The Economic Potential of Stump Wood as an Energy Resource—A Polish Regional Case Study

Abstract

This paper discusses the possibilities of using stump wood as a raw material for energy generation. The research was based on an analysis of the state of knowledge, forest field studies, and participatory observations. A formula was sought to optimise the procurement cost of stump wood appropriate to Polish conditions. Conceptualisation was carried out in a selected area of the Notecka Forest in the Wielkopolska region, located in western Poland. A pilot study was designed to test a computational formula to assess the profitability of harvesting wood from stump wood resources for energy generation. The potential of stump wood is estimated to be around half a million cubic metres per year from the Notecka Forest area alone. This resource provides an opportunity for business development in both forestry and the renewable energy sources (RESs) sector, despite the barriers and risks shown in this study.

1. Introduction

It seems that even in an era dominated by new technologies, wood still remains a valuable raw material, important not only for the present but also for the future of the economy [1,2]. One of the key economic questions is energy resources, possibilities, and directions for their use [3,4,5]. This also applies to the EMEA region, especially Eastern and Central Europe, including Poland [6,7]. In these conditions, an industrial, technological, and business consensus—and, against this background, a legislative criterion—is being sought to determine to what extent the energy use of wood-based resources should be permitted [7,8,9]. Therefore, not only international but also Polish government agencies are working on the optimal possible definition of energy wood [10]. Sector-specific laws in this area are due to be amended in 2025.

The Polish Ministry of Climate and Environment is finalising a regulation on detailed qualitative and dimensional characteristics of energy wood. The new regulation is intended to enable the optimal use of raw wood material, eliminating the phenomenon of burning wood in the power industry that could be used in the wood-based industry. The legal basis for the introduction of this regulation is Article 119a of the Law of 20 February 2015 on Renewable Energy Sources [11].

According to the definition, energy wood is a raw wood material that is not industrially useful or has limited potential for use in sectors other than energy. The new regulation divides raw materials into two main categories: raw roundwood and non-roundwood material. Significantly, the regulation does not allow for the possibility of processing full-grain wood through milling, chipping, or other mechanical or chemical processes to classify it as energy wood. Moreover, the law does not impose a ban on the use of raw materials other than energy wood in the power industry [12,13,14,15].

To date, published research papers on the energy use of wood have been numerous and often interdisciplinary [16,17,18], but they have relatively rarely discussed the management of the often-marginalised root part of the tree: stump wood [19,20,21,22]. This justifies the continuation of not only in-depth theoretical studies but also field, experimental, and mesoeconomic research in forestry [23,24].

2. Materials and Methods

Why study stump wood specifically? Stump wood is a specific wood resource harvested from the roots and stumps left behind after a tree has been felled. It is estimated that stump wood can account for up to several per cent of a tree’s total biomass. Stump wood resources, mainly pine, have so far been used for energy and industrial purposes, with varying intensity. It is noteworthy that by the end of the 20th century, the economic use of stump wood was almost completely dispensed with. Nowadays, mainly due to land deforestation and changing land use—whether for road construction or for agroforestry—the situation has changed [25,26]. Between one and about two thousand cubic metres of stump wood is harvested annually in Poland, of which one thousand is reported in the statistics of the State Forests [27].

This underutilised and undervalued fragment of woody biomass is becoming an increasingly useful resource for the renewable energy sources (RESs) sector [28,29,30]. Of course, an important economic rationale is the limited availability of wood and the exploration of additional valuable resources for the chemical industry [31,32,33].

Furthermore, Poland’s forest resources enable the efficient harvesting of not only timber but also wood by-products. The harvesting of stump wood is favoured by the species structure of Poland’s forests. These include numerous pine stands, which cover more than 60 per cent of the forest area. Pine grows mainly in sandy soils. If the annual total volume of pine felling is estimated at about 30 million cubic metres, it can be estimated (taking into account the specific natural conditions) that the stump wood resource that can be harvested in Poland is more than one million cubic metres, of which about half a million is from the Notecka Forest alone [27,34].

2.1. Justification for the Research and Study Scope

To date, limited research has focused on the assessment and efficiency of stumpage harvesting, followed by the possibility of using this wood-based raw material [35]. However, taking into account the economic, natural, and social aspects related to the activation of rural areas [36,37], stump wood resources should not be neglected despite technological barriers. This is justified by several important reasons:

(1)

First, there is a need to look for additional sources of income and to use underground wood resources in various ways—whether as renewable energy sources (RES), as raw material for industrial production (in the pulp and paperboard sector) [38,39,40], or for the extraction of certain chemical components that chemical and pharmaceutical industries need [41].

(2)

Secondly, the available stump wood resources represent an opportunity to develop a new sector-specific market. This would ensure a continuous—rather than merely incidental—supply of wood to market participants, including potential customers [42].

(3)

Thirdly, there is both an opportunity and increasingly a necessity to develop and implement innovative technologies for stump and roundwood harvesting in forestry, ideally within a single technological operation. The integration of operations can be facilitated through the use of a harvester, particularly due to the manoeuvrability of its cutting heads. This presents a clear opportunity to optimise the utilisation of stump material by enabling lower cutting at the root section (the stump) [43,44].

(4)

Fourthly, and finally, this marginalised economic activity in forestry can paradoxically represent an opportunity to enhance conditions for natural forest regeneration. The depressions left after root removal can effectively prepare the soil, promote water retention, and create micro-irrigation conditions—factors that are particularly beneficial in hilly forested areas.

However, this is not the only aspect to be considered within the scope of this study. Large, flat areas with stump wood may become ecologically homogenised, which is referred to as a non-naturalisation effect. Moreover, stump wood plays a significant role in soil stabilisation and serves important ecological functions. As a form of dead wood, it acts as a biological reservoir, storing nutrients and supporting ecosystem processes [45]. Therefore, in planning stump wood utilisation, a strategy of partial rather than total harvesting should be adopted.

During various energy crises, periodic research was conducted into practical methods for harvesting stump wood, particularly in Scandinavian countries [46,47,48]. However, the high costs associated with harvesting raw wood in this manner rendered the proposed methods economically unviable. Furthermore, the studies did not address potential structural damage to the soil, including risks of disturbance, soil erosion, or nutrient depletion at the stump wood harvesting sites [49].

2.2. Background and Conceptual Data Sources

The initial source of data consisted of literature studies. Although studies were available, thematic research results remain, unfortunately, limited [14,24,32,41]. Subsequently, data from public statistics were verified [27,50,51,52,53]. Pilot field studies and the authors’ own observations conducted in the Notecka Forest located in the Wielkopolska region of western Poland [54] were also included. The authors aimed to integrate these activities into the conceptual framework of interdisciplinary research on the utilisation of stump wood in the energy and wood-processing industries [49].

2.3. Operational Data Sources

The research material was initially obtained from the scientific literature. Additionally, sources from public statistics [27] and regional industry reports [50,51,52,53] were reviewed. The regional focus of the study on the economic- and energy-related utilisation of stump wood was Wielkopolska, a region located in western Poland [55]. In the course of the literature review, the following scientific databases were thoroughly examined in search of research papers addressing the industrial application of stump wood:

(1)

https://www.webofscience.com/wos/woscc/basic-search (accessed on 13 May 2025);

(2)

https://onlinelibrary.wiley.com/ (accessed on 13 May 2025);

(3)

https://www.sciencedirect.com/ (accessed on 13 May 2025);

(4)

https://www.elsevier.com/ (accessed on 13 May 2025);

(5)

https://agro.icm.edu.pl/agro/ (accessed on 13 May 2025);

(6)

https://www.researchgate.net/ (accessed on 13 May 2025);

(7)

https://www.academia.edu/ (accessed on 13 May 2025);

(8)

https://scholar.google.com/ (accessed on 13 May 2025).

The following publication categories were searched in all cases: books, journals, proceedings, magazines, and conference materials.

Only selected sources were included in this study—specifically, those directly related to the topic under investigation and aligned with the objective of the review, namely, the development of a formula for optimising stump wood procurement costs.

2.4. Pilot Study Project in the Notecka Forest

The pilot study, conducted in 2023–2024 in the Notecka Forest, employed a participatory observation approach. The Notecka Forest was selected due to its unique characteristics, both in terms of raw material availability (even-aged stands) and site conditions (a predominance of weak-bearing fine sandy soils). These conditions support the growth of Scots pines (Pinus sylvestris L.) with shallow root systems, resulting in a typical “pine monoculture”. Despite its regional specificity, the findings of this study can be extrapolated to other pine-dominated forest complexes, particularly those with homogeneous stand structure and similar site characteristics.

The present study pursued the following objectives:

-

To conduct a preliminary assessment of the practical feasibility of harvesting stump wood chips (referred to as “bio-chips”), as a potential raw material for the renewable energy sector;

-

To investigate alternative applications of stump wood, including its potential use in the extraction of sources of chemical compounds aimed at mitigating the degradation of raw wood material, as well as in the production of bio-based components for biological filtration systems.

Following the participatory observation phase, the results obtained by various researchers were subsequently compared. Upon aggregating the data [27], it was observed that stump wood sourced from trees growing under challenging natural conditions (e.g., the Notecka Forest) [56,57,58] exhibited superior properties compared to stump wood derived from trees growing on more fertile soils, including post-agricultural land [59,60]. These findings suggest that the end product suitable for the renewable energy sector (RES) may include (a) so-called “bio-stump wood” and (b) “bio-wood”, the latter being a processed form of stump wood [61,62].

As part of our own research activities, field exercises employing the participatory observation method were conducted to assess the practical feasibility of stump wood harvesting in the Notecka Forest [63,64]. The observations were systematically documented with photographic evidence (see Figure 1, Figure 2, Figure 3 and Figure 4).

The following key aspects were documented and analysed during the field study:

(1)

The feasibility of machine-assisted, so-called combined harvesting of roundwood and stump wood was assessed (Figure 1).

(2)

The presence of stump wood as part of raw timber material was observed in forest areas affected by damage or disturbance events (Figure 2).

(3)

Post-harvest forest sites were monitored to evaluate the condition of the area following roundwood and stump wood extraction (Figure 3).

(4)

Finally, the potential for temporary stump wood storage along forest transport roads was examined (Figure 4).

2.5. Profitability Model for Processing Stump Wood in Forestry Production

The assessment of the profitability of stump wood harvesting was carried out using a calculation method to determine its value when processed into wood chips. The foundation for this analysis was the cost optimisation formula originally proposed by Mikołajczak [31,34], which was later verified through field studies conducted by Mikołajczak and Wajszczuk [55], Zajączkowski [59], as well as Kusiak, Wanat et al. [64]. The resulting wood chips produced from stump wood were identified as a “universal resource for further processing”—suitable both for industrial applications (e.g., production of wood-based panels) and for energy purposes, including the manufacture of pellets, wood briquettes and other bioenergy carriers [56,57,58,61].

In this perspective, an attempt was made to develop a formula that is both conceptually sound and computationally applicable for assessing the profitability of stump wood harvesting in forest operations.

3. Results and Discussion

The foundation of this pilot study, conducted primarily at the conceptual level, was the assumption that forest biomass harvesting for industrial and energy purposes should also encompass the underground part of the tree—namely, stump wood.

3.1. Stump Wood Resources and Harvesting in Poland

After the completion of forest logging operations, a substantial volume of organic material remains on the site, including branches, needles, leaves and severed treetops. These are commonly referred to as logging residues and frequently encompass cuttings up to one metre in length, often exhibiting defects that render them unsuitable for industrial processing, along with the root portion of the tree. This raw material is typically characterised by a high level of impurities—primarily sand, reaching up to 10%—as well as elevated chlorine content. The calorific value of this specific type of raw wood material ranges between 7 and 14 GJ per tonne [59,61].

Stump wood refers to the raw timber material derived from the underground part of trees—specifically from stumps that have been processed into slivers or undersized pieces. The precise technical and dimensional specifications of this material are defined by the applicable industry standards in Poland.

Current data on timber harvesting by selected wood assortments in the Polish State Forests from 2011 to 2023, with particular attention to stump wood [thousand m³], are presented in

Table 1. Wood harvesting by selected assortments in the Polish State Forests (2011–2023), including stump wood [thousands m³].

Specification	2011	2012	2013	2014	2015	2020	2022	2023
Total [thousands m3]	37.180	37.045	37,946	39.742	40.247	39.669	44.647	41.662
Timber (including) Fuelwood	34.877	34.978	35.796	37.661	38.327	38.064	42.703	39.846
	3195	3425	3451	3528	2996	3006	4848	5024
Slash (including) Fuelwood	2303	2067	2148	2079	1920	1604	1944	1816
	1785	1619	1693	1655	1512	1437	1799	1627
Fuelwood (total)	4980	5044	5144	5183	4508	4443	6647	6615
Stump wood	0.0	0.1	1.6	2.2	0.3	4.4	0.8	1.0

Source: based on [27,50,51,52,53].

.

Pine stump wood that has remained in the soil for a minimum of seven years may serve as a valuable raw material for resin extraction or decomposition distillation (dry distillation of wood). This type of material—referred to as industrial stump wood—constitutes a primary source for the production of resin and turpentine extraction. Resin-based derivatives from stump wood are obtained through solvent extraction processes involving agents such as petroleum, benzene, and turpentine. Presently, this technology is no longer applied on an industrial scale.

However, as recently as the 1960s, five industrial-scale stump wood extraction facilities operated in Poland, located in Rudnik, Szczebrzeszyn, Kozienice, Spychów, and Czarna Woda. Today, none of these plants remains in operation [34].

Stump wood constitutes a viable raw material for the production of green energy. However, the widespread adoption of this form of biomass utilisation remains limited due to a combination of technological, environmental, and economic constraints. These limitations significantly hinder the efficiency and scalability of stump wood harvesting operations.

Taking into account the annual harvest in the Polish State Forests (41,662 thousand m³), the potential volume of stump wood can be estimated at approximately 4.2 to 6.7 million m³, which represents 10%–16% of the total harvested volume. This estimate is consistent with previous assessments by Zajączkowski [59,60,61], indicating the suitability of this resource for energy purposes. Despite this considerable volume, the potential of stump wood remains largely untapped in Poland [27,62,63].

Based on public forest statistics in Poland, the annual volume of felled pine is estimated at approximately 30 million cubic metres. Using this Figure, and accounting for specific natural conditions, it is estimated that the potential annual yield of harvestable stump wood in Poland averages around one million cubic metres, with approximately half of this volume located in the Notecka Forest alone [27,34].

3.2. Synthesis of the Pilot Study Results in the Notecka Forest

Based on the pilot study project, a field survey was conducted in the Notecka Forest over the period 2023–2024, employing a participatory observation technique. What were the outcomes of the preliminary study? A general research and technology scenario was outlined, laying the groundwork for future investigation. The pilot study provides a basis for the implementation of regular, systematic research in the future. It is assumed that such studies will be conducted in a representative logging area where stump wood harvesting is to be undertaken. In practical terms, this implies that approximately 50%–60% of the total number of stumps can be extracted.

The use of large-scale machinery (harvesters) is envisaged for these operations. This approach represents an innovative technology enabling the simultaneous harvesting of stump wood and roundwood in a single technological cycle. The underlying concept is to optimise the utilisation of stump wood by cutting the underground part of the tree at a lower level, which is facilitated by the enhanced manoeuvrability of the harvester head. A key advantage of these machines lies in their ability to grip the tree (log) at a height of approximately 2 metres during the stump wood extraction process.

This facilitates the tilting of the tree and, consequently, the extraction of the root system along with the stem. In the final phase of the operation, the harvester head is repositioned to the site where the stump wood was severed, enabling the continued processing of roundwood and associated by-products [64]. The extracted stump wood is subsequently transported to a timber yard, where it is subjected to chipping. Ultimately, the chipped material may be delivered to end-users. In scenarios involving only a relatively short transport distance, both roundwood and stump wood may be removed from the forest in an unprocessed form.

It remains essential to assess the regional potential of the stump wood resources in the designated area. The volume of these resources will be estimated by measuring the fresh mass directly using the weighing function integrated into the harvester head.

Based on this approach, a model scenario has been developed for quantifying the stump wood potential within the “Notecka Primeval Forest” Promotional Forest Complex. The proposed methodology includes annual estimation, stump count per unit area, and a 10-year forecast of harvesting potential. Furthermore, it appears feasible to assess labour intensity (in terms of time and cost), machine efficiency, and overall harvesting costs. Naturally, a critical component for achieving these objectives is the formulation of a calculation model that enables the evaluation of the economic profitability of stump wood harvesting.

3.3. Assessing the Viability of Harvesting Stump Wood, i.e., the Feasibility of Its Economic Use

Following a comprehensive review of the state of the art, it was proposed that the profitability of stump wood harvesting be evaluated by determining its value when processed into woodchips. For this purpose, a calculation model, originally developed by E. Mikołajczak and K. Wajszczuk [30,31,55], W. Kusiak et al. [64], and M. Szczawiński [65,66], was adapted.

This line of research demonstrated that the evaluation of sawmill by-products, based on their price in an unprocessed form, can serve as a basis for assessing the profitability of their conversion into biodiesel or energy under various assumptions regarding the net profit margin. Furthermore, the original model [34] was modified to allow verification of key economic thresholds, such as the maximum feasible profit margin, the maximum acceptable processing costs, and the limiting price of the “waste” material, beyond which its acquisition for processing would no longer be economically viable.

Ultimately, the Mikołajczak formula—designed to rationalise the utilisation of the sawmill by-product stream by assigning value to each type of by-product—enables the identification of the most cost-effective wood-based waste management strategy from the perspective of sawmill operations [55]. As such, it serves as a foundation for classifying wood waste according to its optimal end-use. This framework is equally applicable to the assessment of production profitability, especially when wood waste utilisation constitutes a core business activity. Consequently, Mikołajczak’s concept was adopted as the fundamental model for evaluating the economic viability of stump wood-based production as well.

The rationale is as follows: woodchips represent a highly versatile, raw wood-based material suitable for a range of industrial applications, including the production of wood-based panels and the generation of bioenergy (e.g., pellets, wood, and other solid biofuels; see Formulas (1) and (2)). The proposed calculation formula was validated under Polish conditions but employs internationally recognised measurement units.

The conversion of wood by-products, including stump wood, into energy, is economically viable only when it results in a net profit. Referring to the work of Mikołajczak [55], unit profit P_pu was defined using the following equation:

P_pu = R_pu − C_pe − t_r (R_pu − C_pe) [EUR/GJ]

(1)

P_pu = p_u m_u [EUR/GJ]

(2)

where the parameters mean the following:

• P_pu—unit profit from converting stump wood into energy [EUR/GJ];
• R_pu—unit revenue from energy sales (as well as savings from substituting other fuel with energy produced from stump wood) [EUR/GJ];
• C_pe—the production cost of an energy unit [EUR/GJ];
• t_r—corporate income tax rate (CIT) in 2025 = 0.19 (a reduced CIT rate of 9% is also an alternative);
• p_u—unit sales price of woodchips [EUR/GJ];
• m_u—assumed satisfactory net margin entrepreneur level, m_u: {0.01; 0.05; … 0.15}.

After transformation (comparing Equations (1) and (2) side by side), the form was obtained:

p_u m_u = R_pu − C_pe − t_r (R_pu − C_pe)

(3)

p_u m_u = (R_pu − C_pe) (1 − t_r)/(1 – p)

(4)

$${\displaystyle \frac{p_u{m}_u}{1-t_r}}=R_{pu}-C_{pe}$$

(5)

The sales revenue is equal to the unit price multiplied by the quantity of energy units sold: R_pun = p_un. Accordingly, with n = 1, the unit revenue will be:

R_pu = p_u

(6)

Consequently, the unit cost of energy from burning stump wood C_pe can be written with the formula:

$$C_{pe}={\displaystyle \frac{c_{upr}+c_{utr}+c_{mat}}{d{Q}_{drh}}}[EUR/GJ]$$

(7)

where the parameters mean the following:

• C_upr—unit cost of converting stump wood into energy, including other unit operating costs [EUR/m³];
• C_utr—unit cost of transporting stump wood to the conversion to energy site [EUR/m³];
• C_mat—unit cost of energy-burning stump wood, with a value of V_usw [EUR/m³];
• d—bulk density of processed stump wood [t/m³];
• Q_drh—energy (calorific) value of stump wood [GJ/t] with a given relative humidity, denoted h_r.

Formulas (6) and (7) were substituted into Equation (5). This equation was subsequently transformed into Formulas (8) and (9) in order to isolate the value of stump wood converted into energy, denoted as V_usw (Equations (10) and (11)). It was assumed that upon reaching the expected profit margin, the unit cost of combusted fuel corresponds to its value. Accordingly, the equation was reformulated as follows:

$${\displaystyle \frac{p_u{m}_u}{ 1- t_r}}=p_u-{\displaystyle \frac{c_{upr}+c_{utr}+V_{usw}}{d{Q}_{drh}}}\times d{Q}_{drh} \mathrm{and} c_{\mathit{mat}}=V_{usw}$$

(8)

and we subsequently obtained:

$${\displaystyle \frac{p_u{m}_{u}d{Q}_{drh}}{1-t_r}}=p_{u}d{Q}_{drh}-c_{upr}-c_{utr}-V_{usw}$$

(9)

$$V_{usw}=p_{u}d{Q}_{drh}-{\displaystyle \frac{p_u{m}_{u}d{Q}_{drh}}{1-t_r}}-c_{upr}-c_{utr}[EUR/m^3]$$

(10)

$$V_{usw}=p_{u}d{Q}_{drh}\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(11)

Based on the calculations conducted by Mikołajczak and Wajszczuk [55], referencing the foundational work of Krzysik and Szczawiński [65,66], the Q_drh parameter was adjusted using formulas tailored to the specific characteristics of the tested raw material (stump wood). In particular, the following relationships were adopted for pine wood (the dominant wood species in the Notecka Forest) taking into account either the effect of the absolute moisture content h₀ (Equation (12)) or relative humidity h_r (Equation (13)), that is:

$$Q_{drh}={\displaystyle \frac{19.5-2.5h_0}{1+h_0}}[MJ/kg]$$

(12)

or:

$$Q_{drh}=19.5-22h_r[MJ/kg]$$

(13)

Equation (11) takes the following form:

$$V_{usw}=p_{u}d{\displaystyle \frac{19.5-2.5h_o}{1+h_o}}\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(14)

or:

$$V_{usw}=p_{u}d\left(19.5-22h_r\right)\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(15)

Referring to Formula (11), it is therefore assumed that the value of wood-based waste (in this case, stump wood) converted to energy can be determined using the following relationship:

$$V_{ei}=p_{ue}d{\displaystyle \frac{19.5-2.5h_0}{1+h_0}}\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(16)

or, alternatively:

$$V_{ei}=p_{ue}d\left(19.5-22h_r\right)\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(17)

where

• V_ei—value of a specific type of wood-based waste, denoted as “i” (e.g., stump wood)—after conversion into energy [EUR/m³];
• i—index of the type of wood-based waste considered for energy conversion, where: “i” ϵ <1, n>;
• p_ue—unit selling price of the energy derived from the combustion of wood by-products (e.g., stump wood) [EUR/GJ];
• t_r—corporate income tax (CIT) rate, where the rate for the year 2025 was set as 0.19 (a reduced CIT rate of 9% may alternatively apply);
• d—bulk density of the processed stump wood [t/m³],
• m_u—expected net profit margin set by the entrepreneur, possible values, m_u: {0.01; 0.05; … 0.15};
• h_r—relative humidity of the wood-based by-products (e.g., pine stump wood), expressed as a percentage or decimal fraction;
• h₀—the absolute humidity of the wood by-products (e.g., pine stump wood), reported in numerical values;
• C_upr—unit cost of converting stump wood into energy, including associated operational costs [EUR/m³];
• C_utr—unit cost of transporting stump wood to the energy conversion site [EUR/m³].

Alternatively, reference can also be made to Scandinavian studies [46,55], taking into account the use of coniferous and deciduous woodchips, which is included for information purposes but intentionally ignored in the construction of the computational formula.

Finally, the total value of the wood-based raw material intended for energy conversion, denoted as V_ei, specifically in the case of stump wood, at the unit energy price p_ue, can be determined using the following formula:

$$V_{ei}=\sum _{i=1}^n{S}_{\mathit{ui}}{p}_{ue}d{\displaystyle \frac{19.5-2.5h_0}{1+h_0}}\left(1-{\displaystyle \frac{m_u}{1-t_r}}\right)-c_{upr}-c_{utr}[EUR/m^3]$$

(18)

where, aware of the complexity of the energy production process, a parameter denoted S_ui was extracted—representing the share of by-product “i” in the total quantity converted into energy. In the present case, it refers specifically to the share of stump wood. Once the value is determined by Formula (18), it can be compared with the market price of unprocessed stump wood.

3.4. Limitations of the Study and Major Discussion Points

It is noteworthy that in addressing the question of the potential use of stump wood as an energy resource, this study sought not only theoretical insight but also a simple, practical solution. In the case of Poland, despite technological, economic, and operational barriers, the research conducted in the Notecka Forest demonstrated how the marginalised and seemingly useless forest by-products can be utilised.

Through field exercises carried out in the Notecka Forest [64], the potential for harvesting valuable energy wood [55,65,66], specifically from stump wood [67,68], as part of the forest production process was verified. However, the economic viability appears to be the most compelling aspect [49,55,64].

The main limitation of this study is its focus on a relatively uniform (pine monoculture) yet spatially fragmented area, specifically, the Notecka Forest. Another limitation lies in its empirical scope, which is based on field research with a small sample size. As such, this study should be regarded primarily as a conceptual and exploratory overview, with the pilot study serving as a necessary complement to the existing state of the art. Field visits were conducted in a limited section of the forest (randomly selected forest districts of the Notecka Forest) during the 2023–2024 period. This research, based on participatory observation, was therefore predominantly qualitative in nature [54,64].

It is evident that further research is required that is empirical in scope, quantitatively extended, and based on a sample that ensures the representativeness of the study area. Given the theoretical nature of the analysis and the focus on adapting the calculation formula, statistical analysis was not applicable at this stage of the research. The proposed cost-effectiveness formula [30,31,34], following its initial validation, should be applied in an empirical study. This will allow for the verification of various scenarios and the analysis of the formula’s sensitivity to external factors not accounted for in the current study. Despite these limitations in terms of universality, the computational model can serve as a foundation for more detailed investigations. This was, indeed, the principal objective behind its development.

Of course, it is also necessary to consider the environmental risks involved [69,70]. Here, two perspectives intersect. On the one hand, stump wood is regarded as a so-called “problematic material” due to its challenging processing and final utilisation (harvesting difficulties, high mineral content) [71,72,73]. On the other hand, it represents a significant source of forest biomass that can be converted into green energy fuel [74,75,76]. These challenges can be mitigated through the use of appropriate technologies.

An interesting example of the application of an innovative stump wood harvesting technology was developed by the authors and presented in a study conducted in 2020–2021 in the Czech Republic by a team led by Luboš Staněk [77]. The aim of that research was to determine the influence of wood species (coniferous vs. deciduous), trunk diameter, and substrate type on the time required to process stumps. A prototype grubbing head was used, mounted as an adapter on the boom of a remotely controlled harvester. A formula was then developed to predict the time needed to process a single stump. By analysing the economic efficiency of the stump wood harvesting process, this formula may serve as a valuable tool for future empirical research.

On the other hand, the concept of harvesting stump wood raises concerns among foresters and ecologists about potential environmental damage and biodiversity loss resulting from the uprooting of tree roots. Another argument often cited is the hypothetical depletion of forest habitats due to the removal of nutrients along with the extracted biomass. However, these concerns do not appear to be fully substantiated. Research has shown that during the mechanical processing of logging residues, a significant amount of fine branches and assimilative material becomes detached and remains on site. This suggests that the ecological risks associated with stump wood harvesting, at least under Polish conditions, where sustainable forest management is legally mandated, are relatively minor. Moreover, potential issues related to contamination of the harvested stump biomass [78,79,80,81,82], as well as damage to the soil and forest floor, can be effectively addressed through the implementation of modern technologies [83,84,85,86].

Finally, a certain limitation of this study is its exclusive focus on the Polish context of stump wood harvesting, specifically in the case of pine roundwood and pine monoculture forests. However, this does not imply that the findings presented herein cannot serve as a basis for further research and discussion in other countries. This is particularly relevant given that similar challenges are being reported on the international wood market [87,88,89,90,91].

Naturally, this study does not claim to offer a complete or final solution. Rather, it constitutes an invitation for continued research, both within Poland and internationally, advocating that the forest wood value chain should incorporate the principle of certification and the need for comprehensive utilisation of raw wood material [92,93]. This, in turn, may allow at least partial progress towards the concept of integral economics, for which the circular economy model [94,95,96,97] may be a practical implementation within the context of forest economics.

4. Conclusions

Based on the theoretical analysis, followed by a comparative and descriptive evaluation, the following conclusions and recommendations were formulated:

(1)

The harvesting of stump wood as a raw material for the green energy sector was demonstrated through a case study of forest management in the Notecka Forest, located in the Wielkopolska region of Poland.

(2)

The regional example from Poland reveals that stump wood harvesting remains an underestimated and underutilised potential method within the forestry and wood-based sectors.

(3)

Research on the utilisation of forest by-products, particularly those derived from wood and indicated in this pilot study, should be further continued and expanded.

(4)

Although stump wood is a challenging material to process and requires adequate preparation before end use, it constitutes a substantial source of forest biomass sustainable for energy production. The identified technological difficulties can be overcome by implementing appropriate harvesting and processing technologies [98,99].

(5)

The main environmental barriers to stump wood harvesting include concerns over ecological damage and biodiversity loss caused by root extraction, as well as the importance of retaining dead wood in forest ecosystems. However, the application of principles of sustainable forest management should effectively mitigate these risks [100,101].

(6)

Potential concerns regarding stump wood accessibility and the high costs associated with harvesting the underground part of trees can be addressed through robust financial analysis. The proposed formula for calculating the optimal cost of stump wood harvesting may help resolve these challenges in practical business applications.

(7)

Finally, the typically local nature of stump wood energy resources implies that their utilisation should take place as close as possible to the site of production, in order to minimise logistical, economic, and environmental constraints.

Besides green energy, other potential applications for stump wood can be identified [102]. Even within the relatively small study area—the buffer zone of the Notecka Forest—there are wood-based panel production facilities where processed raw material from stump wood harvesting can be effectively recycled. Sectoral forecasts for this region also indicate other valuable directions for application.