Decomposition and nutrient dynamics of stumps and coarse roots of Eucalyptus plantations in southern China

Abstract

Key message

Primary remains of Eucalyptus (E. urophylla × E. grandis) plantations following clear-cutting are the stumps and coarse roots. A chrono-sequence approach revealed that the biomass of coarse roots is higher than that of stumps, while they have a smaller rate of biomass loss than stumps. This implies that coarse roots serve as long-term stock for nutrients (carbon, nitrogen, and phosphorus).

Context

A significant quantity of stumps and coarse roots persist in the forest floor following the clear-cutting of Eucalyptus plantations in China. The decomposition of these stumps and coarse roots is a crucial aspect of the nutrient stocks in plantation ecosystems.

Aims

We described the stock and decomposition dynamics of stumps and coarse roots on Eucalyptus plantations, as well as the nutrient loss rate associated with them.

Methods

We used a chrono-sequence of 0–6-year-old clear-cutting stands of Eucalyptus plantations in southern China. The biomass and nutrient stocks of these samples were assessed at different times of decomposition.

Results

The highest biomass of stump-root system was 33.8 Mg·ha⁻¹, with nutrient stocks were 16.3 Mg·ha⁻¹, and 63.0 kg·ha⁻¹, and 6710.9 g·ha⁻¹, for C, N, and P. The coarse roots accounted for 81–96% of the biomass of the stump-root system. The loss rate of biomass and nutrients from stumps was higher than that of coarse roots. This suggests that the decomposition of coarse roots could serve as a long-term source of nutrients, thereby improving the nutrient status of the plantations.

Conclusion

Stumps and coarse roots serve as significant nutrient stocks that decay at varying rates. The loss of nutrients must be considered while analyzing the decomposition dynamics following clear-cutting in Eucalyptus plantations.

1 Introduction

Coarse woody debris (CWD), comprising logs, branches, stumps, and coarse roots, is a long-term nutrient pool that serves as a vital structural and functional component of forest ecosystems (Harmon et al. 1986; Olajuyigbe et al. 2011). In forests managed for timber production, stumps and coarse roots are produced through clear-cutting or thinning (Rabinowitsch-Jokinen and Vanha-Majamaa 2010; Hautala et al. 2011) and are the primary harvesting residues (Eufrade et al. 2020). However, their decomposition has only been discussed by a few researchers (Didion and Abegg 2022); Palviainen and Finér (2015) found that stumps decompose faster than coarse roots; and Deng et al. (2022) studied the effect of stumps substrates on microbial activity. Furthermore, the measurement of coarse roots is often overlooked owing to the considerable effort required (Palviainen and Finér 2015).

In general, the decomposition of CWD is primarily influenced by the physicochemical properties of the substrate, climatic conditions, and soil biological activity (van Geffen et al. 2010; Bradford et al. 2014; Zhu et al. 2017). The decomposition rate constant of CWD is typically expressed as k, which describes the exponential biomass losses caused by leaching, respiration, and mineralization (Olajuyigbe et al. 2014). CWD is a crucial long-term nutrient stocks due to its slow decomposition rate (Laiho and Prescott 2004). Many studies have reported an increase in the available amounts of N and P during CWD decomposition (Garrett et al. 2012). Therefore, the decomposition of stumps and coarse roots, which are crucial components of the ecosystem in clear-cutting forests, are expected to improve the nutrient status of forest soils (Shorohova et al. 2008; Melin et al. 2009) and provide nutrients for tree growth in the next rotation.

Eucalyptus exhibits a fast growth rate and is highly adaptable to environmental conditions. As a result, China planted over 5.4 million hectares of industrial Eucalyptus, accounting for 7% of the total planted area (National Forestry and Grassland Administration 2019; Wang et al. 2022), making it one of the most abundant tree species in China. Eucalyptus is often planted in areas with poor soil, has a short growth cycle, usually 5 to 7years. Two or three rotations of management are generally performed before clear cutting and replanting. In mature stands of Eucalyptus, stumps and root systems account for 17–38% of tree biomass (Eufrade et al. 2020). In China, the traditional plantation management model retains these stumps and coarse roots. Given the scale of Eucalyptus plantations, it is important to clearly understand the decomposition dynamics of nutrients in stumps and coarse roots to be able to predict nutrient cycling in plantations.

This study aimed to (1) quantify the stocks of biomass and nutrients from Eucalyptus stumps and coarse roots along a chrono-sequence of 0–6-year-old clear-cutting stands and (2) estimate the loss rate of biomass and nutrients, as well as the nutrient dynamics of stumps and coarse roots, to elucidate (1) if the biomass of coarse roots is higher than that of stumps and (2) if there is faster decomposition and nutrient loss rate from stumps from than coarse roots.

2 Materials and methods

2.1 Site characterization

The experiment was conducted in commercially managed Eucalyptus plantations located in the GuangMing mountain forest farm (23°33′ N, 108°13′ E) in southern China, using a chrono-sequence of seven second rotation stands. The area has a southern subtropical monsoon climate with a mean annual temperature of 21.8 °C, a mean precipitation of 1720 mm, and a mean altitude of 400 m. Appendix Table 4 shows the characteristics of each site immediately after clear-cutting. In general, tree characteristics, such as height, diameter at breast height, and crown size, do not differ significantly across all sites before clear-cutting. To prevent competition for nutrients within the stand due to regrowth, the stumps were treated with glyphosate to ensure their death. The same management practices were applied in all replanted stands, and the same Eucalyptus seedlings were planted after clear-cutting. The soil type of all stands were mainly sandy loam, originating from sandstone.

2.2 Field surveys and sampling

We utilized a chronosequence approach to evaluate the decomposition dynamics of stumps and coarse roots. The stands in these seven sites varied in age (with newly planted stands ranging from 0 to 6 years), vegetation cover, and canopy density. In November 2021, four plots (20 m × 30 m) were randomly established at each site, apart by at least 50 m (28 plots in total). Within each plot, all stumps were classified into five decay classes based on the criteria outlined in Appendix Table 5 (Hunter and Schmiegelow 1990; Naesset 1999; Paletto and Tosi 2010). Due to the difficulty in assessing decomposition in deeply buried coarse roots, we assumed that all coarse roots attached to the stumps were at the same stage of decomposition as the stumps. The distribution of stump decay classes at each site is shown in Table 1. Samples were randomly sampled in each plot and the decay class of the samples was determined. All of the coarse roots attached to the stumps as well as the lateral roots were excavated using hand tools. The fresh weights of the stump-root system were measured with a 200-kg maximum load capacity portable dynamometer (Fig. 1). In particular, soil adhering to the root samples may lead to an overestimate of root biomass. Representative stumps and coarse roots were sampled using a hand saw or knife for subsequent indoor physicochemical analyses. These samples for indoor analyses were weighed in the field using an electronic balance (USA.HZ, PTX-FA210S, accuracy ± 0.01 g). For material directly attached to the stumps, 5 to 8 cm discs were cut from the stumps using a chainsaw, and their 30 cm of vertical coarse roots with a diameter of 5 cm at the small end were collected. For decay classes IV and V, a sterilized vinyl bag was used to collect samples. Overall, a total of 256 samples of stumps and coarse roots were transferred to the laboratory for further analysis.

Table 1 Characteristics of the experimental sites

Fig. 1

Excavation and weighing of Eucalyptus stumps and coarse roots in harvested sites

2.3 Stumps and coarse roots biomass calculation

In the laboratory, stumps and coarse roots samples were dried at 85 °C for 2 to 4 days. The volume of each sample was determined by the water displacement method. The density was calculated by dividing the dry mass by the volume of the sample.

The dry weights were calculated based on the moisture content. To obtain the stumps and coarse roots biomass yield (per hectare), the arithmetic mean of the weighted stumps and coarse roots biomass (dry matter) was multiplied by the number of stumps per hectare (Table 1).

2.4 Chemical analysis of samples

Six stumps and coarse roots samples from each decay class were used to determine the chemical composition of wood samples. For each decay class the measured sample is a mixture of at least three subsamples. The moss and soil particles were removed using a brush before grinding the samples. Residual soil dust on the surface of the samples was cleaned using deionized water before measuring the sample volume as described above to avoid contamination from the soil. The dried samples were ground to a powder using a ball mill and filtered through a 1 mm sieve. The samples were then stored in sealed polycarbonate plastic vials until analysis.

The N content using the Kjeldahl method (Uselman et al. 2012). The C and P content were determined using the dichromate oxide ferrous sulfate titration method and the Mo-Sb colorimetric method (Ashagrie et al. 2005), respectively. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) content were determined using an ANKOM A2000i automated fiber analyzer (ANKOM Technology, Macedon, NY, USA). The acid-washed lignin (ADL) content was determined according to the method described by Trofymow et al. (2002). Finally, the cellulose, hemicellulose, and lignin content of the samples was calculated using the following equations:

$$\mathrm{Hemicellulose }=\mathrm{ NDF}-\mathrm{ADF}$$

$$\mathrm{Cellulose }=\mathrm{ ADF}-\mathrm{ADL}$$

$$\mathrm{Lignin }=\mathrm{ ADL}-\mathrm{ash\;content}$$

2.5 The nutrient stocks estimation using the indirect method

The nutrient stocks of stumps and coarse roots during the decay period of 0 to 6 years was obtained by the calculation formula, as follows:

$$\mathrm N\mathrm u\mathrm t\mathrm r\mathrm i\mathrm e\mathrm n\mathrm t\;\mathrm s\mathrm t\mathrm o\mathrm c\mathrm k\mathrm s={\mathrm N}_{\mathrm t}\times{\mathrm B}_{\mathrm t}$$

where B_t (Mg·ha⁻¹) is the biomass in year t; N_t (g·kg⁻¹) is the nutrient content in year t.

2.6 Decay rates of biomass and nutrients

To determine the biomass and nutrient loss rate of stumps and coarse roots, we calculated k values using a single negative exponential decay function (Olson 1963), as follows:

$$\frac{{\text{DW}}_\text{t}}{{\text{DW}}_0}\text{=}\text{e}^{-\textit{k}\text{t}}$$

where t represents the decomposition time, DW₀ and DW_t represent the biomass or nutrient of stumps and coarse roots per year at the initial (0) and final (t) decomposition times, respectively, and k is the annual biomass or nutrient loss rate constant (year⁻¹).

Based on k values, the time required for the loss of 50% (t_0.5) and 95% (t_0.95) of the stumps and coarse roots was calculated as follows:

$${\mathrm t}_{0.5}=\frac{\text{-ln(0.5)}}{\textit{k}}\text{=}\frac{0.693}{\textit{k}}$$

$${\mathrm{t}}_{0.95}=\frac{\text{-ln(0.05)}}{\textit{k}}\text{=}\frac{2.996}{\textit{k}}$$

2.7 Statistical methods

All statistical tests were conducted with a significance level of 0.05 using the Statistical Program for the Social Sciences (SPSS Inc., Chicago, IL, USA). All results were reported as the mean ± standard error of four or six replicates. All data analyzed were normally distributed. The mean comparisons were conducted using Student’s t tests or analysis of variance (ANOVA). Statistically significant differences among different decay classes were analyzed using one-way ANOVA, followed by Duncan’s multiple range test. Differences in all variables between the coarse roots of the stumps were determined by Student’s t test. All figures were plotted using Origin 2021 (Origin Lab Corporation Northampton, MA, USA). The data of Tables 2, 3 and Figs. 2, 3 can be referred to Deng et al. (2023).

3 Results

3.1 Changes in biomass stocks

The biomass of both stumps and coarse roots decreased with increasing decomposition time. The coarse root biomass ranged from 9.3 to 30.7·Mg ha⁻¹ (Table 2), with the highest value observed after 1 year of clear-cutting, while the stump biomass decreased from 6.1 to 0.5 Mg·ha⁻¹. Similar, the stump-root system biomass ranged from 10.2 to 33.8 Mg·ha⁻¹. Across all sites, coarse root biomass accounted for 81–96% of stump-root system biomass.

Table 2 Changes in biomass and nutrient stocks of Eucalyptus stumps and coarse roots during 0–6 years of decomposition time

3.2 Nutrient dynamics

C content of both stumps and coarse roots increased with decay class and then decreased (Fig. 2). The stump C content of decay class I was significantly higher than that of coarse roots (P < 0.05).

Fig. 2

Contents of C, N, and P of stumps and coarse roots in different decay classes (mean ± S.E., n = 6). Different lowercase letters indicate that means are significantly different (P < 0.05) among different decay classes within stumps or coarse roots, whereas different uppercase letters indicate that means are significantly different (P < 0.05) between stumps and coarse roots

N content of stumps in decay class II was significantly higher than the other decay classes. In contrast, the N content of the coarse roots decreased with increasing decay class and reached a minimum in decay class III. Meanwhile, the N content of decay class I and II of coarse roots were significantly higher than those of stumps, while decay class III of coarse roots had a significantly lower N content than that of stumps.

P content of both stumps and coarse roots in decay class I were significantly higher than the other decay classes (Fig. 2). In addition, the P content of stumps of decay class I was significantly lower than that of coarse roots.

3.3 Changes in nutrient stocks

The highest total C, N, and P stocks were found in the stumps at the year of the clear-cutting and then declined to 0.2 Mg·ha⁻¹, 0.7 kg·ha⁻¹, and 36.0 g·ha⁻¹, respectively, as decomposition time increased (Table 2). Coarse roots reached the maximum total C, N, and P stocks at the 1 year after clear-cutting, while they reached a minimum after the 5th year of decomposition. The biomass and nutrient stocks of the coarse roots were significantly higher than that of the stumps in all years investigated.

3.4 Biomass and nutrient loss rate

The single exponential decay model was used to explain the rate of biomass and nutrient loss rate for each of the stumps and coarse roots. The biomass loss rate for stumps and coarse roots were 0.36 and 0.19 year⁻¹, the R² of the exponential model were 0.44 and 0.62, respectively, and the turnover time was 8 and 16 years, respectively (Appendix Table 6, Fig. 3). Among all nutrient element fitting models, the coarse roots were better fitted than the stumps; however, the stumps nutrient loss rate more rapidly (Fig. 3).

Fig. 3

A single exponential decay model fit for biomass and nutrient stocks (n = 28). The colored areas are the 95% confidence intervals of the model. Triangles indicate coarse roots, and circles indicate stumps. For a given year, different numbers of points indicate the remaining biomass or nutrient stocks in the survey sample plots

3.5 Decomposition processes of stumps and coarse roots

Stump density decreased and moisture content increased with increasing decay class, while coarse root density increased and moisture content decreased with increasing decay class (Table 3). The cellulose content of the stumps decreased from 49.79 to 37.45%, and the hemicellulose content decreased from 17.30 to 12.74%, while lignin increased from 19.54 to 29.80%. Cellulose was significantly higher in the stumps than that in coarse roots in decay class I, whereas it was significantly lower than that in coarse roots in decay classes IV and V. Stump hemicellulose was significantly higher than coarse roots in decay classes I and II. However, there were no significant differences in the cellulose and hemicellulose contents of coarse roots of different decay classes. The lignin content in the coarse roots remained relatively consistent, with only decay class IV was significantly lower than decay class III. In addition, lignin was significantly higher in the stumps of decay classes IV and V than that in coarse roots.

Table 3 Variation characteristics of density, moisture, cellulose, lignin, and hemicellulose of the stumps and coarse roots of Eucalyptus from the different decay classes (mean ± S.E., n = 6)

4 Discussion

4.1 Biomass stocks

The biomass of stumps and coarse roots in our study was significantly reduced over the 6-year decomposition period (Table 2), which was similar to the studies conducted by Palviainen and Finér (2015) and Grelle et al. (2012). In addition, the coarse roots accounted for 81–96% of the stump-root system stocks in all the surveyed stands and were the largest stock component of the stump-root system. Our findings regarding coarse root stocks are consistent with studies from other regions. For instance, Merila et al. (2014) estimated that coarse roots accounted for 83% of the stump-root system, with stumps accounting for the remaining 17%. In particular, this study found that coarse root stocks was higher after 1 year of decomposition than the year of clear-cutting. This may be due to the fact that the root system remained alive after cutting, as similarly reported by Garrett et al. (2008) in Pinus radiata roots. However, the stocks of coarse roots after 6 years of decomposition in this study was higher than that at 5 years of decomposition, which may be due to more stumps in the decomposed 6-year-old stand (Table 1). This difference in the number of stumps formed may be due to the natural death of the trees, the differences in the number of branch sprouting in the second-generation budding stands, or the uncertainty of the chrono-sequence approach.

After 6 years of decomposition, the percentage of stump stocks decreased from 19 to 4%, and these results are lower than those reported by Varik et al. (2013), but similar to those observed by Kaarakka et al. (2018) in managed Picea abies forests. Moreover, the stump stocks were higher than those found in commercially managed Sitka spruce forests in northeastern Wicklow Ireland (Kaarakka et al. 2018). Overall, our results highlight the importance of stumps and coarse roots as nutrient reserves in Chinese Eucalyptus plantations.

4.2 Nutrient stocks

Stumps and coarse roots, being the main harvesting residues of plantation forests, are also the main source of nutrients in the forest soil (Hautala et al. 2011). In our study, the stump C stocks are similar to those of Nanjenshan Reserve (3.3 to 4.7 Mg·ha⁻¹) in the tropics (Chao et al. 2017) and lower than commercial Sitka spruce stands (8.0 to 25.0 Mg·ha⁻¹) in Ireland (Tobin et al. 2021). The regional climate and forest type differences between studies may account for the differences observed in C stocks. In addition, Paletto et al. (2014) emphasized that the stocks of C in dead wood were strongly influenced by forest management. At the same time, the C stocks of the coarse roots was higher than that of the stumps. This was expected because coarse roots generally have a higher biomass than stumps (Palviainen and Finér 2015). Nutrient stocks in Eucalyptus differed across structures (Shammas et al. 2003). Our results showed that the coarse roots contained more N and P compared to the stumps (Fig. 2). At the beginning of tree growth, nutrient storage in leaves and branches is higher (Rocha et al. 2019), while the coarse root portion of Eucalyptus provides higher biomass and nutrient storage as the tree grows (Guimaraes et al. 2015).

4.3 Biomass and nutrient loss rate

The decomposition of wood in forest ecosystems is influenced by various factors, including temperature, moisture content of the wood, physicochemical properties of the substrate, and the abundance of decomposing fungi and animals (Beets et al. 2008). The rate of biomass loss from stumps observed in this study was higher than the k value (0.25 year⁻¹) reported by Shammas et al. (2003) for the rapid phase of decomposition of Eucalyptus globulus branches. The rate of biomass loss of coarse roots observed in our study is comparable to those reported by Garrett et al. (2012) for decomposing roots of Pinus radiata in golden down soil types in New Zealand (0.174 year⁻¹). The observed differences in the k values for stumps and coarse roots support the hypothesis that stumps tend to decompose more rapidly compared to coarse roots (Appendix Table 6). However, previous studies comparing the decay rates of stems and roots of the same tree species showed that the decay rate of dead roots was similar to or faster than that of stems (Garrett et al. 2008, 2019). This could be explained by temperature differences as indicated by previous studies claiming that mean annual temperature is the main driver of decomposition (Shorohova et al. 2012). The higher water content of stumps may also contribute to their faster decomposition (Table 3). Some studies have shown that the water content favors the decomposition of microbial activity and accelerates decomposition processes (Jomura et al. 2015). The coarse root stocks increased after 1 year of clear-cutting, while the stump stocks decreased by 50% (Table 2). This indicates that the coarse roots remained alive after cutting and were not immediately decomposed, which also resulted in a higher water content of the coarse roots in decay classes I and II. In addition, due to the fact that we are using a chrono-sequence approach, it may introduce uncertainty in the estimation of k values.

Nutrient loss rate from stumps and coarse roots are often overlooked, although they are probably the most important CWD in managed forests (Harmon et al. 2011). Our findings are consistent with Martinez-Garcia et al. (2015), which reported an annual C loss that ranged from 0.1 to 1.3 Mg·ha⁻¹ year⁻¹, but was less than that reported by Palviainen et al. (2004). Meanwhile, the loss of C from stumps and coarse roots decreased with increasing decomposition times (Table 2). Those are consistent with the findings of Tobin et al. (2021), who found higher C losses in the youngest site plots (4 years of decomposition) than in the oldest site (16 years of decomposition). The N loss from the stumps in this study was similar to the results of Palviainen and Finer (2015) on Norway spruce coarse roots and stumps (0.9 to 1.3 kg·ha⁻¹). However, the loss of N from stumps and coarse roots in our study were faster than that in the study by Palviainen et al. (2010) on Scots pine stumps in Norway. This difference in nutrient dynamics may be attributed to differences in substrate chemistry, decomposition biology, and microenvironmental conditions.

4.4 Decomposition dynamics

The C content may increase slightly (Preston et al. 2012; Lombardi et al. 2013; Romashkin et al. 2018), decrease (Wu et al. 2005; Palviainen and Finér 2015), or even remain constant (Palviainen et al. 2010) during CWD decomposition. Our results are consistent with Garrett et al. (2008), who found that the C concentration in stumps increased during decomposition. Similar, our results are consistent with those of Palviainen et al. (2010), who found that the N content during Scots pine stump decomposition first increased and then decreased. Previous studies have suggested that increased N during CWD decomposition may be due to atmospheric N deposition and other organisms associated with CWD, particularly epiphytes and nitrogen-fixing bacteria (Shorohova et al. 2016; Hu et al. 2018). In this study, the N content of stumps was significantly lower than that of coarse roots during the early stages of decomposition (Fig. 2). And the higher N content also indicated that the coarse roots had more viable cells than the stumps (Apostolov 2006).

5 Conclusions

This study represents the first attempt to quantify the loss rate of biomass and nutrients from stumps and coarse roots in Chinese Eucalyptus plantations during the same decomposition periods. Single exponential decay model confirmed the hypothesis that stumps have a faster loss rate of biomass and nutrient loss rate compared to coarse roots. Additionally, the biomass of coarse roots was significantly higher than that of stumps. Based on the nutrient loss rate estimated by the single exponential decay model, the decomposition of the stumps and coarse roots may meet some of the nutrient requirements in the subsequent rotations. Our results suggest that when conducting studies on the stump-root system of Eucalyptus related to decomposition dynamics, it is necessary to separate the stumps from the coarse roots.

Appendix

Table 4 Description of each Eucalyptus sites before clear-cutting

Table 5 Description of stumps decay classes

Table 6 Single exponential decay model between remaining biomass or remaining nutrient and time for stumps and coarse roots of Eucalyptus