the roots of argan plants aged between 4 and 5 years old was higher than that of other growing years. This variation could be attributed to natural fluctuations in carbon content throughout the growing season and across years, which are influenced by environmental factors such as temperature, rainfall, and nutrient availability. Harvesting at different times (2021 vs. 2023) may capture these variations. Furthermore, differences in management practices may also be a contributing factor. If there had been changes in management practices, such as fertilization or irrigation, between 2021 and 2023, then this could have had a greater impact on the root C content.
The study revealed that the roots of argan trees have a lower carbon concentration (33–55%) compared to the stems and leaves (52–56%). This could be explained by argan plant roots concentrating on acquiring resources and water directing less carbon towards the belowground parts of the plant. Additionally, age-related variations in carbon content can occur, with younger plants investing more carbon in rapid growth and development [
33,
36]. Mahmood et al. [
26] found that the carbon content was between 42% and 47% in leaves and between 40 and 49% in branches, bark, and stems in tropical evergreen forests. Thomas et al. [
37] reported a wide variation in carbon concentration of 45.7–60.7% in subtropical/Mediterranean species, and Ma et al. [
38] found that the carbon content in crop roots was 38.20 ± 5.23%. In the global assessments, carbon content has been assumed to be 50% of tree biomass; however, recent studies indicate that this assumption is not accurate, with substantial variation in carbon content between tree species, as well as between tissue types [
37].
The study results show a non-linear relationship between diameter and height, which suggests that a simple regression (linear model) might not accurately represent the data. These involve using non-linear equations (e.g., power functions, exponential functions) or applying transformations to the data to linearize the relationship. Among the transformation methods, log–log models were most often reported in many studies of different ecosystems [
4,
26,
39].
Several studies in the literature recommend that biomass equations based on both diameter (D) and height (H) are more accurate than equations based only on D [
4,
40,
41]. Also, the precision and the accuracy of allometric equations increased with the level of specificity of equations as found by Paul et al. [
42]. This study included diameter, height, age, and root-to-shoot ratio for the root biomass model as specific independent variables. The results revealed that the best model for leaves included D, H, and age; D and age for stem biomass, and D, H, age, and root–shoot ratio for root biomass. Therefore, the fit model for total biomass included all the mentioned variables with the highest biomass variation (R
2 = 0.95). On the other hand, the accuracy of biomass estimation largely depends on the appropriate selection of allometric models [
26,
43]. In the study, both models 3 and 9 performed well in predicting leaf biomass, with high R-squared adjusted values of 0.95. However, model 1 showed a lower AIC score than model 9, suggesting that it may capture the relationship between predictors and biomass slightly better. Model 3, on the other hand, shows a potential for heteroskedasticity, which means that the error variances may not be consistent across the data. This could have an impact on the reliability of the conclusions drawn from the model. Our findings agree with those of Mokany et al. [
35]. They emphasize the need for reliable root–shoot ratios across various vegetation types to enhance the accuracy of root biomass estimates, particularly for purposes such as National Greenhouse Gas Inventories and carbon accounting and for studies of ecosystem dynamics.
Henry et al. [
14] highlighted the limited consideration of root biomass in current carbon (C) inventories under an arid climate (only 1.3% of equations). They reported the challenges in measuring root biomass as the high cost and time required for the direct measurement of root biomass and the difficulty of extracting the entire root [
14]. In this study, the roots of 5- and 6-year-old argan plants were found to have deeper roots and develop further into the soil compared to the above-ground parts. This limited the extraction of these root samples. This suggests that we should consider alternative methods for estimating root biomass for argan, such as fractal geometry, minirhizotrons, and ground-penetrating radar. The importance of accurate root biomass data is emphasized, as they can contribute to improving carbon inventories and enhancing our understanding of ecosystem dynamics.
In summary, this study formulated biomass allometric equations for each component of young argan plants, including the leaf (Equation (4)), stem (Equation (5)), root (Equation (6)), and total biomass (Equation (7)). The total biomass is calculated by summing the biomass of each component.
The study demonstrates that carbon biomass storage increases with argan plant age, ranging from 0.01 to 0.82 t CO
2 ha
−1. This highlights the significance of conserving older argan trees to improve carbon sequestration and inform conservation strategies. The total biomass of argan plants in the orchards studied was between 0.006 ± 0.002 and 0.47 ± 0.06 tonnes of dry biomass (DB) per hectare. The findings of the current study differ from those of the Belghazi [
16] study in terms of reported biomass values. Belghazi [
16] found that the total dry aboveground biomass of a young argan coppice aged 5 years old was 1.62 t ha
−1 for a density of 75 coppice ha
−1, corresponding to 21.6 kg per coppice [
16]. This value is approximately 21 times higher than the current study’s value (1 kg per plant). The observed difference could be attributed to the disparity in stand characteristics. While Belghazi [
16] studied coppice derived from well-established root systems, our study focused on plants grown from nursery seedlings subsequently transplanted into the field. In coppicing practice, trees are trimmed close to the ground, stimulating vigorous new growth from the well-established root system. Variations in site and plant density, competition for water availability, and nutrient limitations may also affect growth.
Between 2021 and 2023, the average annual carbon sequestration within the studied age range of argan plants was 0.20 t CO
2 per hectare per year, representing the establishment phase of growth. The low rate observed in our findings could be attributed to the species-specific growth phase examined, in contrast to previous studies integrating the exponential growth phase. Environmental conditions, including soil properties, genetics, and arid and semi-arid climates with prolonged dry seasons, particularly those experienced in Morocco during the study period, could account for these differences. The observed carbon sequestration observed is lower than the reported values for some broader agroforestry systems. These systems have been found to have CO
2 removal rates ranging from 0.8 to 15.6 t CO
2 ha
−1 year
−1 during the first 20 years of growth [
44]. Further research is required to investigate the long-term carbon sequestration potential of argan trees as they mature.