Carbon and Nitrogen Content and CO₂ Efflux from Coarse Woody Debris of Norway Spruce, Black Alder, and Silver Birch

Abstract

Coarse woody debris (CWD) is an essential component in forest ecosystems, playing a significant role in enhancing biodiversity, soil formation, and nutrient cycling through decomposition processes. CWD also contributes to greenhouse gas fluxes, particularly through CO₂ emissions. This study investigated the physical and chemical properties of CWD and the CO₂ effluxes from CWD of different decay classes. For this study, a range of CWD—from recently dead to highly decomposed wood—of native tree species such as silver birch (Betula pendula Roth), black alder (Alnus glutinosa (L.) Gaertn.), and Norway spruce (Picea abies (L.) H. Karst.) in hemiboreal forests were investigated. The findings showed that CWD properties significantly differed among tree species and CWD decay classes. Significant variations in wood density and total nitrogen (N) were observed in the early stages of CWD decay, with the highest values found for the deciduous tree species. The concentration of organic carbon (C) increased throughout the decomposition. The lowest CO₂ efflux from CWD was found for spruce CWD from all decay classes and it was the highest for black alder and silver birch, especially for the 3rd and 4th decay classes. CO₂ efflux was mainly influenced by the degree of decomposition, which was represented by the CWD decay class, followed by wood density and C content.

1. Introduction

In forest ecosystems, coarse woody debris (CWD) refers to deadwood components, including tree stems and branches with diameters exceeding 10 cm, lying on the ground [1]. CWD is a recognized component of forest ecosystems, contributing to biodiversity and nutrient cycling through deadwood decomposition and affecting overall soil functioning [2,3]. Deadwood decomposition affects carbon (C) storage and nutrient cycling and simultaneously acts as a source of CO₂ emissions [2,4].

Climate change has increased the scale, frequency, and intensity of natural disturbances such as storms, insect outbreaks, droughts, and forest fires across Europe, followed by higher tree mortality and deadwood production [5,6]. In European Union (EU) forests, deadwood quantities ranging from 5.6 to 33.1 m³ ha⁻¹ with an average value of 15.8 m³ ha⁻¹ are reported [7]. The highest volumes of deadwood retained in forests are in Central Europe, particularly in Slovenia (over 30 m³ ha⁻¹), Germany (29.6 m³ ha⁻¹), the Slovak Republic (27.3 m³ ha⁻¹), Latvia (26.4 m³ ha⁻¹), Austria (23.7 m³ ha⁻¹), and France (22.3 m³ ha⁻¹). In Lithuania, deadwood in forests amounts to approximately 14.2 m³ ha⁻¹ [8]. According to the International Panel of Climate Change (IPCC) Guidelines for National Greenhouse Gas (GHG) Inventories, countries are required to quantify C pools and GHG fluxes in forests, including those associated with deadwood and its decomposition [9]. Like other EU members, Lithuania is obligated by the Kyoto Protocol to report annual GHG emissions from the Land Use, Land Use Change, and Forestry (LULUCF) sector.

Recent studies reported that CWD decomposition is strongly influenced by climate, geographical location, and altitude, as all these factors jointly regulate the activity of microbial and invertebrate wood decomposers [10,11]. Numerous studies have estimated regional decomposition rates of CWD in European boreal forests [12,13,14,15,16,17]. However, knowledge about releasing GHGs, primarily CO₂, from the decay of CWD remains very limited [18]. Long-term experiments on the entire life span of CWD decomposition are crucial for evaluating CO₂ emissions from recently to highly decomposed deadwood [19].

This study aimed to evaluate the physical and chemical properties of CWD such as wood density, moisture content, concentrations of organic carbon (C) and total nitrogen (TN), and C/N ratio of three native tree species—silver birch (Betula pendula Roth), black alder (Alnus glutinosa (L.) Gaertn.), and Norway spruce (Picea abies (L.) H. Karst.) growing on Histosol soil. More specifically, due to the significance of this indicator, the CO₂ efflux from CWD of all three species and across the full range of decay classes was experimentally evaluated. The naturally reforested Norway spruce stand on drained, nutrient-rich organic soil was selected as a base for the CO₂ efflux experiment. This forest type was chosen because Histosols are among the most vulnerable biomes affected by changes in environmental conditions. In Lithuania, Histosols cover about 8%–10% of the terrestrial area and almost 80% are nutrient-rich, mostly drained, organic soils [20,21]. The present study is part of broader research focused on managing nutrient-rich organic soils to mitigate climate change impacts in the region [22].

2. Materials and Methods

2.1. Study Design

This study was conducted in Lithuania, representing the southern part of the hemiboreal forests, where forests cover 33.9% of the country’s territory [8]. Among other species, coniferous stands prevail in Lithuania, covering about 56% of the forest area. The climate transitions between the mild Western European and continental Eastern European climates. Between 1991 and 2020, the average air temperature was 7.4 °C and the mean annual precipitation was 695 mm [23]. The July–August period in 2024 was 1.3–1.4 °C warmer than the 1991–2020 average. July was wetter than average, with 67 mm more precipitation, while August was drier, with 52 mm less precipitation.

This study was performed at the Dubrava Regional Division of State Forest Enterprise in south-eastern Lithuania (54°47′37.5″ N 24°04′50.6″ E). This study was designed to evaluate physical (wood density and moisture content) and chemical (C and TN concentrations) properties of CWD from silver birch, black alder, and Norway spruce grown on Histosols (objective 1) and the CO₂ efflux from CWD representing five decay classes of silver birch, black alder, and Norway spruce on an experimental site established in self-regenerated pure Norway spruce stands on Histosols (objective 2). For objectives 1 and 2, three native tree species were chosen as typical for such forest site types in Lithuania.

For objective 2, an experimental site was established in a 68-year-old pure Norway spruce stand (95% Norway spruce with an admixture of 5% Scots pine) on drained, nutrient-rich organic soil, classified as Sapric Histosol [24]. As known according to land-use history, this forest was drained using a drainage ditch with a depth of 1.6 m more than 50 years ago. During the experiment, the mean groundwater level was about 0.7 m. During the experiment, the Norway spruce stand was described by an average tree height of 19 m, a diameter at breast height (DBH) of 20 cm, a tree density of 860 trees per hectare, and a total stand volume of 259 m³ ha⁻¹. The ground vegetation in the stand was dominated by the red-stemmed feathermoss (Pleurozium schreberi (Willd. ex Brid.) Mitt.) and dwarf shrub blueberry (Vaccinium myrtillus L.).

2.2. Sampling of Experimental Material

For objectives 1 and 2, CWD was sampled from five decay classes to represent silver birch, black alder, and Norway spruce deadwood. Following Bastrup-Birk (2007), CWD was defined as deadwood from stems and branches lying on the ground with a mean diameter of 12 cm [1]. CWD sampling followed the five decay classes defined by Hunter (1990): 1st class (recently dead), 2nd class (slightly decayed), 3rd class (moderately decayed, bark absent), 4th class (heavily decayed), and 5th class (fully decomposed, soft wood) [25]. Experimental material prepared from the deadwood of various decay classes was collected from different fallen stems/thicker branches in the nearby forests at a distance not further than 300 m from the experimental site. The growth conditions in the nearby forests were closely comparable by edaphic and climatic conditions to those in the selected experimental site. The CWD samples, averaging 0.25 m in length and 11 cm in diameter, were cut from tree stems, branches, or their fragments lying on the ground, representing each of the five decay classes for all three species.

The CWD samples were analysed as follows: (i) Physical properties (objective 1): All collected samples were analysed for deadwood density and moisture content; (ii) Chemical properties (objective 1): Samples were grouped by decay class and tree species, resulting in 45 samples (5 decay classes × 3 tree species × 3 replicates) and analysed for C and TN concentrations; and (iii) CO₂ concentrations (objective 2): A total of 75 deadwood samples (5 decay classes × 3 tree species × 5 replicates) were used to measure CO₂ emissions from the CWD.

2.3. CO₂ Measurements and Sample Analysis

To evaluate the physical properties of CWD, the wood samples were oven-dried at 105 °C until constant mass and weighed. Each CWD sample’s wood density (kg m⁻³) was calculated by dividing the dry mass by the sample volume [4]. The volume of all CWD samples was determined using the water displacement method. The 4th and 5th decay class samples were packed in a 6-micron plastic bag before being submerged in the water [26].

The moisture content of each CWD sample was determined immediately before making CO₂ efflux measurements using a portable wood moisture meter (FHM 20, GEO-FENNEL, Baunatal, Germany).

The concentrations of C and TN were determined by dry combustion at 900 °C using an ECS 4010CNS analyser, following LST EN ISO 16948:2015 for C and LST EN 15936:2022 for TN. Chemical properties were analysed once at the beginning of September 2024.

The released CO₂ concentrations (ppm) from CWD samples of studied decay classes and tree species were measured in situ on an experimental site (see Section 2.1) in July–August 2024. The EGM-5 analyser (PP Systems, Amesbury, MA, USA), designed for chamber-based CO₂ measurements, was used to measure CO₂ concentrations from CWD samples. The chamber, with a height of 40 cm and a diameter of 50 cm, was made from transparent plastic and air inside was circulated using a slowly spinning fan to ensure proper CO₂ mixing. Preparation for the CO₂ measurements involved installing five collars in a row in the soil, with about 2 m between them, to measure five CWD samples simultaneously (Figure 1). Before installing the collars, three layers of 200-micron film (2 × 2 m) were laid down, and then the collar was placed and pressed into the soil. Water was added to each collar to prevent the entry of external atmospheric gases and to avoid leakage or additional gas input from the environment—soil or vegetation respiration. For each measurement (each CWD sample), the prepared CWD sample was placed on the film for 10 min and then placed in the same position on the soil. Each fully decomposed CWD (5 decay class) sample was covered with a coarse nylon mesh net (mesh size 30 mm) to prevent material loss. After this, the chamber was placed over the sample and left for 2 min before the start of measurements. Each CO₂ measurement was taken for 180 s. For each of the three tree species and each of the five decay classes, we measured five CWD samples (replicates), processing them sequentially. CO₂ measurements were repeated in three-week intervals (three periods), starting on 1 July 2024 and continuing until the end of August 2024. Measurements were taken between 12 p.m. and 4 p.m. on days without precipitation.

CO₂ efflux was calculated from the linear (R² ≥ 0.95) change of CO₂ concentration (ppm) in time as a function of chamber volume, air temperature, and air pressure during each measurement period according to the equation described in the PP systems methodology [27]. The volume of the chamber was calculated, excluding the volume of the CWD sample. Measured CO₂ efflux was expressed per unit of dry wood mass for one hour (g CO₂ kg DM⁻¹ h⁻¹). Alongside the CO₂ efflux measurements, the air temperature was simultaneously measured using a thermometer (Comet system, Rožnov pod Radhoštěm, Czech Republic). During the CO₂ efflux measurement period, the mean air temperature was 21 ± 2 °C, and the average precipitation in July and August was 92 ± 7 mm. Finally, the average CO₂ efflux was calculated for all three measurement periods, resulting in average values for each tree species and decay class.

2.4. Statistical Analysis

The data’s normality was checked using the Shapiro–Wilk test, confirming that the data followed a normal distribution. Differences in the wood density, moisture content, C, TN, C/N ratio, and CO₂ efflux of CWD among tree species and decay classes were analysed using univariate analysis of variance (ANOVA) with Tukey’s HSD test at a significance level of α = 0.05. The influence of different decay classes on the physical and chemical properties of the studied CWD for each tree species was determined using linear regression analysis. The R² determination coefficient was used to indicate the strength of the relationship between the decay class and the respective properties.

The relationships between CO₂ efflux from the studied CWD were assessed using linear multiple regression. Models were validated for each tree species and variables such as decay classes, as well as the physical and chemical parameters of CWD, were tested to estimate their effect on CO₂ efflux. All statistical analyses were conducted using IBM SPSS 22.0 (IBM, New York, NY, USA).

3. Results and Discussion

3.1. Physical and Chemical Properties

The physical and chemical properties of CWD varied across the studied tree species and decay classes (Figure 2). In general, wood density gradually decreased with the increasing decay class. The highest wood density was found in the 1st decay class and varied from 457 kg m⁻³ (for Norway spruce) to 572 kg m⁻³ (for silver birch). In contrast, the lowest wood density was obtained in the 5th decay class (Figure 2A). The wood density in the 5th decay class, ranging from 165 to 174 kg m⁻³, was similar for all studied tree species. In many cases, the moisture content of CWD samples tended to increase with the advancing decay class (Figure 2B). The lowest moisture content (on average 8%) in all three species was observed for the 1st decay class, while the highest moisture content (except for birch) was recorded for the 5th decay class, varying in a wide range from 12% for silver birch to 62% for black alder CWD. The C concentration slightly increased from 48% in the 1st decay class to 51% in the 5th decay class (Figure 2C). The TN concentration increased with the advancing decay classes of CWD, with the lowest mean value observed in the 1st decay class (ranging from 0.07% in Norway spruce CWD to 0.21% in silver birch CWD). In contrast, the highest TN concentration was found in the 5th decay class, ranging from 0.24% for Norway spruce to 0.3% for black alder (Figure 2D). The highest TN concentrations for the 2nd–4th decay classes were observed in the black alder CWD.

The C/N ratio varied among decay classes and tree species (Figure 2E). In many cases, the highest C/N ratio was observed in spruce CWD, while the lowest was in black alder CWD. Specifically, the mean C/N ratio in the 1st decay class of spruce CWD was C/N 700 ± 200, which decreased with the advancing decay class, reaching C/N 211.18 ± 8 in the 5th decay class. A decreasing trend in the C/N ratio was observed with advancing decay classes of black alder CWD. The highest, C/N 254 ± 4, was determined in the 1st decay class, while the lowest (C/N 169 ± 11) was found in the 5th decay class. Meanwhile, the mean C/N ratio in birch CWD decreased in the following order: in 2nd and 3rd decay classes (C/N 454 ± 56) > in 1st and 4th decay classes (C/N 250 ± 36) > in 5th decay class (C/N 177 ± 23) (Figure 2E). A previous study by Piaszczyk et al. (2019) showed that the least decomposed deadwood had a C/N ratio >200, which decreased to C/N 53-93 due to decomposition and increased N content. This study observed a similar trend for Norway spruce CWD, while the deciduous species responded differently [28].

This study found that decay classes, in many cases, influenced the studied physical (wood density and moisture content) and chemical (C, TN, and C/N) parameters of CWD (Figure 1). Across all species, decay classes explained over 90% of variations in wood density, OC concentration (in silver birch and black alder) and TN concentration only in black alder’s CWD. Decay classes explained from 60% to more than 80% of variations in wood moisture and C/N ratio (in Norway spruce and black alder CWD). The trends obtained in our study well reflected those found by Stakėnas et al. (2016), whose study was focused on C stocks in dead wood of native tree species in hemiboreal forests [29]. That study found that in the early stages of dead wood decay, variations in wood density were mainly influenced by tree species, with higher densities observed in hardwood species such as oak (Quercus robur L.), ash (Fraxinus excelsior L.), and birch species (Betula spp.). In contrast, the lowest CWD densities were recorded in recently deceased CWD of Norway spruce and Scots pine (Pinus sylvestris L.) as well as grey alder (Alnus incana (L.) Moench). Furthermore, at the final stage of CWD decay, differences in wood density among tree species became negligible. That study also found that wood density decreases while C content increases as wood decay progresses [29].

Similar results were also presented in a study by Khanina et al. (2023) in a mesic broad-leaved forest on the East European Plain [4]. That study found that the CWD density decreased, the N concentration increased, and the C concentration remained stable with the increasing stages of CWD decomposition. Other recent studies have found that the increase of N with advancing decay classes of CWD may be influenced by fungal translocation and bacterial N fixation processes [30,31]. Comparable trends in N dynamics were observed in decaying CWD of both coniferous and deciduous tree species [32,33,34]. As the N concentration in decomposing CWD increases, the C/N ratio decreases with advancing decay classes [35]. In addition, a lower C/N ratio indicated an accelerated decomposition rate and was associated with increased CO₂ efflux [18].

3.2. CO₂ Efflux from Coarse Woody Debris

The lowest mean CO₂ efflux in all decay classes was determined in Norway spruce CWD (Figure 3). In more detail, the lowest CO₂ efflux released from Norway spruce CWD was observed in the 1st and the 2nd decay classes (17–18 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹), intermediate mean values of CO₂ efflux were determined in the 3rd and the 4th decay classes (28–38 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹), and the highest efflux value was in the 5th decay class (66.4 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹).

In contrast to the other studied tree species, black alder CWD released the highest CO₂ efflux, with values of 136 and 162 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹ found in the 3rd and 4th decay classes, respectively (Figure 3). CO₂ efflux was, on average, 7.6–9.0 times lower in the 1st decay class of black alder CWD, 2.8–3.4 times lower in the 2nd decay class, and 39–65% lower in the 5th decay class than in the 3rd and 4th decay classes. Meanwhile, CO₂ efflux in birch CWD ranged from 57 to 102 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹ in the 1st–3rd and 5th decay classes. However, the maximum CO₂ efflux of 123 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹ was recorded in the 4th decay class of birch CWD.

Studies conducted in temperate and boreal forests have reported results that align with our study. As an example, comparable results were reported by Bond-Lamberty et al. (2002), who measured CO₂ efflux from three decay classes of black spruce (Picea mariana (Mill.) BSP) CWD [36]. Their study revealed that CO₂ efflux increased with advancing decay classes. The lowest CO₂ efflux was observed in the 1st decay class (0.16 mmol CO₂ kg⁻¹ s⁻¹ or 25.34 mg CO₂ kg⁻¹ h⁻¹), an intermediate value was found in the 2nd decay class (0.37 mmol CO₂ kg⁻¹ s⁻¹ or 58.5 mg CO₂ kg⁻¹ h⁻¹), and the highest CO₂ efflux was in the 3rd decay class (0.44 mmol CO₂ kg⁻¹ s⁻¹ or 69.7 mg CO₂ kg⁻¹ h⁻¹) [36]. Herrman and Bauhus (2013) measured CO₂ efflux in the 2nd decay stage of CWD from European tree species such as common beech (Fagus sylvatica L.), Scots pine, and Norway spruce [37]. Their findings indicated the lowest CO₂ efflux in Scots pine (0.119 g CO₂ kg⁻¹ DM⁻¹ d⁻¹ or 10 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹) and in Norway spruce (0.122 g CO₂ kg⁻¹ DM⁻¹ d⁻¹ or 10.2 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹), and significantly the highest value was recorded in common beech (0.351 g CO₂ kg⁻¹ DM⁻¹ d⁻¹ or 29.3 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹). Furthermore, Mukhortova et al. (2021) measured CO₂ efflux from CWD of birch (Betula tortuosa Ledeb.) and Dahurian larch (Larix gmelinii (Rupr.) Rupr.) across the three decay stages under varying temperature regimes (5 °C, 15 °C, and 25 °C) [18]. Their study reported that CO₂ efflux in the 1st and the 2nd decay stages more strongly depended on temperature than on tree species. However, at 25 °C, in the 3rd decay stage, birch CWD produced a significantly higher CO₂ emission rate (65–140 µg CO₂ g⁻¹ h⁻¹ or 65–140 mg CO₂ kg⁻¹ h⁻¹) compared to Dahurian larch (0–85 µg CO₂ g⁻¹ h⁻¹ or 0–85 mg CO₂ kg⁻¹ h⁻¹).

3.3. Relations Among CWD Properties and CO₂ Efflux

The variation in CO₂ efflux from CWD could be influenced by its physical and chemical properties. In this study, multiple linear regression models were performed separately for each studied tree species to identify significant relationships between CO₂ efflux and the characteristics of CWD across decay classes (ranging from recently dead to highly decomposed CWD), as well as the physical (wood density and moisture) and chemical (C, TN, and C/N ratio) properties of the CWD (

Table 1. Multiple linear regression showing relationships between CO₂ efflux and main characteristics of CWD for Norway spruce, silver birch, and black alder.

Predictor	Betau	SE	Beta *	t	p	Betau	SE	Beta *	t	p	Betau	SE	Beta *	t	p
Norway Spruce						Silver Birch					Black Alder
Constant	−47,922.2	8438.7	—	−5.7	<0.001	−156,215.8	38,737.2	0.0	−4.0	0.00	−162,359.6	37,177.1	0.0	−4.4	0.00
Decay class	4225.9	889.3	2.6	4.8	0.001	9318.2	3389.9	0.9	2.7	0.00	14,037.1	3392.0	1.3	4.1	0.00
Moisture	187.5	96.4	0.5	1.9	0.078	712.1	296.5	0.3	2.4	0.00	−516.3	152.8	−0.5	−3.4	0.00
Density	52.2	10.1	2.7	5.2	<0.001	131.3	35.2	1.2	3.7	0.00	129.1	33.5	1.1	3.9	0.00
TN	−21,555.4	16,292.0	−0.6	−1.3	0.20	73,975.7	63,542.2	0.4	1.2	0.30	103,077.9	61,836.9	0.5	1.7	0.10
C	564.7	128.6	0.9	4.4	0.001	1813.0	770.8	0.3	2.4	0.00	1830.4	730.3	0.3	2.5	0.00
C/N	1.4	4.5	0.098	0.30	0.80	−8.7	25.9	−0.1	−0.3	0.70	10.7	25.7	0.1	0.4	0.70
Model summary	R	R2	R2adj	RMSE		R	R2	R2adj	RMSE		R	R2	R2adj	RMSE
	0.9	0.8	0.8	6503.9		0.8	0.6	0.5	9392.1		0.8	0.6	0.5	8977.6
MLR Equation:	CO2 efflux = −47,922.2 + 4225.9 × Decay class + 187.5 × Moisture + 52.2 × Density − 21,555.4 × TN + 564.7 × OC + 1.4 × C/N					CO2 efflux = −156,215.758 + 9318.202 × Decay class + 712.1 × Moisture + 131.3 × Density + 73,975.746 × TN + 1813.0 × OC − 8.7 × C/N					CO2 efflux = −162,359.563 + 14,037.067 × Decay class − 516.347 × Moisture + 129.112 × Density + 103,077.870 × TN + 1830.364 × OC + 10.736 × C/N

Notes: Beta_u is unstandardized regression Beta coefficient; SE is standard error of Beta_u; Beta * is standardized Beta regression coefficient; t is t test; p is significance level; predictors: (Constant), decay class, moisture, density, TN, C, and C/N; R—correlation coefficient, R² is determination coefficient; R²_adj is determination coefficient adjusted by variables such as decay class, moisture, density, TN, C, and C/N, and RMSE is root mean squared error.

).

The multiple linear regression models validated for each tree species’ CWD, incorporating the physical and chemical factors, explained from 50% (in silver birch and black alder) to 80% (in Norway spruce) of the total variation in CO₂ efflux. The model indicated that CO₂ efflux was primarily influenced by decay class, wood density, and C content (Table 1). However, the effect of wood moisture on CO₂ efflux was inconsistent. While the moisture content of Norway spruce and silver birch CWD had a positive effect, the moisture of black alder CWD had a negative impact, likely due to excessive moisture in the advanced decay class, which may have hindered or reduced microbial activity. Additionally, chemical variables such as TN and the C/N ratio of CWD showed no significant effect on CO₂ efflux.

Previous studies by Herrmann and Bauhus (2013) found that CO₂ efflux from CWD primarily depends on tree species and physical properties such as wood density and moisture [37]. Additionally, CO₂ efflux from CWD can be influenced by chemical composition, including organic compounds such as lignin, cellulose, hemicellulose, oligomers, and secondary metabolites [18,38,39]. Wood with higher cellulose and hemicellulose content, such as birch, decomposes more rapidly due to microbial activity [40], whereas wood with higher lignin content, typical for coniferous species, decomposes slowly [18]. Several studies have also shown that in the early stages of CWD decay, wood has higher density, greater structural integrity, and lower moisture content, leading to lower CO₂ efflux [36,41]. In the final decay stage, CO₂ efflux decreases as high porosity and excessive moisture create overly wet conditions for decomposing microorganisms [41]. Similar to our findings, Herrmann and Bauhus (2013) reported that approximately 80% of the variation in CO₂ efflux from CWD could be explained by factors such as tree species, wood density, moisture content, temperature, and their interactive effects [37].

This study was conducted during the summer months, when CO₂ efflux is typically at its peak. It focused on the CWD of native tree species across different decay classes, covering the entire life span of wood decomposition. The results revealed that CO₂ efflux from CWD in all decay classes was the lowest in Norway spruce (30 ± 15 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹) and significantly higher in both silver birch (84 ± 47 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹) and black alder (92 ± 30 mg CO₂ kg⁻¹ DM⁻¹ h⁻¹) CWD. Furthermore, the previous study by Vigricas et al. (2024) found that the total respiration from Sapric Histosols during summer months was 500–900 mg CO₂ m⁻² h⁻¹ [22]. In our study, CO₂ efflux from studied CWD varied in large ranges, with the lowest values found in recently deceased CWD (162–740 mg CO₂ m⁻² h⁻¹) and the highest (880–1100 mg CO₂ m⁻² h⁻¹) in moderately and highly decayed CWD. We found that CO₂ efflux from slightly decomposed CWD was relatively low, whereas CO₂ efflux from moderately and highly decomposed CWD was comparable to that of Histosols. By comparing our study’s findings with those of Vigricas et al. (2024) [22], we calculated that CO₂ efflux contributed 32% of total soil respiration for slightly decomposed CWD and up to 22% higher than total soil respiration for moderately and highly decomposed CWD.

The present study was conducted in a forest growing on Histosols. Consequently, we assume that the obtained CO₂ efflux values from CWD on this soil type may be site-specific, potentially limiting the applicability of the findings to other soil types. Continued efforts are needed to monitor the decomposition process broadly, especially considering variability in soil properties and climatic conditions. Such studies would improve the evaluation of CO₂ emissions and nutrient fluxes resulting from CWD decay in forest ecosystems.

4. Conclusions

This study evaluated the physical (wood density and moisture content) and chemical (C and TN concentrations) properties of coarse woody debris (CWD) of silver birch, black alder, and Norway spruce on Histosols. Overall, this study highlighted the essential effects of tree species, decay class, and wood properties on CO₂ efflux from CWD. The results showed that CO₂ efflux varied notably between tree species, with Norway spruce having the lowest emissions and black alder the highest, particularly in the middle stages of decay. Moreover, wood density, moisture, and organic carbon (C) content were crucial in shaping CO₂ emissions, with decay class being a key determinant of efflux. Our results also indicated that although chemical factors like nitrogen (N) concentrations and the C/N ratio showed some patterns, their impact on CO₂ efflux was less significant than that of the physical wood properties. CWD is critical in maintaining nutrient balance within forest ecosystems, acting as a carbon sink in the early wood decay classes. However, in later stages of decay, it transforms into a source of CO₂ emissions comparable to those emitted from drained organic soils.

These findings provide a methodological basis for expanding this experiment to evaluate annual CO₂ emissions from different wood decay classes of various tree species while incorporating additional site and wood properties. The expanded results could be applied to National Inventory Reports (NIRs) and used to adjust forest management scenarios under the increasing frequency of natural disturbances.