1. Introduction
The 2023 IPCC (Intergovernmental Panel on Climate Change) report on global climate change shows that the global average temperature in 2023 was 1.45 °C higher than the pre-industrial average temperature [
1]. The global climate is continuously warming, and the CO
2 emissions from human activities are considered the main cause [
2]. As the main body of terrestrial ecosystems, forests have the function of storing carbon (C) and play an important role in mitigating climate warming. It is estimated that forest vegetation can absorb about 2.4 Gt of C per year [
3], reducing human annual C emissions by 21% [
4]. In forests, trees dominate and have important ecological significance, serving as important C sinks. However, as trees grow, mature, and age, the amount of C released will be equal to or even greater than the amount of C stored. At this point, trees will become C sources, which are C emissions outside of fossil fuels [
5]. Harvested wood products originating from forests have delayed the return of their fixed C to the atmosphere due to utilization [
6]. In addition, some of the C contained in wood products at the end of their lifespan can be permanently stored in landfill facilities [
7]. Therefore, wood has the potential to alleviate climate change in the long term.
Five
Rosaceae species, namely,
Sorbus alnifolia (Sieb.et Zucc.) K. Koch,
Pyrus ussuriensis Maxim.,
Malus baccata (L.) Borkh.,
Crataegus pinnatifida var. major N. E. Brown, and
Padus racemosa (Linn.) Gilib., are important economic fruit and timber species in China [
8,
9] and have underlying ecological adaptations to effectively address varying environmental conditions and thus are important greening tree species for addressing climate change [
10,
11]. However, the C sequestration potential of these five
Rosaceae species is unknown. In order to understand the importance of these five
Rosaceae wood species in mitigating climate change, it is necessary to accurately quantify the C content of wood. Understanding wood C density (CD) is the basis for measuring and analyzing C storage in a regional forest ecosystem. Wood CD is usually obtained by an assumed C concentration (0.45 or 0.50 g/g) multiplied by a constant wood density (WD) [
12,
13]. However, the estimated CD has a deviation of about 10% relative to the actual observed values at tree level [
14,
15,
16]. C storage in a regional forest ecosystem is seriously overestimated or underestimated [
17]. Thus, the disentangling drives of C density of these five
Rosaceae species will be an extremely interesting and significant topic.
Many studies highlight the importance of measuring WD to estimate C storage [
17,
18], as it is not only a C investment trait but is also related to various wood properties needed in wood use [
19]. WD is variable among tree species [
20]. The main reason for the interspecific difference in WD is the high heritability. At more refined scales, people may pay more attention to intraspecific variation in WD than to interspecific variation because intraspecific variation is more likely to reflect phenotypic plasticity associated with environmental conditions. For example, Sousa et al. [
21] found that the WD of
Quercus faginea varied from 0.914 g/cm
3 to 1.037 g/cm
3 in two naturally regenerated stands with large environmental differences in Portugal. Even in the same region, WD varies between and within trees due to different site environments [
22]. Silvicultural management leads to rapid growth, producing trees with high yields and relatively low-density wood [
23]. Within a tree, WD usually increases from juvenile to mature wood in the radial direction and decreases from root to branch in the axial direction [
24,
25], but there are exceptions. Petrea et al. [
26] found that branches showed a higher WD than stems for European beech. A serious consequence of ignoring the high variability of WD is reduced accuracy in tree CD estimation.
Variations in WD are an indirect representation of changes in the structure and composition of wood materials [
18]. The main cell wall compounds from wood are cellulose, hemicellulose, and lignin. Lignin is a complex and heterogeneous mixture of polymers, mainly composed of p-hydroxyphenyl, guaiacyl, and syringyl units [
27]. In hardwood, lignin comprises both guaiacyl (C
10H
14O
2) and syringyl (C
11H
16O
3) in a roughly equal proportion [
28], containing a C concentration of 63.22%–72.29%. In the same dry weight of wood, the higher the proportion of lignin, the higher its C sequestration [
29,
30]. Cellulose is the base polymer in wood cell walls, consisting of ring units of D-glucopyranose (C
6H
12O
6) and containing a fixed C concentration of 44.44%. Despite cellulose being a strong, essentially irreversible C pool, the study of [
29] showed that there appears to be no significant correlation between the cellulose concentration (CC) and C concentration in plants. Material consisting of hemicellulose and cellulose is often referred to as ‘holocellulose’, meaning whole cellulosic material. However, the properties of hemicellulose and cellulose are very different. Hemicelluloses are low molecular-weight polysaccharides. In hardwood, hemicellulose mainly consists of D-xylose (C
5H
10O
5) and D-mannose (C
6H
12O
6) units and contains a C concentration of 40%–44.44%. Hemicelluloses could serve as a C pool and could also serve as potential mobile C reservoirs in addition to their structural function as starch and soluble sugar [
31,
32]. They might be better expressed as the uncertainty of the C concentration, thus influencing the uncertainty of CD. Evidence, however, showing correlations between the abundance of wood chemical compounds and CD in C storage in a regional forest ecosystem is limited.
In the present study, we aimed to disentangle CD drives of these five Rosaceae tree species. We examined the (1) WD; (2) CC, hemicellulose concentration (HC), and lignin concentration (LC); and (3) organ carbon concentration in parts of tree wood, including rootwood, stemwood, and branchwood. We calculated wood CD for all five species. We hypothesized that: (1) the CD among species and parts of tree is statistically different, and (2) the abundance of wood chemical compounds affects CD. This study is the first attempt to understand the relationship between CD and chemical components. To the best of our knowledge, no such effort has been made to date.
5. Conclusions
Forest C is an important climate indicator, with wood dominating the forest as a significant sink and source of C. To improve the prediction model of how future climate will affect global C cycling, it is necessary to reduce cognitive uncertainty in existing measurements of wood C storage and identify key factors related to tree growth and wood formation.
Variations in the wood properties, C concentration, and CD among five Rosaceae tree species and their tree parts were investigated in this study. Variations in WD and chemical properties affected the C concentration. CD values based on the SCC were lower than those based on OCC and IPCC default values. Such non-negligible deviations from C in the extract and non-structural carbohydrates are included. Based on SCC calculations, the highest CD was found in Sorbus alnifolia root (0.27 × 106 g/m3), while the lowest was found in Padus racemosa branch (0.22 × 106 g/m3). Considering structural carbohydrates and lignin can help with the accurate estimation of CD.
This is the first study to use structural carbohydrates and the LC to estimate the CD of tree species in temperate deciduous broadleaved forests in Northeastern China, providing important information on C flows and long-term C sequestration for this region. Due to the limitations of the selected region and tree numbers in this study, further research is needed to determine that the estimated patterns revealed are universal. In the future, there is a need for a better understanding of the natural growth process of forests in the region, the process of litter decomposition, and the use of wood products made from logging. Based on changes in C sequestration, reasonable logging strategies should be developed to achieve greater economic and ecological benefits.