Edaphic Determinants of Biomass Hyperdominance in Large Trees of the Amazon

1. Introduction

The Amazon, the largest continuous tropical forest on the planet, harbors unparalleled biological and structural diversity shaped by complex interactions among stable climate, high water availability, and heterogeneous edaphic gradients [1,2]. This combination of factors supports highly specialized plant communities, including mega-trees that stand out not only for their exceptional dimensions but also for their central role in ecological and climatic regulation [3].

Within the Amazonian landscape, large trees [4] are defined as individuals with a diameter at breast height (DBH) ≥ 70 cm [5]. These trees hold cultural significance for local communities, perform essential ecological functions, and sustain diverse forms of life [3]. Furthermore, they store a substantial portion of forest biomass, playing a critical role in carbon storage and in maintaining the structural integrity of Amazonian forests [6].

In this study, large trees were defined as those with DBH ≥ 70 cm. This threshold has been widely adopted in studies of forest structure and biomass distribution in tropical ecosystems and in global monitoring networks, as well as in the protocol proposed by Harris et al. [7] for forest research in the Republic of the Congo. Similarly, recent surveys in the Amazon [6] also employ this threshold, which marks the point above which individuals begin to concentrate a disproportionate fraction of total biomass [8,9]. This definition enables standardized comparisons among studies and reinforces the ecological and functional importance of large trees in maintaining carbon stocks and forest structure.

Recent studies have identified regions with high concentrations of giant trees in the states of Pará and Amapá [6,10]. Ecologically, these trees function as “ecosystem engineers,” influencing both the structure and diversity of plant communities and contributing substantially to forest carbon stocks [3,6]. Their capacity to store carbon further underscores their importance in the context of climate change [6,10].

Large trees account for a significant proportion of aboveground biomass. This concentration exemplifies the phenomenon of biomass hyperdominance, described by Slik et al. [8], in which a small number of individuals or species are responsible for a disproportionate share of carbon storage. This is a universal pattern in Amazonian forests [11], corroborated by pantropical evidence indicating that only about 1% of species can contain up to half of total biomass and carbon productivity [12]. From a functional perspective, trees with DBH ≥ 60–70 cm may represent between one-third and nearly half of living biomass [9,13], making them structural keystones for stability and carbon storage in tropical forests.

Despite their ecological significance, little is known about the environmental conditions that support these monumental trees, particularly regarding the edaphic factors that determine their occurrence and growth. The physical and chemical composition of soils is recognized as one of the main drivers of tropical forest structure, influencing germination, growth, and interspecific competition [2]. However, vast portions of the Amazon remain scientifically unexplored, limiting our understanding of its ecological heterogeneity [14].

In this study, we assess whether the occurrence of large trees in specific regions is explained by edaphic attributes. Specifically, we test the hypothesis that soil characteristics are correlated with the diversity and biomass of large-sized individuals.

2. Materials and Methods

This study was conducted at five sites with a high occurrence of giant trees in the northern Amazon region [6], located in the states of Amapá and Pará. Forest inventory and soil data were collected and analyzed between 2019 and 2024. Three of the study sites are located within the Environmental State Park of the Giant Trees of the Amazon (0°41′29″N, 53°28′41″W), and two sites are situated in the Rio Iratapuru Sustainable Development Reserve (0°19′05″N, 52°43′29″W) (Figure 1).

Both areas are situated within the domain of Amazonian dense ombrophilous forest, under a climate classified as Af (Köppen), characterized by a humid equatorial regime with abundant rainfall throughout the year, mean annual temperatures above 25 °C, and total annual precipitation generally exceeding 2,500 mm [15].

At each study area, three plots were established, each composed of four subplots (north, east, south, and west) measuring 20 × 50 m, with a giant tree serving as the central reference point (Figure 1A). All woody individuals with a diameter at breast height (DBH) ≥ 10 cm were measured. Tree height was estimated using a hypsometer, except for the giant trees that served as central references for the clusters, whose heights were determined using LiDAR (Light Detection and Ranging) data [10].

Botanical identification was performed with the assistance of experienced local parataxonomists, based on dendrological characteristics such as leaves, crown shape, trunk, and bark. Scientific names and families were validated using the Flora and Funga of Brazil database (http://floradobrasil.jbrj.gov.br/). The geographic coordinates of each tree were recorded using a GPS receiver (Garmin 65 CSx).

Each subplot was subdivided into quadrants of 25 m², from which eight were systematically selected for soil sampling in the 0–20 cm layer. The simple samples were homogenized to form a composite sample representative of each subplot [16]. Physical and chemical analyses were conducted at the EMBRAPA Soil Laboratories of Amapá and Pará, following the protocols established by [17].

Biomass estimation was performed using regional allometric models implemented in the BIOMASS package [18]. The equation followed the allometric model developed by Chave et al. [19], which incorporates as independent variables the diameter at breast height (DBH, in cm), tree height (H, in m), and wood density (WD, in g/cm³), expressed as:

AGB = 0.0673 × (WD × DBH² × H) ^0.976,

(1)

where AGB represents the aboveground biomass of each tree (in kg). The total biomass of each sampling unit was converted to biomass per area (Mg/ha), considering the effective sampled area in each plot.

To assess the relationship between the biomass of trees with DBH ≥ 70 cm, overall floristic diversity (species richness and Shannon index), and forest structure (individual density), Spearman’s rank correlation coefficient was applied.

To investigate the association between floristic composition and edaphic attributes across sites, a Redundancy Analysis (RDA) was performed. This multivariate ordination technique combines elements of Principal Component Analysis and multiple linear regression [20], allowing the simultaneous evaluation of how multiple soil attributes explain variation in floristic composition among plots. The species abundance matrix was constructed based on species counts per plot and transformed using the Hellinger method [21].

Continuous edaphic variables were standardized (mean = 0, standard deviation = 1) to ensure comparability among scales. To reduce multicollinearity among predictors, a forward selection procedure based on statistical significance (p < 0.05) was applied using the forward.sel function from the adespatial package [22]. The RDA was conducted using the rda() function of the vegan package [23]. The significance of canonical axes and individual predictor variables was tested using permutation tests (999 iterations). The adjusted coefficient of determination (adjusted R²) was used to quantify the explanatory power of the model.

To evaluate whether the physical and chemical characteristics of the plots’ soils influenced the variation in large-tree biomass, a Generalized Linear Model (GLM) with Gamma distribution and log link function was initially fitted, which is suitable for continuous and strictly positive data. From the full model, edaphic variables were selected through a bidirectional stepwise procedure based on the Akaike Information Criterion (AIC), in order to reduce collinearity among predictors and avoid model overfitting [24]. The functions glm() and step() from the stats package [25] were used.

The variables selected through the stepwise procedure were subsequently employed to fit a Generalized Additive Model (GAM) with Gaussian family, which incorporates smoothing functions (splines) to model nonlinear relationships between soil attributes and tree biomass [26]. This approach does not impose a rigid functional form on the relationships, providing greater flexibility for describing complex ecological patterns [27]. The model was fitted using the gam() function of the mgcv package [26]. The analysis considered only plots with positive biomass values of large trees, excluding 17 plots with zero values to satisfy the assumptions of the Gamma distribution and focus inference on areas with the actual presence of these trees.

3. Results

The results indicate that biomass, particularly that of large trees, depends more strongly on the abundance of these individuals (r = 0.75, p < 0.001) than on overall floristic diversity (Figure 2). This outcome is expected, as biomass accumulation is directly related to the number of large-sized individuals present. A strong correlation was also observed between the biomass of large trees and the total forest biomass (r = 0.99, p < 0.001), demonstrating that a small number of large trees concentrate a substantial proportion of the total biomass stock across the evaluated sites.

The Redundancy Analysis (RDA) revealed that six edaphic variables, potential acidity (H⁺ + Al³⁺), fine sand content, potassium (K⁺), aluminum (Al³⁺), total sand, and soil organic matter (SOM), jointly explained 21.5% of the variation in tree species composition (Figure 3), with an adjusted R² of 12.6%. The canonical axes (RDA1 and RDA2) represent the main edaphic gradients among plots. RDA1 was associated with lower concentrations of H⁺ + Al³⁺ and SOM, and higher levels of Al³⁺, K⁺, total sand, and fine sand. In contrast, RDA2 was related to higher concentrations of H⁺ + Al³⁺ and Al³⁺, and lower values of SOM, K⁺, fine sand, and total sand.

All variables were significant (p < 0.001), indicating the presence of environmental gradients that influence species distribution. Among the analyzed sites, Cupixi stood out for its strong association with the potential acidity gradient (H⁺ + Al³⁺), positioned at the negative extreme of the RDA1 axis. All selected variables showed individual statistical significance, reinforcing that edaphic factors play a key role in shaping the floristic structure and composition of the studied area.

The GAM analysis examining the relationship between edaphic conditions and giant-tree biomass indicated that the soil variables pH, SOM, P, and Al³⁺ exhibited significant nonlinear effects on the biomass of giant trees. The model explained 74.6% of the total variance (adjusted pseudo-R² = 0.631), demonstrating a good fit to the data. Moreover, the analysis revealed nonlinear effects of soil pH, soil organic matter (SOM), phosphorus (P), and aluminum (Al³⁺) on the aboveground biomass of large trees (Mg/ha).

Figure 4. Smoothed effects of the edaphic variables pH (p = 0.00107), SOM (p = 0.01410), P (p = 0.00426), and Al (p = 0.00235) on the biomass of large trees (DBH ≥ 70 cm) in the Eastern Amazon, according to the Generalized Additive Model (GAM) fitted for the states of Pará and Amapá, Brazil. Panel (A) displays the nonlinear effects of each soil attribute on biomass (Mg ha^-1), while panel (B) shows the relationship between observed and predicted values. The model explained 74.6% of the total variance (adjusted pseudo-R² = 0.631).

Figure 4. Smoothed effects of the edaphic variables pH (p = 0.00107), SOM (p = 0.01410), P (p = 0.00426), and Al (p = 0.00235) on the biomass of large trees (DBH ≥ 70 cm) in the Eastern Amazon, according to the Generalized Additive Model (GAM) fitted for the states of Pará and Amapá, Brazil. Panel (A) displays the nonlinear effects of each soil attribute on biomass (Mg ha^-1), while panel (B) shows the relationship between observed and predicted values. The model explained 74.6% of the total variance (adjusted pseudo-R² = 0.631).

4. Discussion

Total biomass was strongly related to the abundance of large trees, confirming the pattern of biomass hyperdominance in the Amazon [8]. This phenomenon reflects the disproportionate contribution of a few individuals to the total aboveground biomass stock, indicating that emergent trees play a central structural role in the stability and functioning of the ecosystem [13,28]. In some sites, a single individual accounted for up to 82% of the living biomass, supporting previous studies that emphasize the importance of large trees as key carbon reservoirs and regulators of forest dynamics [6].

Potential acidity, fine sand, potassium, aluminum, total sand, and soil organic matter explained a significant portion of the variation in total species composition. This pattern reflects the naturally low fertility of Amazonian soils [29], which limits both diversity and growth, consistent with studies highlighting the direct influence of edaphic properties on species composition and distribution [30,31,32]. The strong association of the Cupixi site with potential acidity (H⁺ + Al³⁺) suggests that the higher soil acidity in Cupixi may be related to its lower species richness (Figure S1), reflecting more restrictive edaphic conditions for regeneration and species coexistence.

Among the analyzed attributes, soil pH emerged as one of the most relevant predictors, with a marked increase in biomass observed between values of 3.6 and 6.0. This trend can be explained by the role of pH in regulating nutrient availability [33,34]. Such a pattern aligns with studies reporting an association between lower soil acidity and higher aboveground biomass production [35,36], as pH directly influences the availability of essential nutrients [33,37]. At the Amazonian scale, this suggests that areas with soils within the aforementioned pH range may function as productivity hotspots, indicating that even small variations in acidity can induce substantial biomass responses.

Total phosphorus behaved as a limiting nutrient, with a marked increase in biomass above the threshold of 7.09 mg kg⁻¹, beyond which biomass accumulation rose sharply. This finding suggests the existence of a functional phosphorus threshold. The pattern is consistent with previous studies identifying total phosphorus as the main edaphic factor related to coarse wood production in the Amazon [31]. Therefore, the increase in biomass observed above this threshold indicates that the total phosphorus reservoir is a key determinant of sustained growth in large trees. The study region lies predominantly on dystrophic Oxisols and Ultisols [38,39], typically oxidic soils characterized by low phosphorus availability, low cation-exchange capacity (CEC), and high aluminum content [29]. These conditions constrain nutrient availability and explain the strong influence of phosphorus on biomass variation. The phosphorus range identified in this study therefore represents an optimal availability window, capable of sustaining the growth of large trees even in highly weathered soils.

Aluminum exhibited a positive response to biomass within the range of 0.18 to 1.97 cmolc kg^-1, indicating physiological tolerance or adaptive mechanisms of Amazonian species to acidic soils. Studies have shown that some aluminum-tolerant plants can modify cell wall composition and employ specific transporters that sequester aluminum into vacuoles, where it becomes isolated and loses its toxic effect [40]. This ability may enhance the performance of species adapted to acidic soils, particularly light-demanding species, for which aluminum has been shown to correlate positively with growth [41]. Collectively, these findings suggest that certain Amazonian species have developed adaptive strategies to thrive in acidic soils.

The relationship with soil organic matter (SOM) remained relatively stable along the gradient, indicating that, despite its central role in fertility, it did not emerge as a limiting variable for biomass. SOM functions as a strategic reservoir of nutrients for both plants and microorganisms [42]. Its maintenance strongly depends on litter input and decomposition processes, which are regulated by litter quality and environmental conditions [35]. However, the dynamics of litter production and decomposition in tropical forest ecosystems are complex and still poorly understood [43]. This stability suggests that the biomass of large trees responds less to short-term fluctuations in SOM and more to long-term processes related to nutrient cycling, key mechanisms underpinning the resilience of Amazonian ecosystems.

In summary, the biomass of large trees in the Amazon also results from the interaction between edaphic composition and the functional responses of species along soil gradients. This relationship highlights the role of soil as a structuring component of ecological heterogeneity and reinforces the need to integrate edaphic parameters into predictive models and management policies aimed at conserving high-biomass tropical forests.

5. Conclusions

Amazonian giant trees stand out as key structural elements, accounting for a disproportionate share of the forest’s biomass and carbon stocks. In some sites, these individuals concentrated more than 80% of the total biomass (Figure A1), underscoring their essential role in regulating forest dynamics and maintaining ecosystem stability and services.

Our results also demonstrate that edaphic attributes such as pH, phosphorus, and aluminum are associated with the occurrence and biomass of these trees. This pattern indicates that conserving areas with a high density of large individuals should be a priority in forest management and climate policies, as their loss would compromise the resilience of the Amazon forest.

Appendix A.1

The biomass of all trees and the biomass of large trees also showed significant variations among the evaluated sites, following a similar pattern (Figure 6A). When comparing the two levels within each site, no statistically significant differences were observed between the mean biomass values of all trees and those of large trees alone in Urucupatá, Iratapuru, and Cupixi. This finding reinforces the idea that, in these locations, large trees account for a large proportion of the stored biomass.

Figure A1. The biomass of all trees and that of large trees also showed significant variation among the evaluated sites, following a similar pattern. When comparing the two levels within each site, no statistically significant differences were observed between the mean biomass of all trees and that of large trees alone in Urucupatá, Iratapuru, and Cupixi. This finding reinforces the idea that, in these locations, large trees are responsible for a substantial share of the stored biomass.

Figure A1. The biomass of all trees and that of large trees also showed significant variation among the evaluated sites, following a similar pattern. When comparing the two levels within each site, no statistically significant differences were observed between the mean biomass of all trees and that of large trees alone in Urucupatá, Iratapuru, and Cupixi. This finding reinforces the idea that, in these locations, large trees are responsible for a substantial share of the stored biomass.

The relative importance of large trees in the composition of forest biomass varied considerably among the evaluated sites. In Urucupatá, 84.3% of the total biomass of all trees was attributed to large trees, followed by Iratapuru (71.3%), Cupixi (70.0%), and Ipitinga (52.1%). These results suggest that, in some sites, large trees play a dominant role in the structure and accumulation of forest biomass.